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VYSOKÉ UČENÍ TECHNICKÉ V BRNĚ<br />
FAKULTA CHEMICKÁ<br />
PODKLADY PRO JMENOVACÍ ŘÍZENÍ<br />
v oboru Chemie, technologie a vlastnosti materiálů<br />
2012 DOC. ING. MARTIN WEITER, PH.D.
Obsah<br />
Tento dokument obsahuje Doklady předkládané uchazečem s návrhem na zahájení<br />
jmenovacího řízení profesorem. Jmenovací řízení bude zahájeno v oboru Chemie,<br />
technologie a vlastnosti materiálů na základě návrhu děkana Fakulty chemické VUT<br />
v Brně prof. Ing. Jaromíra Havlicy, DrSc.<br />
Všechny doklady byly zpracována na základě platné Směrnice rektora VUT v Brně<br />
1/2006 v úplném znění z 1.9.2008 (dále Směrnice). Níže předkládané materiály<br />
obsahují zejména životopis uchazeče, vlastní hodnocení uchazeče, které sestává z<br />
tabulky dosažených kvantifikovaných hodnotících kritérií podle odst. 2, Směrnice a se<br />
souhrnného vlastního vyjádření k bodovému hodnocení. Dále je přiložen seznam 35<br />
nejvýznamnějších publikovaných prací a kopie těchto publikací. Na závěr dokumentu<br />
jsou přiloženy další doklady dokládající kvalifikaci, způsobilost a kompetence žadatele.<br />
Životopis .................................................................................................................................................................3<br />
Autoevaluační kritéria......................................................................................................................................7<br />
Upřesnění jednotlivých položek autoevaluačních kritérií............................................................9<br />
Souhrnné vyjádření uchazeče k bodovému hodnocení..................................................................31<br />
Vědecká a odborná činnost .....................................................................................................................31<br />
Pedagogická činnost...................................................................................................................................33<br />
Přehled a kopie nejvýznamnějších publikací: .....................................................................................37<br />
Příloha č. 1 – přiložené publikace<br />
Příloha č. 2 – doklady<br />
1
Živootopis<br />
doc. Inng.<br />
Martiin<br />
Weiter r, Ph.D.<br />
OSOBNÍÍ<br />
ÚDAJE<br />
Datumm<br />
a místo naarození:<br />
Národnnost:<br />
Stav:<br />
Zaměsttnavatel:<br />
Kontakkt:<br />
VZDĚLÁÁNÍ<br />
1994<br />
2002<br />
2006<br />
ZAMĚSTTNÁNÍ<br />
1998 – dosud<br />
Ing. V<strong>Vysoké</strong><br />
<strong>učení</strong><br />
technnické<br />
v Brně, B Faku ulta elektrrotechnická<br />
á, obor<br />
MMikroelektr<br />
ronika<br />
doc. ddocent<br />
v oboru<br />
Fyzikáální<br />
chemie, , FCH VUT v Brně<br />
od 20006<br />
– 2012<br />
od 20110<br />
– dosud<br />
od – 20012<br />
dosud<br />
3.3.19971,<br />
Ostrav va<br />
českáá<br />
ženattý,<br />
2 děti<br />
Vysokké<br />
<strong>učení</strong> tec chnické v Brně B<br />
Antonnínská<br />
548/1,<br />
Brno 60 01 90<br />
weiteer@fch.vutb<br />
br.cz<br />
Ph.D. V<strong>Vysoké</strong><br />
uče ení techniccké<br />
v Brně ě, <strong>Fakulta</strong> <strong>chemická</strong>,<br />
innženýrství<br />
obor Materiálové<br />
asisten nt/odbornýý<br />
asistent/ docent, Úst tav fyzikálnní<br />
a spotřeb bní<br />
chemie,<br />
<strong>Fakulta</strong> c<strong>chemická</strong><br />
VUT V v Brně<br />
proděkan<br />
pro vnnější<br />
vztahy y a IT, <strong>Fakulta</strong><br />
chemickká<br />
VUT v Brně<br />
vedou ucí výzkummného<br />
prog gramu Pokr ročilé orgaanické<br />
mat teriály a<br />
bioma ateriály Cenntra<br />
materi iálové výzk kumu FCH VVUT<br />
v Brně ě<br />
proděkan<br />
pro vzdělávací<br />
činnost<br />
a IT, FCH VUT v Brně<br />
ZAHRANNIČNÍ<br />
PRACCOVNÍ<br />
POBY YTY A STÁŽEE<br />
2002<br />
Institu ut Fyzikálnní<br />
chemie, Phillips Un niversity MMarburg,<br />
Německo N<br />
(roční í pobyt),<br />
2003 – 2005 krátko odobé pobyyty<br />
na zah hraničních pracovištíc p ch (Laboratory<br />
for<br />
Chemi istry of Novvel<br />
Materia als, University<br />
of Monns‐Hainaut<br />
t, Belgie;<br />
Philips s, Eindhovven,<br />
Nizoz zemí; Ph hillips Uniiversity<br />
Marburg, M<br />
2007<br />
Němec cko a další) )<br />
absolu utorium měěsíčního<br />
ce ertifikované ého kurzu EEuropean<br />
Funding F<br />
Academy<br />
– 7th FFramework<br />
k programe<br />
3
ODBORNÉ AKTIVITY<br />
odborné zaměření nové materiály pro organickou elektroniku; příprava a<br />
charakterizace organických polovodičů; optické a elektrické<br />
vlastnosti organických polovodičů; charakterizace<br />
elementárních elektronových jevů a procesů spojených<br />
s fotogenerací a transportem náboje v organických polovodičích;<br />
numerické metody a modelování fotogenerace a transportu<br />
náboje; aplikační využití organických a biologických materiálů v<br />
optických, elektronických a senzorických zařízeních;<br />
publikační činnost autor a spoluautor více než 80 původních vědeckých prací,<br />
z toho<br />
� 35 prací v impaktovaných odborných časopisech,<br />
� 7 prací v recenzovaných odborných časopisech,<br />
� 42 příspěvků uveřejněných ve sbornících na mezinárodních<br />
a národních konferencích konferencích.<br />
Celkem 150 citací uveřejněných článků, h‐index: 7.<br />
grantová činnost spoluřešitel mezinárodního projektu 7. rámcového programu<br />
DEPHOTEX; účast v evropské výzkumné síti ORGANISOLAR;<br />
jeden z řešitelů projektu Centra materiálového výzkumu na<br />
Fakultě chemické VUT v Brně (MŠMT ČR, OP VaVpI); odpovědný<br />
řešitelněkolikagrantůGAČR, GAAV, projektuAVČRvrámci programu Nanotechnologie a společnost, spoluřešitel dvou<br />
aplikačních projektů Ministerstva průmyslu a obchodu. Celková<br />
dotace získaných projektů je 44,5 mil. Kč bez započítání<br />
zahraničních projektů a projektu VaVpI.<br />
PEDAGOGICKÉ AKTIVITY<br />
odborné aktivity e‐learning a jeho využití v distanční a kombinované formě studia,<br />
implementace LCMS systémů pro řízení vzdělávání,<br />
zpřístupnění vzdělávacího obsahu a podporu výuky; ICT<br />
publikační činnost<br />
podpora výuky<br />
autor a spoluautor 12 původních pedagogických prací, 2 skripta,<br />
multimediální učební opory pro výuku<br />
grantová činnost hlavní manažer a ideový tvůrce dvou projektů OP VK cílených na<br />
zvýšení úspěšnosti studentů kombinovaného studia v<br />
bakalářských a navazujících studijních programech v celkové<br />
hodnotě přes 20 miliónů korun; úspěšný řešitel pěti grantů<br />
FRVŠ; řešitel rozvojových projektů MŠMT v oblasti mobility,<br />
internacionalizace výuky a spolupráce s průmyslem<br />
garant předmětů Pokročilé aplikace molekulárních materiálů, Chemická<br />
informatika I, Chemická informatika II, Praktikum z fyziky,<br />
Měřící technika,<br />
vedení studentských prací vedoucí úspěšně obhájených 5 bakalářských, 12<br />
diplomových a 1 dizertační práce; školitel 10 studentů<br />
doktorského studia<br />
4
APLIKOOVANÝ<br />
VÝZKKUM<br />
A DALŠ ŠÍ PROFESNNÍ<br />
ZKUŠENOSTI<br />
Součassná<br />
spoluppráce<br />
s pod dniky a insstitucemi:<br />
��<br />
Indian n Institute of Techn nology Mad dras, Indiee<br />
– vývoj nových<br />
ruthen niových kommplexů<br />
pro o aplikace v senzorechh<br />
a fotovolt taice<br />
��<br />
Centru um organických<br />
syn ntéz, s.r.o., Rybitví, ČČeská<br />
repu ublika –<br />
vývoj multikommponentníc<br />
ch elektro onických ssystémů<br />
na n bázi<br />
��<br />
organi ických slouučenin<br />
pro aplikaci v senzorech s a fotovoltai ice<br />
Výzku umný ústavv<br />
organický ých syntéz a. s., Rybittví<br />
– vývojnových<br />
nanom materiálů a funkcionál l. systémů pro p elektroonické<br />
příst troje<br />
��<br />
GENER RI BIOTECH<br />
s.r.o., Hr radec Králo ové, Česká republika ‐ vývoj<br />
metod dy na elektrronickou<br />
de etekci hybr ridizace DNNA<br />
pro bios senzory<br />
Další pprofesní<br />
zkkušenosti<br />
OSTATNNÍ<br />
AKTIVITYY<br />
A ČLENSTVÍ<br />
od 20004<br />
dosud<br />
2005 ‐ 2006<br />
od 20005<br />
od 20007<br />
od 20110<br />
od 20112<br />
od 20112<br />
JAZYKOOVÉ<br />
ZNALOSSTI<br />
angličttina<br />
aktivnně,<br />
ruština aktivně, něěmčina<br />
pasivně<br />
ZÍSKANNÁ<br />
OCENĚNÍ<br />
2003<br />
V Brně dne 1.září 2012<br />
��<br />
od 20 006 dosudd<br />
‐ proděkan<br />
Fakulty y chemickééVUTvBrně<br />
pro<br />
vnější vztahy zahhrnující<br />
rov vněž spolup práci s průmmyslem<br />
��<br />
od 2009:<br />
hodnottitel<br />
evrops ských proje ektů 7. rámmcového<br />
pr rogramu<br />
pro ob blast Nanottechnologií<br />
í a ICT – fot tonika (EC, Brusel)<br />
��<br />
od 20 004: hodnootitel<br />
proje ektů v obla asti vědy vvýzkumu<br />
a inovací<br />
pro do omácí granttové<br />
agentury<br />
GAČR, GAAV, OP VVK<br />
a další,<br />
��<br />
2002‐ ‐ 2003: úččast<br />
na kurzechvrá<br />
ámci evroppské<br />
sítě Research R<br />
Trainig<br />
Networkk<br />
(Cambrid dge Univer rsity, Univv.<br />
Linköping,<br />
Univ.<br />
Bologn na, Technnical<br />
Univ v. Eindho oven, Uniiv.<br />
M. Hainaut) H<br />
zaměř řeného i naa<br />
aplikaci a komercion nalizaci výsledků<br />
VaV<br />
zaklád dající člen ttýmu<br />
pro ro ozvoj elearningu<br />
na VVUT<br />
předse eda komoryy<br />
akademic ckých pracovníků<br />
Akaademického<br />
senátu u FCH VUT v Brně<br />
člen Americké A chhemické<br />
společnosti<br />
( ACS)<br />
člen vě ědecké raddy<br />
Fakulty chemické c VUT V<br />
člen ob borové raddy<br />
doktorsk kého stud. programu p CChemie,<br />
techno ologie a vlaastnosti<br />
ma ateriálů (FC CH VUT v Brrně)<br />
člen ob borové raddy<br />
Pokročilé<br />
materiály y doktorskéého<br />
stud.<br />
progra amu Pokročilé<br />
materi iály a nanov vědy (MU, CEITEC VU UT)<br />
člen hodnotícíhoo<br />
panelu P2 205 Grantov vé agenturyy<br />
ČR<br />
cena rektora r VUTT<br />
za vynik kající výsled dky v pedaagogické<br />
a vědecké v<br />
práci<br />
5
Autoevaluační kritéria<br />
podle čl. 2 odst. 2 písm. c) Směrnice VUT v Brně pro habilitační řízení<br />
Uchazeč: doc. Ing. Martin Weiter, Ph.D.<br />
Datum narození: 3. března 1971<br />
Bydliště: Lipská 1, Brno – Žabovřesky<br />
Podklad k návrhu na jmenování: profesorem<br />
Souhrn<br />
Kategorie A1‐A6 A7‐A14 A ostatní A celkem B celkem Celkem<br />
A+B<br />
Požadováno 120 80 120 320 80 400<br />
Skutečnost 585 180 434 1199 256 1455<br />
A. Vědecká a odborná činnost<br />
Položka Body<br />
2<br />
Původní vědecká práce ve vědeckém časopisu<br />
s impakt faktorem větším než 0,5<br />
3 Původní vědecká práce ve vědeckém časopisu s IF 0,1– 0,5 45<br />
4<br />
Původní vědecká práce ve vědeckém časopisu s IF menším než 0,100<br />
nebo ve vědeckém časopisu bez IF<br />
6 Citace jiným autorem podle Science Citation Index 225<br />
9<br />
10<br />
11<br />
13<br />
Příspěvek ve sborníku světového nebo evropského kongresu,<br />
sympózia, vědecké konference<br />
Abstrakt ve sborníku světového nebo evropského kongresu,<br />
sympózia, vědecké konference<br />
Příspěvek ve sborníku národního nebo mezinárodního kongresu,<br />
sympózia, vědecké konference<br />
Abstrakt ve sborníku národního nebo mezinárodního kongresu,<br />
sympózia, vědecké konference<br />
20 Členství ve vědecké radě (1 období) 6<br />
22 Členství v programovém výboru národního nebo mezinárodního 20<br />
270<br />
45<br />
45<br />
42<br />
70<br />
23<br />
7
kongresu, sympózia, vědecké konference<br />
23 Získání zahraničního grantu (řešitel, spoluřešitel) 20<br />
24 Získání externího grantu (řešitel, spoluřešitel) 90<br />
26 Členství v grantových komisích, radách výzkumných programů 6<br />
27<br />
Posudek zahraniční publikace nebo projektu, znalecký posudek,<br />
expertíza<br />
29 Posudek domácí publikace nebo projektu 148<br />
Celkem bodů za položky<br />
B. Pedagogická činnost<br />
144<br />
1199<br />
Poř. č Body<br />
1 Pedagogický úvazek 28<br />
3 Zavedení předmětu, který byl vyučován v posledních pěti letech 75<br />
4 Vedoucí obhájené bakalářské/diplomové práce 29<br />
5 Školitel/školitel specialista studenta, který získal Ph.D. 15<br />
8 Skripta 18<br />
9 Vytvoření významné výukové pomůcky (film, video, software) 80<br />
11 Členství v oborové radě doktorského studijního programu 4<br />
12<br />
Členství v komisi pro státní doktorskou zkoušku nebo obhajobu<br />
disertační práce<br />
13 Členství v komisi pro státní závěrečné zkoušky v jednom roce 3<br />
Celkem bodů za položky<br />
4<br />
256<br />
8
Upřesnění jednotlivých položek autoevaluačních kritérií<br />
A. Vědecká a odborná činnost<br />
2. Původní vědecká práce ve vědeckém časopisu<br />
s impakt faktorem větším než 0,5<br />
Poř. č Body<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
9<br />
10<br />
KUČERÍK, J.; DAVID, J.; WEITER, M.; VALA, M.; VYŇUCHAL, J.; OUZZANE, I.;<br />
SALYK, O. Stability and physical structure tests of piperidyl and morpholinyl<br />
derivatives of diphenyl‐diketo‐pyrrolopyrroles (DPP). Journal of Thermal<br />
Analysis and Calorimetry, 2012, roč. 108, č. 2, s. 467‐473. ISSN 1388‐ 6150.<br />
IF(2011)=1,604.<br />
LUŇÁK, S.; VALA, M.; VYŇUCHAL, J.; OUZZANE, I.; HORÁKOVÁ, P.;<br />
MOŽÍŠKOVÁ, P.; WEITER, M. Absorption and fluorescence of soluble polar<br />
diketo‐pyrrolo‐pyrroles. Dyes and Pigments, 2011, 91(1), p. 269 ‐ 278. ISSN<br />
0143‐7208. IF(2010)=2,635.<br />
DAVID, J.; WEITER, M.; VALA, M.; VYŇUCHAL, J.; KUČERÍK, J. Stability and<br />
structural aspects of diketopyrrolopyrrole pigment and its N‐alkyl derivatives.<br />
Dyes and Pigments, 2011, 89(1), p. 137 ‐ 144. ISSN 0143‐7208.<br />
IF(2010)=2,635.<br />
LUŇÁK, S.; HAVEL, L.; VYŇUCHAL, J.; HORÁKOVÁ, P.; KUČERÍK, J.; WEITER, M.;<br />
HRDINA, R. The geometry and absorption of diketo‐pyrrolo‐ pyrroles<br />
substituted with various aryls. Dyes and Pigments, 2010, roč. 85, č. 1‐ 2, s. 27‐<br />
36. ISSN 0143‐ 7208. IF(2010)=2.635.<br />
VALA, M.; VYŇUCHAL, J.; WEITER, M.; TOMAN, P.; LUŇÁK, S. Novel, soluble<br />
diphenyl‐diketo‐pyrrolopyrroles: Experimental and theoretical study. Dyes and<br />
Pigments, 2010, roč. 84, č. 8, s. 176‐182. ISSN: 0143‐ 7208. IF(2010)=2.635.<br />
WEITER, M.; SALYK, O.; BEDNÁŘ, P.; VALA, M.; NAVRÁTIL, J.; ZMEŠKAL, O.<br />
Morphology and properties of thin films of diketopyrrolopyrrole derivatives.<br />
Materials Science and Engineering A, 2009, 165(3), p. 148 ‐ 152. ISSN 0921‐<br />
5093. IF(2009)=1.901.<br />
TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; BARTKOWIAK, W.; MENŠÍK,<br />
M. Model of the influence of energetic disorder on inter‐chain charge carrier<br />
mobility in poly[2‐methoxy‐5‐(2 ‐ethylhexyloxy)‐p‐phenylene vinylene].<br />
Polymers for Advanced Technologies, 2009, 20(3), p. 263 ‐ 267. ISSN 1042‐<br />
7147. IF(2009)=1.532.<br />
WEITER, M.; NAVRÁTIL, J.; VALA, M.; TOMAN, P. Photoinduced reversible<br />
switching of charge carrier mobility in conjugated polymers. European Physical<br />
Journal‐Applied Physics, 2009, 48(1), p. 10401 ‐ 10406. ISSN 1286‐0042.<br />
IF(2009)=0.756.<br />
ZMEŠKAL, O.; VALA, M.; WEITER, M.; ŠTEFKOVÁ, P. Fractal‐cantorian<br />
geometry of space‐time. Chaos, Solitons & Fractals, 2009, 42(3), p. 1878 ‐<br />
1892. ISSN 0960‐0779. IF(2009)=3.315.<br />
ZMEŠKAL, O.; WEITER, M.; VALA, M. Notes to "An irreducibly simple derivation<br />
of the Hausdorff dimension of spacetime" by M.S. El Naschie. Chaos, Solitons &<br />
Fractals. 2009, 42(10), p. 532 ‐ 533. ISSN 0960‐0779. IF(2009)=3.315.<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
9
11<br />
12<br />
13<br />
14<br />
15<br />
16<br />
17<br />
18<br />
19<br />
20<br />
21<br />
22<br />
23<br />
VALA, M.; WEITER, M.; VYŇUCHAL, J.; TOMAN, P.; LUŇÁK, S. Comparative<br />
Studies of Diphenyl‐Diketo‐Pyrrolopyrrole Derivatives for Electroluminescence<br />
Applications. Journal of Fluorescence, 2008, 18(6), p. 1181 ‐ 1185. ISSN 1053‐<br />
0509. IF(2008)=1.880.<br />
VALA, M.; WEITER, M.; VYŇUCHAL, J.; TOMAN, P.; LUŇÁK, S. Experimental and<br />
theoretical study of novel pyrrolopyrroles for luminescence applications.<br />
Luminescence, 2008, 23(4), p. 276. ISSN 1522‐7235. IF(2008)=1.183.<br />
KRATOCHVÍLOVÁ, I.; KRÁL, K.; BUNČEK, M.; VÍŠKOVÁ, A.; NEŠPŮREK, S.;<br />
KOCHALSKA, A.; TODORCIUC, T.; WEITER, M.; SCHNEIDER, B. Conductivity of<br />
natural and modified DNA measured by Scanning Tunneling Micros<strong>copy</strong>. The<br />
effect of sequence, charge and stacking. Biophysical Chemistry, 2008, roč.<br />
2008, č. 11, s. 3‐10. ISSN: 0301‐ 4622. IF (2008)= 2.276<br />
KRATOCHVÍLOVÁ, I.; KRÁL, K.; BUNČEK, M.; NEŠPŮREK, S.; TODORCIUC, T.;<br />
WEITER, M.; NAVRÁTIL, J.; SCHNEIDER, B.; PAVLUCH, J. Scanning Tunneling<br />
Spectros<strong>copy</strong> Study of DNA Conductivity. Central European Journal of Physics,<br />
2008, roč. 6, č. 3, s. 422‐426. ISSN: 1895‐ 1082. IF (2008)= 0,909.<br />
VALA, M.; WEITER, M. Molecular electronics: advances and limitations,<br />
strategies, materials, methods and applications. Chemické listy, 2008, 102(15),<br />
p. s1120 (6 p.). ISSN 1213‐7103. IF(2008)=0.593.<br />
ZMEŠKAL, O.; WEITER, M.; VALA, M.; BŽATEK, T. Mutual relation between<br />
fractal and statistical (random, thermodynamic) phenomena in nature.<br />
Chemické listy, 2008, 102(S), p. s1127 (4 p.). ISSN 1213‐7103.<br />
IF(2008)=0.593.<br />
ZMEŠKAL, O.; NEŠPŮREK, S.; WEITER, M. Space‐charge‐limited currents: An E‐<br />
infinity Cantorian approach. Chaos, Solitons & Fractals, 2007, roč. 34, č. 2, s.<br />
143‐158. ISSN: 0960‐ 0779. IF(2007)=2,980.<br />
TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; SWORAKOWSKI, J.;<br />
BARTKOWIAK, W.; MENŠÍK, M. Influence of dipolar species on charge transport<br />
in poly[2‐methoxy‐5‐(2 '‐ethylhexyloxy)‐p‐phenylene vinylene]. Polymers for<br />
Advanced Technologies, 2006, 17(9‐10), p. 673 ‐ 678. ISSN 1042‐7147.<br />
IF(2006)=1.406.<br />
WEITER, M.; BAESSLER, H. Transient photoconductivity and charge generation<br />
in thin films of pi‐ conjugated polymers. Journal of Luminescence, 2005, roč.<br />
112, č. 1‐ 4, s. 363‐367. ISSN: 0022‐ 2313. IF(2005)=1.314.<br />
SALYK, O.; BROŽA, P.; DOKOUPIL, N.; HERRMANN, R.; KUŘITKA, I.; PRYČEK, J.;<br />
WEITER, M. Plasma polymerisation of methylphenylsilane. Surface and<br />
Coatings Technology, 2005, roč. 200, č. 1‐ 4, s. 486‐489. ISSN: 0257‐ 8972.<br />
IF(2004)=1.432.<br />
MARKHAM, J.; SAMUEL, I.; BURN, S.; WEITER, M.; BAESSLER, H. Charge<br />
transport in highly efficient iridium cored electrophosphorescent dendrimers.<br />
Journal of Aplied Physics, 2004, roč. 95, č. 2, s. 438 ( s.)ISSN: 0021‐ 8979.<br />
IF(2004)=2.171.<br />
NEŠPŮREK, S., SWORAKOWSKI, J., COMBELLAS, C., WANG, G., WEITER, M. A<br />
molecular device based on light controlled charge carrier mobility. Applied<br />
Surface Science, 2004, roč. 234, č. 1–4, s. 395–402. ISSN 0169‐4332. IF (2004)<br />
= 1,284<br />
WEITER, M.; ARKHIPOV, V.; BAESSLER, H. Transient photoconductivity in a<br />
thin film of a polyphenylenevinylene type conjugated polymer. Synthetic Metals,<br />
2004, roč. 141, č. 1‐ 2, s. 165 ( s.)ISSN: 0379‐ 6779. IF(2004)=1.303.<br />
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WEITER, M., BAESSLER, H.; GULBINAS, V;, SCHERF, U. Transient photoconductivity<br />
in a film of ladder‐type poly‐phenylene: Failure of the Onsager approach.<br />
Chemical Physics Letters, 2003, roč. 379, č. 1, s. 117 ( s.)ISSN: 0009‐ 2614.<br />
IF(2003)=2.438.<br />
HORVÁTH, P., SCHAUER, F., WEITER, M., KUŘITKA, I., SALYK, O., DOKOUPIL,<br />
N., NEŠPŮREK, S., FIDLER, V. Luminescence in organic silicons prepared from<br />
organic precursors in plasma discharges. Chemical Monthly, roč. 132, č. 2001,<br />
s. 177‐ 185 ( s.) IF(2008)=1.183.<br />
SCHAUER, F., WEITER, M. Rigorous Modelling of Recombination Under Double<br />
Injection in Amorphous Films ‐ An Example of the Stiff Problem. Journal of<br />
Imaging Science and Technology, 1999, roč. 1999, č. 43, s. 413 ( s.)ISSN: 1062‐<br />
3701. IF(1999)=0.582.<br />
HANDLÍŘ, R., NEŠPŮREK, S., SCHAUER, F., WEITER, M., KUŘITKA, I.<br />
Metastability in poly(methylsilylene) induced by UV radiation and electron<br />
beam. Journal of Non‐ Crystaline Solids, roč. 1998, č. 227‐ 230, s. 669 ( s.)ISSN:<br />
0022‐ 3093. IF(1998)=1.563.<br />
Celkem bodů za položku 270<br />
3. Původní vědecká práce ve vědeckém časopisu s IF 0,1– 0,5<br />
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ČERNÁ, K.; WEITER, M. Organic materials for photovoltaic solar cells.<br />
Chemické listy, 2005, roč. 99, č. 1, s. 562‐564. ISSN: 0009‐ 2770.<br />
IF(2005)=0.445.<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S.; ZMEŠKAL, O. Pi‐conjugated Polymers<br />
Influenced by Permanent Dipole Moment Formation. Chemické listy, 2005,<br />
99(S), p. s627 (2 p.). ISSN 0009‐2770. IF(2005)=0.445.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S. Pi‐conjugated polymers based electronic<br />
devices. Chemické listy, 2005, 99(1), p. 544 ‐ 545. ISSN 0009‐2770.<br />
IF(2005)=0.445.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S.; SWORAKOVSKI, J.; SALYK, O.;<br />
ZMEŠKAL, O. Reversible formation of charge carrier traps in<br />
poly(phenylenevinylene) derivative due to the phototransformation of a<br />
photochromic additive. Molecular Crystals and Liquid Crystals, 2005, 430(1),<br />
p. 227 ‐ 233. ISSN 1058‐725X. IF(2005)=0.468.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S.; SALYK, O. Polymers for Optoelectronic<br />
Applications: Characterization of the Model System. Chemické listy, 2004,<br />
98(8), p. 551 ‐ 551. ISSN 0009‐2770. IF(2004)=0.348.<br />
HANDLÍŘ, R; WEITER, M; SCHAUER, F. Methods of electron structure<br />
spectros<strong>copy</strong> in molecular organic solids based on transient photoconductivity<br />
with space charge. Chemical Papers, 1996, roč. 50, č. 4, s. 199‐205. ISSN: 0366‐<br />
6352 IF(2004)=0.152.<br />
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Celkem bodů za položku 45<br />
4. Původní vědecká práce ve vědeckém časopisu s IF menším než 0,100<br />
nebo ve vědeckém časopisu bez IF<br />
Poř. č Body<br />
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MLADENOVA, D., WEITER, M., STEPANEK, P., OUZZANE, I., VALA, M.,<br />
SINIGERSKY, V., ZHIVKOV, I. Characterization of electrophoretic suspension for<br />
thin polymer film deposition. Journal of Physics: Conference Series.,2012, roč.<br />
356, č. 1, s. 012040. ISSN 1742‐6588.<br />
ŠEDINA, M.; WEITER, M.; VALA, M.; SALYK, O. The role of various transport<br />
layers type for construction of organic solar cells. Journal of Biochemical<br />
Technology, 2011, roč. 2, č. 5, s. S40 ISSN: 0974‐ 2328.<br />
VALA, M.; WEITER, M.; HEINRICHOVÁ, P.; ŠEDINA, M.; OUZZANE, I.;<br />
MOŽÍŠKOVÁ, P. Tayloring of molecular materials for organic electronics.<br />
Journal of Biochemical Technology, 2010, roč. 2, č. 5, s. S44 ISSN: 0974‐ 2328<br />
VALA, M.; WEITER, M.; ZMEŠKAL, O.; NEŠPŮREK, S.; TOMAN, P. Light Induced<br />
Change of Charge Carrier Mobility in Semiconducting Polymers.<br />
Macromolecular symposia, 2008, 268(1), p. 125 ‐ 128. ISSN 1521‐3900.<br />
VALA, M.; WEITER, M.; RAJTROVÁ, G.; NEŠPŮREK, S.; SWORAKOWSKI, J.<br />
Photochromic properties of spiropyran in polymeric pi‐conjugated matrices.<br />
Nonlinear Optics, Quantum optics: Concepts in Modern Optics, 2007, 37(1‐3),<br />
p. 53 ‐ 63. ISSN 1543‐0537.<br />
TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; SWORAKOWSKI, J.<br />
Photoswitching in polymers with photochromic dipolar species. Nonlinear<br />
Optics, Quantum optics: Concepts in Modern Optics, 2007, 37(1‐3), p. 87 ‐ 98.<br />
ISSN 1543‐0537.<br />
WEITER, M.; VALA, M.; ZMEŠKAL, O.; NEŠPŮREK, S.; TOMAN, P. A Molecular<br />
Photosensor Based on Photoswitching of Charge Carrier Mobility.<br />
Macromolecular Symposia, 2007, 2007(247), p. 318 ‐ 325. ISSN 1022‐1360.<br />
BEDNÁŘ, P.; ZMEŠKAL, O.; WEITER, M.; VALA, M.; TOMAN, P. Elecrical devices<br />
based on organic materials. Acta Metallurgica Slovaca, 2007, 13(6), p. 270 ‐<br />
274. ISSN 1335‐1532.<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S. Molecullar current switch: principles and<br />
characterization of the model system. CHEMagazín. 2005. XV(3). p. 28 ISSN<br />
1210‐7409.<br />
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Poř. č<br />
1 ‐<br />
150<br />
6. CCitace<br />
jiný ým autoreem<br />
podle Science Citation C Index<br />
Údaje platnné<br />
k 1.9.201 12 podle Weeb<br />
of Science e<br />
8 Nejcitovaanějších<br />
pra ací:<br />
Celkem bbodů<br />
za položku<br />
Body<br />
225<br />
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9. Příspěvek ve sborníku světového nebo evropského kongresu,<br />
sympózia, vědecké konference<br />
Poř. č Body<br />
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ZMEŠKAL, O.; ŠTEFKOVÁ, P.; VALA, M.; WEITER, M. Study of thermal<br />
properties of sollar cell laminating films by pulse transient method. In<br />
Thermophysics 2008. Kočovce, STU Bratislava. 2006. p. 163 ‐ 168. ISBN 978‐<br />
80‐227‐2968‐0.<br />
TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; MENŠÍK, M. Influence of<br />
Energetic Disorder on Charge Carrier Mobility In Conjugated Polymers. In Book<br />
of Abstract of Conference PAT 2007. Haidelberg. 2007. (4 p.).<br />
WEITER, M.; VALA, M.; ZMEŠKAL, O.; NAVRÁTIL, J.; TOMAN, P.; NEŠPŮREK, S.<br />
Photodetector based on switching of charge carrier mobility in polymers. In<br />
Book of Abstract Conference LOPE‐C. Frankfurt. 2007. p. 1 ‐ 6.<br />
TOMAN, P.; NEŠPŮREK, S.; MENŠÍK, M.; WEITER, M.; VALA, M.;<br />
SWORAKOWSKI, J.; BARTKOWIAK, W. Modeling of charge carrier transport in<br />
conjugated polymers doped by polar species. In Book of Abstracts of<br />
Conference ICFPAM 2007. Krakow, Polsko. 2007. (5 p.).<br />
WEITER, M. Latest achievements in organic semiconductors for future<br />
electronic devices. In Proceedings of IMAPS International Conference. Brno:<br />
IMAPS, 2006. s. XV (XX s.)ISBN: 80‐214‐3246‐ 2.<br />
WEITER, M.; BAESSLER, H. Transient Photoconductivity and Charge Generation<br />
in a Thin Films of Pi‐ Conjugated Polymers. In Proc. of 6th International<br />
Conference on Excitonic Processes in Condensed Matter. 1. Poland: Krakow<br />
Univerity, 2004. ISBN: 83‐921060‐0‐ 8.<br />
SALYK, O.; WEITER, M.; KUŘITKA, I.; DOKOUPIL, N.; OTEVŘEL, M.; BROŽA, P.;<br />
SCHAUER, F.; LUSTIG, F. Physics Laboratory Experiments Using Data Collection,<br />
Evaluation and Simulation. In Proceedings of the Proceedings of conference<br />
Physics Teaching in Engineering Education PTEE 2003. 1. Leuven, Belgie: KU<br />
Leuven, 2002. s. H5 (H5 s.)ISBN: 90‐5682‐359‐ 0.<br />
SALYK, O., HORVÁTH, P., WEITER, M., SCHAUER, F. Photo‐ and<br />
Electroluminescence of Polysilanes Prepared by Vacuum Evaporation and<br />
Plasma Polymerisation. In Procceedings of 10th International School on<br />
Condensed Matter. Varna, Bulgaria: 1999. s. 216 ( s.)<br />
Celkem bodů za položku 45<br />
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10. Abstrakt ve sborníku světového nebo evropského kongresu,<br />
sympózia, vědecké konference<br />
Poř. č Body<br />
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ZMEŠKAL, O.; NEŠPŮREK, S.; WEITER, M. Charge carrier transport in<br />
semiconductors: a fractal approach. International Conference on Recend<br />
Trends in Advanced Materials ICRAM‐ 2012. Vellore, Indie: VIT University,<br />
2012. s. 16‐16.<br />
VALA, M.; WEITER, M.; OUZZANE, I. Diketo‐Pyrrolo‐Pyrroles for Organic<br />
Electronics. Abstract book F‐PI‐10 International Symposium on Functional PI<br />
electron Systems. Beijing, China. 2011. s. 238.<br />
WEITER, M.; VALA, M.; ŠEDINA, M.; OUZZANE, I. Tailoring of properties of<br />
molcular semiconductors for organic electronics and photonics. 10th<br />
|international symposium on functional pi‐ electron systems. Beijing, China:<br />
2011. s. 253.<br />
OUZZANE, I.; WEITER, M.; VALA, M. Optical characterisation of organic diketo‐<br />
pyrrolo‐ pyrroles derivatives and their photo processes study. 10th<br />
International symposium on functional pi‐ electron systems. Beijing, China:<br />
2011. s. 267.<br />
VALA, M.; WEITER M.; LUŇÁK S.; VYŇUCHAL, J. Optical and Electrical<br />
Properties of New Diketo‐pyrrolo‐pyrroles for Organic Electronics. In Book of<br />
Abstracts 12th International conference ERPOS‐2012. Vilnius, Lithuania,<br />
2011. p.120. ISBN 978‐9955‐634‐36‐2<br />
ŠPÉROVÁ, M.; WEITER, M.; KUČERÍK, J. The new materials for organic<br />
electronics and photovoltaics. Book of Abstracts 2nd International Symposium<br />
on Flexible Organic Electronics .Řecko, 2011.<br />
VALA, M.; WEITER M.; LUŇÁK S.; VYŇUCHAL, J. Diketo‐pyrrolo‐pyrroles for<br />
organic electronics. In Book of abstracts 44th Heyrovský Discussion<br />
Nanostructures on Electrodes. Třešť, Czech Republic. 2011. p.41‐42. ISBN<br />
978‐80‐87351‐8<br />
OUZZANE, I.; HEINRICHOVA P.; SEDINA, M.; VALA, M.; WEITER, M. Optical<br />
characterisation studies of novel diketo‐pyrrolo‐pyrroles In Book of abstracts<br />
44th Heyrovský Discussion Nanostructures on Electrodes. Třešť, Czech<br />
Republic. 2011. Ip.57‐58. ISBN 978‐80‐87351‐8<br />
VALA, M.; WEITER M.; LUŇÁK S.; VYŇUCHAL, J. Electrooptical properties of<br />
new diketo‐pyrrolo‐pyrroles. In Book of Abstracts Photonics Prague 2011.<br />
Prague, Czech Republic. 2011. p.57. ISBN 978‐80‐86742‐30‐4<br />
VALA, M.; OUZZANE, I.; LUŇÁK, S.; VYŇUCHAL, J.; WEITER, M. Two Photon<br />
Excited Fluorescence of Novel Diphenyl‐Diketo‐Pyrrolopyrroles. Abstract book<br />
F‐PI‐09 International Symposium on Functional PI electron Systems, Atlanta,<br />
USA. 2010.<br />
WEITER, M.; VALA, M.; SALYK, O.; VYŇUCHAL, J.; LUŇÁK, S. New<br />
Diketopyrrolopyrrole Derivatives for Organic Electronics. Abstract book ECME<br />
2009 ‐ European Conference on Molecular Electronics 2009. Dánsko,<br />
University of Copenhagen. 2009. p. POS1A (1 p.).<br />
DAVID, J.; WEITER, M.; VALA, M.; VYŇUCHAL, J.; KUČERÍK, J. Stability and<br />
physical structure tests of DPP‐ based luminiscent derivates. In Medicta 2009,<br />
Abstracts. France: 2009. s. 92‐92.<br />
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WEITER, M.; VALA, M.; VYŇUCHAL, J. Novel derivatives of<br />
diketopyrrolopyrroles for organic electronics. Book of Abstracts 2nd<br />
International Symposium on Flexible Organic Electronics, 8‐10 July 2009.<br />
Řecko, Galonis. 2009. p. 96 ‐ 96.<br />
VALA, M.; WEITER, M.; VYŇUCHAL, J.; TOMAN, P. Experimental and theoretical<br />
study of novel pyrrolopyrroles for luminescence applications. In XIII<br />
International Symposium on Luminescence Spectrometry. Italy, University of<br />
Bologna. 2008. p. 148 ‐ 148.<br />
WEITER, M.; VALA, M.; ZMEŠKAL, O. Optical and optoelectronic properties of<br />
organic semiconductors for molecular electronic. In XIII International<br />
Symposium on Luminescence Spectrometry. Italy, University of Bologna.<br />
2008. p. 10 ‐ 10.<br />
WEITER, M.; VALA, M.; NAVRÁTIL, J.; TOMAN, P.; NEŠPŮREK, S. Influence of<br />
photoswitchable traps on charge transport in conjugated polymers. In Book of<br />
abstracts. Rakousko. 2008. p. 205 ‐ 205.<br />
TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; BARTKOWIAK, W. Modeling<br />
of charge carrier transport in conjugated polymers doped by polar additives. In<br />
Book of Abstracts. Wroclaw, Polsko. 2008. p. 36 ISBN 978‐83‐7493‐399‐5.<br />
BEDNÁŘ, P.; ZMEŠKAL, O.; WEITER, M.; VALA, M.; SALYK, O.; TOMAN, P.;<br />
VYŇUCHAL, J. Influence of side groups substitution on the optical and electrical<br />
properties of diketopyrrolopyrrole derivatives. In Book of Abstracts. Polsko.<br />
2008. p. 70 ‐ 70. ISBN 978‐83‐7493‐399‐5.<br />
WEITER, M.; VALA, M.; NAVRÁTIL, J.; TOMAN, P.; NEŠPŮREK, S.<br />
Photogeneration and transport of charge carriers in semiconducting polymers<br />
with polar additives. In Book of Abstracts. Polsko. 2008. p. 111 ‐ 111. ISBN<br />
978‐83‐7493‐399‐5.<br />
NAVRÁTIL, J.; WEITER, M.; VALA, M.; ZMEŠKAL, O.; NEŠPŮREK, S.<br />
Photoinduced Reversible Switching of Charge Carrier Mobility in Conjugated<br />
Polymers. In Book of Abstract 1st International Symposium on Flexible<br />
Organic Electronics. Řecko. 2008. p. 90 ‐ 90.<br />
WEITER, M.; NAVRÁTIL, J.; VALA, M.; SALYK, O.; BEDNÁŘ, P.; ZMEŠKAL, O.<br />
Structure Property Relationship of Thin Films of Diketopyrrolopyrrole<br />
Derivatives. In Book of Abstracts 1st International Symposium on Flexible<br />
Organic Electronics.. Řecko. 2008. p. 92 ‐ 92.<br />
WEITER, M.; NAVRÁTIL, J.; VALA, M. Photogeneration of Charge Carriers in<br />
Conjugated Polymers with Polar Additives. In Book of Abstracts. 1st<br />
International Symposium on Flexible Organic Electronics Řecko. 2008. p. 91<br />
VALA, M.; WEITER, M.; TOMAN, P. Novel diphenylpyrrolopyrroles for<br />
electroluminescence applications. In Methods and Applications of<br />
Fluorescence: Spectros<strong>copy</strong>, Imaging and Probes. Regensburg, University of<br />
Regensburg. 2007. p. 100 ‐ 100.<br />
NEŠPŮREK, S.; WEITER, M.; TODORCIUC, T.; SCHNEIDER, B.; BUNČEK, M.<br />
Scanning Tunnelling Spectros<strong>copy</strong> Study of Charge Transport Through DNA<br />
Networks. In Abstracts Booklet of International Conference Nano 07. Brno:<br />
VUTIUM, 2007. s. 34‐34. ISBN: 978‐80‐214‐3460‐ 8.<br />
VALA, M.; WEITER, M.; ZMEŠKAL, O.; NEŠPŮREK, S.; TOMAN, P. Light Induced<br />
Change of Charge Carrier Mobility in Semiconducting Polymers. In Advanced<br />
Polymer Materials for Photonics and Electronics. Praha, IMC AS CR v.v.i. 2007.<br />
p. 108 ‐ 108. ISBN 978‐80‐85009‐56‐9.<br />
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VALA, M.; WEITER, M.; ZMEŠKAL, O.; NAVRÁTIL, J.; OUZZANE, I.; NEŠPŮREK,<br />
S.; TOMAN, P. Photoswitchable Charge Traps in Organic Semiconducting<br />
Matrix. In International Conference on Organic Electronics. Eindhoven,<br />
Netherland. 2007. p. 100 ‐ 101<br />
TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; MENŠÍK, M. Influence of<br />
Energetic Disorder on Charge Carrier Mobility In Conjugated Polymers. In In<br />
Book of Abstract of Conference PAT 2007. Haidelberg: 2007.<br />
TOMAN, P.; NEŠPŮREK, S.; MENŠÍK, M.; WEITER, M.; VALA, M.;<br />
SWORAKOWSKI, J.; BARTKOWIAK, W. Modeling of charge carrier transport in<br />
conjugated polymers doped by polar species. In In Book of Abstracts of<br />
Conference ICFPAM 2007. Krakow, Polsko: 2007.<br />
TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; SWORAKOVSKI, J. Influence<br />
of polar additives on charge transport in MEH‐PPV. In Book of abstracts of<br />
ECME . Okazaki. 2006. p. P‐24‐b (1 p.).<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S.; TOMAN, P. The effect of charge carrier<br />
traps photoswitching on charge transport in molecular materials. In Book of<br />
abstracts of EOC. Eindhoven, Nizozemí. 2006. p. 87 ‐ 87.<br />
WEITER, M.; VALA, M.; NAVRÁTIL, J.; ZMEŠKAL, O.; NEŠPŮREK, S.; TOMAN, P.<br />
Organic semiconductors for future molecular electronic devices. . In Book of<br />
abstracts of Nanoconference. Brno. 2006. p. 50 ‐ 50.<br />
TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; SWORAKOVSKI, J. Influence<br />
of Photochromic Dipolar Species on Electronic Properties of Conjugated<br />
Polymers. In Book of 8th International Symposium Polymers for Advanced<br />
Technologies. 2005.<br />
VALA, M.; WEITER, M.; RAJTROVÁ, G.; NEŠPŮREK, S.; ZMEŠKAL, O.<br />
Photochromic properties of spiropyranes in polymeric pi‐conjugated matrices.<br />
In Proceeding of 10 th International conference of Electrical and Related<br />
Properties of Organic Solids and Polymers. Cargese, France. 2005.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S.; ZMEŠKAL, O. Dopant‐assisted carrier<br />
photogeneration in conjugated polymers. In 10 th International conference of<br />
Electrical and Related Properties of Organic Solids and Polymers. Cargese,<br />
France, Bordeaux University. 2005.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S.; TOMAN, P. The effect of photochromic<br />
dipolar species on charge transport in disordered organic materials. In Proc. of<br />
8th European Conference on Molecular Electronics.Bologna, Italy, CNR Italy.<br />
2005.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S.; TOMAN, P.; SWORAKOVSKI, J. A<br />
molecular device based on light controlled mobility switching: characterization<br />
of the model system. In Proc. of 6th International Conference on Organic<br />
Electronics (ICOE).Eindhoven, Netherland, Philips. 2005.<br />
WEITER, M.; BAESSLER, H. Photoconductivity and Charge Generation in a Thin<br />
Films of Pi‐ Conjugated Polymers. In Book of Abstracts of 6th International<br />
Conference on Excitonic Processes in Condensed Matter. 1. Cracow:<br />
University Cracow, 2005. s. P124 ( s.)ISBN: 83‐921060‐0‐ 8.<br />
SWORAKOVSKI, J., NEŠPŮREK, S., WANG, G., WEITER, M. Light controlled<br />
charge carrier mobilities on polymer chain. Towards a molecular switch. In<br />
Proc. of 7th European Conference on Molecular Electronics ECME‐ 7. Avignon:<br />
ECME, 2003. s. 89<br />
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NEŠPŮREK, S., WANG, G., WEITER, M., SWORAKOVSKI, J. A Molecular Device<br />
Based on Light Controlled Charge Carrier mobility. In Proc. of 9th International<br />
Conference on the Formation of Semiconductror Intrefaces. 1. Madrid: ICFSI,<br />
2003. s. 15 ( s.)ISBN: 90‐5681‐ 4.<br />
SWORAKOVSKI, J., NEŠPŮREK, S., WANG, G., WEITER, M. Light controlled<br />
charge carrier motilities on polymer chain. Towards a molecular switch. In<br />
Proc. of 7th European Conference on Molecular Electronics ECME. 1. Avignon:<br />
PTEE, 2003. s. 48 ISBN: 90‐4872‐ 4.<br />
SALYK, O., KUŘITKA, I., WEITER, M., SCHAUER, F. Oriented Thin Films of<br />
Polysilylenes and their Properties. In Proceedings of the Workshop Electronic<br />
Properties of Molecular Materials and Functional Polymers of EU programme<br />
COST 518 Molecular Materials and Functional Polymers for Advanced<br />
Devices. Brno: Brno University of Technology, Faculty of Chemistry, 2001. s.<br />
168 ( s.)ISBN: 80‐214‐1893‐ 1.<br />
SALYK, O., SCHAUER, F., WEITER, M., HORVÁTH, P. Novel application of<br />
polysilylenes. In Proceeding of 10th International School on Condensed<br />
Matters. Varna, 1998. s. 242<br />
Celkem bodů za položku 42<br />
11. Příspěvek ve sborníku národního nebo mezinárodního kongresu,<br />
sympózia, vědecké konference<br />
Poř. č Body<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
VALA, M.; WEITER, M. Materials for Organic Electronics. In Sborník příspěvků<br />
XI pracovní setkání fyzikálních chemiků a elektrochemiků. Brno, Česká<br />
republika, 2011. p. 301‐304. ISBN 978‐80‐7375‐514‐0<br />
OUZZANE, I.; ŠEDINA, M.; HEINRICHOVÁ, P.; VALA, M.; WEITER, M. Optical<br />
Characterization of Organic Semiconductors. In Sborník příspěvků XI pracovní<br />
setkání fyzikálních chemiků a elektrochemiků. Brno, Česká republika, 2011. p.<br />
200‐202. ISBN 978‐80‐7375‐514‐0<br />
HEINRICHOVÁ, P.; MOŽÍŠKOVÁ, P.; ŠEDINA, M.; WEITER, M. Studium<br />
fotodegradace organických materiálů pro fotovoltaické aplikace. In Z. Remeš,<br />
M. Vaněček, A. Poruba Optical characterization methods in the solar cell<br />
research, Sborník příspěvků z 5. České fotovoltaické konference, 10.‐13. 11.<br />
2010. Brno: 2011. ISBN: 978‐80‐254‐8906‐ 2.<br />
ZMEŠKAL, O.; WEITER, M.; VALA, M. Interaktivní multimediální sbírka příkladů<br />
základního kurzu fyziky. In Sborník příspěvků část 2 ‐ fyzika. Brno, ČR:<br />
Univerzita obrany, 2011. s. 151‐159. ISBN: 978‐80‐7231‐815‐ 5.<br />
WEITER, M.; VALA, M.; VYNUCHAL, J.; KUBAC, L. Development of New Organic<br />
Semiconductors and their Applications in Organic Electronics and Photonnics,<br />
In 2nd NANOCON International Conference. Česká republika, Olomouc. 2010.<br />
p. 114‐119.<br />
ZMEŠKAL, O.; WEITER, M.; VALA, M. The Basics of Fractal Physics, In: 16th<br />
International Conference on Soft Computing, MENDEL 2010. p. 154‐160<br />
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ŠEDINA, M.; HEINRICHOVÁ, P.; WEITER, M.; VALA, M.; SALYK, O. The role of<br />
various transport layers type for construction of organic solar cells. In X.<br />
Pracovní setkání fyzikálních chemiků a elektrochemiků. Brno: Mendelova<br />
univerzita v Brně, 2010. s. 205‐207. ISBN: 978‐80‐7375‐396‐ 2.<br />
HEINRICHOVÁ, P.; MOŽÍŠKOVÁ, P.; ŠEDINA, M.; WEITER, M. The<br />
photodegradation of polymers and small molecular materials applied in organic<br />
optoelectronic devices. In X. Pracovní setkání fyzikálních chemiků a<br />
elektrochemiků & IV. Letní elektro<strong>chemická</strong> škola; Sborník příspěvků 23. ‐ 25.<br />
června 2010. Brno: Mendlova univerzita v Brně, 2010. s. 81‐83. ISBN: 978‐80‐<br />
7375‐396‐ 2.<br />
VALA, M.; NAVRÁTIL, J.; WEITER, M. Podcast učební opory: připravenost<br />
studentů. In Sborník příspěvků z konference a soutěže eLearning 2009. Česká<br />
republika, Gaudeamus UHK. 2009. p. 299 ‐ 302. ISBN 978‐80‐7041‐971‐7.<br />
NAVRÁTIL, J.; VALA, M.; WEITER, M. Podpora výuky formou podcast. In<br />
Sborník příspěvků z konference a soutěže eLearning 2009. Česká republika,<br />
Gaudeamus UHK. 2009. p. 299 ‐ 302. ISBN 978‐80‐7041‐971‐7.<br />
ZMEŠKAL, O.; VALA, M.; CVACHOVEC, F.; KOMÁREK, M. Videopresentace<br />
demonstračních úloh do přednášek z fyziky. In 6. konference o matematice a<br />
fyzice na vysokých školách technických. první. Brno, Univerzita obrany. 2009.<br />
p. 445 ‐ 460. ISBN 978‐80‐7231‐667‐0.<br />
WEITER, M.; VALA, M.; SALYK, O.; ZMEŠKAL, O.; PŘIKRYL, R.; VYŇUCHAL, J.<br />
Low Molecular Materials for Possible Application in Organic Solar Cells. In<br />
Sborník příspěvků. Brno. 2008. p. 111 ‐ 114. ISBN 978‐80‐254‐3528‐1.<br />
RICHTERA, L.; WEITER, M.; VALA, M.; NAVRÁTIL, J. E‐learningová podpora<br />
laboratorní výuky praktik z obecné a anorganické chemie a praktik z fyziky. In<br />
Mezinárodní konference SILESIAN MOODLE MOOT 2008, 5. ročník, sborník<br />
příspěvků. Ostrava, Vysoká škola báňská ‐ Technická univerzita Ostrava.<br />
2008. p. 97 ‐ 102 (272 p.). ISBN 978‐80‐248‐1859‐7.<br />
ZMEŠKAL, O.; VALA, M.; WEITER, M. El Naschie's Golden Field Theory and<br />
Fractal ‐ Cantorian Physics. In Proceeding of New Trends in Physics 2007.<br />
Brno, Novotný, Brno. 2007. p. 193 ‐ 198. ISBN 978‐80‐7355‐078‐3.<br />
WEITER, M.; VALA, M.; ZMEŠKAL, O.; BEDNÁŘ, P.; SALYK, O. Organic Materials<br />
for Future Optoelectronic Devices. In Proceeding of the conference New Trends<br />
in Physics 2007. Brno. 2007. p. 286 ‐ 289. ISBN 978‐80‐7355‐078‐3.<br />
VALA, M.; WEITER, M.; NAVRÁTIL, J. Photoswitchable Devices Based on Organic<br />
Semiconductors. In Proceeding of the conference New Trends in Physics 2007.<br />
Brno, Novotný. 2007. p. 282 ‐ 286. ISBN 978‐80‐7355‐078‐3.<br />
ZMEŠKAL, O.; VESELÝ, M.; VALA, M.; BEDNÁŘ, P.; BŽATEK, T. Image Analysis<br />
Used to Study Physical Properties of Printed Organic Thin Films. In VIII Seminar<br />
in Graphic Arts. Conference Proceedings. Pardubice, Universita Pardubice.<br />
2007. p. 131 ‐ 137.<br />
ZMEŠKAL, O.; WEITER, M. Koncepce výuky fyziky na Fakultě chemické<br />
<strong>Vysoké</strong>ho <strong>učení</strong> <strong>technické</strong>ho v Brně. In 5. konference o matematice a fyzice na<br />
vysokých školách technických s mezinárodní účastí. 1. Brno: JČMF, 2007. s.<br />
523‐532. ISBN: 978‐80‐7231‐274‐ 0.<br />
WEITER, M.; VALA, M.; ZMEŠKAL, O.; NAVRÁTIL, J.; TOMAN, P.; NEŠPŮREK, S.<br />
Polymer photoelectronic devices based on interaction between pi‐conjugated<br />
polymer matrices and photochromic molecules. In Book of Abstract. Praha.<br />
2007. p. 1 ‐ 6. ISBN 80‐214‐3308‐6.<br />
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WEITER, M.; VALA, M.; ZMEŠKAL, O.; NAVRÁTIL, J.; TOMAN, P.; NEŠPŮREK, S.<br />
Polymer optical sensor based on photochromic switching of charge carrier<br />
mobility. In Book of Abstract. Praha. 2007. p. 1 ‐ 5. ISBN 978‐0‐8194‐6729‐4.<br />
WEITER, M.; VALA, M.; NAVRÁTIL, J.; ZMEŠKAL, O. Organic semiconductors for<br />
future molecular electronic devices. In Proceedings of the International<br />
Conference NANO 2006. Brno. 2006. p. 184 ‐ 189. ISBN 80‐214‐3331‐0.<br />
WEITER, M.; VALA, M.; ZABADAL, M. Využití IKT v praktických laboratorních<br />
cvičeních. In Sborník příspěvků. Univerzita Konstantina Filozofa v Nitre. 2006.<br />
p. 25 ‐ 30. ISBN 80‐8094‐032‐0.<br />
WEITER, M. Generace náboje v konjugovaných polymerních materiálech<br />
vhodných pro fotovoltaické aplikace. In Sborník příspěvků 3. České<br />
fotovoltaické konference. Brno: Czech RE agency, 2006. s. 39‐45. ISBN: 80‐<br />
239‐7361‐ 4.<br />
WEITER, M.; ZDÍLNA, P.; ČERNÁ, K.; KRČMOVÁ, M. Připravenost studentů a<br />
zavádění e‐ learningu na Fakultě chemické VUT v Brně. In Sborník 2. ročníku<br />
conference SCO 2005. 1. Brno: MU v Brně, 2005. s. 151‐154. ISBN: 80‐210‐<br />
3699‐ 0.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S.; TOMAN, P. Molecular Current Switch:<br />
Principles and Characterization of the Model System. In sborník. 2005. p. 95 ‐<br />
99. ISBN 80‐214‐3085‐0.<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S.; ZMEŠKAL, O. Optical and electrical<br />
switching in organic semiconductorts. In Juniormat '05. Brno, CZ. 2005. p. 71 ‐<br />
74. ISBN 80‐214‐2984‐4.<br />
WEITER, M.; SALYK, O.; ČERNÁ, K.; BRADA, J.; ZELENÝ, M. Výuka fyzikálního<br />
praktika s podporou LMS Moodle. In Sborník Konference Belcom 05. 1. Praha:<br />
ČVUT v Praze, 2005. s. 67‐71. ISBN: 80‐01‐03203‐ 5.<br />
WEITER, M.; SALYK, O.; VALA, M.; BRADA, J. Utilization of e‐learning in<br />
experimental physics teaching, In Proceedings of Physics Teaching in<br />
Engineering Education PTEE 2005. 2005. p. 245 ‐ 249. ISBN 80‐7355‐024‐5.<br />
SEVEROVÁ, K.; NEŠPŮREK, S.; WEITER, M. Humidity sensors based on organic<br />
semiconductor. In: Česká společnost <strong>chemická</strong>, Brno 2005. s. s610 (s612 s.)<br />
SEVEROVÁ, K.; WEITER, M.; NEŠPŮREK, S. Sensoric properties of soluble<br />
phtalocyanines. In Juniormat 05. 1. Brno: <strong>Vysoké</strong> <strong>učení</strong> <strong>technické</strong>, fakulta<br />
strojního inženýrství, 2005. s. 187‐190. ISBN: 80‐214‐2984‐ 4.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S. Molecular Current Modulator: Principles<br />
and Photoelectronic Characterization of the Model System. In Proc. of<br />
conference New Trends in Physics ‐ NTF 2004. Brno, VUT v Brně. 2004. p. 274<br />
‐ 277. ISBN 80‐7355‐024‐5.<br />
SALYK, O.; WEITER, M. Polysilane Luminescent Materials. In Proc. of 11th<br />
Conference Electronic Device and Systems 2004. 1. Brno: VUT v Brně, 2004. s.<br />
278‐285. ISBN: 80‐214‐2701‐ 9.<br />
SALYK, O.; WEITER, M. Využití eLearningových technologií při výuce fyzikálního<br />
praktika. In sborník konference eLearning ve vysokoškolském vzdělávání<br />
2004. 1. Zlín: UTB Zlín, 2004. s. 135‐139. ISBN: 80‐7318‐190‐ 8.<br />
WEITER, M., SALYK, O., VALA, M. Optoelectronic properties of conjugated<br />
polymers. In Juniormat '03. Brno, Brno University of Technology. 2003. p. 128<br />
‐ 131. ISBN 80‐214‐2462‐1.<br />
Celkem bodů za položku 70<br />
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13. Abstrakt ve sborníku národního nebo mezinárodního kongresu,<br />
sympózia, vědecké konference<br />
Poř. č Body<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
9<br />
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11<br />
12<br />
13<br />
ŠEDINA, M.; OUZZANE, I.; VALA, M.; WEITER, M. Characterization of<br />
Phthalocyanine Derivates for Application in Organic Photovoltaics. In Book of<br />
Abstracts 8th International Conference on Nanosciences & Nanotechnologies<br />
(NN11). Thessaloniki, GREECE. 2011<br />
VALA, M.; WEITER, M.; HEINRICHOVÁ, P.; OUZZANE, I. Optical Properties of<br />
Diketo‐pyrrolo‐pyrroles for Organic Electronic Applications. Chemické listy.<br />
2011. 105(S). p. s.886. ISSN\~0009‐2770. (IF(2010)=0,602).<br />
MLADENOVA, D.; ZHIVKOV, I.; OUZZANE, I.; VALA, M.; WEITER, M.<br />
Characterization of Poly[2‐methoxy‐5‐(3',7'‐dimethyloctyloxy)‐1,4‐<br />
phenylenevinylene] Electrophoretic Suspensions Used for Thin Film<br />
Deposition. Chemické listy. 2011. 105(S). p. s.949. ISSN: 0009‐2770.<br />
(IF(2010)=0,602).<br />
WEITER, M.; HEINRICHOVÁ, P.; ŠEDINA, M.; OUZZANE, I.; FLIMEL, K.; VALA,<br />
M. Development and Characterization of Novel Solar Cells for Organic<br />
Photovoltaics. Chemické listy. 2011. 105(S). p. s.871. ISSN: 0009‐2770.<br />
(IF(2010)=0,602).<br />
ZHIVKOV, I.; MLADENOVA, D.; HEINRICHOVÁ, P.; OUZZANE, I.; VALA, M.;<br />
WEITER, M. Electrophoretic Deposition of Thin Organic Films for Solar Energy<br />
Conversion Purpose. Chemické listy. 2011. 105(S). p. s884. ISSN\~0009‐2770.<br />
(IF(2010)=0,602).<br />
ŠEDINA, M.; OUZZANE, I.; FLIMEL, K.; VALA, M.; WEITER, M. Characterization<br />
of phthalocyanine derivates for application in organic photovoltaic. Chemické<br />
listy. 2011. 105(S). p. s902. ISSN\~0009‐2770.<br />
OUZZANE, I.; WEITER, M.; VALA, M. Novel diketo‐pyrrolo‐ pyrroles organic<br />
derivatives for optical applications. Chemické listy. 2011. 105(S). s. 802‐802.<br />
ISSN: 0009‐ 2770<br />
HEINRICHOVÁ, P.; DZIK, P.; ZHIVKOV, I.; MLADENOVA, D.; WEITER, M.<br />
Development of Organic Solar Cells Based on Conjugated Polymers. .<br />
Chemické listy. 2011. 105(S) s895. ISSN: 0009‐ 2770.<br />
MLADENOVA, D.; WEITER, M.; BUDUROVA, D.; ZHIVKOV, I. Photoelectrical<br />
properties of electrophoretically‐ deposited thin organic films Chemické listy.<br />
2011. 105(S) s. 89‐90. ISSN: 0009‐ 2770.<br />
ŠPÉROVÁ, M.; WEITER, M.; KUČERÍK, J. Optical and electrical properties of<br />
thin layers of humic substances. Chemické listy. 2011. 105(S). s. s934 (s934<br />
s.)ISSN: 0009‐ 2770.<br />
OUZZANE, I.; VALA, M.; WEITER, M. Novel diketo‐pyrrolo‐ pyrroles organic<br />
derivatives for optical applications. Sborník XI. Pracovní Setkání Fyzikálních<br />
Chemiků a Elektrochemiků. Brno 2011.<br />
WEITER, M.; VALA, M.; VYŇUCHAL, J. Molecular design of novel small<br />
molecular semiconductors for molecular electronics. Praha, European<br />
Commission. 2009. p. 224 ‐ 224. ISBN 978‐92‐79‐12973‐5.<br />
WEITER, M.; VALA, M.; SALYK, O.; ZMEŠKAL, O.; PŘIKRYL, R.; VYŇUCHAL, J.<br />
Polymerní a nízkomolekulární materiály pro aplikaci v organických solárních<br />
článcích. In Sborník abstraktů. Brno, ČR. 2008. p. 1 ‐ 4.<br />
0,5<br />
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0,5<br />
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BEDNÁŘ, P.; ZMEŠKAL, O.; WEITER, M.; VALA, M.; VYŇUCHAL, J. Novel<br />
diketopyrrolopyrroles for molecular optical and electrical devices. Chemické<br />
listy, 2008, roč. 102, č. S, s. s959 (s960 s.)ISSN: 1213‐ 7103.<br />
SALYK, O.; WEITER, M.; VYŇUCHAL, J. Structure and morphology of some<br />
diphenyl‐diketo‐pyrrolo‐ pyrrole derivatives pigments. Chemické listy, 2008,<br />
roč. 102, č. 00, s. 1183‐1185. ISSN: 1213‐ 7103.<br />
OUZZANE, I.; HERMANOVÁ, S.; VALA, M.; WEITER, M.; NEŠPŮREK, S. Synthesis<br />
of Substituted Polysililenes Used as Semiconductive Polymers. Chemické listy.<br />
2008. 102(15). p. s1259 (2 p.). ISSN 1213‐7103. (IF(2008)=0.593).<br />
NAVRÁTIL, J.; WEITER, M.; VALA, M. Polymer photoelectronic devices based<br />
on interaction between Pi‐conjugated polymer matrices and photochromic<br />
molecules. Chemické listy (S). 2008. 102(15). p. 1250 ‐ 1251. ISSN 1803‐2389.<br />
(IF(2008)=0.593).<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S. Polymer semiconductors for future<br />
molecular electronic devices. Chemické listy (S). 2008. 102(15). p. 1232 ‐<br />
1234. ISSN 1803‐2389. (IF(2008)=0.593).<br />
RICHTERA, L.; WEITER, M.; VALA, M.; NAVRÁTIL, J. E‐Learning Support for<br />
Laboratory Exercises. In VIRTUAL UNIVERSITY PROCEEDINGS. Huba Mikuláš.<br />
Bratislava, E‐Academia Slovaca. 2008. p. 471 ‐ 473. ISBN 978‐80‐89316‐10‐6.<br />
BEDNÁŘ, P.; ZMEŠKAL, O.; WEITER, M.; VALA, M.; VYŇUCHAL, J. Materals for<br />
organic electroluminiscence devices. In Sborník příspěvků VIII. pracovního<br />
setkámí fyzikálních chemiků a elektrochemiků. Brno, Masarykova univerzita.<br />
2008. p. 14 ‐ 15. ISBN 978‐80‐210‐4525‐5.<br />
BEDNÁŘ, P.; ZMEŠKAL, O.; WEITER, M.; VALA, M. Electrical Devices Based on<br />
Organic Pigments. In Book of Abstracts. Brno. 2007. p. 85 ‐ 85. ISBN 80‐214‐<br />
3331‐0.<br />
WEITER, M.; VALA, M.; ZMEŠKAL, O.; BEDNÁŘ, P.; TOMAN, P.; VYŇUCHAL, J.<br />
Novel Diketopyrrolopyrroles for Molecular Electronics. In Abstract Booklet of<br />
the International Conference. Brno, VUTIUM. 2007. p. 39 ‐ 39. ISBN 978‐80‐<br />
214‐3460‐8.<br />
WEITER, M.; VALA, M.; NAVRÁTIL, J.; ZMEŠKAL, O. Molecular semiconductors<br />
for future electronic devices. In sborník příspěvků VII. pracovního setkámí<br />
fyzikálních chemiků a elektrochemiků. Brno, Masarykova univerzita. 2007. p.<br />
138 ‐ 139. ISBN 978‐80‐210‐4235‐3.<br />
BEDNÁŘ, P.; ZMEŠKAL, O.; WEITER, M.; VALA, M. Utilization of organic<br />
pigments for molecular electrical device. In sborník příspěvků VII. pracovního<br />
setkámí fyzikálních chemiků a elektrochemiků. Brno, Masarykova univerzita.<br />
2007. p. 12 ‐ 13. ISBN 978‐80‐210‐4235‐3.<br />
VALA, M.; KRČMOVÁ, M.; WEITER, M.; KOZÁKOVÁ, Z.; JEŘÁBKOVÁ, P. Optical<br />
characterization and fluorescence quantum yields measurement of novel 1,4 ‐<br />
dioxo‐3,6‐diphenylpyrrolo‐[3,4/C]‐pyrrole derivatives. In sborník příspěvků<br />
VII. pracovního setkámí fyzikálních chemiků a elektrochemiků. Brno,<br />
Masarykova univerzita. 2007. p. 125 ‐ 126. ISBN 978‐80‐210‐4235‐3.<br />
ZMEŠKAL, O.; ŠTEFKOVÁ, P.; VALA, M.; WEITER, M. Pulse transient method<br />
used for analysis of temperature modulated space charge limited currents. In<br />
Thermophysics 2006. Kočovce, STU Bratislava. 2006. p. 1 ‐ 1. ISBN 80‐227‐<br />
2536‐6.<br />
SEVEROVÁ, K.; NEŠPŮREK, S.; WEITER, M.; ZMEŠKAL, O.<br />
Metallophthalocyanines and their sensoric properties. In sborník příspěvků<br />
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31<br />
32<br />
33<br />
34<br />
35<br />
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38<br />
39<br />
40<br />
41<br />
VI. pracovního setkámí fyzikálních chemiků a elektrochemiků. Brno:<br />
Masarykova univerzita, 2006. s. 98‐99. ISBN: 80‐210‐3943‐ 4.<br />
WEITER, M.; VALA, M.; ZMEŠKAL, O. Charge carrier photogheneration in<br />
organic solar cells. In sborník příspěvků VI. pracovního setkání fyzikálních<br />
chemiků a elektrochemiků. Brno: Masarykova univerzita, 2006. s. 114‐115.<br />
ISBN: 80‐210‐3943‐ 4..<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S.; ZMEŠKAL, O. Phototrigered Charge<br />
Traps Formation in Polymer Diodes. In VI. Pracovní setkání fyzikálních<br />
chemiků a elektrochemiků. Brno, Czech Republic. 2006. p. 110 ‐ 111<br />
ZMEŠKAL, O.; VALA, M.; HADERKA, J. Determination of the Image's FractalL<br />
Dimension using Wavelet Transformation. In Mendel 2005. 1. Brno, FSI VUT.<br />
2005. p. 207 ‐ 210. ISBN 80‐214‐2961‐5.<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S.; ZMEŠKAL, O. The influence of<br />
photochromic switching on electro‐optical properties of poly(p‐<br />
phenylenevinylene) derivative. In V. Pracovní setkání fyzikálních chemiků a<br />
elektrochemiků. Brno. 2005. p. 94 ‐ 96. ISBN 80‐210‐3637‐0.<br />
WEITER, M.; VALA, M.; ČERNÁ, K. Optoelectronic properties of conjugated<br />
polymers studied by impedance spectros<strong>copy</strong>. In Proc. of 5th Workshop of<br />
Phys. Chem. and Electrochem. Brno, MU v Brně. 2005. p. 99 ‐ 100. ISBN 80‐<br />
210‐3637‐0.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S.; TOMAN, P. Molecular Current Switch:<br />
Principles and Characterization of the Model System. Abstract booklet of<br />
conference Nano'05. VUT v Brně. 2005. p. 32 ‐ 32. ISBN 80‐214‐3044‐3.<br />
WEITER, M. Photogeneration and charge transport in conjugated polymers.<br />
CHEMagazín, 2005, roč. 15, č. 3, s. 32‐32. ISSN: 1210‐ 7409.<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S.; ZMEŠKAL, O. Organic semiconductors<br />
influenced by photochromic transformation of spiropyrane dye. ChemZi. SK.<br />
2005. 1(1). p. 244 ‐ 244. ISSN 1336‐7242.<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S.; ZMEŠKAL, O. Kinetika fotochromních<br />
přeměn spiropyranu dopovaného v tenkých polymerních filmech. In CHLSAC<br />
98. Symposium. Ostrava, ČSCH. 2004. p. 736 ‐ 736.<br />
VALA, M.; WEITER, M.; NEŠPŮREK, S. Molecular current switch: principles and<br />
photoelectronic characterization of the model system. In IV.Pracovní setkání<br />
fyzikálních chemiků a elektrochemiků. Brno, Masarykova univerzita v Brně.<br />
2004. p. 43 ‐ 43. ISBN 80‐210‐3319‐3.<br />
WEITER, M.; VALA, M.; NEŠPŮREK, S.; SALYK, O.; ZMEŠKAL, O. New Functional<br />
Conjugated Polymers for Optoelectronic Applications. In IV. Pracovní setkání<br />
fyzikálních chemiků a elektrochemiků. Brno, Masarykova Univerzita v Brně.<br />
2004. p. 45 ‐ 45. ISBN 80‐210‐3319‐3.<br />
WEITER, M.; ŠORMOVÁ, H.; SALYK, O.; ZMEŠKAL, O. Využití e‐ learningových<br />
technologií při výuce fyziky na FCH VUT. In e‐ learning v česke a slovenské<br />
republice. Praha: ČVUT v Praze, 2004. s. 91 ( s.)ISBN: 80‐01‐02923‐ 9.<br />
WEITER, M. Transient photoconductivity and charge generation in Pi‐<br />
conjugated polymers. Sborník 10. konference Optické vlastnosti pevných látek<br />
v základním výzkumu a aplikacích. 1. 2003.<br />
WEITER, M.; NEŠPŮREK, S.; SCHAUER, F. Metastable states in<br />
poly(methylphenylsilylene). In Proc. 3rd. Seminary on Physics and Chemistry<br />
of Molecular Systems. Chemical Faculty of the TU, Brno: VUT v Brně, 2000. s.<br />
43 ( s.)ISBN: 80‐214‐1118‐ X.<br />
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42<br />
43<br />
44<br />
WEITER, M., KRČMA, F., ZMEŠKAL, O. The Electric Conductivity of Polymers –<br />
a General Approach. In Proceedings of Dielectric Analysis of Polymers and<br />
Composites, and Related Phenomena (Workshop of Physical Chemistry<br />
Fundamentals). Brno: FCH VUT v Brně, 1999. s. 9 ( s.)ISBN: 80‐214‐1372‐ 7.<br />
SALYK, O., HORVÁTH, P., WEITER, M., SCHAUER, F. Physical Propertie of<br />
Plasmatic and Evaporated Polysilylenes. Interantional Workshop on<br />
Diagnostic of Solid State Surfaces. 1998. Bratislava: Ústav fyziky SAV, 1998. s.<br />
24 ( s.)<br />
HANDLÍŘ, R., SCHAUER, F., WEITER, M. Metastabilní stavy<br />
polymetylfenylsilylsilylenu. In Proceding of 3rd. Seminary on Physics na d<br />
Chemistry Systems. 1998. Brno: FCH VUT, s. 43 ( s.)ISBN: 80‐214‐ 111.<br />
Celkem bodů za položku<br />
20. Členství ve vědecké radě (1 období)<br />
Poř. č Body<br />
1 člen vědecké rady Fakulty chemické VUT v Brně v období 2007 ‐ 2010 3<br />
2 člen vědecké rady Fakulty chemické VUT v Brně v období 2010 ‐ dosud 3<br />
Celkem bodů za položku<br />
22. Členství v programovém výboru národního nebo mezinárodního<br />
kongresu, sympózia, vědecké konference<br />
Poř. č Body<br />
1<br />
2<br />
člen výboru konference Electronic Properties of Molecular Materilas and<br />
Functional polymers, Brno, 2000<br />
člen programového výboru Electronic Properties of Molecular Materilas and<br />
Functional polymers, Brno, 2001<br />
3 člen programového výboru konference Chemistry and Life 2008, Brno 5<br />
4 člen programového výboru konference MoodleMoot 2011, Brno 5<br />
Celkem bodů za položku<br />
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0,5<br />
23<br />
6<br />
5<br />
5<br />
20<br />
24
23. Získání zahraničního grantu (řešitel, spoluřešitel)<br />
Poř. č Body<br />
1<br />
2<br />
Project No 214459 DEPHOTEX ‐ Development of Photovoltaic Textiles<br />
Based on Novel Fibres, 2008‐2011, spoluřešitel mezinárodního projektu 7.<br />
rámcového programu v rámci výzvy FP7‐NMP‐2007‐SME‐1<br />
SIGA885 ‐ OPTIMOLEL – Optimisation of thin film deposition for the<br />
molecular electronic devices, 2010‐2011, projekt FP7 COFUND – MCurie /<br />
SOMOPRO<br />
Celkem bodů za položku<br />
24. Získání externího grantu (řešitel, spoluřešitel)<br />
Poř. č Body<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
9<br />
10<br />
P205/10/2280, Grantová agentura ČR, 1.1 2010‐31.12 2013, Fotogenerace<br />
a transport náboje v molekulárních polovodičích pro organickou fotovoltaiku<br />
CZ.1.05/2.1.00/01.0012, MŠMT, OP VaVpI, Centra materiálového výzkumu<br />
na Fakultě chemické VUT v Brně, MŠMT ČR, :, 2008‐2013, vedoucí<br />
výzkumného programu<br />
KAN401770651, AV ČR, Program Nanotechnologie a společnost, 1.6.2006 –<br />
31.12.2010, Molekulární nanosystémy a materiály pro nanoelektroniku,<br />
hlavní řešitel<br />
CZ.1.07/2.2.00/15.0154, MŠMT, OP VK, 1.11.2010 – 30.8.2013,<br />
ChemLearning ‐ zvýšení úspěšnosti studentů kombinovaného studia, hlavní<br />
řešitel<br />
CZ.1.07/2.2.00/28.0330, MŠMT, OP VK, 1.9.2012 – 30.6.2015, IngLearning ‐<br />
zvýšení úspěšnosti studentů magisterského kombinovaného studia, hlavní<br />
řešitel<br />
Centralizovaný rozvojový projekt MŠMT, 1.1. 2011 – 31.12.2011 Pracoviště<br />
pro zpracování historie chemického průmyslu a aplikované chemie s cílem<br />
prohloubení <strong>technické</strong>ho vzdělávání studentů vysokých škol <strong>technické</strong>ho a<br />
přírodovědného typu, včetně oborů pro přípravu učitelů, spoluřešitel<br />
Centralizovaný rozvojový projekt MŠMT, 1.1. 2010 – 31.12.2010 Pracoviště<br />
pro zpracování historie chemického průmyslu a aplikované chemie, včetně oborů pro<br />
přípravu učitelů, spoluřešitel<br />
1042/Aa/2009, FRVŠ, 1.1.2009 – 31.12.2009, Rozvoj a inovace laboratoří<br />
pro praktickou výuku studentů v základních povinných předmětech, hlavní<br />
řešitel<br />
203/06/0285, Grantová agentura ČR, 1.1 2006‐31.12 2009, Fotoaktivní<br />
molekulární elektronické prvky, spoluřešitel<br />
IAA401770601, GAAV, 1.1.2006 — 31.12.2009 Elektronové procesy na<br />
molekulární úrovni v látkách vhodných pro organické fotocitlivé součástky,<br />
hlavní řešitel<br />
10<br />
10<br />
20<br />
6<br />
6<br />
6<br />
6<br />
6<br />
6<br />
6<br />
6<br />
6<br />
6<br />
25
11<br />
12<br />
13<br />
14<br />
15<br />
2006/2999/F1d, FRVŠ ,Tvorba interaktivních učebních opor pro výuku<br />
matematiky, fyziky a fyzikální chemie, hlavní řešitel<br />
GA203/03/63D133, Grantová agentura ČR, 1.9 2003‐31.8 2006, Název:<br />
Světlem řízený molekulární proudový spínač, hlavní řešitel<br />
1662/F1 d/2004, FRVŠ, 1.1.2004 – 31.12.2004, Internetový multimediální<br />
učební text pro výuku fyziky, hlavní řešitel<br />
01301/A/2001, FRVŠ, 1.1.2001 – 31.12.2001,Vybudování pracoviště<br />
informačních technologií pro výuku fyziky, hlavní řešitel<br />
0132/F1/2001, FRVŠ, 1.1.2001 – 31.12.2001, Inovace fyzikálního praktika<br />
pro potřeby fakuty chemické, spoluřešitel<br />
Celkem bodů za položku<br />
26. Členství v grantových komisích, radách výzkumných programů<br />
Poř. č Body<br />
1 Člen hodnotícího panelu P205 Grantové agentury ČR 2<br />
2<br />
Vedoucí výzkumného programu Pokročilé organické materiály a biomateriály<br />
Centra materiálového výzkumu FCH VUT v Brně<br />
3 Člen grantové komise Interní grantové agentury VUT 2<br />
Celkem bodů za položku<br />
27. Posudek zahraniční publikace nebo projektu, znalecký posudek,<br />
expertíza<br />
Poř. č Body<br />
1<br />
2<br />
3<br />
Evropská komise, Brusel: od roku 2009 působím jako hodnotitel<br />
evropských projektů 7. rámcového programu pro oblast Nanotechnologií a<br />
ICT – fotonika (EC, Brusel), v rámco hodnocení jsem doposud vypracoval<br />
odborné posudky a expertízy pro 25 projektů 7. rámcového programu<br />
posudky zahraničních publikací pro mezinárodní časopisy Sensors (ISSN:<br />
1424‐8220), Chemical Papers (ISSN: 0336‐6352), Nanoformulation 2011,<br />
Chemical Physics (ISSN: 0301‐0104), celkem minimálně 20 posudků<br />
Agentúra na podporu výskumu a vývoja : externí posuzovatel projektů,<br />
celkem hodnoceny 3 projekty<br />
Celkem bodů za položku<br />
6<br />
6<br />
6<br />
6<br />
6<br />
90<br />
2<br />
6<br />
75<br />
60<br />
9<br />
144<br />
26
29. Posudek domácí publikace nebo projektu<br />
Poř. č Body<br />
1<br />
Člen hodnotícího panelu P205 Grantové agentury ČR, vrámci panelu jsem<br />
zpracoval posudky na 12 projektů, dalších<br />
2 Externí posuzovatel GA ČR, GAAV: celkem hodnoceno 9 projektů 18<br />
3 Hodnotitel TAČR: celkem zpracováno 10 posudků 20<br />
4<br />
Hodnotitel projektů OPVK v oblasti vědy, výzkumu a vzdělávání (prioritní<br />
osy 2.2, 2.3, 2.4), celkem zpracováno 24 posudků projektů<br />
5 Grantová agentura Univerzity Karlovy: celkem zpracovány 3 posudky 6<br />
6 Interní grantová agentura VUT v Brně: celkem zpracovány 4 posudky 8<br />
7 Posuzovatel projektů FRVŠ, celkem zpracováno 12 posudků 24<br />
Celkem bodů za položku<br />
B. Pedagogická činnost<br />
1. Pedagogický úvazek<br />
Poř. č Body<br />
1<br />
Na FCH VUT v Brně působím jako odborný asistent(později docent) na plný<br />
úvazek od 16.2.1998.<br />
Celkem bodů za položku<br />
3. Zavedení předmětu, který byl vyučován v posledních pěti letech<br />
Poř. č Body<br />
1<br />
2<br />
3<br />
Pokročilé materiálové technologie: přednášky, vyučován od roku 2007/08<br />
v navazujícím studijním programu Spotřební chemie.<br />
Advanced Applications of Molecular Materials : přednášky, předmět<br />
s výukou v angličtině, vyučován od roku 2009/10 v navazujícím studijním<br />
programu Spotřební chemie.<br />
Měřící technika: přenášky a cvičení, vyučován od roku 2006/07 ve všech<br />
bakalářských studijních programech FCH.<br />
4 Chemická informatika I: předmět byl zaveden a je vyučován od školního 15<br />
24<br />
48<br />
148<br />
28<br />
28<br />
15<br />
15<br />
15<br />
27
5<br />
roku 2005/06 ve všech bakalářských studijních programech FCH..<br />
Chemická informatika II: předmět byl zaveden a je vyučován od školního<br />
roku 2005/06 ve všech bakalářských studijních programech FCH..<br />
Celkem bodů za položku<br />
4. Vedoucí obhájené bakalářské/diplomové práce<br />
Poř. č Body<br />
1 vedoucí úspěšně obhájených 5 bakalářských prací 5<br />
2 vedoucí úspěšně obhájených 12 diplomových prací 24<br />
Celkem bodů za položku<br />
5. Školitel/školitel specialista studenta, který získal Ph.D.<br />
Poř. č Body<br />
1<br />
Školitel 1 studenta, který obhájil Ph.D. (v současné době další 3 studenti před<br />
obhajobou)<br />
Celkem bodů za položku<br />
8. Skripta<br />
Poř. č Body<br />
1<br />
2<br />
Salyk O., Weiter M., FYZIKA ‐ Laboratorní cvičení, skripta FCH VUT v Brně,<br />
2003 ISBN 80‐214‐2467‐2 – 7 autorských archů<br />
Weiter, M., Elektronika a měřící technika návody pro laboratorní cvičení. 1.<br />
vydání, FCH VUT, Brno, 2000 – 2 autorské archy<br />
Celkem bodů za položku<br />
15<br />
75<br />
29<br />
15<br />
15<br />
14<br />
4<br />
18<br />
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9. Vytvoření významné výukové pomůcky (film, video, software)<br />
Poř. č Body<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
Chemická informatika I: Vytvoření komplexních učebních opor pokrývající<br />
výuku celého tematického obsahu předmětu včetně elearningového kurzu pro<br />
podporu zejména kombinované ale i prezenční formy studia. Kromě<br />
studijních m,ateriálů opory zahrnují testy a autotesty, zpětnovazebné a<br />
komunikační prvky, a multimediální návody pro řešení vybraných úloh.<br />
Chemická informatika II: Vytvoření komplexních učebních opor pokrývající<br />
výuku celého tematického obsahu předmětu včetně elearningového kurzu pro<br />
podporu zejména kombinované ale i prezenční formy studia. Kromě<br />
studijních m,ateriálů opory zahrnují testy a autotesty, zpětnovazebné a<br />
komunikační prvky, a multimediální návody pro řešení vybraných úloh.<br />
Praktikum z fyziky: Vytvoření komplexních učebních opor pokrývající výuku<br />
celého tematického obsahu předmětu včetně elearningového kurzu pro<br />
podporu prezenční formy studia. Kromě standardních materiálů k úlohám<br />
materiály zahrnují i multimediální návody k jednotliovým úlohám a<br />
k metodice zpracování a vyhodnocení experimentálních dat. Podpora výuky<br />
zahrnuje rovněž implementaci systému LabView využívaného pro řízení<br />
experimentu, sběr a vyhodnocení dat.<br />
Pokročilé materiálové technologie: Vytvoření komplexních učebních opor<br />
pokrývající výuku celého tematického obsahu předmětu včetně<br />
elearningového kurzu pro podporu zejména kombinované ale i prezenční<br />
formy studia.<br />
Advanced Applications of Molecular Materials : Vytvoření komplexních<br />
učebních opor pokrývající výuku celého tematického obsahu předmětu včetně<br />
elearningového kurzu pro podporu zejména kombinované ale i prezenční<br />
formy studia.<br />
Měřící technika: Vytvoření komplexních učebních opor pokrývající výuku<br />
celého tematického obsahu předmětu včetně elearningového kurzu pro<br />
podporu zejména kombinované ale i prezenční formy studia.<br />
Podíl na implementaci vzdělávacího systému Moodle pro podporu výuky<br />
na celém VUT vBrně, metodická podpora jeho využívání (od roku 2005 do<br />
současnosti).<br />
Návrh a implementace LCMS (Learning content management system)<br />
systému MOODLE pro e‐learningovou podporu výuky na Fakultě chemické,<br />
metodická a jiná podpora jeho využívání (od roku 2005 do současnosti).<br />
Celkem bodů za položku<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
80<br />
29
Poř. č<br />
1<br />
2<br />
Poř. č<br />
1<br />
Poř. č<br />
1<br />
11. Člen<br />
člen oboroové<br />
rady do oktorského sstudijního<br />
programu p Chemie, C techhnologie<br />
a<br />
vlastnosti mmateriálů<br />
FC CH VUT v Brrně,<br />
od 2010 0 doposud<br />
člen oboroové<br />
rady Pokročilé P mmateriály<br />
do oktorského studijního programu<br />
Pokročilé mmateriály<br />
a nanovědy n (MMU,<br />
CEITEC VUT), od 20 012 doposudd<br />
Celkem bbodů<br />
za položku<br />
122.<br />
Členstv<br />
V létech 20010‐2012<br />
čle en 4 komisí pro SDZ a obhajoby o diz zertační prááce<br />
Celkem bbodů<br />
za položku<br />
113.<br />
Členst<br />
V létech 20006<br />
‐ 2012 člen č 3 komissí<br />
pro SZZ<br />
Celkem bbodů<br />
za položku<br />
V Brně ddne<br />
1.září. 22012<br />
nství v obo orové raddě<br />
doktors ského stu udijního pprogramu<br />
u<br />
ví v komis si pro státtní<br />
doktor rskou zko oušku nebbo<br />
obhajo obu<br />
diseertační<br />
pr ráce<br />
tví v komisi<br />
pro stáátní<br />
závěr rečné zko oušky v jeddnom<br />
roc ce<br />
Body<br />
doc. Ing. Martin Weiter,<br />
PhD.<br />
2<br />
2<br />
4<br />
Body<br />
4<br />
4<br />
Body<br />
3<br />
3<br />
30
Souhrnné vyjádření uchazeče<br />
k bodovému hodnocení<br />
podle čl. 3 odst.1 písm. b) Směrnice rektoraVUT v Brně 1/2006<br />
(v úplném, znění z 1.9.2008)<br />
V předchozích tabulkách autoevaluačních kritérií jsou sumarizovány výsledky a výstupy<br />
mého 14‐letého působení na Fakultě chemické v roli akademického pracovníka.<br />
Sumárně lze konstatovat, že všechny kritéria byly spolehlivě naplněny, přičemž jejich<br />
stanovená minimální hodnota byla vždy několikanásobně překročena. Jak vyplývá<br />
z uvedeného bodového hodnocení, byly dominantními faktory, které nejvíce přispěly<br />
k dosaženému bodovému hodnocení následující výstupy:<br />
a) Publikace v impaktovaných časopisech (kategorie A2+A3, celkem 315 bodů tj<br />
22% z celkového počtu),<br />
b) Citace jiným autorem (kategorie A6, celkem 225 bodů tj .16% z celkového počtu),<br />
c) Výstupy pedagogické činnosti (kategorie B, celkem 220 bodů tj .18% z celkového<br />
počtu),<br />
d) Výstupy z grantové činnosti včetně posudků projektů a publikací (kategorie A23 ‐<br />
29, celkem 402 bodů, tj .28% z celkového počtu).<br />
V následujících odstavcích jsou výsledky bodového hodnocení uchazeče v souladu se<br />
Směrnicí blíže komentovány.<br />
Vědecká a odborná činnost<br />
Výsledky zahrnuté v hodnocení odborné činnosti vychází především z mého odborného<br />
působení na Fakultě chemické VUT v Brně, základem části publikovaných výsledků byly<br />
také zkušenosti a výstupy získané během mého 9‐měsíčního pracovního<br />
postdoktorandského pobytu na Institutu Fyzikální, Makromolekulární a Nukleární<br />
Chemie, Philipps University Marburg v Německu v pracovní skupině prof. Bässlera.<br />
Publikované výsledky z poslední doby byly příznivě ovlivněny vybudováním Centra<br />
materiálového výzkumu FCH VUT v Brně (podpořené projektem OP VaVpI), v rámci<br />
kterého působím jako Vedoucí výzkumného programu Pokročilé organické materiály a<br />
biomateriály, do kterého je zapojeno více než 30 pracovníků.<br />
Většina výsledků publikovaných v impaktovaných časopisech byly vytvořena na základě<br />
rozsáhlé spolupráce s předními domácím i zahraničními výzkumnými pracovišti a<br />
firmami, mezi které patří například The Dyson Perrins Laboratory na Oxford University,<br />
IMEC v Belgii, Ústav makromolekulární chemie AV ČR, Fyzikální ústav AV ČR, Bergishe<br />
Universitat Wuppertal v Německu, Univerzita ve Vilniusu v Litvě, Institute of Physical<br />
31
and Theeoretical<br />
Chhemistry,<br />
Wroclaw W Unniversity<br />
of<br />
Technology,<br />
firma GGENERI<br />
BIO OTECH a<br />
dalších celkem vícce<br />
než 25 pracovišť,<br />
kkteré<br />
participovaly<br />
na publikovanných<br />
pracíc ch<br />
uvedenných<br />
v autoevaluačních<br />
kritériíchh.<br />
Za klíčoové<br />
doložiteelné<br />
výsled dky ve vědeecké<br />
a odbo orné oblast ti, které se nnásledně<br />
odrazily o<br />
na výši jednotlivých<br />
hodnotí ících kritériií,<br />
považuji i zejména:<br />
Vytvoření<br />
nové pracovní<br />
skuppiny<br />
•sezaměřřením<br />
výzk kumu na Pookročilé<br />
materiály m pro<br />
organicckou<br />
elektronniku<br />
a foto oniku včetnně<br />
jejího od dborného vedení, v perssonálního<br />
a<br />
experimmentálního<br />
zabezpečen z ní.<br />
Nové matteriály<br />
a no ové poznattky<br />
•voblastiistudia<br />
elementárníchh<br />
elektrono ových jevů a procesů sspojených<br />
předevšíím<br />
s fotoge enerací a traansportem<br />
m náboje v organických<br />
o h<br />
polovodičích.<br />
Aplikovanné<br />
výsledk ky výzkummu<br />
•dosažennézejména<br />
vrámciřeššení<br />
projek ktů 7. rámco ového proggramu<br />
a<br />
MPO.<br />
Zapojení do meziná árodního vvýzkumu,<br />
•participaace<br />
v evrop pských koopperativních<br />
h projektec ch a platforrmách<br />
(např.<br />
FP7 – Deephotex,<br />
Or rganisolar, Organic Ele ectronic As ssociation a další).<br />
Podíl na zzaložení<br />
a stabilizacci<br />
Centra materiálové<br />
m ého<br />
výzkumuu<br />
FCH VUT v Brně,<br />
• vrámcikterého<br />
pů ůsobím jakoo<br />
Vedoucí výzkumnéh v ho programmu<br />
Pokročil lé<br />
organickké<br />
materiály<br />
a biomatteriály,<br />
do kterého k je zapojeno<br />
vííce<br />
než 30<br />
pracovníků.<br />
Naprostá<br />
většina ppublikovan<br />
ných obornných<br />
prací vznikala v ve vynikající tvůrčí atmosféře<br />
mé praccovní<br />
skuppiny<br />
zaměře ené na Pokkročilé<br />
mat teriály pro o organickkou<br />
elektro oniku a<br />
fotonikku.<br />
Tuto zppočátku<br />
nef formální prracovní<br />
sku upinu jsem založil po ssvém<br />
návra atu ze<br />
zahraniičního<br />
pobyytu<br />
v roce 2003. 2 Skuppina<br />
se pers sonálně sta abilizovala a rozrůstal la<br />
předevšším<br />
na základě<br />
granto ových a dallších<br />
výzku umných pro ojektů a úkoolů,<br />
do kter rých<br />
bylo zappojováno<br />
sstále<br />
více sp polupracovvníků<br />
a stud dentů. (Celkem<br />
bylo v rámci sku upiny<br />
řešeno v létech 20003<br />
‐2012 cca 17 projjektů<br />
s celk kovým rozp počtem (přřipadajícím<br />
m na<br />
pracoviiště)<br />
převyyšujícím<br />
30 miliónů Kčč..<br />
V souvislosti<br />
se vzn nikem Centtra<br />
materiálového<br />
výzkummu<br />
FCH VUTT<br />
v Brně (d dále jen CMMV)<br />
se potom m tato prac covní skupiina<br />
etablov val i po<br />
formálnní<br />
stránce a je pevnou u součástí vvýzkumnéh<br />
ho programu<br />
CMV Pokkročilé<br />
orga anické<br />
32
materiály a biomateriály, za jehož vedení jsem odpovědný. Odborné kompetence,<br />
erudice a profesionální přístup mých spolupracovníků (včetně studentů a doktorandů)<br />
jakožto spoluautorů publikací tak výrazně přispěly k naplnění odborných hodnotících<br />
kritérií1 .<br />
Vznik publikací podpořila i řada a výzkumných grantových projektů, jejichž výčet je<br />
uveden v rámci hodnotících kritérií. Jednalo se o projekty různých poskytovatelů.<br />
V první fázi se jednalo především o projekty základního výzkumu zaměřené na<br />
charakterizaci elementárních elektronových jevů a procesů v organických polovodičích<br />
s důrazem fotogenerací a transportem náboje v organických polovodičích.<br />
V posledních zhruba 6 letech na tento základní výzkum, kterému je stále v naší skupině<br />
věnována velká pozornost, navazuje výzkum aplikovaný, zaměřený do oblasti vývoje<br />
nových materiálů a jejich aplikace v organické elektronice a fotonice. Toto je spojeno i<br />
s vývojem nových technologií pro depozice organických materiálů. V současné době je<br />
připravována průmyslová právní ochrana dvou nových technologií pro<br />
elektroforetickou depozici a sprejové nanášení organických materiálů pro organickou<br />
elektroniku.<br />
Příkladem úspěšně řešeného komplexního projektu může být projekt 7. Rámcového<br />
programu DEPHOTEX ‐ Development of Photovoltaic Textiles based on novel Fibres ,<br />
který byl zaměřen na vývoj fotovoltaických textilií za použití materiálů a technologií,<br />
které by byly snadno průmyslově realizovatelné. Výsledkem projektu potom byly vzorky<br />
fotovoltaických textilií, z nichž některé byly připraveny i na našem pracovišti. Na řešení<br />
se podílelo 15 partnerů, kteří byli převážně SME nebo technologické firmy a centra. Ze<br />
zajímavých partnerů stojí za zmínku výzkumné centrum FIAT nebo textilky GRADO<br />
ZERO ESPACE a TÊXTEIS PENEDO, které mají zájem aplikovat fotovoltaické textilie ve<br />
výrobcích pro domácí a hotelový sektor (záclony, závěsy, apod.) respektive ve výrobcích<br />
pro sport a zábavu (bundy, batohy, kabelky apod.).<br />
Pedagogická činnost<br />
Pedagogické hodnocení vychází především z mé 14‐leté pedagogické činnosti na Fakultě<br />
chemické VUT v Brně. Do výuky jsem byl zapojen jako asistent/odborný asistent<br />
v předmětech bakalářských studijních programů FCH VUT, od akademického roku<br />
2006/07 potom také jako docent a garant předmětů v navazujícím studijním programu<br />
Spotřební chemie. Do tohoto období spadá přechod na třístupňový vzdělávací systém a<br />
s ním spojená restrukturalizace a inovace studijních programů na FCH VUT v Brně, na<br />
které jsem se významně podílel. Od září 2012 působím rovněž jako proděkan pro<br />
1<br />
V souladu se Směrnicí je tento faktor v hodnocení reflektován tím, že publikace jsou<br />
hodnoceny pouze polovičním počtem bodů.<br />
33
pedagoogickou<br />
činnnost.<br />
Pro účely ú vyjádřření<br />
k bodo ovému hod dnocení jsemm<br />
výstupy své<br />
pedagoogické<br />
činnoosti<br />
rozděli il do dvou kkategorií<br />
so ouvisejících h se zabezppečením<br />
výuky v a<br />
rozvojeem<br />
výuky.<br />
Z hledisska<br />
mého zzapojení<br />
do o výuky a zabezpeče ení výuky jsou j výše uuvedené<br />
hodnoty<br />
autoevaaluačních<br />
kkritérií<br />
dané<br />
předevšímm<br />
těmito fa aktory:<br />
Zavedeníí<br />
nových př ředmětů vvčetně<br />
přípr ravy učebn ních<br />
opor pro ttyto<br />
předm měty, garantt<br />
předmětů ů s pravideln nou<br />
výukou v těchto před dmětech:<br />
• Měřící ttechnika:<br />
přenášky p a cvičení, vy yučován od d roku 20077<br />
ve všech<br />
bakalářsských<br />
studi ijních proggramech<br />
FC CH.<br />
• Pokročiilé<br />
materiá álové technnologie:<br />
př řednášky, vyučován v odd<br />
roku 200 08<br />
v navazuujícím<br />
studijním<br />
progrramu<br />
Spotř řební chem mie.<br />
• Advanceed<br />
Applica ations of MMolecular<br />
Materials M : přednáškyy,<br />
předmět<br />
s výukouu<br />
v angličtin ně, vyučováán<br />
od roku 2010 v nav vazujícím sstudijním<br />
programmu<br />
Spotřebn ní chemie.<br />
• Chemickká<br />
informa atika I a III:<br />
předměty y byl zavede eny a jsou vvyučovány<br />
od školnního<br />
roku 2005/06.<br />
• Praktikuum<br />
změří ící technikky:<br />
vyučová án 1999‐2000<br />
ve všechh<br />
bakalářsských<br />
studi ijních proggramech<br />
FC CH, s reorga anizací studia<br />
zrušen.<br />
Podstatnná<br />
inovace stávajícíchh<br />
předmět tů včetně<br />
inovace uučebních<br />
op por, garant ppředmětů<br />
s pravideln nou<br />
výukou v těchto předmětech::<br />
• Chemickká<br />
informa atika I: v ppodstatně<br />
inovované i formě f vyuččován<br />
od<br />
roku 20110/11<br />
ve všech<br />
bakaláářských<br />
stu udijních pr rogramech FCH.<br />
• Chemickká<br />
informa atika II: v ppodstatně<br />
inovované formě vyuučován<br />
od<br />
roku 20110/11<br />
ve všech<br />
bakaláářských<br />
stu udijních pr rogramech FCH.<br />
• Praktikuum<br />
zfyzik ky: v inovovvané<br />
formě ě vyučován od roku 20006/07<br />
ve<br />
všech baakalářských<br />
h studijníchh<br />
programech<br />
FCH.<br />
Vedení sttudentskýc<br />
ch kvalifikkačních<br />
pra ací:<br />
• Bakalářřské<br />
práce:<br />
vedoucí 5 obhájenýc ch prací.<br />
• Diplomoové<br />
práce: vedoucí 122<br />
obhájený ých prací, 2 práce v řeššení.<br />
• Dizertačční<br />
práce: vedoucí 1 oobhájené<br />
práce, p 3 dok ktorandi přřed<br />
obhajobou,<br />
7 prací í v řešení.<br />
34
Rozvoj výuky<br />
Roli vysokoškolského pedagoga nespatřuji pouze v předávání a rozvíjení poznatků<br />
v daném oboru, ale jsem přesvědčen, že velkou pozornost je nutno věnovat rovněž<br />
samotnému pedagogickému procesu, který musí reagovat na zásadní změny v oblasti<br />
vysokoškolského vzdělávání v posledních létech. Kromě samotného přechodu<br />
k Boloňskému systému a s tím související modifikaci studijních programů nelze<br />
pominout ani rapidně vzrůstající počtem VŠ studentů, významné zvyšování podílu<br />
studentů v kombinované formě studia, dominantní roli informačních technologií<br />
v oblasti zpřístupňování vzdělávacího obsahu studentům, měnící se profil současné<br />
generace (tzv. generace Y) a další faktory, které musí současná VŠ pedagogika<br />
reflektovat.<br />
Osobně jsem se k tomuto snažil přispět tím, že jsem již od počátku své pedagogické<br />
kariéry snažil vysokoškolské pedagogice a didaktice porozumět (prostřednictvím 2‐<br />
letého doplňujícího pedagogického studia a samovzděláváním v oblasti VŠ pedagogiky,<br />
didaktiky a andragogiky) a následně využívat v praxi. V roce 2002 jsem na naší fakultě<br />
implementovat a začal používat ve výuce elearningový LCMS systém (Learning contents<br />
management system), který nejen umožnil nahradit nepřehledné a málo efektivní<br />
ukládání a předávání studijních podkladů pro studenty na různých discích a disketách,<br />
ale zároveň je možno nástroje tohoto systému (nástroje pro testy a autoevaluační testy,<br />
příklady s průvodcem řešení, multimediální prvky a mnoho dalších) využít pro podporu<br />
výuky. Na základě našich zkušeností byl potom tento systém v roce 2005<br />
implementován v rámci celého VUT v Brně, na čemž jsem se rovněž podílel<br />
nezanedbatelnou měrou. VUT se tak stalo první univerzitou v ČR, která měla jednotný<br />
elearningový systém a dodnes patří ve světovém měřítku k provozovatelům tohoto<br />
systému s největším počtem uživatelů – více než 22 000 studentů VUT v Brně.<br />
Výše uvedená opatření technologického charakteru by byla samoúčelná a neefektivní<br />
bez souběžné metodické podpory. Za tímto účelem jsou pravidelně pořádány semináře a<br />
školení s cílem vyzvednout a ozřejmit pedagogické aspekty využívání elarningového<br />
systému. V této oblasti intenzivně spolupracuji s našimi předními odborníky<br />
z pedagogických dalších fakult (např. doc. Hrbáček, PedF MU; doc. Bauerová, VŠB TU<br />
Ostrava, dr. Vejvodová ZČU Plzeň a mnoho dalších). Od roku 2009 (respektive 2012) je<br />
tato činnost realizována v rámci dvou projektů OP VK mířených na zvýšení úspěšnosti<br />
studentů kombinovaného studia v bakalářských (respektive navazujících) studijních<br />
programech. U obou projektů v celkové hodnotě přes 20 miliónů korun jsem jejich<br />
ideovým tvůrcem a zároveň i hlavním řešitelem.<br />
35
Za klíčoové<br />
doložiteelné<br />
výsled dky v této ooblasti,<br />
kter ré se násled dně odrazilly<br />
na výši<br />
jednotliivých<br />
autoeevaluačních<br />
h kritérií (zzejména<br />
př říspěvky na a konferenccích<br />
a významné<br />
výukovvé<br />
pomůckyy),<br />
považuji i zejména:<br />
Implemenntace<br />
vzdě ělávacího pprostředí<br />
na n FCH VU UT<br />
(systém MMoodle)<br />
a jeho rozvooj,<br />
• včetněppedagogick<br />
ké podporyy<br />
jeho využí ívání (meto odické mateeriály,<br />
seminářře,<br />
školení a další). V současné<br />
do obě je systé ém využíváán<br />
ve výuce e<br />
více než 100 předm mětů a kurzzů.<br />
Participaace<br />
na impl lementaci vzdělávac cího prostř ředí<br />
na VUT v Brně ( sys stém Mooddle),<br />
•včetněppedagogické<br />
é podpory jjeho<br />
využív vání. V souč časné doběě<br />
je systém<br />
využívánn<br />
ve výuce více v než 4000<br />
předmět tů, kterých se účastní více než<br />
15 000 sstudentů.<br />
Zabezpeččení<br />
ICT, te echnologiccké<br />
a metodické<br />
(pedagoggické)<br />
podp pory výukky<br />
na FCH VUT V v Brně ě,<br />
•vybudovvání<br />
pracov viště pro tvoorbu<br />
multimediálních<br />
h vzdělávaccích<br />
opor,<br />
technoloogická<br />
a ICT T podpora vvýuky<br />
(pod dcasty, zázn namy přednnášek<br />
a<br />
cvičení, mmultimediá<br />
ální návodyy<br />
do praktik k, interakti ivní výuka a další).<br />
36
Přehled a kopie nejvýznamnějších publikací:<br />
Jako příloha této práce jsou uvedeno celkem 30 odborných recenzovaných publikací<br />
publikovaných v časopisech s impakt faktorem zahrnutých v rámci databází Web of<br />
Science, Scopus a dalších.<br />
Seznam přiložených publikací:<br />
1. LUŇÁK, S.; VALA, M.; VYŇUCHAL, J.; OUZZANE, I.; HORÁKOVÁ, P.; MOŽÍŠKOVÁ, P.;<br />
WEITER, M. Absorption and fluorescence of soluble polar diketo‐pyrrolo‐pyrroles. Dyes and<br />
Pigments, 2011, 91(1), p. 269 ‐ 278.<br />
2. DAVID, J.; WEITER, M.; VALA, M.; VYŇUCHAL, J.; KUČERÍK, J. Stability and structural<br />
aspects of diketopyrrolopyrrole pigment and its N‐alkyl derivatives. Dyes and Pigments,<br />
2011, 89(1), p. 137 ‐ 144.<br />
3. LUŇÁK, S.; HAVEL, L.; VYŇUCHAL, J.; HORÁKOVÁ, P.; KUČERÍK, J.; WEITER, M.; HRDINA,<br />
R. The geometry and absorption of diketo‐pyrrolo‐ pyrroles substituted with various aryls.<br />
Dyes and Pigments, 2010, roč. 85, č. 1‐ 2, s. 27‐36.<br />
4. VALA, M.; VYŇUCHAL, J.; WEITER, M.; TOMAN, P.; LUŇÁK, S. Novel, soluble diphenyl‐<br />
diketo‐pyrrolopyrroles: Experimental and theoretical study. Dyes and Pigments, 2010, roč.<br />
84, č. 8, s. 176‐182.<br />
5. KUČERÍK, J.; DAVID, J.; WEITER, M.; VALA, M.; VYŇUCHAL, J.; OUZZANE, I.; SALYK, O.<br />
Stability and physical structure tests of piperidyl and morpholinyl derivatives of diphenyl‐<br />
diketo‐pyrrolopyrroles (DPP). Journal of Thermal Analysis and Calorimetry, 2012, roč.<br />
108, č. 2, s. 467‐473.<br />
6. MLADENOVA, D., WEITER, M., STEPANEK, P., OUZZANE, I., VALA, M., SINIGERSKY, V.,<br />
ZHIVKOV, I. Thin polyphenylene vinylene electrophoretically and spin‐coated films –<br />
photoelectrical properties. Surface and Coatings Technology, 2012, accepted.<br />
7. MLADENOVA, D., WEITER, M., STEPANEK, P., OUZZANE, I., VALA, M., SINIGERSKY, V.,<br />
ZHIVKOV, I. Characterization of electrophoretic suspension for thin polymer film<br />
deposition. Journal ofPhysics:Conference Series, 2012, roč. 356, s. 012040.<br />
8. WEITER, M.; SALYK, O.; BEDNÁŘ, P.; VALA, M.; NAVRÁTIL, J.; ZMEŠKAL, O. Morphology<br />
and properties of thin films of diketopyrrolopyrrole derivatives. Materials Science and<br />
Engineering A, 2009, 165(3), p. 148 ‐ 152.<br />
9. TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; BARTKOWIAK, W.; MENŠÍK, M. Model<br />
of the influence of energetic disorder on inter‐chain charge carrier mobility in poly[2‐<br />
methoxy‐5‐(2 ‐ethylhexyloxy)‐p‐phenylene vinylene]. Polymers for Advanced Technologies,<br />
2009, 20(3), p. 263 – 267.<br />
10. WEITER, M.; NAVRÁTIL, J.; VALA, M.; TOMAN, P. Photoinduced reversible switching of<br />
charge carrier mobility in conjugated polymers. European Physical Journal‐Applied<br />
Physics, 2009, 48(1), p. 10401 ‐ 10406. ISSN 1286‐0042.<br />
11. ZMEŠKAL, O.; VALA, M.; WEITER, M.; ŠTEFKOVÁ, P. Fractal‐cantorian geometry of space‐<br />
time. Chaos, Solitons & Fractals, 2009, 42(3), p. 1878 ‐ 1892.<br />
37
12. ZMEŠKAL, O.; WEITER, M.; VALA, M. Notes to "An irreducibly simple derivation of the<br />
Hausdorff dimension of spacetime" by M.S. El Naschie. Chaos, Solitons & Fractals. 2009,<br />
42(10), p. 532 ‐ 533.<br />
13. VALA, M.; WEITER, M.; VYŇUCHAL, J.; TOMAN, P.; LUŇÁK, S. Comparative Studies of<br />
Diphenyl‐Diketo‐Pyrrolopyrrole Derivatives for Electroluminescence Applications. Journal<br />
of Fluorescence, 2008, 18(6), p. 1181 ‐ 1185.<br />
14. KRATOCHVÍLOVÁ, I.; KRÁL, K.; BUNČEK, M.; VÍŠKOVÁ, A.; NEŠPŮREK, S.; KOCHALSKA, A.;<br />
TODORCIUC, T.; WEITER, M.; SCHNEIDER, B. Conductivity of natural and modified DNA<br />
measured by Scanning Tunneling Micros<strong>copy</strong>. The effect of sequence, charge and stacking.<br />
Biophysical Chemistry, 2008, roč. 2008, č. 11, s. 3‐10.<br />
15. KRATOCHVÍLOVÁ, I.; KRÁL, K.; BUNČEK, M.; NEŠPŮREK, S.; TODORCIUC, T.; WEITER, M.;<br />
NAVRÁTIL, J.; SCHNEIDER, B.; PAVLUCH, J. Scanning Tunneling Spectros<strong>copy</strong> Study of<br />
DNA Conductivity. Central European Journal of Physics, 2008, roč. 6, č. 3, s. 422‐426.<br />
16. VALA, M.; WEITER, M.; ZMEŠKAL, O.; NEŠPŮREK, S.; TOMAN, P. Light Induced Change of<br />
Charge Carrier Mobility in Semiconducting Polymers. Macromolecular symposia, 2008,<br />
268(1), p. 125 ‐ 128.<br />
17. VALA, M.; WEITER, M.; RAJTROVÁ, G.; NEŠPŮREK, S.; SWORAKOWSKI, J. Photochromic<br />
properties of spiropyran in polymeric pi‐conjugated matrices. Nonlinear Optics, Quantum<br />
optics: Concepts in Modern Optics, 2007, 37(1‐3), p. 53 ‐ 63.<br />
18. WEITER, M.; VALA, M.; ZMEŠKAL, O.; NEŠPŮREK, S.; TOMAN, P. A Molecular Photosensor<br />
Based on Photoswitching of Charge Carrier Mobility. Macromolecular Symposia, 2007,<br />
2007(247), p. 318 ‐ 325.<br />
19. WEITER, M.; VALA, M.; ZMEŠKAL, O.; NAVRÁTIL, J., TOMAN, P. NEŠPŮREK, S.;. Polymer<br />
optical sensor based on photochromic switching of charge carrier mobility. Optical Sensing<br />
Technology and Applications, 2007, č. 6585, s. 658519.<br />
20. ZMEŠKAL, O.; NEŠPŮREK, S.; WEITER, M. Space‐charge‐limited currents: An E‐ infinity<br />
Cantorian approach. Chaos, Solitons & Fractals, 2007, roč. 34, č. 2, s. 143‐158.<br />
21. TOMAN, P.; NEŠPŮREK, S.; WEITER, M.; VALA, M.; SWORAKOWSKI, J.; BARTKOWIAK, W.;<br />
MENŠÍK, M. Influence of dipolar species on charge transport in poly[2‐methoxy‐5‐(2 '‐<br />
ethylhexyloxy)‐p‐phenylene vinylene]. Polymers for Advanced Technologies, 2006, 17(9‐<br />
10), p. 673 ‐ 678.<br />
22. WEITER, M.; BAESSLER, H. Transient photoconductivity and charge generation in thin<br />
films of pi‐ conjugated polymers. Journal of Luminescence, 2005, roč. 112, č. 1‐ 4, s. 363‐<br />
367.<br />
23. WEITER, M.; VALA, M.; NEŠPŮREK, S.; SWORAKOVSKI, J.; SALYK, O.; ZMEŠKAL, O.<br />
Reversible formation of charge carrier traps in poly(phenylenevinylene) derivative due to<br />
the phototransformation of a photochromic additive. Molecular Crystals and Liquid<br />
Crystals, 2005, 430(1), p. 227 ‐ 233..<br />
24. SALYK, O.; BROŽA, P.; DOKOUPIL, N.; HERRMANN, R.; KUŘITKA, I.; PRYČEK, J.; WEITER,<br />
M. Plasma polymerisation of methylphenylsilane. Surface and Coatings Technology, 2005,<br />
roč. 200, č. 1‐ 4, s. 486‐489.<br />
38
25. MARKHAM, J.; SAMUEL, I.; BURN, S.; WEITER, M.; BAESSLER, H. Charge transport in<br />
highly efficient iridium cored electrophosphorescent dendrimers. Journal of Aplied Physics,<br />
2004, roč. 95, č. 2, s. 438 ( s.)<br />
26. NEŠPŮREK, S., SWORAKOWSKI, J., COMBELLAS, C., WANG, G., WEITER, M. A molecular<br />
device based on light controlled charge carrier mobility. Applied Surface Science, 2004,<br />
roč. 234, č. 1–4, s. 395–402.<br />
27. WEITER, M.; ARKHIPOV, V.; BAESSLER, H. Transient photoconductivity in a thin film of a<br />
polyphenylenevinylene type conjugated polymer. Synthetic Metals, 2004, roč. 141, č. 1‐ 2, s.<br />
165 ( s.)<br />
28. WEITER, M. BAESSLER, H.; GULBINAS, V;, SCHERF, U. Transient photoconductivity in a<br />
film of ladder‐type poly‐phenylene: Failure of the Onsager approach. Chemical Physics<br />
Letters, 2003, roč. 379, č. 1, s. 117 ( s.).<br />
29. HORVÁTH, P., SCHAUER, F., WEITER, M., KUŘITKA, I., SALYK, O., DOKOUPIL, N.,<br />
NEŠPŮREK, S., FIDLER, V. Luminescence in organic silicons prepared from organic<br />
precursors in plasma discharges. Chemical Monthly, roč. 132, č. 2001, s. 177‐ 185 ( s.)<br />
30. HANDLÍŘ, R., NEŠPŮREK, S., SCHAUER, F., WEITER, M., KUŘITKA, I. Metastability in<br />
poly(methylsilylene) induced by UV radiation and electron beam. Journal of Non‐ Crystaline<br />
Solids, roč. 1998, č. 227‐ 230, s. 669 ( s.).<br />
39
Příloha č. 1 – přiložené publikace<br />
41
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
Absorption and fluorescence of soluble polar diketo-pyrrolo-pyrroles<br />
Stanislav Lunák Jr. a , Martin Vala b, *, Jan Vynuchal a,c,d , Imad Ouzzane b , Petra Horáková a ,<br />
Petra Mozísková b , Zdenek Eliás a , Martin Weiter b<br />
a Faculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-530 09 Pardubice, Czech Republic<br />
b Faculty of Chemistry, Centre for Materials Research, Brno University of Technology, Purkynova 464/118, CZ-612 00 Brno, Czech Republic<br />
c Research Institute of Organic Syntheses, Rybitví 296, CZ-533 54 Rybitví, Czech Republic<br />
d Synthesia a.s., Pardubice, Semtín 103, CZ-532 17 Pardubice, Czech Republic<br />
article info<br />
Article history:<br />
Received 26 October 2010<br />
Received in revised form<br />
29 April 2011<br />
Accepted 4 May 2011<br />
Available online 19 May 2011<br />
Keywords:<br />
Diketo-pyrrolo-pyrrole<br />
Fluorescence<br />
Solvatochromism<br />
Solid-state fluorescence<br />
Twisted intramolecular charge transfer<br />
Organic electronics<br />
1. Introduction<br />
abstract<br />
Although originally developed as high performance organic<br />
pigments [1,2,3], various structural modifications made diketopyrrolo-pyrroles<br />
(DPPs) interesting as advanced materials for<br />
modern optical and electronic technologies. The devices based on<br />
DPPs copolymerized with e.g. carbazoles [4] and especially with<br />
oligothiophenes [5,6] reached the promising efficiencies as organic<br />
field-effect transistors (OFET) [5] and bulk heterojunction (BHJ)<br />
photovoltaic solar cells (OPV) [4,6]. Aside from DPP copolymers,<br />
which form rather specific area with dramatically increasing<br />
number of references, DPP monomers have been recently found<br />
also to be perspective in photovoltaics, either in the BHJ OPV [7],or<br />
in dye-sensitized solar cells (DSSC) [8] type. The common structural<br />
features of DPP derivatives designed for these purposes are:<br />
The substituted (usually alkylated, in some cases acylated [9])<br />
pyrrolinone nitrogens, changing the insoluble pigments to<br />
molecules enabling wet solution based processing, and the<br />
presence of electron-donating groups as a counterpart to diketopyrrolo-pyrrole<br />
core with an electron-accepting character.<br />
* Corresponding author. Tel.: þ420 541 149 411; fax: þ420 541 149 398.<br />
E-mail address: vala@fch.vutbr.cz (M. Vala).<br />
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.<br />
doi:10.1016/j.dyepig.2011.05.004<br />
Dyes and Pigments 91 (2011) 269e278<br />
Contents lists available at ScienceDirect<br />
Dyes and Pigments<br />
journal homepage: www.elsevier.com/locate/dyepig<br />
Six soluble derivatives of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione N-alkylated on<br />
pyrrolinone ring with polar substituents in para positions of pendant phenyl rings were synthesized; five<br />
of them are reported for the first time. Absorption and fluorescence spectra were studied in solvents of<br />
different polarity. The compounds show small solvatochromism of absorption and a moderate positive<br />
solvatochromism of fluorescence, especially when substituted by strong electron-donating piperidino<br />
substituent. A significant decrease of fluorescence quantum yields and its biexponential decay for dipolar<br />
derivatives in polar solvents was tentatively ascribed to the formation of twisted intramolecular charge<br />
transfer (TICT) excited state. All six compounds show fluorescence in polycrystalline solid-state with the<br />
maxima covering a range over 200 nm in visible and near infrared region.<br />
Ó 2011 Elsevier Ltd. All rights reserved.<br />
We have recently published the syntheses and spectral properties<br />
of parent DPP chromophore 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione<br />
(I) and its electron-donor (piperidino)<br />
and electron-acceptor (cyano) symmetrically and unsymmetrically<br />
substituted derivatives (IIeVI, Fig. 1) [10]. The derivatives have<br />
shown bathochromic and hyperchromic shift of absorption and<br />
bathofluoric shift of fluorescence with respect to parent compound<br />
I invoked above all by piperidino electron-donating substituent.<br />
Dimethyl sulfoxide (DMSO) was found to be the only common<br />
solvent able to dissolve all these pigments. In order to make these<br />
compounds better treatable we have decided to substitute them<br />
on pyrrolinone nitrogens and so eliminate the intermolecular<br />
hydrogen bonding [1]. On the contrary to more usual N-alkylation<br />
by alkylhalogen, used in our previous studies [11,12], we applied<br />
ethyl bromoacetate in this case (Fig. 1). Such substitution was reported<br />
only once resulting in compound VII with highly distorted<br />
structure in crystal according to X-ray diffraction [13] giving thus<br />
a good chance to be highly soluble because of the absence of pep<br />
stacking, another insolubility implicating phenomenon aside from<br />
COeNH hydrogen bonding [14]. Compounds VIIIeXII were never<br />
reported, thus they are fully characterized in Experimental. This is<br />
the first case, to the best of our knowledge, when simple push-pull<br />
substituted well soluble DPP derivative (XII) is studied.
270<br />
DPP derivatives are known for a long time to be strongly fluorescent<br />
in solution [15]. The only reported exceptions to the best<br />
of our knowledge are the compounds, in which the pyrrolinone<br />
carbonyl group underwent a nucleophilic substitution by heteroarylacetonitrile<br />
[16] or bis(trimethylsilyl)carbodiimide [17] forming<br />
the products loosing the true DPP character. The aim of this study<br />
was thus first to investigate in detail the dependence of absorption<br />
and fluorescence spectra and fluorescence efficiencies in solution<br />
on the character of pendant phenyl substitution in VIIIeXII<br />
with respect to on-phenyl unsubstituted VII and N-unsubstituted<br />
precursors IeVI. Contrary to hardly soluble pigments IeVI, itwas<br />
possible also to measure a solvatochromism on VIIeXII, which was<br />
never studied in detail before.<br />
As we observed the luminescence of some of these N,N 0 -<br />
dialkylated DPPs in solid-state just by naked eye during the<br />
samples handling (opposite to totally non-luminescent precursors<br />
IeVI), we studied also this not particularly common behaviour but<br />
being of growing interest [18,19,20]. Since the pioneering work<br />
of Langhals [21], the solid-state fluorescence of several DPP derivatives<br />
was mentioned in literature [9,22,23], but the full understanding<br />
of this phenomenon requires more systematic studies on<br />
the representative series of derivatives.<br />
2. Experimental<br />
2.1. Syntheses and analyses<br />
HN<br />
O<br />
R1<br />
The synthesis of the starting derivatives IeVI was described in<br />
Ref. [10]. Compound VII was synthesized from compound I in<br />
R2<br />
O<br />
NH<br />
Symbol R1 R2<br />
I H H<br />
II CN H<br />
III CN CN<br />
IV H N<br />
V N N<br />
VI CN N<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278<br />
BrCH 2 COOCH 2 CH 3<br />
K 2 CO 3 ,NMP<br />
CH 3 CH 2 OOCCH 2<br />
O<br />
N<br />
R1<br />
R2<br />
N<br />
O<br />
Symbol R1 R2<br />
VII H H<br />
VIII CN H<br />
IX CN CN<br />
X H N<br />
XI N N<br />
XII CN N<br />
Fig. 1. General synthetic route and notation of the compounds under study.<br />
CH 2 COOCH 2 CH 3<br />
a similar way as described earlier and confirmed by melting point<br />
210e212 C (lit. [13]. 207e208 C). N-methyl-2-pyrrolidone (NMP),<br />
ethyl bromoacetate and potassium carbonate were purchased from<br />
Sigma-Aldrich, so as the three solvents used in the spectral studies.<br />
2.1.1. Preparation of diethyl-3-(phenyl)-6-(4-cyanophenyl)-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione-diacetate<br />
(VIII)<br />
Compound II (8 g, 0.0256 mol), potassium carbonate (35.4 g,<br />
0.256 mol) and NMP (480 ml) were charged to a three-necked flask.<br />
Reaction mixture was heated to 120 C. Ethyl bromoacetate (42.8 g,<br />
0.256 mol) in NMP (185 ml) was added to the reactor dropwise<br />
during 40 min. Then the reaction was stirred and heated to 120 C<br />
for 2 h. After cooling, the reaction was slowly poured onto ice-cold<br />
water (1400 ml). The obtained precipitate was filtered off and<br />
washed with water until neutral washings. The crude product was<br />
recrystallized from a mixture of methanol and water (2:1). Yield:<br />
2.02 g (16.29%) of compound VIII. m.p. 139e141 C.<br />
Calculated: C (66.90), H (4.78), N (8.66), Found: C (66.84), H<br />
(4.73), N (8.46)<br />
MW ¼ 485 Da; Positive-ion APCI-MS: m/z 486 [M þ H] þ , (100%)<br />
1 H chemical shifts: 8.12 (2H, m, ArH); 7.99 (2H, m, ArH); 7.84 (2H,<br />
m, ArH); 7.66 (3H, m, ArH); 4.64 (2H, s, eNCH2,); 4.63 (2H, s, eNCH2,);<br />
4.115 (2H, q, J ¼ 7.1 Hz, eOCH2); 4.110 (2H, q, J ¼ 7.1 Hz, eOCH2); 1.142<br />
(3H, t, J ¼ 7.1 Hz, eCH2CH3); 1.140 (3H, t, J ¼ 7.1 Hz, eCH2CH3)<br />
13 C chemical shifts: 168.37 (1C, >C]O); 168.29 (1C, >C]O);<br />
161.30 (1C, >C]O); 161.03 (1C, >C]O); 149.52 (1C, ArC); 145.37<br />
(1C, ArC); 133.02 (2C, ArC); 132.09 (1C, ArC); 131.33 (1C, ArC);<br />
129.26 (2C, ArC); 129.21 (2C, ArC); 128.68 (2C, ArC); 126.88 (1C,<br />
ArC); 118.26 (1C, eC^N); 113.52 (1C, ArC); 109.90 (1C, ArC); 108.47
(1C, ArC); 61.41 (1C, NeCH2); 61.38 (1C, NeCH2); 43.38 (1C, eOCH2);<br />
43.26 (1C, eOCH2); 13.92 (2C, eCH2CH3)<br />
2.1.2. Preparation of diethyl-3,6-di-(4-cyanophenyl)-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione-diacetate<br />
(IX)<br />
Compound III (5 g, 0.0148 mol), potassium carbonate (20.42 g,<br />
0.148 mol) and NMP (272 ml) were charged into a three-necked<br />
flask. Reaction mixture was heated to 120 C and ethyl bromoacetate<br />
(24.7 g, 0.148 mol) in NMP (116 ml) was added dropwise to the<br />
reactor during 40 min. Then the reaction was stirred and heated to<br />
120 C for 2 h. After cooling, the reaction was slowly poured onto<br />
ice-cold water (760 ml). The obtained precipitate was filtered off<br />
and washed with water until neutral washings. The crude product<br />
was recrystallized from methanol. Yield: 0.81 g (10.7%) of orange<br />
compound IX, m.p. 177e179 C.<br />
Calculated: C (68.49), H (6.12), N (7.73), Found: C (68.49), H<br />
(6.04), N (7.63)<br />
1 H chemical shifts: 8.14 (4H, m, ArH); 8.01 (4H, m, ArH); 4.64<br />
(4H, s, eNCH2); 4.10 (4H, q, J ¼ 7.1 Hz, eOCH2); 1.13 (6H, t, J ¼ 7.1 Hz,<br />
eCH2CH3)<br />
13 C chemical shifts: 168.23 (2C, >C]O); 160.97 (2C, >C]O);<br />
146.92 (2C, ArC); 133.05 (4C, ArC); 131.06 (2C, ArC); 129.33 (4C,<br />
ArC); 118.36 (2C, eC^N); 113.82 (2C, ArC); 109.82 (2C, ArC); 61.45<br />
(2C, NeCH2); 43.40 (2C, eOCH2); 13.90 (2C, eCH2CH3)<br />
2.1.3. Preparation of diethyl-3-(phenyl)-6-(4-piperidinophenyl)-<br />
2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione-diacetate (X)<br />
Dry and pure NMP (150 ml), compound IV (3.7 g, 0.01 mol) and<br />
potassium carbonate (15.2 g, 0.11 mol) were charged to a threenecked<br />
flask. Reaction mixture was heated to 120 C. Ethyl bromoacetate<br />
(18.4 g, 0.11 mol) in NMP (80 ml) was added dropwise to<br />
the reactor during 40 min. Then the reaction mixture was stirred<br />
and heated to 120 C for 2 h. After cooling, the reaction was slowly<br />
poured onto ice-cold water (500 ml). The obtained precipitate<br />
was filtered off and washed with water until neutral washings. The<br />
crude product was recrystallized from methanol. Yield: 3.2 g (59%)<br />
of compound X, m.p. 207e214 C.<br />
Calculated: C (65.88), H (4.34), N (10.97), Found: C (65.47), H<br />
(4.44), N (10.82)<br />
MW ¼ 510 Da; Positive-ion APCI-MS: m/z 511 [M þ H] þ (100%)<br />
1 H chemical shifts: 7.83 (2H, m, ArH); 7.78 (2H, m, ArH); 7.61<br />
(3H, m, ArH); 7.1 (2H, m, ArH); 4.66 (2H, s, eNCH2); 4.61 (2H, s,<br />
eNCH2); 4.17 (2H, q, J ¼ 7.1 Hz, eOCH2); 4.12 (2H, q, J ¼ 7.1 Hz,<br />
eOCH2); 3.48 (4H, m, eCH2CH2CH2N); 1.64 (6H, m, eCH2CH2CH2N<br />
and eCH2CH2CH2N); 1.19 (6H, t, J ¼ 7.1 Hz, eCH2CH3); 1.15 (6H, t,<br />
J ¼ 7.1 Hz, eCH2CH3)<br />
13 C chemical shifts: 168.65 (1C, >C]O); 168.61 (1C, >C]O);<br />
161.99 (1C, >C]O); 161.06 (1C, >C]O); 152.74 (1C, ArC); 149.20 (1C,<br />
ArC); 144.33 (1C, ArC); 131.05 (1C, ArC); 130.70 (2C, ArC); 129.03 (2C,<br />
ArC); 128.45 (2C, ArC); 127.59 (1C, ArC); 114.51 (1C, ArC); 113.47 (2C,<br />
ArC); 108.75 (1C, ArC); 105.91 (1C, ArC); 61.29 (1C, NeCH2); 61.21<br />
(1C, NeCH2); 47.69 (2C, eCH2CH2CH2N); 43.88 (1C, eOCH2); 43.34<br />
(1C, eOCH2); 25.00 (2C, eCH2CH2CH2N); 24.07 (1C, eCH2CH2CH2N);<br />
14.02 (1C, eCH2CH3); 13.97 (1C, eCH2CH3)<br />
2.1.4. Preparation of diethyl-3,6-di-(4-piperidinophenyl)-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione-diacetate<br />
(XI)<br />
Compound V (5 g, 0.011 mol), potassium carbonate (15.2 g,<br />
0.11 mol) and NMP (205 ml) were charged into three-necked flask.<br />
Reaction mixture was heated to 120 C and ethyl bromoacetate<br />
(18.38 g, 0.11 mol) in NMP (80 ml) was added dropwise to the<br />
reactor during 40 min. Then the reaction was stirred and heated to<br />
120 C for 2 h. After cooling, the reaction was slowly poured onto<br />
ice-cold water (600 ml). The obtained precipitate was filtered off<br />
and washed with water until neutral washings. The crude product<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278 271<br />
was recrystallized from methanol. Yield: 3.57 g (54.3%) compound<br />
XI, m.p. 213e217 C.<br />
Calculated: C (68.99), H (6.75), N (8.94), Found: C (68.80), H<br />
(6.85), N (8.78)<br />
MW ¼ 626 Da; Positive-ion APCI-MS: m/z 627 [M þ H] þ (100%)<br />
1 H chemical shifts: 7.75 (4H, m, ArH); 7.01 (4H, m, ArH); 4.64<br />
(4H, s, eNCH2); 4.16 (4H, q, J ¼ 7.2 Hz, eOCH2); 3.41 (8H, m,<br />
eCH2CH2CH2N); 1.64 (12H, m, eCH2CH2CH2N and eCH2CH2CH2N);<br />
1.18 (6H, t, J ¼ 7.2 Hz, eCH2CH3)<br />
13 C chemical shifts: 130.27 (4C, ArC); 113.67 (4C, ArC); 61.19 (2C,<br />
NeCH2); 47.83 (4C, eCH2CH2CH2N); 43.80 (2C, eOCH2); 24.97 (4C,<br />
eCH2CH2CH2N); 14.04 (2C, eCH2CH3); Remaining signals were not<br />
detected due to low solubility of the sample.<br />
2.1.5. Preparation of diethyl-3-(4-cyanophenyl)-6-(4piperidinophenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-<br />
1,4-dione-diacetate (XII)<br />
Compound VI (3 g, 0.0076 mol), potassium carbonate (10.5 g,<br />
0.076 mol) and NMP (140 ml) were charged into three-necked flask<br />
(500 ml). Reaction mixture was heated to 120 C and ethyl bromoacetate<br />
(14 g, 0.082 mol) in NMP (60 ml) was added dropwise to<br />
the reactor during 40 min. Then the reaction was stirred and heated<br />
to 120 C for 2 h. After cooling, the reaction was slowly poured onto<br />
ice-cold water (400 ml). The obtained precipitate was filtered off<br />
and washed with water until neutral washings. The crude product<br />
was recrystallized from the mixture of n-hexan and methanol (7:3).<br />
Yield: 2.26 g (70.4%) of compound XII, m.p. 192e195 C.<br />
Calculated: C (67.59), H (5.67), N (9.85), Found: C (67.40), H<br />
(5.72), N (9.65)<br />
MW ¼ 568 Da; Positive-ion APCI-MS: m/z 569 [M þ H] þ (100%)<br />
1 H chemical shifts: 8.08 (2H, m, ArH); 8.01 (2H, m, ArH); 7.87<br />
(2H, m, ArH); 7.10 (2H, m, ArH); 4.69 (2H, s, eNCH2); 4.65 (2H, s,<br />
eNCH2); 4.17 (2H, q, J ¼ 7.0 Hz, eOCH2); 4.11 (2H, q, J ¼ 7.1 Hz,<br />
eOCH2); 3.50 (4H, s, eCH2CH2CH2N); 1.65 (6H, s, eCH2CH2CH2N<br />
and eCH2CH2CH2N); 1.19 (3H, t, J ¼ 7.1 Hz, eCH2CH3); 1.15 (3H, t,<br />
J ¼ 7.0 Hz, eCH2CH3)<br />
13 C chemical shifts: 168.62 (1C, >C]O); 168.57 (1C, >C]O);<br />
162.07 (1C, >C]O); 160.80 (1C, >C]O); 152.95 (1C, ArC); 150.68<br />
(1C, ArC); 141.33 (1C, ArC); 132.92 (2C, ArC); 131.85 (1C, ArC);<br />
131.07 (2C, ArC); 129.04 (2C, ArC); 118.44 (1C, eC^N); 114.04 (1C,<br />
ArC); 113.36 (2C, ArC); 112.73 (1C, ArC); 110.35 (1C, ArC); 105.81<br />
(1C, ArC); 61.39 (1C, NeCH2); 61.32 (1C, NeCH2); 47.63 (2C,<br />
eCH2CH2CH2N); 44.05 (1C, eOCH2); 43.37 (1C, eOCH2); 25.05 (2C,<br />
eCH2CH2CH2N); 24.10 (1C, eCH2CH2CH2N); 14.04 (1C, eCH2CH3);<br />
13.97 (1C, eCH2CH3)<br />
2.2. Structure and purity characterization<br />
2.2.1. Mass spectrometry<br />
The ion trap mass spectrometer MSD TRAP XCT Plus system<br />
(Agilent Technologies, USA) equipped with APCI probe was used.<br />
Positive-ion and negative-ion APCI mass spectra were measured in<br />
the mass range of 50e1000 Da in all the experiments. The ion trap<br />
analyzer was tuned to obtain the optimum response in the range of<br />
the expected m/z values (the target mass was set from m/z 289<br />
to m/z 454). The other APCI ion source parameters: drying gas<br />
flow rate 7 dm 3 min 1 , nebulizer gas pressure 60 psi, drying gas<br />
temperature 350 C, nebulizer gas temperature 350 C. The samples<br />
were dissolved in a mixture of DMSO/acetonitrile and methanol in<br />
various ratios. All the samples were analyzed by means of direct<br />
infusion into LC/MS.<br />
2.2.2. Elemental analysis<br />
Perkin-Elmer PE 2400 SERIES II CHNS/O and EA 1108 FISONS<br />
instruments were used for elemental analyses.
272<br />
2.2.3. Nuclear magnetic resonance<br />
Bruker AVANCE 500 NMR spectrometer operating at<br />
500.13 MHz for 1 H was used for measurements of the 1 H NMR<br />
spectra. The compounds were dissolved in hexadeuteriodimethyl<br />
sulfoxide used as deutered standard. The 1 H chemical shifts were<br />
referred to the central signal of the solvent (d ¼ 2.55).<br />
2.3. Optical characterization<br />
2.3.1. UVeVIS absorption and fluorescence spectros<strong>copy</strong><br />
The referred UVeVIS absorption spectra in solution were<br />
recorded using Varian Carry 50 UVeVIS spectrometer. The fluorescence<br />
spectra in solution were measured in 90 configuration at<br />
Aminco Bowmann S2 fluorimeter. The solid-state luminescence<br />
spectra were recorded on Perkin-Elmer LS 55 equipped with an<br />
accessory for solid-state measurements from the same producer.<br />
Polycrystalline samples were placed under quartz plate and the<br />
emission spectra were recorded using front face geometry.<br />
2.3.2. Fluorescence quantum yields<br />
The fluorescence quantum yields (4F) in solution were calculated<br />
according to the comparative method [24], where for each<br />
test sample gradient of integrated fluorescence intensity versus<br />
absorbance F ¼ f(A) is used to calculate the 4F using two known<br />
standards. The standards were previously cross-calibrated to verify<br />
the method. This calibration revealed accuracy about 5%. As the<br />
reference we used Rhodamine B and Rhodamine 6G with 4Fl 0.49<br />
[25] and 0.950 0.005 [26], respectively. The excitation wavelength<br />
was chosen to be the same as for the laser excited experiments<br />
i.e. 532 nm. Since the VII has very low absorption coefficient<br />
at this exciting wavelength, we used Fluorescein (0.91 0.02) [20]<br />
and Coumarin 6 (0.78) [27] because of the better spectral overlap.<br />
The excitation wavelength in this case was 460 nm.<br />
2.3.3. Fluorescence lifetimes<br />
The fluorescence lifetime sF was measured using Andor Shamrock<br />
SR-303i spectrograph and Andor iStar ICCD camera. The<br />
samples were excited by third harmonic of EKSPLA PG400<br />
Nd:YAG laser (355 nm) with light pulse time duration of w30 ps.<br />
The temporal resolution of the system is approximately 25 ps. In<br />
order to avoid chromatic aberrations, the emitted light from the<br />
sample was collected by two off-axis mirrors.<br />
2.4. Computational procedures<br />
All theoretical calculations for compounds VIIeXII were carried<br />
out on exactly same level as for previously reported precursors IeVI<br />
[10], in order to be directly comparable. The ground state (S0)<br />
geometry was optimized using quantum chemical calculations<br />
based on DFT. Hybrid three-parameter B3LYP functional in combination<br />
with 6-311G(d,p) basis was used. No constraints were<br />
preliminary employed, but, as the nonconstrainted computations<br />
converged to symmetrical structures for compounds VII, IX and<br />
XI, the final computations were carried out with Ci symmetry<br />
constraint. No imaginary frequencies were found by vibrational<br />
analysis, confirming that the computed geometries were real<br />
minima on the ground state hypersurfaces.<br />
TD DFT computations of the vertical excitation energies were<br />
carried out on the computed S0 geometries. The same exchangecorrelation<br />
functional (B3LYP) was used in TD DFT calculations with<br />
rather broader basis set (6-311þG(2d,p)). Solvent effect of DMSO<br />
was involved by non-equilibrium PCM.<br />
All methods were taken from Gaussian09W program suite [28],<br />
and the default values of computational parameters were used. The<br />
results were analyzed using GaussViewW from Gaussian Inc, too.<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278<br />
3. Results and discussion<br />
3.1. Syntheses<br />
Although looking quite simple (Fig. 1), N,N-dialkylation of DPPs<br />
is a two-step process complicated by extremely low solubility<br />
of starting pigments, which sometimes leads to the presence of<br />
mono-alkylated intermediate in reaction mixture, even if an excess<br />
of both alkylating agent and HBr neutralizing potassium carbonate<br />
is used [11]. In-depth study of this reaction was recently carried out<br />
[29]. The reports on the syntheses targeted directly on N-monoalkylated<br />
DPPs are relatively rare, but there was recently shown,<br />
that these compounds can be highly sensitive and selective<br />
fluorescent sensors for fluoride anions [30]. All six DPP derivatives<br />
VIIeXII in the presented study were prepared with moderate to<br />
high yields and the special purification procedures like chromatography<br />
or fractional crystallization were not necessary in order<br />
to obtain the product of the desired quality. We ascribe this fact to<br />
higher reactivity of bromine on ethyl bromoacetate as compared<br />
to e.g. n-butyl bromide. The compounds show good solubility over<br />
a wide range of solvents, so we have carried out the spectral<br />
measurements in highly polar DMSO, in order to have direct<br />
comparison with our previous results obtained for N-non-alkylated<br />
pigment precursors IeVI [10] or N-butylated analogues of VII and<br />
XI [11,12], in acetonitrile, as a representative of less polar solvents,<br />
and in almost non-polar but easily polarizable toluene.<br />
3.2. DFT computed structure<br />
As expected, DFT optimized geometries of VIIeXII predict<br />
non-zero dihedral angles, describing phenyl-pyrrolinone rotation<br />
(Table 1), on the contrary to strictly planar precursors IeVI [10]. The<br />
computed values of dihedral angles are the result of a compromise<br />
between sterical effect of methylene and ortho phenyl hydrogens<br />
(more or less the same for all six derivatives), invoking nonplanarity,<br />
and conjugation effect (dependent primarily on para phenyl<br />
substituent of each derivative), maximal for planar arrangement.<br />
The effect of the substituent on opposite pendant phenyl is marginal<br />
(less than 1 ). Average values are thus 26 ,36 and 38 for piperidino,<br />
cyano and unsubstituted phenyls, respectively.<br />
As these dihedral angles are crucial for the interpretation of<br />
absorption spectra, they should be verified by comparison with<br />
the experiment. The only known X-ray diffraction structure of<br />
compound VII [13] is a bit problematic from this point of view.<br />
The molecule is highly unsymmetrical in crystal with a ¼ 36.5 , i.e.<br />
quite close to the computed value 38.2 , and b ¼ 68.8 , i.e. totally<br />
out of a reasonable agreement. These results show a dramatic role<br />
of packing forces in DPP crystal. As discussed earlier [31], there are<br />
always some perturbation of planarity even for theoretically planar<br />
DPP pigments, bearing evidence of relatively flat minima of ground<br />
state geometry with respect to phenyl-pyrrolinone rotation. One<br />
can expect, that the equilibrium between sterical and conjugation<br />
Table 1<br />
DFT computed phenylepyrrolinone dihedral angles and PCM (DMSO) TD DFT<br />
computed excitation energies converted to wavelengths.<br />
Compound a [ ] b [ ] l00 [nm] fosc<br />
VII 38.2 38.2 465 0.512<br />
VIII 34.7 38.2 495 0.563<br />
IX 36.3 36.3 509 0.615<br />
X 38.0 26.1 518 0.835<br />
XI 26.6 26.6 546 1.138<br />
XII 35.2 25.5 561 0.923<br />
a [ ] ¼ R1-phenyl-pyrrolinone dihedral angle; b [ ] ¼ R2-phenyl-pyrrolinone dihedral<br />
angle; l00 [nm] ¼ theoretical excitation energy; fosc ¼ oscillator strength.
effects may be even more fragile for N-alkylated derivatives and<br />
thus the effect of packing forces may be more dramatic. That is<br />
probably the case of the X-ray structure of VII. Unfortunately, such<br />
distorted molecular structure can give only limited evidence on the<br />
accuracy of the computed structures. We have tested the relevance<br />
of DFT method to predict the dihedral angles of N,N-disubstituted<br />
compound I on another two known X-ray structures, in which<br />
Ci molecular symmetry in crystal is retained. The results were quite<br />
encouraging. The computed value for N,N-dimethyl I (30.3 )<br />
matches well the experimental one (30.4 [32]), and an agreement<br />
for N,N-diallyl I (theor. 31.2 , exp. 35.8 [13]) is also acceptable.<br />
There are no X-ray data for N,N-di-n-butyl I available, but a lot of<br />
them exist, for its derivatives (see bellow), so we computed also the<br />
dihedral angle for this compound. Its value (26.4 ) is considerably<br />
lower, than for compound VII (38.2 ).<br />
We searched the Cambridge structural database (CSD) using<br />
ConQuest procedure [33] in order to find the structures similar to the<br />
derivatives VIIeXII. There were found four files with X-ray structures<br />
of DPP derivatives with both pyrrolinone nitrogen substituted<br />
by n-butyl and at least one pendant phenyl either unsubstituted,<br />
or substituted with amino or cyano groups in para position: KAKMAL<br />
(R1 ¼ CN, R2 ¼ H, a ¼ 45.2 , b ¼ 32.6 ) [34], XATKIN(R1 ¼ R2 ¼ CN,<br />
a ¼ b ¼ 43.1 ), NAWREJ (R1 ¼ H, R2 ¼ diphenylamino, a ¼ 32.8 ,<br />
b ¼ 30.5 ) and XATKEJ (R1 ¼ H, R2 ¼ 4-MeO-phenyl, a ¼ 41.0 ,<br />
b ¼ 42.3 ) [35]. Another relevant data come from recently published<br />
N,N-dibenzylated DPPs. Such group recall probably similar steric<br />
hindrance with respect to phenyl, as dihedral angles of chlorinated<br />
derivative (R1 ¼ R2 ¼ Cl, a ¼ 41.6 , b ¼ 43.9 ) [23] are very similar to<br />
N,N-dibutylated dibromo derivative found in CSD in XATJAE file<br />
(R1 ¼ R2 ¼ Br, a ¼ b ¼ 45.0 ) [35]. So the results for N,N-dibenzylated<br />
dimorfolino derivative (R1 ¼ R2 ¼ morfolino, a ¼ b ¼ 28.1 ) [23] are<br />
close to those obtained for diphenylamino substituent (file NAWREJ)<br />
in accordance with an interpretation based of a mixing of two<br />
resonance structures for dimorfolino (and generally diamino) DPP<br />
derivatives [23].<br />
Finally, DFT predicted decrease of the phenyl-pyrrolinone<br />
dihedral angle accompanying p-piperidino substitution is in<br />
accordance with the relevant experimental data, at least qualitatively.<br />
On the other hand, small lowering of this dihedral angle<br />
connected with p-cyano substituent is inconsistent not only with<br />
the data coming from both above mentioned p-cyano substituted<br />
derivatives, but even with a general trend represented by other<br />
electron-accepting substituents, i.e. halogens.<br />
In order to test the sensitivity of dihedral angles of CN<br />
substituted derivatives on a quantum chemical method, we carried<br />
out the calculations on HartreeeFock (HF) level with the same<br />
6-311G(d,p) basis set. We obtained the dihedral angles considerably<br />
higher (a ¼ 52.4 , b ¼ 52.1 for VIII, a ¼ b ¼ 52.4 for IX and,<br />
a ¼ 52.5 , b ¼ 47.9 for XII) compared to those ones coming from<br />
DFT calculations (Table 1). But these computations also do not<br />
considerably distinguish 4eCNephenyl-pyrrolinone and phenylpyrrolinone<br />
dihedral angles.<br />
3.3. Absorption and fluorescence spectra in DMSO<br />
The absorption maxima of VIIeXII in DMSO show hypsochromic<br />
and hypochromic shifts (Table 2a, Fig. 2) with respect to corresponding<br />
precursors IeVI [10]. Hypsochromic shift is in fact a net<br />
effect of three contributions: 1) An increase of excitation energy<br />
due the less efficient conjugation because of the loss of molecular<br />
planartity, 2) the redistribution of the intensities of vibronic<br />
sub-bands from 0e0 maximum of IeVI in favour of 0e1inVIIeXII,<br />
as shown by a successive N-alkylation of I and V [11,12] and 3) the<br />
opposite effect e the decrease of excitation energy due to the<br />
increase of electron-donating strength of a pyrrolinone nitrogens in<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278 273<br />
Table 2<br />
The spectroscopic properties of VIIeXII in a) DMSO, b) acetonitrile and c) toluene.<br />
Compound lA (nm) 3 (lA)<br />
(l mol 1 cm 1 )<br />
lF(nm) 4F DlStokes<br />
(cm 1 )<br />
sF (ns)<br />
a) DMSO<br />
VII 460 15,800 519 0.56 2470 7.34 0.08<br />
VIII 471 16,000 547 0.51 2950 7.07 0.03<br />
IX 480 13,300 557 0.72 2880 6.50 0.04<br />
X 516 32,200 601 0.12 2740 1.17 0.02<br />
6.7 1.2<br />
XI 540 45,100 603 0.45 1940 3.45 0.02<br />
XII 541 42,300 654
274<br />
Relative intensity<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0.0<br />
substituted IX and XI. Although soluteesolvent interaction may<br />
play some role, we relate this behaviour mainly to the dependence<br />
of the dihedral angle describing the phenyl-pyrrolinone rotation on<br />
a nature of para substituent of this pendant phenyl. The conclusion<br />
is thus clear: piperidino substitution dramatically decreases<br />
the phenyl-pyrrolinone dihedral angles in accordance with DFT<br />
predictions, while cyano substitution increases these angles<br />
considerably, contrary to theory.<br />
PCM TD DFT computed data relate to the experimental maxima<br />
only qualitatively. More precisely, the series can be divided into two<br />
groups of derivatives. The first one (VII, X and XI) show the deviation<br />
between computed l00 (Table 1) and experimental lA (Table 2a)<br />
2e6 nm, while the difference for cyano substituted derivatives<br />
is significantly bigger (20e22 nm for mono-cyano substituted<br />
VIII and XII and even 29 nm for di-cyano IX). As there were no such<br />
discrepancies between theory and experiment for planar derivatives<br />
IeVI [10], we ascribe this inconsistency also to underestimated<br />
dihedral angles in DFT calculated geometries.<br />
Finally, the suspicion revealed in part 3.2. filled up and the<br />
absorption spectra clearly show, that the dihedral angle between<br />
p-cyano-phenyl and pyrrolinone rings should be relatively higher<br />
than for unsubsubstituted phenyl-pyrrolinone case, contrary<br />
for the DFT prediction. PCM (DMSO) TD DFT 6-311 þ G(2d,p)<br />
calculations carried out on the above mentioned more distorted HF<br />
geometry resulted in a considerable blue shift (l00 ¼ 426.nm for<br />
VIII, 435 nm for IX and 477 nm for XII), compared the values obtained<br />
on DFT geometry (Table 1), i.e. the values of l00 are in this<br />
case much lower compared to the experimental lA (Table 2a)<br />
showing an evidence on nonrealistic distortion coming from HF<br />
geometries.<br />
The hypsochromic shift of VII with respect to N,N-dibutylated<br />
I (7 nm) [12] and opposite bathochromic shift of XI with respect<br />
to N,N-dibutylated V ( 4 nm) [12] do not support, that<br />
eCH2COOCH2CH3 grouping is sterically significantly more efficient<br />
than eCH2CH2CH2CH3 as it would relate to the computed difference<br />
of phenyl rotation in VII (38.2 ) and N,N-dibutylated I (26.4 ).<br />
The absorption spectra do not corfirm that unsymmetrical highly<br />
distorted X-ray structure of VII [13] is retained in solution.<br />
The relation between fluorescence maxima of corresponding<br />
members of N-alkylated and non-alkylated sets is different from<br />
absorption and much less clear on the first view (Table 2a, Fig. 3).<br />
The maxima of unsubstituted I with respect to VII are almost the<br />
same (þ2 nm on behalf of VII), while IX shows hypsofluoric<br />
shift with respect to III ( 8 nm). The fluorescence maximum of<br />
compound VIII shows also a hypsofluoric shift with respect to II<br />
( 3 nm, i.e. exactly between þ2 nm and 8 nm for symmetrical<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278<br />
a b<br />
400 500 600<br />
Wavelength (nm)<br />
VII<br />
VIII<br />
IX<br />
X<br />
XI<br />
XII<br />
Relative intensity<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0.0<br />
pairs I/VII and III/IX). N,N-dialkylated electron-donor substituted<br />
derivatives X (þ14 nm) and XI (þ15 nm) show moderate<br />
bathofluoric shifts as compared to IV and V, respectively. Push-pull<br />
derivative XII shows almost identical maximum as VI (the difference<br />
is þ1 nm), i.e. its value lies between III/IX and V/XI pairs as in<br />
absorption. As a consequence the Stokes shift of compound XII is<br />
absolutely the highest one (3190 cm 1 ), i.e. significantly higher<br />
than for non-alkylated compound VI (2060 cm 1 ). An increment<br />
of a Stokes shift increase connected with N,N-disubstitution is<br />
relatively similar (1090e1270 cm 1 ) for all three pairs of piperidino<br />
substituted compounds and rather higher (1930e2070 cm 1 ) for<br />
unsubstituted or only cyano substituted pairs.<br />
The main reason for the general increase of the Stokes shift<br />
due to N-alkylation is caused by the fact, that it is considered as<br />
a difference between absorption and fluorescence maxima. The<br />
maxima correspond to 0e0 vibronic transition in fluorescence<br />
spectra for all twelve compounds, to 0e0 vibronic transition in<br />
absorption for IeVI [10] and to 0e1 transition for VIIeXII (Fig. 3). An<br />
increase of Stokes shift caused by a redistribution of vibronic bands<br />
intensities in absorption may not be strictly constant, but probably<br />
quite similar for all six pairs. The rest of the changes in Stokes shift<br />
goes on account of the differences in internal (geometrical) and<br />
external (solvent) relaxation, when going from vertical Franck-<br />
Condon (FC) state to relaxed excited state. Internal relaxation is<br />
probably mainly connected with the changes of above discussed<br />
dihedral angles. In order to have a better view on external contribution,<br />
it was necessary to carry out the spectral measurements in<br />
other solvents with different (lower) polarity.<br />
3.4. Solvatochromism<br />
500 600 700<br />
Wavelength (nm)<br />
Fig. 3. Normalized absorption (a) and fluorescence (b) spectra of VIIeXII in DMSO.<br />
VII<br />
VIII<br />
IX<br />
X<br />
XI<br />
XII<br />
It was impossible to measure the absorption and fluorescence<br />
solvatochromism of IeVI because of their insolubility in other than<br />
highly polar solvents able to form H-complexes with solute. The<br />
spectral data for N-alkylated derivatives in toluene and acetonitrile<br />
are summarized in Table 2b, c and the spectra are shown on Figs. 4<br />
and 5. The shape of the absorption spectra is the same in all three<br />
solvents, i.e. the vibronic structure is completely unresolved even in<br />
toluene, while the vibronic structure of fluorescence spectra is<br />
best resolved for all compounds in toluene, in which clearly 0e0<br />
vibronic transition is the absolute maximum, and its resolution<br />
decreases in acetonitrile and is almost lost in DMSO.<br />
Compound VII shows small negative solvatochromism, when<br />
going from toluene to acetonitrile ( 8 nm) and almost the same<br />
positive shift from acetonitrile to DMSO (þ6 nm). The shifts of its<br />
fluorescence maxima are almost identical, thus Stokes shift is the
same in all three solvents. The solvent induced shifts of IX are<br />
nearly the same within 1 nm. The spectral shifts when going from<br />
toluene to acetonitrile markedly evoke the situation found for<br />
BODIPY dyes [36] and the interpretation is the same. While the<br />
absorption maximum of their hybrid VIII lies exactly between VII<br />
and IX in all solvents, the fluorescence maximum is generally closer<br />
to IX and a small growth of Stokes shift with solvent polarity from<br />
toluene (2800 cm 1 ) to acetonitrile (2960 cm 1 ) is observed. Thus<br />
VIII is only a bit more polar in relaxed than in FC excited state.<br />
The introduction of piperidino group brings two general trends<br />
with respect to excited state relaxation in compounds XeXII: The<br />
contribution of internal relaxation is decreased, which well relates<br />
with lower rotation of p-piperidino-phenyl in FC state, while the<br />
external contribution is increased, as the electron-donating<br />
substitution changes the intramolecular charge distribution. The<br />
former statement can be documented by lower Stokes shift of XI<br />
(1840 cm 1 ) with respect to VII (2410 cm 1 ) in non-polar toluene.<br />
The latter sentence is generally proved by a dependence of Stokes<br />
shift on a solvent polarity, i.e. its increase is less than 160 cm 1 for<br />
VIIeIX and notably higher for XI (330 cm 1 ), X (680 cm 1 ) and<br />
especially XII (1020 cm 1 ) when going from toluene to acetonitrile.<br />
Thus, the largest observed Stokes shift of XII (3190 cm 1 in DMSO)<br />
is in fact mainly done by donor-acceptor substitution (2060 cm 1 in<br />
DMSO for VI [10]) and the additional effect of N-alkylation goes<br />
mainly on account of a redistribution of vibronic intensities in<br />
absorption spectrum of XII.<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
a b<br />
Relative intensity<br />
Relative intensity<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
VII<br />
VIII<br />
IX<br />
X<br />
XI<br />
XII<br />
0.0<br />
300 400 500 600<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
Wavelength (nm)<br />
Relative intensity<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0.0<br />
500 600 700<br />
Wavelength (nm)<br />
Fig. 4. Normalized absorption (a) and fluorescence (b) spectra in acetonitrile.<br />
VII<br />
VIII<br />
IX<br />
X<br />
XI<br />
XII<br />
0.0<br />
300 400 500 600<br />
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278 275<br />
a b<br />
Wavelength (nm)<br />
VII<br />
VIII<br />
IX<br />
X<br />
XI<br />
XII<br />
The solvent induced shifts in absorption of XeXII are less than<br />
4 nm when going from toluene to acetonitrile, reflecting very<br />
similar polarity of the ground and excited FC states. Positive<br />
solvatochromism of fluorescence is moderate: 15 nm for symmetrical<br />
XI, and 24 and 35 nm for asymmetrical IX and XII, respectively,<br />
that can be ascribed to relaxed excited state solvent stabilization.<br />
The positive solvatochromism in absorption of XeXII when going<br />
from acetonitrile to DMSO is almost constant (16e17 nm), very<br />
similar to the corresponding shift in fluorescence (13e16 nm) and<br />
a bit higher than for VIIeIX (6e7 nm in absorption and 7e9 nmin<br />
fluorescence). It means, that further increase of solvent polarity<br />
does not lead to additional excited state relaxation and the<br />
energy of the excited FC and relaxed states is lower only because<br />
the excited state charge distribution interacts more favourably than<br />
the ground state itself with the reaction field of more polar solvent<br />
(induced by ground state distribution) [37].<br />
The highest solvent induced rise of Stokes shift, i.e. for compound<br />
XII between toluene and acetonitrile (þ1020 cm 1 ), is significantly<br />
lower than for 4-(dimethylamino)-4 0 -cyano-1,4-diphenylbutadiene<br />
(DCB, þ4640 cm 1 between n-hexane and acetonitrile [38]), in<br />
which only central 1,4-diphenyl-butadiene backbone of XII is pushpull<br />
substituted. This difference goes exclusively on account of much<br />
lower value of positive fluorescence solvatochromism of compound<br />
XII (þ35 nm) with respect to DCB (þ129 nm), while the solvatochromism<br />
of absorption is almost the same ( 3nmforXII<br />
and þ2 nm for DCB), i.e. negligible between above mentioned<br />
Relative intensity<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0.0<br />
500 600 700<br />
Wavelength (nm)<br />
Fig. 5. Normalized absorption (a) and fluorescence (b) spectra in toluene.<br />
VII<br />
VIII<br />
IX<br />
X<br />
XI<br />
XII
276<br />
Relative intensity<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0.0<br />
VII<br />
VIII<br />
IX<br />
X<br />
XI<br />
XII<br />
500 600 700 800 900<br />
Wavelength (nm)<br />
Fig. 6. Solid-state fluorescence spectra of DPP derivatives.<br />
non-polar and polar solvents. Thus, although compounds X and XII<br />
behave qualitatively like push-pull like chromophores, quantitatively<br />
their fluorescence solvatochromism is quite limited.<br />
Compound XI is formally a quadrupolar molecule with D-p-A-p-<br />
D character. Such compounds may or may not undergo an excited<br />
state symmetry-breaking in highly polar solvents [39]. The shifts of<br />
absorption (3 nm) and fluorescence (15 nm) maxima of XI, when<br />
going from toluene to acetonitrile, is relatively small (although not as<br />
small as for e.g. squaraines [39]), indicating very small (if any) excited<br />
state perturbation. Compound XI is thus an intermediate quadrupolar<br />
chromophor with expected high two-photon absorption<br />
cross-section. Their relatively high values estimated for N-octylated<br />
p-diphenylamino substituted DPPs confirm this idea [40].<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278<br />
3.5. Photophysical behaviour<br />
Fluorescence quantum yields (4F) and lifetimes (sF) for VIIeXII<br />
in all three solvents are summarized in Table 2. Corresponding<br />
photophysical data for non-alkylated precursors I (4F ¼ 0.74,<br />
sF ¼ 6.21 ns) and V (4F ¼ 0.58, sF ¼ 3.66 ns) in DMSO were reported<br />
previously [12] and we now supply them by 4F and sF for further<br />
two piperidino substituted precursors IV (4F ¼ 0.48, sF ¼ 3.49 ns)<br />
and VI (4F ¼ 0.14, sF ¼ 3.90 ns) also in DMSO and not published in<br />
Ref. [10]. Both these compounds show monoexponential fluorescence<br />
decay.<br />
All six compounds VIIeXII also show a relatively high 4F in<br />
toluene and the fluorescence decay is strictly monoexponential<br />
with lifetimes similar to corresponding non-alkylated precursors<br />
in DMSO. However, there is a dramatic change for compounds<br />
X and XII when going to polar solvents. The quantum yields<br />
of fluorescence are significantly decreased, especially for XII<br />
(Table 2b, c), and the decay is biexponential. It implies, that some<br />
specific process connected with the excited state intramolecular<br />
charge transfer (ICT) must be present. Such behaviour may be<br />
connected with a conformational change in excited state known<br />
as twisted intramolecular charge transfer (TICT). 4-piperidinobenzonitrile<br />
is known to undergo such process even more willingly<br />
than 4-dimethylamino-benzonitrile, a prototype molecule<br />
with respect to TICT [41]. Nevertheless, it is generally very difficult<br />
to prove this idea, if not supported by two emission bands in<br />
steady-state fluorescence. According to our opinion, the observable<br />
fluorescence of XII in polar solvents comes from the minor<br />
portion of excited molecules, for which the charge separating<br />
twist did not pass. Nevertheless this explanation is only speculative<br />
and the final solution of this problem would require further<br />
sophisticated photophysical experiments that are out of the scope<br />
of this article.<br />
Fig. 7. Polycrystalline samples of IeXII under daylight (top) and fluorescence of the samples of VIIeXII under UV irradiation (365 nm) with the same settings of Panasonic DMC-FZ7<br />
camera (bottom).
3.6. Solid-state fluorescence<br />
All six studied DPP derivatives show pronounced solid-state<br />
fluorescence, on the contrary to any of their non-alkylated<br />
precursors IeVI. The spectra are shown on Fig. 6. The fluorescence<br />
of symmetrical VII, IX and XI is strong, easily observable by<br />
naked eye under UV irradiation (Fig. 7). The fluorescence of<br />
unsymmetrical VIII and X is less intense, but observable. Solid-state<br />
fluorescence of push-pull derivative XII is almost not observable<br />
(and totally undetectable by a camera e Fig. 7) partly because of its<br />
significantly lower intensity and also as it falls almost fully into the<br />
infrared region. The values of emission maxima in polycrystalline<br />
phase are 568 nm (VII), 639 nm (VIII), 648 nm (IX), 708 nm (X),<br />
670 nm (XI) and 784 nm (XII), and hence, their sequence corresponds<br />
to that one in polar solvent (Table 2a) except the changeover<br />
of X and XI pair. The corresponding changeover of dipolar VIII<br />
and formally quadrupolar IX did nor occur, but the fluorescence<br />
maximum of hybrid VIII in solid state is significantly closer to<br />
parent IX than to VII. The spectral maxima are shifted bathochromically<br />
with respect to even the most polar solvent (DMSO).<br />
The smallest shift was found for compound VII (49 nm) without<br />
any polar substituent, the moderate one for centrosymmetric<br />
compounds XI (67 nm) and IX (91 nm), and the highest one for<br />
polar VIII (92 nm) and X (107 nm) and, especially, for push-pull<br />
substituted XII (130 nm).<br />
Although the intermolecular interactions in highly organized<br />
crystal phase cannot be in principle described by simple soluteesolvent<br />
terminology, the red shifts probably predicate about the<br />
polarity inside the crystal environment, i.e. 1) it is effectively more<br />
polar than DMSO in all cases and 2) the highest effect is of course<br />
in crystals composed from the molecules with non-zero dipole<br />
moment. The changeover of solid-state fluorescence maxima of X<br />
and XI is thus a logical continuation of the trend observed in solvents<br />
with increasing polarity (Table 2aec). Although the crystal of XII has<br />
evidently more polar environment than in DMSO solution, the<br />
fluorescence of XII does not definitely diminishes in it. We consider<br />
this phenomenon as further support of TICT role in the deactivation<br />
cascade of XII in DMSO, while the excited state twisting is disabled<br />
in rigid crystal environment. Almost identical behaviour and interpretation<br />
was reported for push-pull substituted 1,6-diphenyl-1,3,5hexatriene<br />
[42].<br />
Generally, the solid-state fluorescence of organic pigments is<br />
considered as a property of individual molecule conserved in crystal<br />
phase [43]. In other words, the quenching process connected<br />
with electron-phonon coupling in crystals of IeVI, eliminated by<br />
N,N-dialkylation in their derivatives VIIeXII, relate very probably to<br />
impossibility of pep stacking in crystal [21]. Such disabled stacking<br />
may come either from intramolecular sterical hindrance, i.e.<br />
molecular nonplanarity, or the intermolecular one, e.g. disabled<br />
proximity of molecular planes due to large volume side chains. We<br />
consider the former contribution as crucial in this case.<br />
4. Conclusions<br />
Six N-alkylated soluble DPP derivatives with polar substituents<br />
in para positions of pendant phenyls were synthesized, in order to<br />
make original non-alkylated pigment precursors better treatable.<br />
The compounds are non-planar with phenyl rings rotated<br />
out of diketo-pyrrolo-pyrrole plane. The degree of this rotation is<br />
decreased by the electron-donating substituents, while increased<br />
by the electron-withdrawing substituents, contrary to the DFT<br />
predictions. The compounds show small solvatochromism of<br />
absorption and a moderate positive solvatochromism of fluorescence,<br />
if substituted by strong electron-donating substituent.<br />
The significant decrease of fluorescence quantum yields and its<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278 277<br />
biexponential decay for dipolar derivatives in polar solvents was<br />
tentatively ascribed to the formation of non-fluorescent TICT<br />
excited state. All compounds show fluorescence in polycrystalline<br />
solid-state with the maxima covering a range over 200 nm in visible<br />
and near infrared region, where the solid-state fluorescence is quite<br />
rare [18].<br />
Acknowledgement<br />
The research was supported by the Academy of Sciences of the<br />
Czech Republic via project KAN401770651, by the Ministry of<br />
Industry and Trade of the Czech Republic via 2A-1TP1/041 project,<br />
by the Czech Science Foundation via P205/10/2280 project and by<br />
the project “Centre for Materials Research at FCH BUT” No. CZ.1.05/<br />
2.1.00/01.0012 from ERDF.<br />
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<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
Stability and structural aspects of diketopyrrolopyrrole pigment<br />
and its N-alkyl derivatives<br />
Jan David a , Martin Weiter a , Martin Vala a , Jan Vynuchal b ,Jirí Kucerík a, *<br />
a Faculty of Chemistry, Brno University of Technology, Purkynova 118, CZ-612 00 Brno, Czech Republic<br />
b Research Institute of Organic Syntheses, VUOS, Rybitví 296, CZ-533 54 Rybitví, Czech Republic<br />
article info<br />
Article history:<br />
Received 29 September 2009<br />
Received in revised form<br />
29 September 2010<br />
Accepted 3 October 2010<br />
Available online 12 October 2010<br />
Keywords:<br />
Diphenyl-diketo-pyrrolo-pyrroles (DPPs)<br />
Pigments<br />
Organic electronics<br />
Thermal stability<br />
Thermogravimetry<br />
Calorimetry<br />
1. Introduction<br />
abstract<br />
Nowadays, a strong effort can be seen in seeking for highperformance<br />
and simultaneously photo- and oxidative-thermally<br />
stable materials used in organic electronics. Printing technology<br />
seems to be a leading technique for cheap large scale production of<br />
such devices. Using this technique, the layers of electronic devices<br />
are printed employing solutions containing the active materials.<br />
However, the increase in solubility can reversibly act on the<br />
stability of the materials. Derivatives of 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione,<br />
commonly referred as DPPs, are<br />
potentially attractive materials for organic electronics. They belong<br />
to important industrial high-performance pigments [1] showing<br />
significantly high melting points with respect to other low molecular<br />
pigment standards. They also exhibit remarkable resistance<br />
against chemical, heat, light and climate influences. There is a wide<br />
range of possible applications which have been already investigated<br />
covering e.g. latent pigments [2], solid state dye lasers [3], gas<br />
detectors [4,5] or electroluminescent devices [6,7].<br />
The DPPs are usually insoluble in common solvents [8]. One of<br />
the reason for their low solubility is the existence of hydrogen<br />
* Corresponding author. Tel.: þ420 5 41 14 94 85; fax: þ420 5 41 21 16 97.<br />
E-mail address: kucerik@fch.vutbr.cz (J. Kucerík).<br />
URL: http://www.fch.vutbr.cz/home/kucerik<br />
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.<br />
doi:10.1016/j.dyepig.2010.10.001<br />
Dyes and Pigments 89 (2011) 137e143<br />
Contents lists available at ScienceDirect<br />
Dyes and Pigments<br />
journal homepage: www.elsevier.com/locate/dyepig<br />
An unsubstituted sample, three symmetrically N-substituted samples (methyl, butyl and heptyl) and two<br />
asymmetrically N-substituted samples (butyl and heptyl) of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]<br />
pyrrole-1,4-dione (DPP) were investigated using thermogravimetry and differential scanning calorimetry<br />
to reveal the influence on physical-chemical properties of different alkyl chains and symmetry of Nsubstitution.<br />
Stability tests revealed that in all cases the substitution brought significant destabilization<br />
of the structure in comparison with the unsubstituted DPP molecule. It was demonstrated that the length<br />
of the substituting alkyl chain is a crucial factor in the stability of N-alkyl derivatives; the shorter the<br />
alkyl chain was, the less stable was the derivative. Further, the symmetrical derivates were less stable<br />
than the asymmetrical ones. Unlike the unsubstituted DPP molecule, all the derivatives showed<br />
remarkable sensitivity to different cooling regimes which lead to the revealing of monotropical polymorphism<br />
in the symmetrical butyl and heptyl derivatives crystalline structure.<br />
Ó 2010 Elsevier Ltd. All rights reserved.<br />
bondings between the H atom of the nitrogen functional group and<br />
oxygen. The unsubstituted DPP is perfectly planar, the pep electrons<br />
stacking occurs in solid state and also contributes to their<br />
insolubility. In order to modify the solubility, it is necessary to carry<br />
out either the N-substitution and/or the breaking of the molecule<br />
planarity [8]. In our previous work [7], we discussed the influence<br />
of N-alkylation on optical properties, and the results were correlated<br />
with molecule geometry calculated by quantum chemical<br />
methods. It was found that while the parent DPP molecule<br />
(abbreviated as DPP in this work) is perfectly planar, the Nsubstitution<br />
causes rotation of the adjacent phenyl rings (see Fig. 1)<br />
and thus reducing the effective conjugation extent. This, in turn,<br />
causes hypsochromic shift of the absorption spectrum. Simultaneously,<br />
the fluorescence spectra move to the longer wavelength<br />
region increasing Stokes shifts. The effect is more pronounced for<br />
the double N-alkylated derivatives. Since the angle of distortion is<br />
not dependent on the length of the substituent, no differences<br />
between butyl substituted (DPP-B, DPP-BB) and heptyl substituted<br />
derivatives (DPP-H, DPP-HH) were found [7].<br />
The stability and behavior of physical structure of photo-sensitive<br />
materials which were exposed to different thermal history is<br />
very important. In order to produce high quality devices in<br />
a controlled way, knowledge about the crystallinity, and polymorphism<br />
seems to be crucial. Since physical properties of<br />
polymorphs can significantly differ, polymorphism can cause
138<br />
Fig. 1. The basic structure of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione<br />
(DPP), also known as DPP and the respective derivatives used in this study. The definition<br />
of calculated torsion angles a and b can be found in [7].<br />
troubles in technological applications [15]. Essentially, the difference<br />
in crystal structure leads to different optical properties,<br />
structural stability, solubility, melting temperature, enthalpy etc.<br />
Since the crystallization of one specific polymorph is controlled by<br />
a combination of thermodynamic and kinetic factors [9], it is<br />
important to determine the relationship between such fraction and<br />
the others in the polymorphs mixture.<br />
In this study, using thermogravimetrical analysis (TGA), our<br />
research was extended by the thermal and thermo-oxidative<br />
stability tests and by distinguishing between processes of evaporation<br />
or degradation. In fact, materials DPP and DPP-MM have<br />
already been investigated by TGA [10]; however, the large set of<br />
DPP materials and comparison of their properties can bring deeper<br />
insight into the physical chemistry of such interesting pigments.<br />
Furthermore, employing differential scanning calorimetry (DSC)<br />
we would like to shed light on the structural variability of investigated<br />
samples after different thermal history. The obtained results<br />
are discussed with respect to the chemical structure of the studied<br />
molecules.<br />
2. Experimental<br />
2.1. Materials<br />
Samples of the studied derivatives were synthesized in VUOS,<br />
a.s. (Research Institute of Organic Syntheses, Inc., Rybitvi, Czech<br />
Republic) according to the procedures described in [7,8]. One<br />
unsubstituted sample (DPP), three symmetrically N-substituted<br />
samples (DPP-MM e methyl group substituted, DPP-BB e butyl<br />
group substituted and DPP-HH e heptyl group substituted) and two<br />
asymmetrically substituted samples (DPP-B with butyl group and<br />
DPP-H with heptyl group) were investigated in this study. Molecular<br />
structures of investigated samples are depicted in the Fig. 1.<br />
Before all the experiments had been performed, the compounds<br />
were carefully milled in the agate mortar in order to maintain<br />
uniform heat flow to the whole dosing of sample in both thermal<br />
analysis methods.<br />
2.2. Methods<br />
2.2.1. Thermogravimetric analysis<br />
Thermogravimetric studies were performed using TA Instruments<br />
TGA Q5000 (New Castle, DE, U.S.A.) device in 100 mL open<br />
platinum pans. The samples, typically 5 mg, were heated by using<br />
thermal ramp of 10 C min 1 from 40 C to 650 C in either dynamic<br />
atmosphere of nitrogen (thermal stability) or air (thermo-oxidative<br />
stability). Flow rate of both gases was 25 mL min 1 .<br />
2.2.2. Differential scanning calorimetry<br />
Calorimetric analyses were carried out employing TA Instruments<br />
DSC Q200 equipped with an external cooler RCS90 allowing<br />
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J. David et al. / Dyes and Pigments 89 (2011) 137e143<br />
experimental temperature range from 90 to 400 C. Experiments<br />
were conducted in open TA TzeroÔ aluminum pans. The first<br />
heating run was conducted at 10 C min 1 from 40 C to the<br />
temperature 5 C lower than the degradation temperature onset<br />
(Tonset) previously determined by using thermogravimetry under<br />
nitrogen. In order to simulate the moderate temperature decrease,<br />
cooling ramp of 0.5 C min 1 was applied to reach 90 C, followed<br />
by 1 min of isothermal stage. The first calorimetric measurement<br />
was performed using heating ramp of 10 C min 1 from 90 Cto<br />
the temperature (Tonset 5) C. The second experiment was performed<br />
using the same heating ramp after the rapid equilibration of<br />
the sample down to 90 C and 1 min of isothermal stage. It is<br />
necessary to point out that in this case, the averaged temperature of<br />
cooling exceeded 20 C min 1 . All DSC experiments were made<br />
under 50 mL min 1 of nitrogen purge. The device was calibrated for<br />
temperature and enthalpy using indium, tin and zinc standards<br />
(Perkin Elmer). Additional measurement of sample DPP was carried<br />
out also by the TA Instruments Q600 (simultaneous DSC and TGA)<br />
under the same conditions as described above. All the records were<br />
assessed by TA Universal Analysis 2000 software version 4.4A.<br />
3. Results<br />
3.1. Stability tests e thermogravimetric analysis (TGA)<br />
TGA measurements were conducted to test the temperatures of<br />
mass loss occurrence. These temperatures correspond either to the<br />
temperature of degradation (TD), evaporation (Tev) or sublimation<br />
(Ts). The character of two latter phenomenons was revealed by DSC.<br />
In principle, if the sample shows an endotherm on DSC record, the<br />
sample is melted and therefore, the temperature can be associated<br />
to evaporation. On the contrary, if DSC record does not show any<br />
melting, the sample is solid and the process can be attributed to the<br />
sublimation.<br />
In order to distinguish between Ts and Tev or Ts, the mass loss and<br />
its first (DTG) and second temperature derivative curves were used.<br />
The clarification of such an approach is explained further in the<br />
text. The typical TGA record and its DTG curve were obtained by<br />
using conditions described in Experimental part for sample DPP-<br />
MM and is reported in Fig. 2. In this work, the DTG is plotted in an<br />
inverse mode to improve the illustrative quality of Figures. Since<br />
the TGA record provides an integral piece of information regarding<br />
the mass loss, DTG or potential second derivation allows a closer<br />
look into the dynamics of the processes and frequently helps to<br />
distinguish the change in mass loss mechanism.<br />
Sample DPP showed no event on the DSC curve when we used<br />
the temperature program reported in Experimental section.<br />
Therefore, in order to discover potential melting at temperatures<br />
above sublimation, the simultaneous DSC and TGA was carried out<br />
(Fig. 3). As it can be seen on the DSC curve, no overlapping of<br />
enthalpy processes occurred, and thus the sample cannot be melted<br />
under conditions used in this work. The respective 1st and 2nd<br />
derivative curves of TGA of DPP are reported in Fig. 4.<br />
The results obtained from DTG records are summarized in<br />
Table 1. All of the samples in both atmospheres showed one or two<br />
degradation steps. If occurred, the second degradation step was<br />
only minor and was not taken for further assessment. The onset of<br />
a process in DTG is traditionally determined from the 1st derivative<br />
mass loss curve. However, it is clear that the determined temperature<br />
does not indicate whether the temperature corresponds to<br />
the degradation or to the evaporation. The determined onset only<br />
indicates the beginning of mass changes which can be attributed to<br />
both above-mentioned processes. Therefore, the 2nd derivative was<br />
carried out in order to enable the separation of possible overlapping<br />
processes. An example of such a separation for sample DPP is
Fig. 2. TGA heating run for the DPP-alkyl derivate DPP-MM in the atmosphere of<br />
nitrogen. Onset temperatures are summarized in Table 1.<br />
reported in Fig. 4. Therefore, Table 1 reports the two temperatures,<br />
i.e. Tev (mass loss without degradation) and TD (mass loss connected<br />
with degradation). The only exception is the sample DPP, which<br />
showed no melting, and therefore, had no Tev but a temperature of<br />
sublimation Ts. All Tev and Ts occurred in the temperature range from<br />
238 C to 383 C. As it can be seen in Table 1, thermal-oxidative<br />
stability results (experiments in air) are similar to the thermal<br />
stability results (experiments in N2), but generally shows a slight<br />
shift to lower temperatures. As expected, the thermal stability of<br />
investigated materials is slightly higher than the thermal-oxidative<br />
Fig. 3. Simultaneous DSC and TGA record of sample parental sample DPP.<br />
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J. David et al. / Dyes and Pigments 89 (2011) 137e143 139<br />
Fig. 4. Determination of the onset1 and the onset2, i.e., distinguishing between<br />
sublimation and degradation of sample DPP. Other samples exhibited melting and<br />
therefore, the onset1 stands for evaporation temperature in this case.<br />
stability. Comparison of TD showed the thermal stability of samples<br />
in order of DPP > DPP-HH > DPP-B > DPP-MM > DPP-BB; sample<br />
DPP-H could not be taken into account since the whole sample was<br />
completely evaporated at 350 C. In dynamic air atmosphere, the<br />
thermo-oxidative stability showed following order of DPP > DPP-<br />
HH > DPP-B ¼ DPP-H > DPP-MM > DPP-BB. DPP-H sample was<br />
included in this order since the thermo-oxidative stability showed<br />
lower temperature than the Tev, which suggests that the degradation<br />
under air occurs earlier than the evaporation. As stated in the<br />
Experimental part, the temperatures of Tev and onsets1 (see Fig. 4)<br />
were used to design the DSC experiments (to suggest upper<br />
temperature limit). The Ts and Tev, indicating the temperature at<br />
which the molecular vibrations overcame the weak intermolecular<br />
interactions stabilizing structures either in solid state (DPP4) or in<br />
melted (the rest), are ordered as follows: DPP4 > DPP-H > DPP-<br />
HH > DPP-B > DPP-BB > DPP-MM.<br />
3.2. Phase transitions e differential scanning calorimetry (DSC)<br />
DSC experiments revealed additional differences in physical<br />
structure of investigated materials. Most of the derivatives, when<br />
heated from 90 C expressed several phase transitions which<br />
varied depending on the chemical structure and thermal history of<br />
the respective samples. DSC records of selected samples are given in<br />
Figs. 5 and 6, for samples DPP-BB and DPP-HH, respectively. Table 2<br />
summarizes main phase transitions such as melting, crystallization<br />
and glass transition of measured samples. Minor transitions, which<br />
Table 1<br />
Thermogravimetric analyses results. The onset1 suggests the temperature of evaporation,<br />
the onset2 indicates possible beginning of degradation.<br />
Sample Purge gas Onset1 [ C] Onset2 [ C] Char [%wt] Steps<br />
DPP N2 383 396 0.4 1<br />
DPP air 356 0 1<br />
DPP-MM N2 238 262 3.8 2<br />
DPP-MM air 237 0 2<br />
DPP-B N2 269 281 0.1 1<br />
DPP-B air 267 0 2<br />
DPP-BB N2 241 246 0 1<br />
DPP-BB air 234 0 2<br />
DPP-H N2 290 E 0 1<br />
DPP-H air 267 0 2<br />
DPP-HH N2 282 290 0 1<br />
DPP-HH air 271 0 1<br />
E e sample was completely evaporated, no degradation was observed.
140<br />
Fig. 5. Comparison of DSC records for sample DPP-BB after different cooling regime.<br />
Moderate cooling regime was 0.5 C per min from 200 to 90 C, heating 10 C per<br />
min.<br />
are not important for further discussion, are only described in the<br />
next paragraphs.<br />
DSC record of DPP sample after both moderate and rapid cooling<br />
did not show any event at lower temperatures (Fig. 3). In contrast,<br />
at elevated temperatures the simultaneous DSC/TGA record<br />
showed an intensive endothermal peak with no shoulder accompanied<br />
by a massive mass loss which can be attributed to the<br />
sublimation of the sample followed by degradation (cf. Table 1).<br />
After moderate and rapid cooling DSC record of DPP-MM sample<br />
showed endothermal melting peaks around 234 C with melting<br />
enthalpy around 112 J g 1 , which indicates no influence on the<br />
sample thermal history.<br />
Moderate cooling ramp caused a tiny exotherm probably corresponding<br />
to the cold crystallization of DPP-B with onset at 19 C<br />
(enthalpy 0.43 J g 1 ) and followed by an endotherm corresponding<br />
to the melting at 8 C (peak temperature 5 C, melting enthalpy<br />
0.26 J g 1 ). Further, at 10.7 C an endotherm appeared again indicating<br />
the structure melting (peak temperature 17 C, melting<br />
enthalpy 0.18 J g 1 ). A glass transition with midpoint at e18 Cwas<br />
observed in the record after rapid cooling ramp.<br />
Fig. 6. Comparison of DSC records for sample DPP-HH after different cooling regime.<br />
Moderate cooling regime was 0.5 C per min from 200 to 90 C, heating 10 C per<br />
min.<br />
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J. David et al. / Dyes and Pigments 89 (2011) 137e143<br />
One of the most complex calorimetric results was given by the<br />
measurement of the sample DPP-BB. DSC records after the<br />
moderate cooling ramp resulted in three consecutive and intensive<br />
phase transitions; melting at 120 C (melting enthalpy 72.3 J g 1 ),<br />
followed by cold crystallization at 124 C (peak temperature 125 C,<br />
enthalpy of crystallization 38.5 J g 1 ) and subsequent second<br />
melting peak at 135.2 C (melting enthalpy 70.9 J g 1 ). Calorimetric<br />
results from the experiment after the rapid cooling ramp showed<br />
a glass transition with midpoint of 5 C, a cold crystallization at<br />
56 C (enthalpy of crystallization 69 J g 1 ). After that, a slight<br />
melting peak at 108 C was detected (melting enthalpy 6.1 J g 1 )<br />
followed by intensive melting at 136 C (melting enthalpy 84 J g 1 ),<br />
which was observed in the experiment after moderate cooling rate<br />
as well.<br />
Heating run after moderate cooling of sample DPP-H resulted in<br />
a cold crystallization at 41 C with peak temperature of 26 C<br />
and crystallization enthalpy of 1.85 J g 1 followed by melting at 0 C<br />
and peak temperature at 2 C and melting enthalpy of 1.09 J g 1 .<br />
After rapid cooling, the DPP-H sample showed two consequent<br />
melting peaks at 19 C (peak temperature at 16 C, melting<br />
enthalpy of 0.04 J g 1 ) and at 12 C (melting enthalpy of<br />
0.24 J g 1 ), respectively. Both temperature regimes performed<br />
similar melting peaks around 212 C (enthalpies around 100 J g 1 ).<br />
Complicated results were obtained from calorimetric experiments<br />
in the sample DPP-HH as well. After moderate cooling,<br />
significant melting at 115 C with peak temperature of 116 C and<br />
melting enthalpy of 96.3 J g 1 was detected. After a rapid cooling,<br />
a glass transition with midpoint at 22 C was observed followed<br />
by two cold crystallization peaks. The first crystallization peak<br />
occurred at 25 C, with temperature of the peak at 31 C and<br />
crystallization enthalpy of 37.4 J g 1 . The second cold crystallization<br />
event was less prominent and appeared at 62 C with peak<br />
temperature of 71 C and crystallization enthalpy of 30.7 J g 1 . The<br />
same melting peak, as observed in the experiment after moderate<br />
cooling, was detected also after rapid cooling, i.e., the onset at<br />
115 C, peak temperature at 116 C and melting enthalpy of<br />
95.4 J g 1 .<br />
In contrast to the order of Tev, the temperatures of crystal<br />
melting followed reverse order, i.e., DPP-MM > DPP-BB > DPP-<br />
HH > (DPP). Similar order can be seen for asymmetrical samples<br />
onsets DPP-B < DPP-H and for melting temperatures of DPP-<br />
B > DPP-H.<br />
4. Discussion<br />
Table 1 indicates significant distinctions in stability and structural<br />
feature of investigated samples. First of all, there has been<br />
observed a great difference between substituted derivates and<br />
unsubstituted DPP. In fact, all the substituted samples were less<br />
stable thermally (except DPP-H where stability could not be<br />
determined) than the unsubstituted parent sample DPP. The onset<br />
temperature of degradation 238 C found for the least stable<br />
derivative DPP-MM is about 145 C lower compared to the DPP<br />
(383 C). Table 1 summarizes the data obtained from TGA under<br />
inert atmosphere and oxidative atmosphere of air. As can be seen in<br />
several cases, the temperatures of evaporation in nitrogen are<br />
similar to those obtained in air (i.e., DPP-MM, DPP-B and possibly<br />
DPP-BB). That suggests that in those samples, oxygen did not play<br />
an important role in their degradation.<br />
The highest stability of DPP can be easily explained with respect<br />
to its structure. As already mentioned, overlapping of pep electrons<br />
between adjacent molecules occurs due to the planarity of the<br />
molecule [8]. The compact and rigid structure is stabilized by four<br />
intermolecular hydrogen bonds per molecule between the eNH<br />
group of one molecule and the oxygen atom of the neighboring one
along the b axis [11]. This interaction further contributes to the<br />
insolubility of the non-N-substituted DPPs. In DPP, the number of<br />
atomic contact along the stacking axis is 9 [12]. That number<br />
strongly depends on the character of the N-substitution of the<br />
derivate [13]. Therefore, the existence of hydrogen bonds between<br />
the eNH group and oxygen on the central pyrrolo-pyrrole core<br />
additionally increases the energy needed for molecule defragmentation<br />
as confirmed by Tev or Ts or degradation TD. On<br />
the contrary, it has been stated that the torsion angles between the<br />
central pyrrolo-pyrrole part and the phenyls are independent of<br />
the alkyl length [7], therefore, the difference between derivates<br />
with different alkyl length has to be related to different properties.<br />
Remaining char observed on TGA for DPP sample confirms the<br />
hypothesis about the degradation which follows the sublimation or<br />
evaporation. Traditionally, the DPP pigments are deposited on<br />
a surface at elevated temperatures. The char formation indicates<br />
that choosing an inappropriate temperature can lead to the<br />
pigment degradation since Ts and TD of this sample are not significantly<br />
different. In TGA experiments the linear heating is employed<br />
which implies that the sample has not enough time to sublimate<br />
completely from the pan. In technological (industrial) practise this<br />
problem can be (and usually is) solved by pressure reduction [12].It<br />
is clear that the processes of evaporation are relatively moderate<br />
and since the temperature of degradation is shifted only 15 C<br />
higher, the deposition under vacuum is necessary.<br />
The introduction of two methyl groups to the nitrogen atoms<br />
breaks the planarity of the molecule [7] decreasing the strength of<br />
H-bonds and consequently causes easier defragmentation of the<br />
molecule. That can be identified in the sample DPP-MM which<br />
showed the lowest onset on DTG plot. Since Tev and TD in air are<br />
similar, it could be expect that the temperature is associated with<br />
sample’s evaporation. However, as suggested by TGA analysis in<br />
nitrogen, that temperature is again either close or identical to the<br />
temperature of sample degradation since there was a char observed<br />
at the end of the analysis at 650 C. In contrast, it can be seen that<br />
N-substitution by methyl groups in sample DPP-MM caused relatively<br />
high melting temperatures of present crystals (very close to<br />
the temperature of evaporation/degradation) with the highest<br />
melting enthalpy in comparison with other samples. Unlike the<br />
other samples, DPP-MM was not able to be measured after different<br />
thermal history due to the vicinity of both temperatures. The value<br />
of melting enthalpy reported in Table 2 can also be biased by<br />
processes occurring simultaneously.<br />
A similar situation was observed for sample DPP-B. Unlike the<br />
sample DPP-MM, DPP-B presents an asymmetrical derivate of DPP<br />
which seems to increase both temperatures of evaporation and<br />
crystal melting. Again, the temperatures of degradation determined<br />
using TGA in both atmospheres were very close to each other<br />
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J. David et al. / Dyes and Pigments 89 (2011) 137e143 141<br />
Table 2<br />
Comparison of results obtained by DSC measurement for samples which underwent different thermal history. For better understanding only major events are given, the rest<br />
can be found in the text. DPP is not reported due to the absence of significant transitions.<br />
Thermal history Sample Crystallization I Crystallization II Melting I Meting II Glass transition<br />
Tonset [ C] DH [J g 1 ] Tonset [ C] DH [J g 1 ] Tonset [ C] DH [J g 1 ] Tonset [ C] DH [J g 1 ] Tmidpoint [ C]<br />
moderate cooling DPP-MM e e e e 234.4 109.4 e e e<br />
rapid cooling DPP-MM n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.<br />
moderate cooling DPP-B e e e e 245.4 93.8 e e e<br />
rapid cooling DPP-B e e e e 244.6 92.5 e e 18.3<br />
moderate cooling DPP-BB 123.9 38.5 e e 120.1 72.3 135.2 70.9 e<br />
rapid cooling DPP-BB 55.5 69.0 e e 108.3 6.1 135.6 83.9 5.1<br />
moderate cooling DPP-H e e e e 211.9 75.8 e e e<br />
rapid cooling DPP-H e e e e 212.1 74.9 e e e<br />
moderate cooling DPP-HH e e e e 115.2 96.3 e e e<br />
rapid cooling DPP-HH 25.3 37.4 62.1 30.7 114.7 95.4 e e 22.2<br />
suggesting the same explanation as given for DPP-MM sample, i.e.<br />
sample is simultaneously evaporated and decomposed above Tev.<br />
That observation is confirmed by the presence of char at 650 C<br />
(Table 1).<br />
An increase in the onset temperature was observed also for the<br />
second asymmetrical sample DPP-H. The significant difference in<br />
the onset temperatures obtained under different conditions indicates<br />
the decrease of thermo-oxidative resistance with the<br />
increasing length of alkyl chain. This can be interrelated to the<br />
decrease in temperature of crystal melting in the sample. It is our<br />
hypothesis that alkyl chains have higher molecular motion after the<br />
crystalline structure melting and reactive gas can easily diffuse<br />
through the sample. It is well known that aliphatic chains are<br />
relatively unstable under a thermo-oxidative attack [14], hence, the<br />
stability of the derivatives under study decreases with the<br />
increasing length of alkyl chain.<br />
Table 2 reports that asymmetrical molecules, i.e. samples DPP-B<br />
and DPP-H did not show any response to the different thermal<br />
history which implies a relatively easy molecular transport to form<br />
crystals and no tendency to polymorphism. In contrast, symmetrical<br />
molecules with longer alkyl chains DPP-BB and DPP-HH seem<br />
to have tendency to form different crystalline forms resulting in<br />
occurrence of multiple peaks in DSC signal. In contrast to the<br />
sample DPP-HH, when the DPP-BB sample was cooled rapidly,<br />
the enthalpy of melting dropped down which indicates that the<br />
molecular motion during the crystal formation of both samples was<br />
disturbed by different extraneous factors. Moderate cooling of<br />
sample DPP-BB gave melting-crystallization-melting sequence<br />
while DPP-HH simply melted in one step and at lower temperature<br />
than DPP-BB. When cooled quickly, DPP-HH pre-crystallized in two<br />
steps while the onset of the first melting was at lower temperature<br />
than that for sample DPP-BB. That implies that in the first stage the<br />
longer alkyl chains in sample DPP-HH needed lower energy input to<br />
re-crystallize rather than the shorter chain in sample DPP-BB.<br />
Accordingly, inspired by melting temperatures and enthalpies of<br />
pure alkanes, it is supposed that there is a hindrance in molecular<br />
motion in C4H9 alkyl chain reducing the rate of crystallization.<br />
There is a possible explanation that the DPP molecular skeleton has<br />
an influence on the alkyl chain via van der Waals interactions. In<br />
fact, the first carbons of alkyl chain can be affected by the presence<br />
of neighboring electron acceptor atom having a great influence on<br />
the molecular motion of the whole chain. The other possibility is<br />
that the molecular motion is influenced by the whole DPP skeleton<br />
and the C4H9 chain length is not long enough to overcome the<br />
electrostatic interactions between adjacent DPP molecules. Since<br />
there is a strong intermolecular interaction between oxygen lone<br />
electron pair and hydrogen of eNH group [18], it seems that the H<br />
atom can be substituted in its role by short alkyl chain since the
142<br />
melting of DPP-MM sample is relatively high; on the contrary, the<br />
temperature of degradation/evaporation is very close, as a result<br />
the H-bonds formed by eNH group is weaker and consequently<br />
destabilizes the structure at relatively low temperatures.<br />
Using the nomenclature of Lehman [15], enantiotropic polymorphism<br />
pertains two crystal forms that undergo reversible phase<br />
exchanges between one another, while monotropic polymorphism<br />
corresponds to two crystal forms where one e a kinetically trapped,<br />
metastable form e undergoes an irreversible phase change to the<br />
other thermodynamically more stable form [16]. Fig. 5 indicates<br />
two crystalline structures present in sample DPP-BB represented by<br />
two endothermal melting peaks. Clearly, the existence of the less<br />
stable form depends on the rate of cooling, so it is kinetically<br />
governed and therefore the sample crystalline structure is monotropically<br />
polymorphous. In the case of sample DPP-HH (Fig. 6),<br />
after cooling the kinetically most favorable polymorph crystallized<br />
first and subsequently transformed into thermodynamically stable<br />
one. Again, such behavior can be attributed to the monotropic<br />
polymorphism.<br />
Fruitful discussion can be held after comparison of monosubstituted<br />
and bi-substituted derivatives. The TDs obtained for the<br />
mono-substituted derivatives were always higher than those for the<br />
bi-substituted. A possible explanation implies the electron donating<br />
character of the alkyl groups. Due to the mesomeric effect between<br />
the nitrogen and oxide atoms, the electron density is unequally<br />
distributed over the molecule and creates a dipole. The alkylation<br />
even increases the mesomeric effect and consequently increases the<br />
polarity of the molecule. This can be experimentally observed as<br />
a blurring of vibrational structure of electron spectra [7,16,17].<br />
Therefore, the electron donating character of the substituted alkyls<br />
increases reactivity and consequently causes easier defragmentation<br />
of the molecule. As a result the mono-substituted derivatives<br />
are thermally more stable because of their lower reactivity.<br />
In order to support our statement about the evaporation followed<br />
by degradation, the DPP-MM sample was thermally treated<br />
at specific temperatures and the sample composition was followed<br />
by FTIR analysis (KBr pellet technique). As shown in Fig. 7, at<br />
temperature 260 C, i.e., 3 C below the predicted decomposition<br />
temperature, the sample still resembled the molecular character of<br />
DPP-MM. At 280 C the FTIR spectra showed a shift in intensities at<br />
several wave numbers, and the shift continued with increasing<br />
temperature (not shown). The attribution of peaks wave numbers<br />
Fig. 7. Comparison of FTIR records of sample DPP-MM, and DPP-MM heated up to 260<br />
and 280 C. The Figure shows only selected wavenumbers range.<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
J. David et al. / Dyes and Pigments 89 (2011) 137e143<br />
to bond vibrations can be found for example in [3]. It was impossible<br />
to determine the exact temperature at which the change of<br />
structure occurred since the degradation did not take part in the<br />
whole volume of the sample. Indeed, the changes were observable<br />
at first sight, the color of the pigment changed from red to orange at<br />
280 C and greenish above 300 C.<br />
5. Conclusion<br />
One unsubstituted sample, three symmetrically N-substituted<br />
samples (methyl, butyl and heptyl) and two asymmetrically Nsubstituted<br />
samples (butyl and heptyl) of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione<br />
were investigated to observe<br />
the influence on physical-chemical properties of different alkyl<br />
chains and symmetry of substitution. From the point of view of<br />
their evaporation temperature, thermal stability and thermooxidative<br />
stability, it was revealed that substitution brought about<br />
significant destabilization of the structure in comparison with<br />
parental molecule. It was demonstrated that the length of<br />
the substituting alkyl chain is a crucial factor in their stability; the<br />
shorter chain the less stable derivate was obtained while<br />
the symmetrical derivates were less stable than asymmetrical ones.<br />
As revealed by DSC, unlike the other samples, the symmetrical<br />
derivates indicated monotropical polymorphism. On the contrary,<br />
DSC experiments did not reveal the presence of different crystalline<br />
structures in DPP, although the presence of two crystalline forms<br />
was reported [19]. It seems that the thermal agitation (and therefore<br />
DSC) is not a suitable manner for transformation of those<br />
phases, it is likely that the aforementioned weak interactions are<br />
strong enough to resist their vibration upon heating.<br />
Acknowledgements<br />
The financial support of the Ministry of Education of the Czech<br />
Republic e project MSM 0021630501 and the Academy of Sciences<br />
of the Czech Republic e project KAN401770651 are acknowledged.<br />
References<br />
[1] (a) Rochat AC, Cassar L, Iqbal A. EP 94911; 1983. (b) Iqbal A, Pfenninger J,<br />
Rochat AC, Babler F. EP 181290; 1989. (c) Pfenninger J, Iqbal A, Rochat AC,<br />
Wallquist O. USP 4778899; 1986. (d) Surber W, Iqbal A, Stern C. EP 302018;<br />
1989. (e) Wooden G, Schloeder I, Wallquist O. EP 672729; 1995. (f) Hendi SB.<br />
EP 962499; 1999.<br />
[2] Zambounis JS, Hao Z, Iqbal A. Latent pigments activated by heat. Nature<br />
(London) 1997;388(6638):131e2.<br />
[3] Fukuda M, Kodama K, Yamamoto H, Mito K. Evaluation of new organic<br />
pigments as laser-active media for a solid-state dye laser. Dyes and Pigments<br />
2004;63(2):115e25.<br />
[4] Mizuguchi J, Imoda T, Takahashi H, Yamakami H. Polymorph of 1,4-diketo-3,6bis-(4’-dipyridyl)-pyrrolo-[3,4-c]pyrrole<br />
and their hydrogen bond network: A<br />
material for H2 gas sensor. Dyes and Pigments 2006;68(1):47e52.<br />
[5] Hoki T, Takahashi H, Suzuki S, Mizuguchi J. Hydrogen gas sensor based upon<br />
proton acceptors integrated in copper-tetra-2,3-pyridinoporphyradine. IEEE<br />
Sensors Journal 2007;7(5):808e13.<br />
[6] Beyerlein T, Tieke B, Forero-Lenger S, Brütting W. Red electroluminescence<br />
from a 1,4-diketopyrrolo[3,4-c]pyrrole (DPP)-based conjugated polymer.<br />
Synthetic Metals 2002;130(2):115e9.<br />
[7] Vala M, Weiter M, Vynuchal J, Toman P, Lunák Jr S. Comparative studies of<br />
diphenyl-diketo-pyrrolopyrrole derivatives for electroluminescence applications.<br />
Journal of Fluorescence 2008;18(6):1181e6.<br />
[8] Potrawa T, Langhals H. Fluorescent dyes with large stokes shifts e soluble<br />
dihydropyrrolopyrrolediones. Chemische Berichte 1987;120(7):1075e8.<br />
[9] Aret E, Meekes H, Vlieg E, Deroover G. Polymorphic behavior of a yellow<br />
isoxazolone dye. Dyes and Pigments 2007;72:339e44.<br />
[10] Mizuguchi J, Wooden G. A large bathochromic shift from the solution to the<br />
solid state in 1,4-diketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole. Berichte der<br />
Bunsen-Gesellschaft für Physikalische Chemie 1991;95:1264e74.<br />
[11] Mizuguchi J. Correlation between crystal and electronic structures in diketopyrrolopyrrole<br />
pigments as viewed from exciton coupling effects. Journal of<br />
Physical Chemistry A 2000;104:1817e21.
[12] Mizuguchi J, Grubenmann A, Wooden G, Rihs G. Structures of 3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4-dione<br />
and 2,5-dimethyl-3,6-diphenylpyrrolo<br />
[3,4-c]pyrrole-1,4-dione. Acta Crystallographica 1992;B48:696e700.<br />
[13] Weiter M, Salyk O, Bednár P, Vala M, Navrátil J, Zmeskal O, Vynuchal J, Lunák<br />
Jr. S. Morphology and properties of thin films of diketopyrrolopyrrole derivatives,<br />
Materials Science and Engineering: B, submitted for publication.<br />
[14] Válková D, Kislinger J, Pekar M,Kucerík J. The kinetics of thermo-oxidative<br />
humic acids degradation studied by isoconversional methods. Journal of<br />
Thermal Analysis and Calorimetry 2007;89:957e64.<br />
[15] Lehman O. Die krystallanalyse. Leipzig: Wilhelm Engelmann; 1891.<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
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[16] Hauser MR, Zharakov L, Doxsee KM, Tonglei L. Polymorphism of a simple<br />
organic amide. Crystal Growth and Design 2008;8(12):4428e31.<br />
[17] Lunák S, Vynuchal J, Vala M, Havel L, Hrdina R. The synthesis, absorption and<br />
fluorescence of polar diketo-pyrrolo-pyrroles. Dyes and Pigments<br />
2009;82:102e8.<br />
[18] Vala M, Vynuchal J, Toman P, Weiter M, Lunák Jr S. Novel soluble diphenyldiketo-pyrrolopyrroles.<br />
Experimental and theoretical study. Dyes and<br />
Pigments 2009;89:102e8.<br />
[19] Hao Z, Iqbal A. Some aspects of organic pigments. Chemical Society Reviews<br />
1997;26:203e13.
The geometry and absorption of diketo-pyrrolo-pyrroles<br />
substituted with various aryls<br />
Stanislav Luňák Jr. a , Lukásˇ Havel a , Jan Vyňuchal b,c, *, Petra Horáková a ,<br />
Jirˇí Kučerík d , Martin Weiter d , Radim Hrdina a<br />
a<br />
Department of Organic Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-532 10 Pardubice, Czech Republic<br />
b<br />
Research Institute of Organic Syntheses, Rybitví 296, CZ-533 54 Rybitví, Czech Republic<br />
c<br />
Synthesia a.s., Pardubice, Semtín 103, CZ-532 17 Pardubice, Czech Republic<br />
d<br />
Faculty of Chemistry, Brno University of Technology, Purkyňova 118, CZ-612 00 Brno, Czech Republic<br />
article info<br />
Article history:<br />
Received 30 June 2009<br />
Received in revised form<br />
25 September 2009<br />
Accepted 28 September 2009<br />
Available online 1 October 2009<br />
Keywords:<br />
Diketo-pyrrolo-pyrroles (DPP)<br />
Time dependent density functional theory<br />
(TD DFT)<br />
Absorption<br />
Fluorescence<br />
1. Introduction<br />
abstract<br />
The present authors have recently shown that calculations based<br />
on time dependent density functional theory (TD DFT) could predict<br />
the absorption maxima of diketo-pyrrolo-pyrrole (DPP) pigments<br />
which had been substituted with various combinations of strong<br />
electron-donating and electron-withdrawing substituents in the<br />
para positions of both pendant phenyl rings [1]. A necessary<br />
component of such computation was the introduction of solvent<br />
effects by a polarized continuum model (PCM). The aim of this paper<br />
is to challenge this methodology by interpreting the relationship<br />
between structure and absorption of a series of symmetrical and<br />
unsymmetrical DPPs that comprise different combinations of aryl<br />
substituents (Table 1).<br />
3,6-Diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (BPPB<br />
in our notation) is a basic chromophore of this set; it has been<br />
patented by CIBA [2]. BPPB itself (C.I. Pigment Red 255) and at least<br />
* Corresponding author at: Research Institute of Organic Syntheses, Rybitví 296,<br />
CZ-533 54 Rybitví, Czech Republic. Tel.: þ420 466 823 351; fax: þ420 466 822971.<br />
E-mail address: jan.vynuchal@vuos.com (J. Vyňuchal).<br />
0143-7208/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.<br />
doi:10.1016/j.dyepig.2009.09.014<br />
Dyes and Pigments 85 (2010) 27–36<br />
Contents lists available at ScienceDirect<br />
Dyes and Pigments<br />
journal homepage: www.elsevier.com/locate/dyepig<br />
A series of five symmetrical and four unsymmetrical diaryl substituted diketo-pyrrolo-pyrroles was<br />
synthesized; two unsymmetrical derivatives are reported for the first time. The relationship between the<br />
theoretical excitation energies of the S0 / S1 transition, computed by time dependent density functional<br />
theory and the experimental positions of 0–0 vibronic bands in the visible absorption (or fluorescence<br />
excitation) spectra was studied. Experimental data were obtained from either solution or from low<br />
temperature organic solvent glass, in which the progressions of the vibrational structure enabled correct<br />
assignment of vibronic sub-bands in some cases. Theoretical calculations predicted that a linear bathochromic<br />
and hyperchromic shift would accompany substitution of each phenyl in the parent 3,6-diphenyl-<br />
2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione by providing a more extensively conjugated aryl centre<br />
(2-naphthyl, biphenyl, stilbenyl) for the ensuing planar derivatives. Qualitatively, the experimental<br />
bathochromic shifts of the 0–0 vibronic sub-bands were in exact agreement with theory, whilst hyperchromic<br />
shifts were affected by the very low solubility of the planar, symmetrical derivatives. Deviations<br />
from this ideal behaviour were observed for non-planar derivatives (aryl ¼ stilbenyl or 1-naphthyl), for<br />
which the dihedral angles describing the aryl, out-of-plane torsions were probably underestimated by DFT.<br />
Ó 2009 Elsevier Ltd. All rights reserved.<br />
five its symmetrical derivatives substituted on phenyl form<br />
a commercially very successful family of high performance organic<br />
pigments [3]. One of them, BiPPBi (C.I. Pigment Red 264) was also<br />
used in this study. These two pigments are the only ones from the<br />
model set, for which the structure was estimated experimentally<br />
using X-ray diffraction [4,5]. The other compounds under study were<br />
not commercialised at the time of manuscript preparation.<br />
Extensive literature exists on DPPs in general and, more specifically,<br />
in relation to commercial examples; in this context, >279 references<br />
exist for BPPB and 138 for BiPPBi were found as at 08/06/2009. In<br />
contrast, there is a paucity of citations relating to other symmetrical<br />
derivatives, with just 6 references to 1NPP1N,7for2NPP2N and 2 for<br />
StPPSt being found in patent literature. Four patents discuss the<br />
asymmetrical compound BiPPB and 1 patent describes 1NPPB;asno<br />
references relating to the 2-naphthyl compound 2NPPB and the stilbene<br />
analogue StPPB were found, the syntheses of these two particular<br />
compounds was described in detail. Styryl (phenyl–ethenyl) PePPB<br />
and PePPPe derivatives were handled only theoretically, as a synthetic<br />
route to theses compounds is not known [6].<br />
With exception of the authors’ previous work [1], noab initio<br />
calculations of DPP pigments have been published and references
28<br />
Table 1<br />
Notation of the compounds studied.<br />
Ar<br />
to only semiempirical PPP [7] and INDO/S studies on BPPB and its<br />
aggregates [8] were found. The latter computations were based on<br />
X-ray geometry [7] which was later revised [4] and which underestimated<br />
the visible absorption maximum compared to its<br />
experimental counterpart [9].<br />
2. Experimental<br />
2.1. Syntheses and analysis<br />
Ar<br />
HN NH<br />
O<br />
BPPB was synthesized from benzonitrile and diisopropyl<br />
succinate [10]. Other symmetrical derivatives (BiPPBi, 1NPP1N,<br />
2NPP2N and StPPSt) were synthesized in an analogous manner<br />
from corresponding nitriles purchased from Aldrich, except for<br />
4-cyano-stilbene, which was synthesized as described bellow. The<br />
purity of all compounds was determined using elemental analysis,<br />
MS and 1 H NMR.<br />
The unsymmetrical derivatives (BiPPB, 1NPPB and StPPB)were<br />
synthesized by the base-catalysed condensation of ethyl 4,5dihydro-5-oxo-2-phenyl(1H)pyrrole-3-carboxylate(phenyl-pyrrolinone<br />
ester) as described previously [11] with the above<br />
mentioned nitriles. 2NPPB was synthesized from benzonitrile and<br />
ethyl 4,5-dihydro-5-oxo-2-(2-naphthyl)-(1H)pyrrole-3-carboxylate<br />
(2-naphthyl-pyrrolinone ester), which synthesis is described<br />
bellow. The syntheses of two till unknown unsymmetrical derivatives<br />
(2NPPB and StPPB) is presented in detail.<br />
2.1.1. Syntheses of 4-cyano-stilbene<br />
Triethanolamine (200 ml), 4-bromobenzonitrile(18.2 g, 0.1 mol),<br />
styrene (10.4 g, 0.1 mol) and palladium acetate (0.23 g,1 mmol) were<br />
O<br />
Ar<br />
symmetrical<br />
BPPB –<br />
BiPPBi BiPPB<br />
2NPP2N 2NPPB<br />
1NPP1N 1NPPB<br />
StPPSt StPPB<br />
PePPPe PePPB<br />
S. Luňák Jr. et al. / Dyes and Pigments 85 (2010) 27–36<br />
HN NH<br />
O<br />
O<br />
Ar<br />
unsymmetrical<br />
charged into a 0.5 l Keller flask equipped with a stirrer, reflux<br />
condenser, thermometer and a nitrogen inlet. The reaction mixture<br />
was heated out at 100 C for 10 h. After a cooling to room temperature<br />
the product was extracted with ether (2 300 ml). The ether<br />
layers were washed with water, dried with magnesium sulphate and<br />
evaporated. The yellow product was dissolved by stirring in methanol<br />
(which removed triethanolamine), filtered and dried. Yield:<br />
19.47 g (95%). The melting point was 116–118 C (lit. [12] 117–119 C).<br />
2.1.2. Syntheses of ethyl 4,5-dihydro-5-oxo-2-(2-naphhtyl)-<br />
(1H)pyrrole-3-carboxylate (2-naphthyl-pyrrolinone ester)<br />
2.1.2.1. Preparation of ethyl 2-naphthoate. 2-Naphthoic acid<br />
(Aldrich, 60 g, 0.348 mol), ethanol (226 ml) and sulphuric acid<br />
(7 ml) were added to the two-necked flask equipped with thermometer,<br />
stirrer and refluxing condenser. The reaction mixture was<br />
heated to reflux for 10 h and after that the mixture was cooled to<br />
room temperature; excess ethanol was distilled off. The ensuing<br />
product was extracted with diethyl ether; the organic layer was<br />
separated, washed with sodium carbonate (5%) and water and then<br />
dried over Na2SO4. The solvent was removed under reduced pressure.<br />
The amount of the oily product was 52 g (yield 75%).<br />
2.1.2.2. Preparation of ethyl 3-(2-naphthyl)-3-oxopropanoate. A<br />
mixture of sodium hydride (caution: reacts violently with water,<br />
liberating hydrogen; incompatible with water, acids, alcohols, strong<br />
oxidizing agents; Aldrich 60%, 16.8 g, 0.42 mol), ethyl 2-naphthoate<br />
obtained by Section 2.1.2.1 (42 g, 0.21 mol) and 1-butoxybutane<br />
(360 ml) was added to the three neck flask equipped with thermometer,<br />
stirrer, dropping funnel and refluxing condenser. The<br />
reaction mixture was heated to 100 C and ethyl acetate (27.6 g,<br />
0.313 mol) in 1-butoxybutane (20 g) was added to the mixture by<br />
means of dropping funnel within about 1.5 h. The suspension was<br />
heated to reflux for 6 h, then cooled to room temperature and<br />
diluted with diethyl ether (100 ml). The obtained salt was filtrated<br />
off, washed with diethyl ether (300 ml). Subsequently, the suspension<br />
was poured onto 300 g ice and acidified by hydrochloric acid to<br />
pH ¼ 2. The product was extracted with diethyl ether, organic layer<br />
was separated, washed with sodium carbonate (5%) and water. Then<br />
dried over Na2SO4. The solvent was removed under reduced pressure.<br />
The amount of oily product was 32.4 g (64% of theory).<br />
2.1.2.3. Preparation of ethyl 4,5-dihydro-5-oxo-2-(2-naphthyl)<br />
(1H)pyrrole-3-carboxylate. Ethyl 3-(2-naphthyl)-3-oxopropanoate<br />
(32.4 g, 0.134 mol) obtained by Section 2.1.2.2, ethyl bromoacetate<br />
(24.6 g, 0.147 mol), sodium carbonate (19.3 g), acetone (32 ml) and<br />
ethyleneglycol dimethylether (32 ml) were added to the threenecked<br />
flask equipped with thermometer, stirrer and refluxing<br />
condenser. The reaction mixture was refluxed for 14 h. Acetone was<br />
continuously added into the reaction mixture to keep the constant<br />
volume due to evaporating off. Subsequently, the mixture was<br />
cooled down at room temperature and inorganic salt was filtrated<br />
and washed by acetone. The filtrate and acetone from the washing<br />
process was combined and acetone was distilled off with other<br />
volatile fractions up to 150 C under nitrogen. Acetic acid (101 ml)<br />
and ammonium acetate (58 g) were added to the dark liquid<br />
distilling residue. The mixture was kept under reflux at 120 C for<br />
4 h. 16.1 g (yield 43%) of a product was obtained by filtration from<br />
the mixture after the cooling. A sample for analysis was recrystallized<br />
from toluene. The melting point was 188–193 C.<br />
Calculated: C (72.58), H (5.37), N (4.98). Found: C (72.50),<br />
H (5.35), N (5.11).<br />
MW ¼ 281 Da; positive-ion APCI-MS: m/z 282 [M þ H] þ (100%)<br />
1 H chemical shifts: 10.83 (1H, s, NH); 8.21–7.61 (7H, m, ArH);<br />
4.05 (2H, q, CH2); 3.49 (2H, s, CH2); 1.10 (3H, t, CH3).
2.1.3. Syntheses of 3-(2-naphthyl)-6-phenyl-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione<br />
(2NPPB)<br />
tert-Amyl alcohol (33 ml) and sodium metal (caution: reacts<br />
violently with water, liberating hydrogen; flammable solid;<br />
incompatible with water, strong oxidizing agents; air sensitive;<br />
0.7 g, 30 mmol) were charged into a 100 ml three-necked flask<br />
equipped with a stirrer, reflux condenser and thermometer. The<br />
sodium metal was dissolved under reflux in the presence of catalytic<br />
amount of FeCl3 (which approximately took 1 h), whereupon<br />
benzonitrile (1.4 g, 13.6 mmol) was added. 2-Naphthyl-pyrrolinone<br />
ester (2.5 g, 8.9 mmol), obtained according to Section 2.1.2.3 was<br />
continuously introduced in small amounts over 0.5 h. The ensuing<br />
mixture was stirred under reflux for 1 h and the hot suspension was<br />
then filtered, the filter cake suspended in 100 ml tert-amyl alcohol<br />
and then 10 ml acetic acid was added to protolyse the salt. Protolysis<br />
was carried out at 100 C for 2 h. The resulting hot<br />
suspension was filtered, and the filter cake was washed with hot<br />
water to neutral washings. The filter cake was suspended in 300 ml<br />
methanol. The suspension was heated to boiling and refluxed for<br />
2 h. The hot suspension was filtered, washed with ethanol and hot<br />
water Yield: 1.5 g (54%).<br />
Calculated: C(78.09), H(4.17), N(8.28). Found C(77.19), H(4.20),<br />
N(8.06)<br />
MW ¼ 338 Da; Negative-ion APCI-MS: m/z 337 [M H] (100%)<br />
1<br />
H chemical shifts: 11.45 (2H, br s, NH); 9.03 (1H, m); 8.7 (1H,<br />
m); 8.55(2H, m); 8.14(1H, m); 8.04 (1H, m); 8.00 (1H, m); 7.56–<br />
7.72 (5H, m).<br />
13<br />
C chemical shifts were not determined due to a very low<br />
solubility of the sample.<br />
2.1.4. Syntheses of 3-(4-stilbenyl)-6-phenyl-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione<br />
(StPPB)<br />
tert-Amyl alcohol (67 ml), sodium metal (1.43 g, 97 mmol) and<br />
a catalytic amount of FeCl3 were charged into a 250 ml threenecked<br />
flask equipped with a stirrer, reflux condenser, thermometer<br />
and nitrogen inlet. The sodium metal was dissolved under the<br />
reflux, whereupon 4-cyano-stilbene (6.4 g, 31 mmol) obtained<br />
according to Section 2.1.1 was added. After that phenyl-pyrrolinone<br />
ester (7.27 g, 31 mmol) was added within 0.5 h. Finally, this mixture<br />
was stirred and refluxed for 2 h. The reaction mixture was cooled to<br />
a60 C and infused into 200 ml distilled water. The mixture was<br />
carried out at 80 C for 2 h. The resulting hot suspension was<br />
filtered and filter cake was washed with 35 ml of hot isopropyl<br />
alcohol and than with hot water to neutral washings. The filter cake<br />
was suspended into methanol. The suspension was heated to<br />
boiling and refluxed 0.5 h. The hot suspension was filtered, washed<br />
with methanol and dried. Yield: 6.66 g (54%).<br />
Calculated: C(79,98), H(4,65), N(7,17). Found C(79,61), H(4,62),<br />
N(7,07)<br />
MW ¼ 390 Da; Positive-ion APCI-MS: m/z 391 [M þ H] þ (100%)<br />
1<br />
H chemical shifts: 11.37 (2H, br s, NH); (7.54 (1H, d, J ¼ 16.7 Hz);<br />
7.38 (1H, d, J ¼ 16.7 Hz); 8,53 (4H, m); 7.86 (2H, d), 7.63 (3H, m))<br />
signals of Ar–CH ¼ CH–Ar; 7.70 (2H, ortho, ArH); 7.47 (2H, meta,<br />
ArH), 7.38 (1H, para, ArH)<br />
13<br />
C chemical shifts were not determined due to a very low<br />
solubility of the sample.<br />
2.2. Instrumental equipment<br />
The room temperature (DMSO) absorption measurements were<br />
carried out on a Perkin–Elmer Lambda 9 absorption spectrometer.<br />
The room temperature (DMSO, MTHF) and low temperature (MTHF,<br />
S. Luňák Jr. et al. / Dyes and Pigments 85 (2010) 27–36 29<br />
77 K) fluorescence emission and excitation spectra were measured<br />
on Perkin Elmer LS 35 fluorescence spectrometer equipped with<br />
commercial low temperature accessory.<br />
Thermogravimetric studies (TGA) were performed using TA<br />
Instruments TGA Q5000 (New Castle, Delaware, USA) device in<br />
100 ml open platinum pans. The samples, typically 5 mg, were<br />
heated by thermal ramp of 10 C min 1 from 40 C to 650 Cin<br />
dynamic nitrogen atmosphere (25 ml min 1 ). Calorimetric analyses<br />
(DSC) were carried out employing TA Instruments DSC Q200<br />
calorimeter equipped with external cooler RCS90 allowing experimental<br />
temperature range from 90 to 500 C. Experiments were<br />
conducted in open TA TzeroÔ aluminum pans. Thermal history of<br />
all samples was set up to be the same using of heating ramp of<br />
3 C min 1 from 40 Cto 90 C and then 10 C/min 1 to the<br />
temperature determined by TGA as Ts (Table 2). All DSC experiments<br />
were made under 50 ml min 1 nitrogen purge. Before<br />
analyses device was calibrated for temperature and enthalpy using<br />
indium, tin and zinc standards (Perkin Elmer, Waltham, Massachussets,<br />
USA). All the records were assessed by TA Universal<br />
Analysis 2000 software version 4.4A.<br />
The equipment for other analytical measurements and the<br />
analytical procedures are the same as described in Ref. [1].<br />
2.3. Computational procedures<br />
The geometry of all eleven compounds was optimized using<br />
quantum chemical calculations based on DFT. Hybrid threeparameter<br />
B3LYP functional [13] in combination with<br />
6-311G(d,p) basis was used. No constraints were preliminary<br />
employed, but, if the non-constrainted computations converged<br />
to symmetrical structures, the final computations were carried<br />
out with these symmetry constraints. No imaginary frequencies<br />
were found after diagonalization of Hessian matrix, confirming<br />
that the computed geometries were real minima on the ground<br />
state hypersurfaces.<br />
The TD DFT method was used for a computation of vertical<br />
excitation energies on the computed geometry. The same<br />
exchange-correlation functional (B3LYP) was used with rather<br />
broader basis set (6-311 þ G(2d,p)) particularly efficient for TD<br />
modeling of organic dyes [14]. Solvent effect of dimethylsulfoxide<br />
(DMSO) was involved by non-equilibrium PCM [15].<br />
All the methods were taken from Gaussian03W program suite<br />
[16], and the default values of computational parameters were<br />
used. The results were analyzed using GaussViewW from Gaussian<br />
Inc., too.<br />
Table 2<br />
Experimental absorption and fluorescence excitation and emission maxima and<br />
sublimation temperatures Ts determined by TGA.<br />
Comp. Absorption Fluorescence Ts [ C]<br />
DMSO (20 C) MTHF (77 K)<br />
labs 3max lem lex lem<br />
BPPB 505 34200 517 512 515 383<br />
2NPP2N 529 30400 547 – – 463<br />
2NPPB 517 37200 534 523 527 372<br />
1NPP1N 471 7900 567 498 527 278<br />
1NPPB 493 21500 558 503 511 347<br />
BiPPBi 535 17300 549 – – 477<br />
BiPPB 520 40100 535 526 531 415<br />
StPPSt 565 33600 578 – – ?410<br />
StPPB 535 44300 554 548 550 ?428<br />
labs, absorption maximum [nm]; lex, excitation maximum [nm]; lem, emission<br />
maximum [nm]; 3max, molar absorptivity [l mol 1 cm 1 ], values in italics come from<br />
insufficiently soluble compounds.
30<br />
3. Results and discussion<br />
3.1. Syntheses and sublimation<br />
The symmetrical derivatives were synthesized from diisopropyl<br />
succinate and corresponding aromatic nitrile. The synthesis of<br />
unsymmetrical derivatives was carried out from ethyl 4,5-dihydro-<br />
5-oxo-2-phenyl(1H)pyrrole-3-carboxylate (phenyl-pyrrolinone<br />
ester) and corresponding aromatic nitrile (Scheme 1 for BiPPB),<br />
S. Luňák Jr. et al. / Dyes and Pigments 85 (2010) 27–36<br />
Scheme 1.<br />
except 2NPPB, which was synthesized from ethyl 4,5-dihydro-5oxo-2-(2-naphthyl)-(1H)pyrrole-3-carboxylate(2-naphthyl-pyrrolinone<br />
ester) and benzonitrile (Scheme 2). The reaction yields were<br />
usually over 50%. The purity of the compounds was checked by<br />
elemental analysis, 1 H NMR and mass spectros<strong>copy</strong>. For details of<br />
the syntheses and analytics see Experimental.<br />
Synthesized compounds were characterized by sublimation<br />
temperature Ts (Table 2) determined by thermogravimetry (TGA).<br />
In fact, the temperature Ts stands for the temperature at which
mass loss starts; kinetics of sublimation is very slow and thus<br />
progressive increase in temperature causes later the degradation of<br />
the sample. The char which could be seen at the end of the TGA<br />
experiment typically reached from 10 to 40% of original sample<br />
mass. Experiments carried out using differential scanning calorimetry<br />
(DSC) revealed no phase transitions within 90 C and Ts.<br />
TGA of StPPB and especially StPPSt show the slowest kinetics of<br />
a sublimation process, for which Ts was determined only tentatively<br />
(Table 2).<br />
The synthesized compounds are pigments highly insoluble in<br />
common solvents, except 1NPPB and especially 1NPP1N. The<br />
unsymmetrical pigments (BiPPB, 2NPPB and StPPB) are slightly<br />
better soluble than the corresponding symmetrical ones (BiPPBi,<br />
2NPP2N and StPPSt).<br />
3.2. DFT ground state geometry and energy<br />
The restricted B3LYP calculations resulted in a strictly planar<br />
geometry for BPPB and thus C2h symmetry [1]. The dihedral angle<br />
between phenyl and pyrrolinone rings in unsymmetrical biphenyl<br />
substituted compound BiPPB was also near to zero, while between<br />
both phenyls it was about 38 . The same angle between phenyls in<br />
biphenyl moiety was found for both rotamers (C2, Ci) ofBiPPBi,<br />
which differ by a mutual orientation of rotated terminal phenyl<br />
rings. The ground state energy of Ci rotamer is negligibly higher<br />
(Table 3).<br />
Two (three) minima of energy were found for 2NPPB (2NPP2N),<br />
all of them corresponding to planar geometry (Fig. 1). The most<br />
stable rotamers are 2NPP2N (180–180) and 2NPPB (0–180), but the<br />
energy differences are marginal (Table 3) as the steric interaction in<br />
all two (three) rotamers is almost the same.<br />
The situation for 1-naphthyl substituted DPPs is different. They are<br />
non-planar, as both 1-naphthyl and phenyl are always rotated out<br />
from a plane of central heterocycle. Such rotation of phenyl ring of<br />
1NPPB, induced by the rotation of 1-naphthyl bonded to the opposite<br />
ring of a central heterocycle, is analogical to the case of N-monoalkylated<br />
BPPB [10]. If planar, there should exist two (three) minima<br />
for 1NPPB (1NPP1N) by analogy with 2-naphthyl conformers,<br />
differing in this case by a steric interaction of a naphthalene ring<br />
S. Luňák Jr. et al. / Dyes and Pigments 85 (2010) 27–36 31<br />
Scheme 2.<br />
Table 3<br />
DFT computed relative ground state energy and PCM TD DFT computed excitation<br />
energies (converted to lmax [nm]) and oscillator strengths (fosc) in DMSO.<br />
Compound Symmetry Dihedral<br />
angles a<br />
Relative<br />
internal<br />
energy b<br />
S0 / S1<br />
transition<br />
characteristics<br />
lmax fosc<br />
BPPB C2h 0–0 – 494 0.641<br />
2NPP2N C2h 0–0 0.21 523 1.156<br />
Cs 0–180 0.15 534 0.952<br />
C2h 180–180 0.00 542 0.840<br />
2NPPB Cs 0–0 0.09 509 0.886<br />
Cs 0–180 0.00 519 0.718<br />
1NPP1N Ci 134–134 12.85 493 0.584<br />
C2 136–136 12.12 497 0.599<br />
C1 28–135 11.46 516 0.666<br />
C1 29–136 10.95 513 0.679<br />
Ci 29–29 10.29 537 0.755<br />
C2 29–29 9.64 532 0.767<br />
1NPPB C1 7–136 6.19 495 0.611<br />
C1 7–29 4.87 515 0.703<br />
BiPPBi Ci 0–0 0.01 531 1.168<br />
C2 0–0 0.00 531 1.170<br />
BiPPB C1 0–0 – 513 0.894<br />
StPPSt C2h 0–0 0.15 600 2.012<br />
Cs 0–180 0.12 601 1.959<br />
C2h 180–180 0.00 601 1.923<br />
StPPB Cs 0–0 0.06 550 1.355<br />
Cs 0–180 0.00 550 1.296<br />
PePPPe C2h 0–0 1.89 574 1.402<br />
Cs 0–180 0.89 588 1.128<br />
C2h 180–180 0.00 598 0.974<br />
PePPB Cs 0–0 0.90 534 1.016<br />
Cs 0–180 0.00 547 0.789<br />
a The first value is the dihedral angle between phenyl and pyrrolinone, the second<br />
is between non-phenyl aryl and pyrrolinone for unsymmetrical derivatives.<br />
b Relative to the bolded energy of most stable isomer [kcal. mol 1 ].
32<br />
non-bonded to pyrrolinone ring with either N–H or C]O group.<br />
Furthermore, each of them can theoretically exist in two rotameric<br />
forms differing by a mutual sense of aryl rotations. In summary, 4 (6)<br />
minima should exist for 1NPPB (1NPP1N).<br />
Only two of the theoretical four minima were found for 1NPPB,<br />
showing the naphthyl-pyrrolinone dihedral angle 29 (136 ) and<br />
phenyl-pyrrolinone angle 7 in both cases. Even if the starting<br />
geometry favoured the rotamer with opposite sense of phenyl<br />
rotation (173–136, 173–29), the geometry converged to the above<br />
S. Luňák Jr. et al. / Dyes and Pigments 85 (2010) 27–36<br />
Fig. 1. Rotamers of 2NPP2N, 1NPP1N, StPPSt and PePPPe.<br />
mentioned ones (7–136, 7–29). The energy difference is not<br />
negligible in this case: the 1NPPB (7–136) is 1.32 kcal/mol higher in<br />
energy than 1NPPB (7–29), so the steric interaction in an<br />
arrangement, where the naphthalene ring non-bonded to pyrrolinone<br />
is closer to carbonyl group, is more pronounced.<br />
All six possible rotamers of 1NPP1N were found as the local<br />
minima at this level of calculation. Three rotamers with the same<br />
sense of aryl rotations corresponding to Ci symmetry for (29–29)<br />
and (134–134) dihedral angle sets are shown in Fig. 1, too. The
energy difference between the pair of rotamers with opposite sense<br />
of aryl rotations is low (less than 1 kcal mol 1 ), preferring C2<br />
symmetry. The energy difference among three rotamers differing<br />
by a mutual orientation of naphthalene with respect to carbonyl<br />
group are more pronounced: (29–136) and C2 (136–136) rotamers<br />
are higher in energy with respect to the most stable one C2 (29–29)<br />
for 1.31 kcal mol 1 and 2.48 kcal mol 1 , respectively.<br />
Generally, any rotamer of 2-naphthyl derivatives is always more<br />
stable than the most stable 1-naphthyl isomer. The differences<br />
between the most stable ones are 4.87 kcal mol 1 between 1NPPB<br />
(7–29) and 2NPPB (0–180), and 9.64 kcal mol 1 between 1NPP1N<br />
(C2 (29–29)) and 2NPP2N (180–180).<br />
Stilbenyl derivatives (only trans arrangement was taken into<br />
account) are completely planar by theory, so as the styryl ones<br />
(Fig. 1). The former ones evoke the 2-naphthyl DPPs with<br />
a marginal differences of the ground state energy between all<br />
rotamers, while the latter ones are closer to 1-naphthyl DPPs with<br />
indispensable differences among the rotamers’ energies (Table 3).<br />
The experimental X-ray structures are available only for BPPB<br />
[4] and BiPPBi [5]. The computed geometry of both compounds<br />
well predicts aromatic character of phenyl rings and bond lengths<br />
alternation in diketo-pyrrolo-pyrrole moiety. The main difference<br />
between computed geometry and that obtained from X-ray<br />
diffraction on crystals is in planarity. According to X-ray diffraction<br />
the pendant phenyl groups on both compounds are twisted with<br />
respect to the heterocycle plane. The dihedral angle is reported to<br />
be equal to 7 for BPPB [4] and the molecule is of Ci symmetry in<br />
crystal. The dihedral angle phenyl-pyrrolinone is 18–19 for BiPPBi,<br />
the phenyl–phenyl one is 33 and the molecule is of C1 symmetry in<br />
crystal, because the Ci symmetry is slightly distored through a nonplanarity<br />
(4 ) on central dipyrrolinone heterocycle.<br />
We have discussed the non-planarity of further DPP pigments,<br />
whose X-ray structures are available, in our previous study [1] and<br />
ascribed the non-zero dihedral angles between phenyl and pyrrolinone<br />
planes to packing effects in crystal phase. The phenyl–<br />
phenyl dihedral angle in biphenyl moiety is less important for<br />
a good prediction of spectral behaviour, so the difference about 5<br />
between computed and experimental values is acceptable.<br />
3.3. PCM TD DFT vertical excitation energies in DMSO<br />
The vertical excitation energies of the lowest electronic transition<br />
recomputed to wavelengths (lmax) and oscillator strengths (fosc)ofall<br />
the compounds computed using PCM TD DFT (B3LYP/6-<br />
311 þ G(2d,p)) on B3LYP/6-311G(d,p) geometry in dimethylsulfoxide<br />
(DMSO) are summarized in Table 3. Table S1 in Supplementary<br />
information contains the computed spectral characteristics of four<br />
lowest S0 / Sn transitions. The lowest excited state is always of<br />
S1(pp * ) character, and the transition S0 / S1 is symmetry allowed of<br />
HOMO / LUMO type, significantly (always more than 90 nm)<br />
separated from the nearest forbidden state (either on carbonyl<br />
localized np * or HOMO / LUMO þ 1delocalizedpp * ).<br />
The difference in lmax and fosc between C2 and Ci rotamers of<br />
BiPPBi is negligible for the first three transitions. Each phenyl<br />
substitution brings the same bathochromic shift of S0 / S1 transition<br />
(19 nm), when going from BPPB through BiPPB to BiPPBi,<br />
accompanied by almost linear hyperchromic shift (Table 3). On the<br />
other hand the energies of S0 / S1 transition of 2-naphthyl<br />
derivatives depends on a rotamer type strongly. The increment in<br />
predicted lmax values is about 10 nm per one naphthyl rotation, the<br />
most bathochromically shifted rotamers are 2NPP2N (180–180)<br />
and 2NPPB (0–180). Rather surprisingly, this bathochromic trend<br />
among the rotamers is accompanied by the hypochromic one.<br />
Bathochromic and hyperchromic shift is also linear when going<br />
from BPPB through 2NPPB to 2NPP2N and the rotamers with<br />
S. Luňák Jr. et al. / Dyes and Pigments 85 (2010) 27–36 33<br />
corresponding arrangement of 2-naphthyl substituent with respect<br />
to pyrrolinone are taken into account: e.g. the increment in lmax is<br />
15 nm for an arrangement with dihedral angles equal to 0 and the<br />
values of fosc are almost the same as for biphenylyl set.<br />
The rotamers in stilbenyl set show marginal differences of lmax<br />
values and moderate ones for fosc. On the other hand the styryl<br />
derivatives resemble the 2-naphthyl set; the dependence of these<br />
spectral features on a rotameric arrangement is even more<br />
pronounced, including the coupled batho and hypochromic trends<br />
among the rotamers. Generally, any increase of a conjugated<br />
system of pendant aryls leads to bathochromic shift with respect to<br />
parent BPPB for planar DPPs. The most efficient is stilbene, followed<br />
closely by styryl. The position of biphenyl and 2-naphthyl in<br />
this sequence depends on a rotameric form of 2-naphthyl DPPs. The<br />
S0 / S1 transition energies of unsymmetrical derivatives lie<br />
approximately in the middle of the interval between the corresponding<br />
values of symmetrical analogues and BPPB, oscillator<br />
strengths are close to their arithmetic mean.<br />
The excitation energies predicted for 1-naphthyl derivatives are<br />
strongly dependent on a degree of non-planarity. The position of<br />
lmax covers a wide range from the values close to the parent BPPB<br />
for the most distored rotamers to the values similar as for their<br />
planar 2-naphthyl isomers. Rather surprisingly, fosc depends on<br />
a geometry only moderately, probably because the hyperchromic<br />
trend, caused by an increase of conjugated system, is compensated<br />
by a hypochromic trend, resulting from a planarity perturbation.<br />
3.4. Experimental absorption and fluorescence excitation spectra<br />
All compounds, except 1NPP1N and 1NPPB, show very limited<br />
solubility even in DMSO at room temperature. The worst soluble<br />
compounds are BiPPBi, 2NPP2N and StPPSt, all showing the<br />
spectral artefacts. The spectrum of 2NPP2N (and BiPPBi, too)<br />
shows a long wavelength tailing caused by a light scattering of the<br />
unsoluted particles of a pigment. The spectrum of StPPSt has<br />
furthermore a long wavelength spectral band at 612 nm, that can be<br />
probably attributed to dimer or higher aggregate (Fig. 2). Both types<br />
of spectral artefacts are not detected in fluorescence excitation<br />
spectra of these compounds. Other derivatives are better soluble<br />
and a complete agreement between absorption and fluorescence<br />
excitation is observed.<br />
The absorption spectra of all compounds except 1NPP1N and<br />
1NPPB (Fig. 3) show the spectral shape very similar to parent BPPB.<br />
Their vibronic structure, well resolved even in DMSO at room<br />
temperature, enables the identification of 0–0 vibronic sub-band,<br />
that can be compared with the theoretical value. This sub-band<br />
Relative intensity (-)<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0.0<br />
300 350 400 450 500 550 600 650 700 750 800<br />
(nm)<br />
2NPP2N<br />
StPPSt<br />
Fig. 2. The room temperature absorption spectra of DMSO solutions of StPPSt and<br />
2NPP2N.
34<br />
(l.cm-1.mol-1)<br />
forms also an absolute spectral maximum. All compounds fluoresce<br />
in DMSO, showing approximately a mirror symmetry relation<br />
between excitation and emission. The wavelengths of absorption<br />
and emission 0–0 vibronic sub-bands are summarized in Table 2.<br />
Stokes shift lies between 10 and 20 nm. The vibronic structure is<br />
further sharpened in low temperature (77 K) 2-methyl-tetrahydrofuran<br />
(MTHF) glass, at which the fluorescence emission and<br />
excitation spectra were measured. The typical behaviour is shown<br />
in Fig. 4 for StPPB. The excitation maxima are shifted bathochromically,<br />
as MTHF glass is even more polar environment than<br />
DMSO solution [17], and the emission maxima hypsochromically, as<br />
the excited state relaxation processes are limited in rigid glass.<br />
Finally, Stokes shift is decreased to 2–5 nm (Table 2). BiPPBi,<br />
2NPP2N and StPPSt were not sufficiently soluble in MTHF even at<br />
concentrations lower than 10 6 M to measure their low temperature<br />
fluorescence emission and excitation spectra.<br />
1-Naphthyl derivatives show partially (1NPPB) or fully<br />
(1NPP1N) blurred vibronic structure (Fig. 3) in DMSO, as a consequence<br />
of non-planarity, so an identification of 0–0 vibronic transition<br />
is difficult, especially for 1NPP1N. Their low temperature<br />
fluorescence excitation spectra show quite well resolved vibronic<br />
structure for 1NPPB (Fig. 5) and rather worse resolution for<br />
1NPP1N (Fig. 6). Evolution of the spectra when going from room to<br />
low temperature may be considered as an evidence, that the<br />
absolute spectral maximum corresponds to 0–0 vibronic transition<br />
for 1NPPB and probably to 0–1 transition for 1NPP1N.<br />
If we do not consider both non-planar 1-naphthyl DPPs for this<br />
moment, we can see several clear trends. Larger conjugated system<br />
of an aryl substituent causes a bathochromic shift; labs of unsymmetrical<br />
derivative is exactly an arithmetic mean of parent BPPB and<br />
Relative intensity (-)<br />
35000.0<br />
30000.0<br />
25000.0<br />
20000.0<br />
15000.0<br />
10000.0<br />
5000.0<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0.0<br />
300 350 400 450 500 550 600<br />
(nm)<br />
0.0<br />
350 400 450 500 550 600 650 700 750<br />
(nm)<br />
BPPB<br />
1NPP1N<br />
1NPPB<br />
Fig. 3. The room temperature absorption spectra of DMSO solutions of BPPB, 1NPPB<br />
and 1NPP1N.<br />
Abs. 300K<br />
Fl.300K<br />
Ex.77K<br />
Fl. 77K<br />
Fig. 4. The room temperature absorption and fluorescence (DMSO) and low temperature<br />
(MTHF) fluorescence emission and excitation spectra of StPPB.<br />
S. Luňák Jr. et al. / Dyes and Pigments 85 (2010) 27–36<br />
Relative intensity (-)<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0.0<br />
350 400 450 500 550 600 650 700 750<br />
(nm)<br />
Abs.300K<br />
Fl.300K<br />
Ex.77K<br />
Fl. 77K<br />
Fig. 5. The room temperature absorption and fluorescence emission and low<br />
temperature fluorescence emission and excitation spectra of 1NPPB in MTHF.<br />
corresponding symmetrical derivative as predicted by theory. When<br />
going from BPPB to all three unsymmetrical derivatives, a hyperchromic<br />
shift is observed, also in a qualitative agreement with<br />
computational results. But molar absorptivities of all three<br />
symmetrical derivatives are lower than those ones of corresponding<br />
unsymmetrical ones, on the contrary to the theoretical results, predicting<br />
approximately linear increase. The main reason for this<br />
discrepancy is an extremal insolubility of symmetrical pigments,<br />
making impossible a correct estimation of 3max. The difference in<br />
3max values between BPPB and BiPPB is 5900 l mol 1 cm 1 ,sothe<br />
similar value should be expected according to theoretical predictions<br />
between BiPPB and BiPPBi. Thus3max value of BiPPBi, extrapolated<br />
from better soluble BPPB and BiPPB, is46000lmol 1 cm 1 ,while<br />
the experimental one is 17 300 l mol 1 cm 1 .Thevalueof2NPP2N of<br />
3max is also lower (30 400 l mol 1 cm 1 ) than the one derived by<br />
extrapolation from BPPB and 2NPPB (40 200 l mol 1 cm 1 ),<br />
although the difference is not so dramatic as for biphenylyl set. The<br />
same type of inconsistency between the value (54 400 l mol 1 cm 1 )<br />
extrapolated from BPPB and StPPB and experimental (33<br />
600 l mol 1 cm 1 ) one for StPPSt is observed, too, from the same<br />
reasons.<br />
There is quite a good agreement between computed and<br />
experimental maxima for BPPB, BiPPB and BiPPBi, for which<br />
either there is only one possible geometry or both rotamers give the<br />
same theoretical lmax. The experimental maxima are always at<br />
rather longer wavelengths, but the differences are acceptable<br />
(11 nm, 7 nm and 4 nm). The situation seems to be similar to that<br />
one described in Ref. [1]: the para substituted derivatives of BPPB<br />
are computed rather better than the parent chromophore itself.<br />
Relative intensity (-)<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0.0<br />
350 400 450 500 550 600 650 700 750<br />
(nm)<br />
Abs.300K<br />
Fl.300K<br />
Ex.77K<br />
Fl.77K<br />
Fig. 6. The room temperature absorption and fluorescence emission and low<br />
temperature fluorescence emission and excitation spectra of 1NPP1N in MTHF.
Relative intensity (-)<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0.0<br />
350 400 450 500 550 600 650 700 750<br />
(nm)<br />
Ex.300K<br />
Fl.300K<br />
EX.77K<br />
Fl.77K<br />
Fig. 7. The room and low temperature fluorescence emission and excitation spectra of<br />
2NPPB in MTHF.<br />
No dependence of the fluorescence emission spectrum on the<br />
excitation wavelength (and opposite) was observed for any sample,<br />
either at room or low temperature. As the difference in absorption<br />
maxima of rotamers of 2-naphthyl derivatives should be about<br />
10 nm, a similar value as experimentally obtained for rotamers of 2naphthyl-ethylenes,<br />
we can conclude, that only one rotamer of<br />
2NPPB and 2NPP2N is present in solution [18], although their<br />
ground state energy is almost the same. Furthermore, a mixture of<br />
spectrally shifted rotamers could hardly produce the spectra with<br />
such sharp vibronic structure as shown in Fig. 7. Thus the question<br />
arises, whether the rotamer present in solution can be estimated by<br />
a comparison of the experimental and theoretical lmax. The<br />
experimental values are closer for 2NPPB (0–0) rotamer and lie<br />
between 2NPP2N (0–0) and 2NPP2N (0–180) rotamers. Thus, if we<br />
suppose, that the same orientation of naphthalene ring is preferred<br />
in both compounds, the rotamers characterized by dihedral angle<br />
equal to 0 (Fig. 1) can be concluded as more probable. This<br />
assignment is also consistent with a comparison of 2NPPB (0–0)<br />
with BiPPB: the difference in lmax is 4 (3) nm by theory (experiment)<br />
and 3max are very close as predicted.<br />
On the contrary to 2-naphthyl and biphenylyl derivatives the<br />
experimental absorption maxima of stilbenyl ones are considerably<br />
blue shifted with respect to the theoretical values. Furthermore,<br />
molar absorptivity of StPPB is only a bit higher than for BiPPB,<br />
although the theoretical prediction supposes a higher value of 3max.<br />
We consider two sources of this discrepancy. First, trans-stilbene,<br />
although planar in crystal [19] and by high level quantum chemical<br />
calculations [20], is often considered as non-planar in solution with<br />
both phenyls partially rotated out-of-plane [21]. Such rotation<br />
could cause a considerable hypsochromic and hypochromic shift, if<br />
present in StPPB and StPPSt. Second, both stilbenyl derivatives<br />
show the longest conjugated chain in one direction, that can be<br />
a problem for TD DFT method, because of its known incorrect<br />
asymptotic behaviour with respect to the long distance between<br />
charges [22]. Both these effect can be less significant in styryl<br />
derivatives PePPB and PePPPe, so it is possible, that they could be<br />
experimentally bathochromically shifted with respect to stilbenyl<br />
analogues, although the theory predicts the opposite relation. Thus<br />
it is desirable to find a synthetic alternative for them [6].<br />
The most controversial results were obtained for non-planar<br />
well soluble 1-naphthyl derivatives. They show significantly higher<br />
Stokes shift in DMSO, dramatically decreased in MTHF glass, mainly<br />
as a consequence of limited ability of excited state geometrical<br />
relaxation in rigid environment. The experimental values of 3max<br />
are significantly decreased with respect to parent chromophore or<br />
to 2-naphthyl isomers as qualitatively expected for the lost<br />
planarity, but not quantitatively predicted by TD DFT. Furthermore,<br />
S. Luňák Jr. et al. / Dyes and Pigments 85 (2010) 27–36 35<br />
the value extrapolated from BPPB and 1NPPB (8800 l mol 1 cm 1 )<br />
is close to the experimental one (7900 l mol 1 cm 1 ) for 1NPP1N,<br />
that is only qualitatively in accordance with theory and only for the<br />
most distored rotamer set, e.g. BPPB / 1NPPB (7–136) /<br />
1NPP1N (Ci, 134–134). Even if we take into account, that the<br />
absorption maximum of 1NPP1N corresponds to 0–1 vibronic<br />
transition, experimental lmax of both 1-naphthyl derivatives show<br />
a hypsochromic shift with respect to parent BPPB on the contrary<br />
to the theoretical predictions for any rotamer. Simply said, the<br />
computations underestimate the excitation energies and overestimate<br />
the probability of spectral transitions. We do think, that<br />
the explanation of this discrepancy comes from the underestimation<br />
of dihedral angles by DFT calculations of the ground state<br />
geometry. The synthesis of further non-planar symmetrical and<br />
unsymmetrical till unknown DPPs, with N-heteroaryles coming<br />
from 4-cyano-quinoline and 1-cyano-isoquinoline, is now in<br />
progress. Thus, we return to the detailed discussion of 1-naphthyl<br />
DPPs with similar steric hindrance after collecting more experimental<br />
data from their N-heteroanalogues.<br />
4. Conclusions<br />
The effect of a size of a conjugation system of aryl substituents<br />
on the absorption spectra of diketo-pyrrolo-pyrroles was studied.<br />
Two new unsymmetrical derivatives were synthesized in order to<br />
complete the investigated set. PCM TD DFT calculations predict<br />
a linear increase of both lmax and 3max with a substitution of each<br />
phenyl in parent BPPB by larger aryl for planar derivatives. Qualitatively,<br />
the experimental bathochromic shifts of 0–0 vibronic subbands<br />
are in exact agreement with theory, while the hyperchromic<br />
shifts are affected by extremally low solubility of planar symmetrical<br />
derivatives. Quantitatively, the theory only slightly overestimates<br />
the excitation energies in this case. The deviations from<br />
this ideal behaviour were observed for non-planar derivatives<br />
(aryl ¼ stilbenyl or 1-naphthyl), for which the dihedral angles<br />
describing the aryl out-of-plane torsions are probably underestimated<br />
by DFT.<br />
Acknowledgments<br />
Authors acknowledge the support of the grant project No.<br />
2A-1TP1/041 sponsored by the Ministry of Trade and Industry of<br />
the Czech Republic.<br />
Appendix. Supplementary information<br />
Supplementary data associated with this article can be found, in<br />
the online version, at doi:10.1016/j.dyepig.2009.09.014.<br />
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Novel, soluble diphenyl-diketo-pyrrolopyrroles: Experimental<br />
and theoretical study<br />
M. Vala a, *, J. Vyňuchal b , P. Toman c , M. Weiter a , S. Luňák Jr. d<br />
a Faculty of Chemistry, Brno University of Technology, Purkyňova 464/118, CZ-612 00 Brno, Czech Republic<br />
b Research Institute of Organic Syntheses, Rybitví 296, CZ-533 54 Rybitví, Czech Republic<br />
c Institute of Macromolecular Chemistry v. v. i., Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, CZ-162 00 Prague, Czech Republic<br />
d Faculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-530 09 Pardubice, Czech Republic<br />
article info<br />
Article history:<br />
Received 16 March 2009<br />
Received in revised form<br />
29 July 2009<br />
Accepted 30 July 2009<br />
Available online 14 August 2009<br />
Keywords:<br />
Diketo-pyrrolo-pyrrole<br />
DPP<br />
Photoluminescence<br />
Fluorescence<br />
Absorption<br />
1. Introduction<br />
abstract<br />
Derivatives of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-<br />
1,4-dione, commonly referred to as DPPs, constitute an important<br />
class of high-performance pigments [1–13] (parent molecule I in<br />
Fig. 1). The compounds are endowed with brilliant shades (ranging<br />
from yellow-orange to red-violet) and exhibit exceptional resistance<br />
to chemical, heat, light, and weather. Furthermore, some of<br />
their physical properties such as high melting points are exceptional<br />
in view of their very low Mr, in conventional pigment<br />
molecule terms. It has been shown that DPP units introduced into<br />
various materials, such as polymers [14–23] dendrimers [24],<br />
polymer–surfactant complexes [25], and oligomers [26] results in<br />
deeply coloured, highly photoluminescent [15–26] and electroluminescent<br />
[19,20] materials. Owing to their interesting properties,<br />
there is wide range of possible applications which have been<br />
already investigated covering for example latent pigment [27],<br />
charge generating materials for laser printers and information<br />
storage systems [28–33], solid-state dye lasers [30] or gas detectors<br />
[34,35].<br />
* Corresponding author. Tel.: þ420 541 149 411; fax: þ420 541 149 398.<br />
E-mail address: vala@fch.vutbr.cz (M. Vala).<br />
0143-7208/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.<br />
doi:10.1016/j.dyepig.2009.07.014<br />
Dyes and Pigments 84 (2010) 176–182<br />
Contents lists available at ScienceDirect<br />
Dyes and Pigments<br />
journal homepage: www.elsevier.com/locate/dyepig<br />
Derivatives of diphenyl-diketo-pyrrolopyrrole, possessing electron-donating or withdrawing groups in<br />
the p-position of the phenyl, were synthesized and studied using optical characterization (absorption,<br />
fluorescence, time-resolved fluorescence) and quantum chemical calculation. An increase in absorption<br />
coefficient 10 5 dm 3 mol 1 cm 1 was observed using electron–donor groups; a bathochromic shift in<br />
both absorption and luminescence peaks was observed as a result of increased conjugation. Soluble<br />
derivatives were obtained by the introduction of alkyl groups (by N-alkylation) in the central pyrrolopyrrole<br />
unit. Calculated phenyl torsion angles using HF and B3LYP methods showed that the loss of<br />
molecule planarity reduced the extent of overlap between the p-orbitals of the central pyrrolopyrrole<br />
unit and phenyls after N-alkylation. This treatment thus reduced both the bathochromic shift and<br />
increased absorption coefficient. The presence of the donor or acceptor groups in itself does not influence<br />
molecule planarity.<br />
Ó 2009 Elsevier Ltd. All rights reserved.<br />
The key issue for the construction of any device either in the<br />
form of thin film composed of DPP or incorporation of the DPP unit<br />
into supporting matrix, is solubility. DPPs are insoluble in the<br />
majority of common solvents. Whilst this is favourable for many<br />
applications, the ability to solubilize the compounds would offer<br />
the possibility of using solution-based techniques (spin-coating,<br />
drop-casting, inject printing, etc.) to prepare devices from DPPs.<br />
One reason for their insolubility is the existence of H-bonds<br />
between the –NH group and oxygen. Since the basic DPP core is<br />
perfectly planar, p–p electron overlap occurs in the solid state and<br />
also contributes to their insolubility. These interactions can be so<br />
strong as to impart colour change between the solid and dissolved<br />
forms and influence other properties, such as fluorescence and<br />
Stokes shift [36]. It is therefore clear, that modified solubility can be<br />
achieved either through N-substitution and/or disruption of<br />
molecular planarity [8].<br />
A previous contribution by the present workers [37] discussed<br />
the influence of N-alkylation on optical properties and employed<br />
quantum chemical calculations to correlate the results with molecule<br />
geometry. In this paper, electron-donating (IV–VIII) or<br />
accepting (IX, X) groups were introduced at the p-position on the<br />
phenyl so as to influence the electronic spectra of the compounds.<br />
Further, some of these derivatives were N-alkylated in order to<br />
modify their solubility (VII–X). These derivatives were compared
with the parent (I) and N-alkylated (II, III) 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione<br />
(Fig. 1). The observed optical<br />
properties of both, the donor or acceptor substituted only and<br />
substituted and N-alkylated derivatives are explained according to<br />
the results obtained by quantum chemical calculations.<br />
2. Experimental<br />
The UV–VIS absorption and photoluminescence spectra were<br />
recorded in dimethylsufoxide (DMSO) purchased from Aldrich. The<br />
molar absorption coefficients were calculated from the linear part<br />
of the concentration dependence of absorption A ¼ f(c) measured in<br />
5 cm quartz cuvette. The photoluminescence spectra and quantum<br />
yields (PLQYs) were obtained according to the comparative method<br />
[38], where for each test sample gradient of integrated fluorescence<br />
intensity vs. absorbance PL ¼ f(A) is used to calculate the PLQY using<br />
two known standards. The standards were previously crosscalibrated<br />
to verify the method. This calibration revealed accuracy<br />
better than w3%. The photoluminescence lifetime sPL was<br />
measured using spectrograph and fast ICCD camera. The samples<br />
were excited by second or third harmonic of Nd:YAG laser (532 or<br />
355 nm) with light pulse time duration of w30 ps. The temporal<br />
resolution of the system is approximately 25 ps.<br />
3. Results and discussion<br />
3.1. Absorption<br />
The spectral properties of the prepared derivatives are<br />
compared in Figs. 2 and 3 and summarised in Table 1. The Influence<br />
of N-substitution on the parent derivative I on absorption is<br />
depicted in Fig. 2a. It can be seen, that insertion of an alkyl group<br />
decreases molar absorption coefficient (hypochromic shift) and<br />
simultaneously the longer wavelength maximum is shifted towards<br />
higher energy region (hypsochromic shift). Furthermore, the<br />
vibration structure is less pronounced. As was pointed out in our<br />
previous paper [37], this is caused by torsion between pyrrolinone<br />
central part and phenyl adjacent to the alkyl group and consequently,<br />
is caused by loss of molecule planarity which is in turn<br />
responsible for loss of effective conjugation. Since the addition of<br />
second alkyl rotates also the second phenyl group, this effect is even<br />
M. Vala et al. / Dyes and Pigments 84 (2010) 176–182 177<br />
Fig. 1. The basic structure of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (I), also known as DPP and the respective derivatives used in this study. The definition of<br />
calculated torsion angles a and b.<br />
more pronounced. The loss of vibration structure can be attributed<br />
to the increased dipole moment interacting with polar DMSO<br />
solvent [39]. The dipole–dipole interaction of bi-substituted<br />
derivatives with the completely non-planar structure is the most<br />
pronounced. No dependency on the length of the alkyl used was<br />
found in our previous work [37].<br />
Introduction of electron-donating groups has significant effect<br />
on the DPP absorption [40]. Increase of the absorption coefficient<br />
from 3 ¼ 3.4 10 4 dm 3 mol 1 cm 1 of the parent compound I to<br />
3 ¼ 1.3 10 5 dm 3 mol 1 cm 1 for compound V bi-substituted by<br />
N,N-dimethylamin in p-position on phenyls, accompanied with<br />
strong bathochromic shift (up to 55 nm), was observed (Fig. 2b).<br />
This behaviour implies that charge separation occurs via electron<br />
delocalization leading to creation of permanent dipole moment.<br />
Therefore, the central part composed of H-chromophores behaves<br />
as an electron-accepting group. Blurring of vibration structure in<br />
case of the mono-substituted derivative (IV) was observed,<br />
whereas for the bi-substituted (V) was not. We explain this finding<br />
by the fact that due to symmetry of the bi-substituted derivative is<br />
its resulting dipole moment smaller than for the mono-substituted<br />
asymmetric derivative.<br />
Similar increase of the absorption coefficient 3 value and change<br />
of the peak position was achieved using piperidine group (VI)<br />
(Fig. 2c). For this derivative, we studied also the impact of<br />
N-alkylation. Introduction of one alkyl (VII) decreased the 3 only<br />
slightly. After addition of second alkyl (VIII) we observed almost<br />
the same value as for the parent molecule (I). This was accompanied<br />
by the hypsochromic shift and blurring of the vibration<br />
structure. We propose the same mechanism as for the N-alkylated<br />
only derivatives: the N-alkylation causes rotation of the phenyls<br />
and consequently breaks the molecule symmetry and hence the<br />
effective conjugation and increases the polarity.<br />
To describe the influence of groups with the electron-withdrawing<br />
character, derivatives with the halogen element (chloride)<br />
were prepared. Since the aim was to prepare soluble derivatives we<br />
will discuss the alkylated only derivatives. Fig. 2d compares the<br />
observed absorption coefficients 3. The introduction of the chloride<br />
did not caused increase of 3. This is further evidence for the electron-accepting<br />
character of the central part. For the alkylated<br />
derivatives we observed hypso- and hypochromic shift with the<br />
loss of vibration structure again.
178<br />
a b<br />
(dm 3 mol -1 cm -1 )<br />
4x10 4<br />
2x10 4<br />
1x10 5<br />
5x10 4<br />
I<br />
II<br />
III<br />
0<br />
300 400 500 600<br />
wavelength (nm)<br />
2x10 5 c d<br />
(dm 3 mol -1 cm -1 )<br />
I<br />
VI<br />
VII<br />
VIII<br />
0<br />
300 400 500 600<br />
wavelength (nm)<br />
(dm 3 mol -1 cm -1 )<br />
(dm 3 mol -1 cm -1 )<br />
2x10 5<br />
1x10 5<br />
5x10 4<br />
4x10 4<br />
2x10 4<br />
I<br />
IV<br />
V<br />
0<br />
300 400 500 600<br />
wavelength (nm)<br />
I<br />
IX<br />
X<br />
0<br />
300 400 500 600<br />
wavelength (nm)<br />
Fig. 2. The influence of N-alkyl substitution (a), and various donor (b, c) and acceptor (d) groups on the DPP central part on molar absorption coefficient.<br />
a 1,0<br />
b<br />
0,8<br />
I<br />
II<br />
PL<br />
0,6<br />
0,4<br />
0,2<br />
0,0<br />
0,6<br />
0,4<br />
0,2<br />
0,0<br />
500 600 700<br />
wavelength (nm)<br />
500 600 700<br />
wavelength (nm)<br />
1,0<br />
0,8<br />
0,6<br />
0,4<br />
0,2<br />
0,0<br />
c 1,0<br />
d 1,0<br />
0,8<br />
I<br />
VI<br />
0,8<br />
PL<br />
M. Vala et al. / Dyes and Pigments 84 (2010) 176–182<br />
III<br />
VII<br />
VIII<br />
PL<br />
PL<br />
0,6<br />
0,4<br />
0,2<br />
0,0<br />
500 600 700<br />
wavelength (nm)<br />
500 600 700<br />
wavelength (nm)<br />
Fig. 3. Comparison of photoluminescence spectra scaled using the PL quantum yields (PLQY$PL/PLmax) of N-alkylated (a), donor (b, c) and acceptor (d) substituted DPP derivatives.<br />
I<br />
IV<br />
V<br />
I<br />
IX<br />
X
Table 1<br />
Experimental values for molar absorption coefficient 3, position of absorption lAbs<br />
and luminescence lPL peak maximums, Stokes shift DlStokes, and photoluminescence<br />
quantum yield PLQY and lifetime sPL of the prepared DPPs in dimethylsulfoxide.<br />
3<br />
(dm 3 mol 1 cm 1 )<br />
3.2. Photoluminescence<br />
lAbs<br />
(nm)<br />
lPL<br />
(nm)<br />
DlStokes<br />
(nm)<br />
PLQY<br />
( )<br />
sPL<br />
(ns)<br />
I 34 300 507 519 12 0.74 6.21<br />
II 21 300 493 525 32 0.69 6.67<br />
III 18 500 467 530 63 0.69 6.57<br />
IV 86 500 544 580 36 0.49 2.88<br />
V 126 000 562 580 18 0.57 3.46<br />
VI 108 000 562 582 20 0.58 3.66<br />
VII 100 600 553 592 39 0.42 3.38<br />
VIII 42 400 536 599 63 0.41 3.37<br />
IX 36 000 501 534 33 0.60 6.79<br />
X 24 500 473 538 65 0.51 6.63<br />
The photoluminescence (PL) spectra are compared in Fig. 3. The<br />
PL intensity is recalculated using PL quantum yield (PLQY): the<br />
measured spectra were normalised to its maximum and multiplied<br />
by PL quantum yield (PL ¼ PLQY$PLl/PLMax). After this transformation,<br />
the heights of the curves give direct evidence of the<br />
efficiency of the light emission similarly to the 3 in the case of light<br />
absorption.<br />
For the N-alkylated derivatives, we found practically the same<br />
PLQY in the accuracy range of the method. The Stokes shift was<br />
increased from 12 nm for the parent derivative to 32 nm and 63 nm<br />
for the monoalkylated and dialkylated derivatives, respectively. The<br />
observed spectra (Fig. 3a) were characteristic by graduate loss of<br />
mirror symmetry of absorption-fluorescence and vibronic structure.<br />
As was pointed out, the phenyl torsion due to the N-alkylation<br />
is the main mechanism for this behaviour in polar DMSO.<br />
Fig. 3b and c show the influence of electron-donating groups. The<br />
mono-substituted derivative shows a smaller PLQY compared to<br />
the di-substituted one. This is in accordance with the values of the<br />
absorption coefficient 3: the polarity of the mono-substituted<br />
molecule (IV) is higher compared to the symmetric V and VI. The<br />
observed Stokes shifts confirm this hypothesis: for the monosubstituted<br />
IV reaches the value 63 nm whereas for di-substituted V<br />
and VI is only 18 nm and 20 nm, respectively. The N-alkylation<br />
causes further decrease of PLQYaccompanied with increasing Stokes<br />
shift similarly to the N-alkylated only derivatives. The electronwithdrawing<br />
group (IX and X) causes analogous behaviour (Fig. 3d).<br />
We also determined fluorescence lifetime sPL of the prepared<br />
derivatives. Table 1 summarises the obtained results. All of the<br />
observed fluorescence decays followed first order kinetics, i.e.<br />
showed monoexponencial decay, see Fig. 4. The N-alkylation only<br />
Counts<br />
R i<br />
10000<br />
1000<br />
1<br />
0<br />
-1<br />
0 5 10<br />
Time (ns)<br />
15 20<br />
Fig. 4. Typical photoluminescence decay of DPP (here V) after 30 ps excitation at<br />
532 nm (on top) with weighted residuals to monoexponential fit (on bottom).<br />
M. Vala et al. / Dyes and Pigments 84 (2010) 176–182 179<br />
caused slight increase of the lifetime. As for the PLQY, the lifetime of<br />
the donor-substituted derivatives decreased relative to the parent I.<br />
Subsequent alkylation caused further reduction of the observed<br />
lifetime. The longest lifetime was observed for chlorine substituted<br />
derivative with broken symmetry (IX). The shortest lifetime was<br />
observed for the compound IV mono-substituted by N,N-dimethylamin<br />
in p-position on phenyl. The lifetime of its bi-substituted<br />
counterpart V was found to be higher.<br />
3.3. Quantum chemical study<br />
3.3.1. Methods<br />
The molecular conformations and absorption and luminescence<br />
spectra of the synthesized derivatives were characterized by means<br />
of the quantum chemical methods in order to better understand<br />
the observed behaviour.<br />
The molecular conformations were optimized by means of both<br />
the Hartree-Fock (HF) and the hybrid HF/density functional theory<br />
method B3LYP [41,42] at the 6–31G* level. The HF method usually<br />
provides a fairly good description of the ground state molecular<br />
conformations and is used for a major part of the present-day<br />
calculations of the electronic properties. The latter method was<br />
successfully used for the conformation study of many different<br />
p–conjugated systems [43–45]. Although the B3LYP method<br />
requires more computational time than the HF method, its<br />
computational requirements are much lower than demands of other<br />
‘‘correlated’’ methods. The main difference of the HF and B3LYP<br />
methods is that usually the former underestimates and the latter<br />
slightly overestimates the electron delocalization and the degree of<br />
conjugation in the conjugated molecules. The B3LYP conformations<br />
are usually close to the conformations either calculated by the<br />
Møller–Plesset method or obtained experimentally, see e.g. [46–48].<br />
First excited states S1 of the studied derivatives were calculated<br />
using time-dependent B3LYP (TD–B3LYP) method at the optimized<br />
HF geometry. Time-dependent density functional methods recently<br />
became an effective and rather accurate tool for single point calculations<br />
of electronic excitations in various, namely conjugated,<br />
molecular systems [49–51]. However, this method is not suitable for<br />
the excited state conformation optimization necessary for luminescence<br />
spectra calculations. For this reason, relaxed (exciton)<br />
conformations of the S1 state were optimized by means of ab initio<br />
configuration interaction method with single-excitation (CIS)<br />
method. The exciton conformations were subsequently used for the<br />
luminescence spectra calculations using TD–B3LYP method.<br />
Keeping the same level of the calculations enabled determination of<br />
the Stokes shift DEStokes and deformation energy Edef of the relaxed<br />
exciton state.<br />
3.3.2. Results<br />
The calculated molecular parameters of the studied derivatives<br />
are presented in Table 2. Comparison of the phenyl torsion angles<br />
calculated by means of HF and B3LYP methods, respectively, shows<br />
the same trends determined by both methods. The differences in<br />
absolute values can be explained by the different electron delocalization<br />
predicted by these methods. The results show that the Nalkylated<br />
derivatives possess significantly rotated phenyl groups of<br />
the central DPP unit in the position adjacent to the alkyl, while the<br />
basic I molecule is completely planar. Phenyl group rotation<br />
significantly decreases the overlap between p-orbitals of the<br />
central DPP unit and the respective phenyl. As a result, the frontier<br />
orbital (HOMO and LUMO) energetic levels are shifted and consequently,<br />
absorption and luminescence spectra are modified. The<br />
absorption peak energies ES0/S1 of the N-alkylated derivatives<br />
show a hypsochromic shift strongly correlated with the phenyl<br />
torsion angles a and b (for the definition of the angles see Fig. 1).
180<br />
Table 2<br />
Phenyl torsion angles calculated by the HF and B3LYP methods. Lowest absorption<br />
energy ES0/S1, oscillator strength fS0/S1 of this transition, 1st luminescence peak<br />
Elum, Stokes shift DEStokes, and deformation energy Edef of the relaxed excited state<br />
calculated by the ab initio CIS method.<br />
B3LYP<br />
method<br />
HF<br />
method<br />
Simultaneously, the oscillator strengths of the absorption peaks of<br />
the molecules IV–VIII are notably higher than that of the basic I<br />
molecule. On the other hand, the absorption peaks of the molecules<br />
II, III, and X are reduced. These findings are in a good agreement<br />
with the experimentally measured molar absorption coefficients<br />
shown in Fig. 2. The calculated results further show, that Stokes<br />
shifts DEStokes and deformation energies Edef of the excited state of<br />
the N-alkylated derivatives are considerably increased. The calculated<br />
luminescence peak Elum depends only slightly on the phenyl<br />
torsion angles a and b.<br />
The substitution of phenyls by donor or acceptor groups has<br />
almost no influence on the phenyl torsion angles and the molecular<br />
conformation of the central DPP unit. However, it leads to the<br />
bathochromic shift of the absorption and luminescence peaks due<br />
to the increased effective extent of the conjugation.<br />
3.4. Syntheses and analyses<br />
ES0/S1<br />
(eV)<br />
fS0/S1 Elum<br />
(eV)<br />
DEStokes<br />
(eV)<br />
Edef<br />
(eV)<br />
a ( ) b ( ) a ( ) b ( )<br />
I 0.0 0.0 0.0 0.0 2.837 0.49 2.427 0.410 0.339<br />
II 36.0 6.7 46.9 16.9 2.935 0.40 2.424 0.511 0.437<br />
III 36.5 36.5 46.1 46.1 3.012 0.37 2.441 0.571 0.482<br />
IV 0.0 0.0 0.0 0.0 2.691 0.79 2.365 0.327 0.309<br />
V 0.0 0.0 0.0 0.0 2.640 1.03 2.325 0.315 0.294<br />
VI 3.1 3.1 10.5 10.5 2.624 1.13 2.293 0.331 0.326<br />
VII 27.5 6.3 42.6 12.5 2.714 0.95 2.296 0.417 0.394<br />
VIII 29.7 29.7 41.6 41.6 2.786 0.78 2.262 0.482 0.459<br />
IX 34.1 6.7 45.7 16.2 2.875 0.49 2.366 0.508 0.439<br />
X 34.4 34.4 44.9 44.9 2.949 0.43 2.382 0.567 0.483<br />
The synthesis of the starting ethyl 4,5-dihydro-5-oxo-2-phenyl(1-<br />
H)pyrrole-3-carboxylate (pyrrolinone ester) was described previously<br />
[52], as was the syntheses of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4c]pyrrole-1,4-dione<br />
(I), 3,6-Diphenyl-2-butyl-2,5-dihydro-pyrrolo<br />
[3,4-c]pyrrole-1,4-dione (II) and 3,6-Diphenyl-2,5-dibutyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione<br />
(III) [37]. The 4-fluorobenzonitrile,<br />
diisopropyl succinate, natrium, tert-amyl alcohol and the<br />
remaining common solvents were obtained from the Research Institute<br />
of Organic Syntheses.<br />
3.4.1. Synthesis of 3-(phenyl)-6-(4-dimethylamino-phenyl)-2,<br />
5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (IV)<br />
80 ml of tert-amyl alcohol and 2.8 g (0.12 mol) of sodium metal<br />
(caution: extremely dangerous and corrosive; highly unstable;<br />
reacts violently with water; can form flammable hydrogen in<br />
contact with air.; incompatible with water, oxygen, carbon dioxide,<br />
carbon tetrachloride, halogens, acetylene, metal halides, ammonium<br />
salts, oxides, oxidizing agents, acids, alcohols, chlorinated<br />
organic compounds, many other substances) (in three portions)<br />
were charged into the 250 ml three-necked flask equipped with<br />
a magnetic stirrer, a reflux condenser, a thermometer and<br />
a nitrogen inlet. The sodium metal was dissolved under reflux in<br />
the presence of a catalytic amount of FeCl3 (approximately 2 h) and<br />
5.8 g (0.04 mol) of 4-dimethylamino-benzonitrile was added. After<br />
that, pyrrolinone ester [44] (9.2 g, 0.04 mol) was continuously<br />
introduced in small portions within 0.5 h. Subsequently, this<br />
mixture was stirred under reflux for 2 h and the ensuing hot salt<br />
was filtered. The protolysis was made separately in 120 ml of 2propanol<br />
and 80 ml of distilled water under reflux for 2 h. The<br />
M. Vala et al. / Dyes and Pigments 84 (2010) 176–182<br />
product was filtered and the filter cake washed with hot water to<br />
neutral washings. The filter cake was dried and suspended in<br />
100 ml of methanol. The suspension was heated to boiling and<br />
refluxed for 1 h; the ensuing hot suspension was filtered, washed<br />
with methanol and hot water. Yield: 2 g (30%) of IV compound.<br />
Calculated: C (72.49), H (5.17), N (12.68). Found: C (71.96), H<br />
(5.18), N (12.43). MW ¼ 331 Da; Negative-ion APCI-MS: m/z 330<br />
[M H] (100%). 1 H chemical shifts: chemical shifts were not<br />
determined due to a very low solubility of the sample.<br />
3.4.2. Synthesis of 3,6-bis-(4-dimethylamino-phenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione<br />
(V)<br />
182 ml of tert-amyl alcohol and 11.4 g (0.50 mol) of sodium<br />
metal (in three portions) were charged into the 0.5 dm 3 Keller flask<br />
equipped with a stirrer, a reflux condenser, a thermometer and<br />
a nitrogen inlet. The sodium metal was dissolved under the reflux<br />
in the presence of catalytic amount of FeCl3 (approximately 2 h),<br />
and 25 g (0.7 mol) of 4-dimethylamino-benzonitrile was added.<br />
Diisopropyl succinate (16.9 g, 0.08 mol) dissolved in 16.9 g of tertamyl<br />
alcohol was introduced over 3 h and the mixture stirred at<br />
reflux for 1 h. The reaction mixture was cooled to 60 C and then<br />
500 ml of distilled water was added to protolyse the salt. The<br />
protolysis was carried out at 80 C for 2 h. The resulting hot<br />
suspension was filtered and the filter cake washed with hot water<br />
to neutral washings. The filter cake was dried and suspended in<br />
500 ml of acetone. The suspension was heated to boiling and<br />
refluxed for 1 h; the hot suspension was filtered, washed with<br />
acetone and hot water. Yield: 3.1 g (10%) of V compound.<br />
Calculated: C (70.57), H (5.92), N (14.96). Found: C (69.14), H<br />
(5.95), N (14.40). MW ¼ 374 Da; Negative-ion APCI-MS: m/z 373<br />
[M H] (100%). 1 H chemical shifts: chemical shifts were not<br />
determined due to a very low solubility of the sample.<br />
3.4.3. Synthesis of 3,6-di-(4-piperidinophenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione<br />
(VI)<br />
Synthesis of 4-piperidine-1-yl-benzonitrile (starting nitrile):<br />
Dry, pure N,N-dimethylacetamide (400 ml), 47.8 g (0.4 mol) p-fluorobenzonitrile<br />
(caution: incompatible with strong oxidizing<br />
agents, strong acids, strong bases) and 84 g (0.99 mol) of piperidine<br />
were charged into a 1 dm 3 Erlenmeyer flask equipped with stirrer<br />
and condenser. Reaction was carried out at 100O110 C for 8 h, the<br />
gases from the reaction being exhausted via the fume-chamber.<br />
Subsequently, the reaction mixture was poured onto 1 kg ice and<br />
the crude product was collected by filtration and recrystallized<br />
from 80% ethanol. Yield: 43.5 g (60%) of 4-piperidine-1-yl-benzonitrile<br />
(m.p. 53O55 C).<br />
Synthesis of VI: 390 ml of tert-amyl alcohol and 24.4 g (1 mol) of<br />
sodium metal (in three portions) were charged to a 1.5 dm 3 Keller<br />
flask equipped with stirrer, reflux condenser, thermometer and<br />
a nitrogen inlet. The sodium metal was dissolved under reflux in<br />
the presence of a catalytic amount of FeCl3 (which took approximately<br />
2 h), and 67 g (0.36 mol) of 4-piperidine-1-yl-benzonitrile<br />
was added. After that, diisopropyl succinate (36.3 g, 0.18 mol) dissolved<br />
in 36.3 g of tert-amyl alcohol was added over 3 h. Subsequently,<br />
the mixture was stirred at reflux for 1 h and the ensuing<br />
mixture was cooled to 60 C, and then 700 ml distilled water was<br />
added to protolyse the salt. Protolysis was carried out at 80 C for<br />
2 h. The resulting hot suspension was filtered and the filter cake<br />
was washed with hot water to neutral washings. The filter cake was<br />
dried and suspended in 800 ml acetone. The suspension was heated<br />
to boiling and refluxed for 1 h. The hot suspension was filtered,<br />
washed with acetone and hot water. Yield: 12 g (15%) of VI<br />
compound.<br />
Calculated: C (73.98), H (6.65), N (12.33). Found: C (73.23), H<br />
(6.55), N (12.11). MW ¼ 454 Da; Negative-ion APCI-MS: m/z 453
[M H] (100%). 1 H chemical shifts: 10.93 (2H, br s, NH), 8.36 (4H,<br />
m, ArH); 7.07 (4H, m, ArH); 3.43 (8H, m, –CH2 CH2CH2N); 1.65 (12H,<br />
m, –CH2CH2 CH2N and –CH2CH2CH2N)<br />
3.4.4. Synthesis of 3,6-di-(4-piperidinophenyl)-2-butyl-2,<br />
5–dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (VII) and 3,6-di-<br />
(4-piperidinophenyl)-2,5-butyl-2,5–dihydro-pyrrolo[3,4-c]pyrrole-<br />
1,4-dione (VIII)<br />
13.7 g (0.03 mol) of the intermediate VI and 90 g of dry DMF were<br />
charged into an evaporating flask and stirred. 12.2 g of 30% methanolic<br />
solution of sodium methylate was added. The fine slurry of<br />
sodium salt of VI was stirred for 20 min, methanol was distilled<br />
under vacuum (t < 40 C, p ¼ 40 mbar) and then 12.6 g (0.093 mol) of<br />
n-butyl bromide was added. The mixture was heated to 80 C and<br />
stirred for 20 h after which time, the temperature was increased to<br />
80–100 C and this was maintained with stirring for a further 2 h.<br />
The reaction mixture was added to 1000 ml of distilled water and the<br />
ensuing solution boiled. The final mixture was cooled to 10 C and<br />
the product filtered. The filter cake was reslurried in 200 ml of<br />
methanol, the suspension boiled and then filtered. In this step all byproducts<br />
from alkylation were removed by TLC (mobile phase:<br />
acetone:n-hexane (2:3), thin layer: Alugram Sil G/UV). The filter cake<br />
from the previous step (a mixture of mono-, di-substituted product<br />
and starting material) was reslurried in 300 ml of acetone. The<br />
suspension was boiled and filtered. After cooling, the di-substituted<br />
product was obtained. This extraction procedure was repeated twice<br />
after which, the raw, di-substituted product was recrystallized from<br />
acetone. In this way 1.7 g of dark blue VIII was obtained. The filter<br />
cake from the previous step (a mixture of mono-alkylate product<br />
and starting material) was reslurried in 300 ml of acetone together<br />
with 10 g silica gel for column chromatography (0.06–0.2 mm, pore<br />
diameter 6 nm). The suspension was boiled and filtered; acetone<br />
from the filtrate was evaporated and 0.22 g of dark blue VII was<br />
obtained after the filtration.<br />
VII: Calculated: C (75.26), H (7.50), N (10.97). Found: C (74.21), H<br />
(7.23), N (10.83). MW ¼ 510 Da; Negative-ion APCI-MS: m/z 511<br />
[M H] (100%). 1 H chemical shifts: 10.90 (1H, br. s, NH); 8.38 (2H,<br />
m, H-ortho); 7.81 (2H, m, H-ortho); 7.08 (4H, m, H-meta); 3.86 (4H,<br />
t, NCH2); 3.43 (8H, m, –CH2 CH2CH2N); 1.65 (12H, m, -CH2CH2 CH2N<br />
and –CH2CH2CH2N); 1.55 (2H, m, CH2); 1.26 (2H, m, CH2); 0.87 (3H,<br />
t, CH3).<br />
VIII: Calculated: C (76.29), H (8.18), N (9.89). Found: C (76.08), H<br />
(8.09), N (9.84). MW ¼ 566 Da; Pozitive-ion APCI-MS: m/z 567<br />
[M þ H] þ (100%). 1 H chemical shifts: 7.84 (4H, m); 7.1 (4H, m); 3.81<br />
(4H, t, NCH2); 3.40 (8H, m, -CH2 CH2CH2N); 1.65 (12H, m, -CH2CH2<br />
CH2N and –CH2CH2CH2N) 1.50 (4H, m, CH2); 1.25 (4H, m, CH2); 0.86<br />
(6H, t, CH3).<br />
3.4.5. Syntheses of 3,6-bis-(4-chloro-phenyl))-2-butyl-2,5–<br />
dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (IX) and 3,6-bis-(4-chlorophenyl))-2,5-butyl-2,5–dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione<br />
(X)<br />
The starting 3,6-Bis-(4-chloro-phenyl)-2,5–dihydro-pyrrolo-<br />
[3,4-c]pyrrole-1,4-dione was purchased from Synthesia as the<br />
commercial pigment Versal Red DP3G. The sample was refluxed in<br />
2-propanol before for 1 h and the ensuing hot suspension was<br />
filtered, washed with 2-propanol and hot water. 46.6 g (0.13 mol) of<br />
this intermediate and 500 ml of dry DMF were charged into an<br />
evaporating flask and stirred. Subsequently, a methanolic solution<br />
of sodium methylate prepared from 6 g (0.26 mol) of sodium metal<br />
and 50 ml of methanol was added. The fine slurry of the sodium salt<br />
of I was stirred for 20 min, methanol was distilled under vacuum<br />
(t < 40 C, p ¼ 40 mbar) and then 100 g (0.735 mol) of n-butyl<br />
bromide was added. The mixture was heated to 60 C and stirred<br />
for 20 h, after which time the temperature was increased to 100 C<br />
and maintained at this temperature for a further 2 h with stirring.<br />
M. Vala et al. / Dyes and Pigments 84 (2010) 176–182 181<br />
The reaction mixture was added to 4500 ml of distilled water and<br />
boiled; the final mixture was cooled to 10 C and the product was<br />
filtered<br />
The filter cake was reslurried in the mixture of methanol<br />
(300 ml) and acetone (300 ml) and the ensuing suspension was<br />
boiled and filtered. The filter cake (10.8 g) was identified as<br />
a starting material. After cooling of the filtrates, a reddish substance<br />
(a mixture of IX and X) was obtained which was recrystallized from<br />
ethanol. The more soluble X was separated from the filtrate by<br />
evaporation of ethanol and the less-soluble derivative IX was<br />
obtained from the filter cake. Derivatives IX a X were three times<br />
recrystallized from ethanol.<br />
IX: Calculated: C (63.93), H (4.39), N (6.78), Cl (17.16). Found: C<br />
(63.51), H (4.5), N (6.70), Cl (N/N). MW ¼ 412 Da; Negative-ion<br />
APCI-MS: m/z 411 [M H] (100%). 1 H chemical shifts: 11.30 (1H, br.<br />
s, NH); 8.53 (2H, m, H-ortho); 7.91 (2H, m, H-ortho); 7.73 (4H, m, Hmeta);<br />
3.83 (2H, t, NCH2); 1.45 (2H, m, CH2); 1.20 (2H, m, CH2); 0.80<br />
(3H, t, CH3).<br />
X: Calculated: C (66.53), H (5.58), N (5.97), Cl (15.11). Found: C<br />
(66.42), H (5.73), N (5.92), Cl (15.81). MW ¼ 468 Da; Positive-ion<br />
APCI-MS: m/z 469 [M H] (100%). 1 H chemical shifts: 7.83 (4H,<br />
m); 7.65 (4H, m); 3.67 (4H, t, NCH2); 1.35 (4H, m, CH2); 1.11 (4H, m,<br />
CH2); 0.73 (6H, t, CH3).<br />
3.5. Experimental equipment<br />
3.5.1. Mass spectrometry<br />
The ion trap mass spectrometer MSD TRAP XCT Plus system<br />
(Agilent Technologies, USA) equipped with APCI probe was used.<br />
Positive-ion and negative-ion APCI mass spectra were measured in<br />
the mass range of 50–1000 Da in all the experiments. The ion trap<br />
analyzer was tuned to obtain the optimum response in the range of<br />
the expected m/z values (the target mass was set from m/z 289 to<br />
m/z 454). The other APCI ion source parameters: drying gas flow<br />
rate 7 dm 3 min 1 , nebulizer gas pressure 60 psi, drying gas<br />
temperature 350 C, nebulizer gas temperature 350 C.<br />
The samples were dissolved in a mixture of DMSO/acetonitrile<br />
and methanol in various ratios. All the samples were analyzed by<br />
means of direct infusion into LC/MS.<br />
3.5.2. Elemental analysis<br />
Perkin Elmer PE 2400 SERIES II CHNS/O and EA 1108 FISONS<br />
instruments were used for elemental analyses.<br />
3.5.3. Nuclear magnetic resonance<br />
Bruker AVANCE 500 NMR spectrometer operating at<br />
500.13 MHz for 1H was used for measurements of the 1H NMR<br />
spectra and A Bruker AMX 360 NMR spectrometer, operating at<br />
360.13 MHz for 1H and 90,56 MHz for 13C, was used for<br />
measurements of 1H and 13C NMR spectra. The compounds were<br />
dissolved in hexadeuteriodimethyl sulfoxide. The 1H and 13C<br />
chemical shifts were referred to the central signal of the solvent<br />
(d ¼ 2.55 (1H) and 39.60 (13C)). Positive values of chemical shifts<br />
denote shifts to higher frequencies.<br />
3.5.4. IR spectrometry<br />
IR spectra were determined by FT-IR spectrometer Nicolet<br />
Magna-IR 760. Samples were measured by KBr pellet technique.<br />
KBr pellet (diameter 13 mm) was prepared from 1 mg sample and<br />
300 mg KBr.<br />
4. Conclusions<br />
Novel diphenyl-diketo-pyrrolopyrroles with electron-donating<br />
or withdrawing groups were prepared. To increase their solubility,
182<br />
we introduced N-alkylation on the central DPP part. The prepared<br />
materials were studied experimentally and by the quantum<br />
chemical calculations. One of the key-parameter governing<br />
absorption and luminescence is the effective extent of the conjugation.<br />
This is maximised when the molecule is perfectly planar.<br />
Substitution of electron-donating or withdrawing groups affected<br />
the molecule planarity only slightly. However, hyperchromic and<br />
bathochromic shift in absorption was observed using electrondonating<br />
groups suggesting electron-accepting character of the<br />
central part. On the other hand, N-alkylation introduced to increase<br />
the solubility of the DPP derivatives broke the planarity and thus<br />
reduced this effect.<br />
Acknowledgement<br />
The research was supported by the Ministry of Industry and<br />
Trade of the Czech Republic via Tandem project No FT-TA3/048 and<br />
by the Grant Agency of the Academy of Sciences of the Czech<br />
Republic via project A401770601. The access to the METACentrum<br />
supercomputing facilities provided under the research intent<br />
MSM6383917201 as well as the computer time on the servers Luna/<br />
Apollo in the Institute of Physics of the AS CR, v.v.i., Prague is highly<br />
appreciated.<br />
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[32] Mizuguchi J, Rochat CA. A new near-infrared photoreceptor based on 1,4dithioketo-3,6-diphenylpyrrolo[3,4-c]-pyrrole.<br />
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1988;32(3):135–40.<br />
[33] Mizuguchi J. Phase-change of 3,6-diphenzlpyrrolo [3,4-c]pzrrole-1,4-dithiones<br />
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[34] Mizuguchi J, Imoda T, Takahashi H, Yamakami H. Polymorph of 1,4-diketo-3,6bis-(4’-dipyridyl)-pyrrolo-[3,4-c]pyrrole<br />
and their hydrogen bond network:<br />
a material for H2 gas sensor. Dyes and Pigments 2006;68(1):47–52. 2005.<br />
[35] Hoki T, Takahashi H, Suzuki S, Mizuguchi J. Hydrogen gas sensor based upon<br />
proton acceptors integrated in copper-tetra-2,3-pyridinoporphyradine. IEEE<br />
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[37] Vala M, Weiter M, Vyňuchal J, Toman P, Luňák Jr S. Comparative studies of<br />
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J Therm Anal Calorim<br />
DOI 10.1007/s10973-011-1896-8<br />
Stability and physical structure tests of piperidyl and morpholinyl<br />
derivatives of diphenyl-diketo-pyrrolopyrroles (DPP)<br />
Jirˇí Kučerík • Jan David • Martin Weiter •<br />
Martin Vala • Jan Vyňuchal • Imad Ouzzane •<br />
Ota Salyk<br />
3 rd Joint Czech-Hungarian-Polish-Slovak Thermoanalytical Conference Special Chapter<br />
Ó Akadémiai Kiadó, Budapest, Hungary 2011<br />
Abstract Crystalline structure, thermo-oxidative and<br />
thermal stability of symmetrical and asymmetrical piperidyl<br />
and morpholinyl derivatives of both N-substituted and non-<br />
N-substituted butyl diphenyl-diketo-pyrrolopyrrole (DPP)<br />
pigments were studied using differential scanning calorimetry<br />
(DSC) and thermogravimetry (TG). Except for the<br />
asymmetrical morpholine DPP derivative, all the samples<br />
showed melting peaks which were relatively close to their<br />
degradation temperatures (from 260 to 430 °C). Using DSC,<br />
monotropic polymorphism was revealed in the symmetrical<br />
piperidyl-N-butyl-derivative which confirmed earlier<br />
observation about tendency of symmetrical N-alkyl DPP<br />
derivates to form several crystalline structures. TG carried<br />
out under nitrogen atmosphere served for distinguishing of<br />
evaporation/sublimation and degradation temperatures.<br />
Temperatures of evaporation/sublimation were typically<br />
10–30 °C lower in comparison with temperatures of thermal<br />
degradation. The highest thermal (450 °C) and thermo-oxidative<br />
stability (around 360 °C) showed the DPP derivatives<br />
J. Kučerík (&)<br />
Institute of Environmental Sciences, University of Koblenz-<br />
Landau, Fortstrasse 7, 768 29 Landau, Germany<br />
e-mail: kucerik@uni-landau.de<br />
J. Kučerík J. David M. Weiter M. Vala O. Salyk<br />
Faculty of Chemistry, Brno University of Technology,<br />
Purkyňova 118, Brno 612 00, Czech Republic<br />
J. David M. Weiter M. Vala I. Ouzzane<br />
Faculty of Chemistry, Centre for Materials Research CZ.1.05/<br />
2.1.00/01.0012, Brno University of Technology, Purkyňova<br />
464/118, Brno 612 00, Czech Republic<br />
J. Vyňuchal<br />
Research Institute of Organic Syntheses, Rybitví 296,<br />
533 54, Czech Republic<br />
containing morpholine moieties with no alkyl substitution on<br />
NH-group of DPP core. The presence of the latter was found<br />
to be the most destabilizing factor. Piperidyl group showed<br />
more stabilizing effect due to its polar character and its<br />
influence on p–p intermolecular interactions of neighbouring<br />
phenyl groups. The highest stabilizing effect of morpholine<br />
moiety on DPP structure was explained based on the<br />
presence of polar oxygen atom in that group. The preparations<br />
of 3,6-di-(4-morpholinophenyl)-2,5-dihydro-pyrrolo-<br />
[3,4-c]pyrrole-1,4-dione and 3-(phenyl)-6-(4-morpholinophenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione<br />
are reported.<br />
Keywords Derivatives Diphenyl-diketo-pyrrolopyrrole<br />
(DPP) Electroluminescent devices Polymorphism<br />
Thermal stability Thermo-oxidative stability<br />
Introduction<br />
Currently, a strong effort to produce organic electroluminescent<br />
devices (OLED) usable as a new generation of<br />
lamps and full colour flat panel displays, can be recognized.<br />
The potential advantages of these devices are their<br />
high efficiency, low driving voltage, versatility in their<br />
application (e.g. flexibility and transparency), low mass<br />
and relatively low production costs [1]. In order to fulfil all<br />
the requested parameters, the interest is focused not only<br />
on their optical properties, but also on physical–chemical<br />
characteristics such as solubility, resistance against various<br />
influences such as oxygen or temperature fluctuation.<br />
3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione<br />
known as DPP and its whole family of derivates are<br />
nowadays of a great interest [1–10]. Similarly, as other<br />
organic pigments they consist of centrosymmetric molecule<br />
having zero dipole moment [7]. Despite being of a<br />
123
elatively low molecular weight, DPP is an insoluble,<br />
crystalline material, thermally stable up to approximately<br />
400 °C [8]. Physical properties are related to the presence<br />
of strong intermolecular bonds, which stabilize the crystal<br />
structure [1].<br />
The derivatization of DPP is essential since resulted<br />
materials have improved physical–chemical properties<br />
such as for example, solubility, keeping the same or even<br />
better application properties in comparison with the<br />
parental DPP. Recently, it has been demonstrated that the<br />
introduction of piperidino (donor) and cyano (acceptor)<br />
groups into the DPP molecule had a strong influence on its<br />
absorption maxima. While both types of substitution<br />
resulted in bathochromic and hyperchromic shifts with<br />
respect to the parental DPP, the influence of piperidino<br />
group appeared to have the strongest effect [7].<br />
Another example is a symmetrical dipyridyl derivate<br />
(DPPP) which shows a high proton affinity and, therefore,<br />
represents a good candidate for H2 gas sensors [9]. However,<br />
only one of two possible crystal phases of DPPP was<br />
shown to be sensitive for H2 detection; the reason of<br />
inactivity of one form is the interaction between N atom of<br />
pyridyl rings and NH-group(s) of DPP skeleton [9].<br />
Thermal analysis represents a powerful tool in analysis of<br />
inorganic and organic pigments. The most important techniques<br />
are thermogravimetric analysis (TGA) and differential<br />
scanning calorimetry (DSC), applied either separately,<br />
simultaneously or in combination with other techniques.<br />
Recently, those techniques were used in order to study the<br />
stability of DPP N-alkyl derivatives and polymorphism<br />
occurring upon fast and moderate cooling of the material [8].<br />
More recently, the thermogravimetrical analysis of DPP<br />
derivatives of red-emitting diketopyrrolopyrrole-alt-phenylenevinylene<br />
[11] polymers and poly(arylene ethynylene)s<br />
derived from 1,4-diketo-3,6-diphenylpyrrolo[3,4-c] pyrrole<br />
[12] were reported. The thermal stability of the products<br />
were assessed in nitrogen atmosphere as ranging from 260<br />
to 400 °C [11, 12].<br />
The main aim of this study is the evaluation of the<br />
thermal and thermo-oxidative stability of DPP derivates<br />
with para-substituted phenyl groups and partially substituted<br />
NH-groups. Furthermore, DSC is employed in order<br />
to reveal possible phase transitions in heating run upon<br />
different cooling regimes. This should imitate the possible<br />
fluctuation of environmental conditions, which can hamper<br />
Fig. 1 Molecular structure of<br />
investigated DPP derivatives<br />
123<br />
N N<br />
O<br />
O<br />
processing and possible applications of DPP derivatives.<br />
All parameters determined using thermo-analytical techniques<br />
are discussed with respect to the molecular structure<br />
of studied samples.<br />
Experimental<br />
Thermal analysis<br />
TG studies were performed using TA Instruments TGA<br />
Q5000 (New Castle, DE, USA) device in 100-lL open<br />
platinum pans. The samples, typically around 5 mg, were<br />
heated by thermal ramp of 10 °C min -1 from 40 to 650 °C<br />
in a dynamic atmosphere of either nitrogen (thermal stability)<br />
or air (thermo-oxidative stability) of 25 mL min -1 .<br />
Calorimetric analyses were carried out employing TA<br />
Instruments DSC Q200 calorimeter equipped with an<br />
external cooler RCS90 allowing experimental temperature<br />
range from -90 to 500 °C. Experiments were conducted in<br />
open TA Tzero TM<br />
open aluminium pans. Thermal history of<br />
all samples was set up to be the same using a heating ramp<br />
of 10 °C min -1 from 40 °C to the temperature of 5 °C<br />
lower than the degradation onset, which was previously<br />
determined using thermogravimetry. Next, the moderate<br />
cooling ramp (0.5 °C min -1 ) was applied to reach -90 °C<br />
followed by 1 min of an isothermal stage. Next segment<br />
was performed using a heating ramp of 10 °C min -1 from<br />
-90 °C to the temperature of 5 °C lower than degradation<br />
onset. The last segment included a 10 °C min -1 heating<br />
run followed after the rapid equilibration of the sample<br />
down to -90 °C (quenching) and 1 min of isothermal<br />
stage. All DSC experiments were made under dynamic<br />
atmosphere of nitrogen, flow rate 50 mL min -1 . Before the<br />
analyses, the device was calibrated for temperature and<br />
enthalpy using deionized water (own production), indium<br />
and tin standards (Perkin Elmer, Waltham, MA, USA.).<br />
Samples<br />
NH NH<br />
HN HN<br />
O<br />
N<br />
Five DPP derivates were investigated in this study. The<br />
molecular structures of tested DPP derivatives are reported<br />
in Fig. 1. The preparation of DPP1, DPP4 and DPP5 is<br />
reported in Ref. [9]. Preparation of DPP2 and DPP3 is<br />
given in following paragraphs.<br />
O<br />
O<br />
N<br />
O<br />
N<br />
DPP1 DPP2 DPP3 DPP4 DPP5<br />
O<br />
HN<br />
O<br />
NH<br />
N<br />
O<br />
N<br />
H C N<br />
9 4<br />
O<br />
O<br />
N C H<br />
4 9<br />
J. Kučerík et al.<br />
N<br />
H C N<br />
9 4<br />
O<br />
O<br />
NH<br />
N
Stability and physical structure tests of piperidyl and morpholinyl derivatives of DPP<br />
Synthesis of 4-morpholine-1-yl-benzonitrile (starting<br />
nitrile)<br />
Dry N,N-dimethylacetamide (p.a., 400 mL), p-fluorobenzonitrile<br />
(47.8 g; 0.4 mol) and morpholine (85 g;<br />
0.98 mol) were charged into a 1-dm 3 Erlenmeyer flask<br />
equipped with a stirrer and condenser. The reaction was<br />
carried out at 100–110 °C for 8 h. The reaction gases from<br />
the reaction were let out to fume-chamber. Subsequently,<br />
the reaction mixture was poured onto 1 kg ice. The crude<br />
product was collected by filtration and recrystallized from<br />
80% ethanol. Yield was 38 g (50%) of 4-morfoline-1-ylbenzonitrile<br />
(m.p. 53–55 °C).<br />
Synthesis of DPP2 (3,6-di-(4-morpholinophenyl)-<br />
2,5–dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione)<br />
Tert-amyl alcohol (390 mL) and sodium metal (24.4 g,<br />
1 mol, in three portions) were charged into an 1.5-L Keller<br />
flask equipped with a stirrer, reflux condenser, thermometer<br />
and nitrogen inlet. The sodium metal was dissolved under<br />
reflux in the presence of catalytic amount of FeCl 3 (which<br />
took approximately 2 h), and 4-morpholine-1-yl-benzonitrile<br />
(69 g, 0.37 mol) was added. After that, diisopropyl<br />
succinate (36.2 g, 0.18 mol) dissolved in tert-amyl alcohol<br />
(36.3 g) was added drop-wise within 3 h. Subsequently,<br />
this mixture was stirred under reflux for 1 h. The reaction<br />
mixture was cooled to 60 °C, and then 1000 mL of distilled<br />
water was added to protonate the salt. The protolysis was<br />
carried out at 80 °C for 2 h. The resulting hot suspension<br />
was filtered, and the filter cake was washed with hot water<br />
to neutral pH. The filter cake was dried and suspended in<br />
800 mL acetone. The suspension was boiled and refluxed<br />
for 1 h. The hot suspension was filtered, washed with<br />
acetone and hot water. Yield of the DPP2 was 19.5 g<br />
(23.9%). Theoretical values of elemental analysis: C<br />
(68.11), H (5.72), N (12.22). Determined values of elemental<br />
analysis: C (67.89), H (5.75), N (12.05). 1 H<br />
chemical shifts: (500 MHz, DMSO) 10.99 (2H, s, NH);<br />
8.40 (4H, m, ArH); 7.11 (4H, m ArH); 3.78 (8H, t,<br />
H morpholine); 3.39 (8H, t, H morpholine).<br />
Synthesis of DPP3 (3-(Phenyl)-6-(4-morpholinophenyl)-<br />
2,5–dihydropyrrolo[3,4-c]pyrrole-1,4-dione)<br />
Tert-amyl alcohol (80 mL) and sodium metal (2.8 g,<br />
0.12 mol) were charged into a 250-cm 3 flask equipped with<br />
a stirrer, reflux condenser, thermometer and a nitrogen<br />
inlet. The sodium metal was dissolved under the reflux in<br />
the presence of catalytic amount of FeCl3 (which approximately<br />
took 2 h), whereupon 4-morpholine-1-yl-benzonitrile<br />
(7.5 g, 0.04 mol) was added. After that, pyrrolinone<br />
ester (9.2 g, 0.04 mol, in small portions) was continuously<br />
added within 0.5 h. Finally, this mixture was stirred and<br />
refluxed for 2 h. The hot suspension of sodium salt of<br />
compound DPP3 was collected by suction. The filter cake<br />
was charged into 200 mL propan-2-ol/water mixture (3/2<br />
by vol), and the suspension was heated to boiling and<br />
refluxed for 2 h. The product was collected by filtration,<br />
and the filter cake was charged into 100 mL methanol and<br />
refluxed for a short time. The hot suspension was filtered,<br />
the filter cake was washed with methanol, and finally with<br />
hot water. Yield: 5.3 g (36%) compound DPP3. Theoretical<br />
values of elemental analysis: C (70.76), H (5.13), N<br />
(11.25). Determined values of elemental analysis: C<br />
(70.06), H (5.10), N (10.98). 1 H chemical shifts: 500 MHz,<br />
DMSO) 11.19 (1H, s, NH); 11.16 (1H, s, NH); 8.46 (4H, m,<br />
ArH); 7.57 (3H, m ArH); 7.14 (2H, m ArH); 3.78 (8H, t,<br />
H morpholine); 3.39 (8H, t, H morpholine).<br />
Results and discussion<br />
The derivative TG results, i.e. DTG of samples measured<br />
under nitrogen and air are reported in Figs. 2 and 3,<br />
respectively. As it can be seen, the DTG curves are<br />
reported in the reverse direction then calculated in order to<br />
simplify the data reading. Table 1 summarizes the data<br />
obtained from TG, DTG and DSC analysis of DPP derivates.<br />
DTG–air and DTG–nitrogen–evaporation onset<br />
results show the onsets determined from DTG records by<br />
the approach indicated in the small frame in Fig. 2. While<br />
DTG–air has the meaning of thermo-oxidative stability, the<br />
DTG–nitrogen–evaporation onset indicates the temperature<br />
of intensive mass loss onset, i.e. it indicates either evaporation<br />
or sublimation. The character of the process, if either<br />
evaporation or sublimation takes place, is discussed in<br />
following paragraphs with respect to the obtained DSC<br />
results. The DTG–nitrogen–degradation onset stands for<br />
Derivative mass/% °C –1<br />
3.5<br />
3.0<br />
2.5<br />
2.0<br />
1.5<br />
1.0<br />
0.5<br />
DPP1<br />
DPP2<br />
DPP3<br />
DPP4<br />
DPP5<br />
T onset<br />
280 300 320 340 360 380<br />
0.0 250 275 300 325 350 375 400 425 450 475 500<br />
Temperature/°C<br />
Fig. 2 Comparison of the first derivative of mass loss (DTG) of DPP<br />
derivatives in nitrogen, heating rate 10 °C min -1<br />
123
the thermal stability of a sample and it was determined<br />
from the second derivative as demonstrated in Fig. 4. This<br />
approach was developed and tested in Ref. [8]. Unlike for<br />
nitrogen experiments, the amount of char after air TG<br />
experiment is not reported since there was no rest after this<br />
kind of analysis. Regarding to the DSC results, Table 1<br />
reports the enthalpies of melting and onset temperatures of<br />
respective events. For sample DPP2, the enthalpy of<br />
melting is not reported since the melting was immediately<br />
followed by the sample’s evaporation and, therefore, peak<br />
area of melting was overlapped by the peak of evaporation.<br />
For the sample DPP3, no melting peak before the DTG<br />
onset was recorded, therefore, the nitrogen DTG onset<br />
indicates the temperature of sublimation.<br />
By applying TG [8], it was showed the onset temperature<br />
of sublimation of parental non-substituted DPP under<br />
nitrogen (sublimation) as high as 383 °C (for a heating rate<br />
10 °C min -1 ). That process was followed by the thermal<br />
degradation at 408 °C and the thermo-oxidative stability<br />
was determined around 356 °C (under the same conditions<br />
as used in this study). Despite the fact that two crystallographic<br />
forms in parental DPP sample may occur, the DSC<br />
tests disclosed no significant phase transition such as<br />
melting or glass transition in the temperature interval from<br />
Dreivative mass/% °C –1<br />
1.25<br />
1.00<br />
0.75<br />
0.50<br />
0.25<br />
0.00 250 300 350 400 450 500 550 600 650<br />
Temperature/°C<br />
DPP1<br />
DPP2<br />
DPP3<br />
DPP4<br />
DPP5<br />
Fig. 3 Comparison of the first derivative of mass loss (DTG) of DPP<br />
derivatives in air, heating rate 10 °C min -1<br />
Table 1 Summary of TGA and DSC results, for detail explanation see the text<br />
-90 to 383 °C. In contrast, the alkyl derivates (substitution<br />
of H in NH-group in parental DPP molecule) were less<br />
stable (a difference more than 100 °C) and their physical<br />
structure showed a strong dependence on the thermal pretreatment<br />
(thermal history). That was explained as a consequence<br />
of a molecular symmetry disturbance caused by<br />
the presence of alkyl chains in the structure of DPP<br />
derivatives. As stated previously, pure DPP is stabilized by<br />
means of 3D intermolecular hydrogen bonds based on<br />
–NH O=C interaction, by p–p intermolecular interactions<br />
between adjacent phenolic groups, by electrostatic interactions<br />
of charged groups and partly also by van der Waals<br />
forces [10]. Substitution of one or both H atoms in the NHgroups<br />
by alkyl chains in DPP molecule caused the<br />
destabilization of crystalline structure of derivatives under<br />
study. In fact, significant differences were found in physical<br />
structures of derivates depending on the symmetry<br />
and/or asymmetry of derivatization. Symmetrical derivatives<br />
exhibited a strong dependency on the thermal history,<br />
i.e. differences were recorded in the DSC heating run of the<br />
sample which was previously cooled either quickly or<br />
slowly. Such approach revealed the polymorphism of<br />
symmetrical derivates with alkyl chains –C 4H 9 and –C 7H 15<br />
but not for –CH3. On the contrary, the asymmetrical<br />
Sample DPP1 DPP2 DPP3 DPP4 DPP5<br />
DTG–air/°C 332 357 378 292 326<br />
DTG–nitrogen–evaporation onset/°C 440 440 416 330 374<br />
DTG–nitrogen–degradation onset/°C 446 459 441 373 383<br />
Char in nitrogen/% 16.4 17.1 11.7 10.0 10.8<br />
Tmelting/°C 431 435 – 263 361<br />
DHmelting/J/g 213.7 n.d. – 77.1 a<br />
91.8<br />
a Only main melting peak is reported, minor ones are described in the text<br />
123<br />
1st derivative mass/% °C –1<br />
3.0<br />
2.5<br />
2.0<br />
1.5<br />
1.0<br />
0.5<br />
0.0<br />
360 380 400 420 440 460<br />
Temperature/°C<br />
J. Kučerík et al.<br />
Fig. 4 The determination of degradation onset from the second<br />
derivative mass curve of sample DPP3<br />
0.1<br />
0.0<br />
–0.1<br />
–0.2<br />
–0.3<br />
2nd derivative mass/% °C –2
Stability and physical structure tests of piperidyl and morpholinyl derivatives of DPP<br />
derivates showed practically no response in physical<br />
structure when exposed to a different thermal history. It is<br />
noteworthy that glass transition was observed only in some<br />
samples regardless to the symmetry of the molecule.<br />
Figure 2 reports the DTG of investigated samples in<br />
nitrogen. As already mentioned, the tests in nitrogen were<br />
carried out to obtain the temperatures of evaporation or<br />
sublimation; however, due to the intermolecular interactions<br />
stabilizing the DPP physical structure, processes of<br />
evaporation or sublimation are relatively slow and, therefore,<br />
the programmed continuous increase in the oven’s<br />
temperature causes later the degradation of the rest of the<br />
sample which was not removed yet and remained on the<br />
pan. In practise, when the sample is evaporated to be<br />
deposited on a surface, this problem can be solved by<br />
pressure reduction and keeping constant temperature during<br />
the deposition [10]. However, it can be seen that except<br />
for sample DPP5, all samples gave only one intensive DTG<br />
peak which indicates the one-step process. For this reason,<br />
the second derivative of TG signal was calculated in order<br />
to reveal some possible overlapping processes. Indeed, two<br />
separated processes were disclosed and attributed to the<br />
evaporation followed by the degradation, similarly as<br />
proved and published recently [8]. A typical determination<br />
is reported in Fig. 4.<br />
The highest onset in nitrogen TG experiments showed<br />
samples DPP1 and DPP2 followed by samples DPP3 and<br />
DPP5. The lowest onset was observed for sample DPP4. In<br />
contrast, experiments in oxidative atmosphere of air<br />
brought about a different order. The highest stability was<br />
observed for sample DPP3, further for DPP2. Under oxidative<br />
conditions, sample DPP1 showed a lower stability<br />
than DPP2 (difference 25 °C) and it was only slightly<br />
higher than the stability of sample DPP5. Again, the lowest<br />
stability was observed for sample DPP4.<br />
Differences in stabilities under different conditions were<br />
accompanied also by differences in physical structures as<br />
revealed by DSC measurement. It can be identified in<br />
Table 1 that melting temperatures of samples DPP1, DPP2<br />
and DPP5 are higher than DTG–air onset temperatures<br />
which suggest that samples underwent thermo-oxidative<br />
degradation before their melting. On the contrary, all the<br />
samples (except DPP3) were pre-melted before the onset<br />
determined by DTG under inert conditions.<br />
In general, it can be stated that all DPP derivates<br />
investigated in this study showed higher stability than Nalkyl<br />
derivatives tested in Ref. [8]. In contrast, as follows<br />
from the above-statements, the parental DPP showed<br />
higher thermal stability than DPP5 and DPP4; the thermooxidative<br />
stability was higher than DPP5, DPP4, DPP1 and<br />
similar to DPP2.<br />
Unlike the degradation in nitrogen, the degradation in<br />
oxidative atmosphere (Fig. 3) proceeded in several steps.<br />
The lowest stability showed samples with aliphatic chains<br />
in the structure. In fact, the mass loss extracted from TG<br />
curve in the temperature range corresponding to the first<br />
degradation step in sample DPP4 was about 20% which is<br />
equal to the molar fraction of alkyl part in that molecule.<br />
Analogous result can be obtained also for sample U5, i.e.<br />
detected mass loss was 11%. Therefore, it seems that the<br />
less stable part of the derivative molecule is the aliphatic<br />
chain attached to the N-molecular skeleton of DPP. The<br />
piperidine part of a molecule has less de-stabilizing effect,<br />
probably due to its polar character and its influence on p–p<br />
intermolecular interactions of phenyl groups. Employing<br />
the same approach as for aliphatic derivates for sample<br />
DPP1, the piperidyl group is decomposed in several steps.<br />
Figure 3 shows two identical peaks at 340 and 349 °C<br />
corresponding to 9 and 20% mass loss, respectively. At<br />
37% of degradation (i.e. when both piperidyl groups are<br />
degraded) at 450 °C which corresponds to the minimum<br />
between two main degradation stages. Piperidyl group is<br />
more resistant against thermo-oxidative attack than simple<br />
aliphatic chain. This can be easily identified also when the<br />
most stable sample DPP1 (with no alkyl chain) is compared<br />
with DPP4 (symmetrical N-alkyl derivatization), which is<br />
less stable and with DPP5 (asymmetrical N-alkyl derivatization),<br />
which is only slightly less stable in air, however,<br />
significantly less stable in nitrogen. Such order also supports<br />
the observation which was done in the study with Nalkyl<br />
derivates that asymmetrical derivates of DPP have<br />
higher stability than the symmetrical ones.<br />
Comparison of samples DPP1 and DPP2, i.e. samples<br />
with different para-substituents on phenyl groups of DPP<br />
showed only slight difference in thermo-oxidative stability<br />
while thermal stability was identical. The piperidyl groups<br />
showed lower stabilizing effect against oxidation than<br />
morpholine group which can be explained as a stabilizing<br />
effect of polar O atom in morpholine group. However, as<br />
indicated by DSC (Table 1) there is a similar influence of<br />
both substituents on melting temperature in both derivatives.<br />
Since parental DPP did not show any melting before<br />
the evaporation [8] it can be assumed that both substituents,<br />
donors of electrons, have an influence on the weakening<br />
or opening of the structure allowing the oxygen<br />
molecules to penetrate inside the structure and decompose<br />
the material. The degradation of morpholine groups in<br />
sample DPP2 above-mentioned approach, based on mass<br />
loss calculation, failed, which implies that the breaking<br />
down of that group proceeds in a more complicated way<br />
than aliphatic chains or piperidyl groups. Interestingly,<br />
such approach was successful in sample DPP3 where the<br />
mass loss of morpholine group corresponded to the minimum<br />
in DTG after the first degradation step.<br />
Comparison of DSC records of DPP1 and DPP4 can<br />
shed an additional light on the influence of aliphatic chains<br />
123
on the stability of DPP derivatives structure. As it can be<br />
seen, the alkyl chains destabilize significantly the structure<br />
and cause the lower stability of crystals in sample DPP4.<br />
Significantly higher melting enthalpy of sample DPP1<br />
indicates the formation of completely different crystalline<br />
structure than in the case of N-alkyl derivates (their<br />
enthalpy of melting) was reported as significantly lower<br />
than those reported in Ref. [8].<br />
Differences between samples DPP3 (asymmetrical substitution<br />
of DPP’s phenyls) and DPP2 (symmetrical substitution)<br />
brought the insight into the influence of dipole<br />
moment on their thermal and thermo-oxidative stability. In<br />
contrast to DPP2, due to its asymmetry, the molecule DPP3<br />
has non-zero dipole moment [13] stabilizing the structure<br />
against oxygen agitation. It is noteworthy that DPP3 is the<br />
only sample which had no melting before the DTG onset.<br />
Therefore, the present dipole moment destabilizes the<br />
structure and the sublimation (thermal stability) of sample<br />
DPP3 occurs at lower temperatures than for DPP 2.<br />
As reported in Fig. 5, sample DPP4 showed a strong<br />
dependency on the thermal pre-treatment. When heated up<br />
without any pre-treatment, indicated as ‘C’ record in Fig. 4<br />
(i.e. measured as received), there can be seen three melting<br />
events (in Table 1 only the last peak, the most intensive<br />
one is reported). Small peaks occurred at 153 and 166 °C<br />
with respective enthalpies of 0.7 and 11.7 J g -1 . On the<br />
other hand, when cooled quickly, indicated as ‘A’ record, a<br />
glass transition with midpoint at 58 °C accompanied by a<br />
slight enthalpic recovery appeared followed by two exotherms<br />
attributable to crystallization. Peaks occurred at 110<br />
and 164 °C with enthalpies 39.8 and 5.1 J g -1 , respectively.<br />
Melting of present crystalline structure was<br />
observed at 263 °C. Finally, when cooled slowly, only the<br />
melting at 254 °C was observed with no pre-crystallization<br />
event (‘B’ record).<br />
DSC signal/W g –1<br />
0 20 40 60 80<br />
25 50 75 100 125 150 175 200 225 250 275<br />
Temperature/°C<br />
Fig. 5 Polymorphism of sample DPP4 as revealed by DSC measurement.<br />
A heating after rapid cooling, B heating after moderate<br />
cooling, C heating run without thermal pre-treatment<br />
123<br />
A<br />
B<br />
C<br />
Such behaviour suggests the possible polymorphism of<br />
sample DPP4. As mentioned in previous paragraphs, similar,<br />
but not the same, temperature-dependent behaviour<br />
was observed for symmetrical N-alkyl DPP derivates,<br />
namely C4H9 and C7H15 derivatives [8]. That was attributed<br />
to the melting of two different crystalline forms<br />
implying the monotropic polymorphism. The possible<br />
existence of several crystalline structures was observed<br />
only in the first ‘C’ record. The run ‘A’ which shows also<br />
the glass transition can be explained as a consequence of<br />
quick cooling; the movement of molecules otherwise<br />
forming crystals is decelerated and instead, they form<br />
amorphous structures. Heating up of the sample causes<br />
molecular segments relaxation (glass transition) followed<br />
by two steps of crystal perfection. Since aliphatic groups<br />
are involved in the crystalline structure, two phases may<br />
imply the progressive formation of both aliphatic and<br />
piperidyl moieties containing crystalline domains.<br />
Conclusions<br />
Thermal analysis has already showed its potential to study<br />
physical–chemical properties of pigments and dyes, e.g.<br />
refs. [8, 14, 15]. In this study, the physical–chemical<br />
properties of DPP derivates were investigated employing<br />
thermal analysis methods. The determined temperatures,<br />
especially the distinguishing between evaporation/sublimation<br />
and degradation temperatures is important for the<br />
designing of experimental conditions for deposition of such<br />
materials in the form of thin layers. The high thermal and<br />
thermo-oxidative stability of DPP materials which was<br />
determined using TG is a promising factor supporting their<br />
future application. Furthermore, the knowledge on the<br />
physical structure, as revealed by DSC showed the potential<br />
problem in manipulating with some DPP derivates<br />
since only an appropriate crystalline structure can be<br />
requested for the specific application.<br />
Acknowledgements The financial support of the Ministry of Education<br />
of the Czech Republic - project MSM 0021630501, Academy<br />
of Sciences of the Czech Republic project KAN401770651 and Czech<br />
Science Foundation project GACR 203/08/1594 are acknowledged.<br />
This study was also supported by the project ‘‘Centre for Materials<br />
Research at FCH BUT’’ No. CZ.1.05/2.1.00/01.0012 from ERDF.<br />
References<br />
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studies of diphenyl-diketo-pyrrolopyrrole derivatives for<br />
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2. Mizuguchi J. Correlation between crystal and electronic structures<br />
in diketopyrrolopyrrole pigments as viewed from exciton<br />
coupling effects. J Phys Chem A. 2000;104:1817–21.
Stability and physical structure tests of piperidyl and morpholinyl derivatives of DPP<br />
3. Hoki T, Takahashi H, Suzuki S, Mizuguchi J. Hydrogen gas<br />
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3-pyridinoporphyradine. IEEE Sensors J. 2007;7:808–13.<br />
4. Beyerlein T, Tieke B, Forero-Lenger S, Brütting W. Red electroluminescence<br />
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conjugated polymer. Synthetic Metals. 2002;130:115–9.<br />
5. Potrawa T, Langhals H. Fluorescent dyes with large Stokes shifts<br />
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6. Fukuda M, Kodama K, Yamamoto H, Mito K. Evaluation of new<br />
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7. Luňák S, Vyňuchal J, Vala M, Havel L, Hrdina R. The synthesis,<br />
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11. Qiao Z, Xu Y, Lin S, Peng J, Cao D. Synthesis ands characterization<br />
of red-emitting diketopyrrolopyrrole-alt-phenylenevinylene<br />
polymers. Synthetic Metals. 2010;160:1544–50.<br />
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Patri M. Synthesis and characterization of alternative donoracceptor<br />
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1, 4-diketo-3, 6-bis-(4 0 -dipyridyl)-pyrrolo-[3, 4-c]pyrrole and<br />
their hydrogen bond network: A material for H2 gas sensor. Dyes<br />
Pigments. 2006;68:47–52.<br />
14. Rotaru A, Moanta A, Popa G, Rotaru P, Segal. E. Thermal<br />
decomposition kinetics of some aromatic azomonoethers. full<br />
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123
Thin polyphenylene vinylene electrophoretically and spin-coated films –<br />
photoelectrical properties<br />
D. Mladenova a,b,∗ , I. Zhivkov a,b,∗∗ , I. Ouzzane a , M. Vala a , D. Budurova c , M. Weiter a<br />
a Brno University of Technology, Faculty of Chemistry, Centre for Materials Research, Purkynova 118, 612 00 Brno, Czech Republic<br />
b Institute of Optical Materials and Technologies ”Acad. J. Malinowski”, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. bl. 101/109,<br />
1113 Sofia, Bulgaria<br />
c Institute of Polymers, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. bl. 103A, 1113 Sofia, Bulgaria<br />
Abstract<br />
Thin poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) films were prepared<br />
by electrophoretic deposition (EPD). Absorption spectra measured in a toluene solution and a toluene/<br />
acetonitrile suspension with the same MDMO-PPV concentration of 0.0033 g.l -1 were analysed. An observed broadening<br />
of the characteristic absorption peak could be related to the formation of tightly-folded polymer chains in the<br />
suspension.<br />
Thin films of about 300 nm thickness were prepared by EPD from suspension and spin coating (SC) from solutions<br />
with a concentration of 0.0033 g.l -1 and 8.95 g.l -1 , respectively.<br />
Atomic force micros<strong>copy</strong> study of surface morphology shows that the SC technique produces films with smooth<br />
and flat surfaces. The surfaces of the films obtained by the EPD method are rougher.<br />
The fine film structure of the SC films does not result in better photovoltaic properties.<br />
The ITO|MDMO-PPV|Al structure with EPD MDMO-PPV films behaves as a photovoltaic cell, while the<br />
same sample configuration with SC MDMO-PPV films acts more like a photoresistor. EPD and SC films exhibit the<br />
same charge-carrier photogeneration mechanism.<br />
Keywords: Molecular electronics, Organic solar cells, Electrophoretic deposition, Spin coating,<br />
Poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene]<br />
1. Introduction<br />
Conjugated polymers are a class of organic semiconductors<br />
that have attracted much interest in recent<br />
decades from both fundamental and practical points of<br />
view.[1] The π-electron system on their main chain provides<br />
unique electronic and optical properties that could<br />
not be predicted by conventional solid state theories.<br />
Much effort has been devoted to developing electronic<br />
devices that use conjugated polymers as electroluminescent<br />
diodes, photovoltaic cells, gas sensors,<br />
and field-effect transistors.[2] In contrast to traditional<br />
∗ Corresponding author<br />
∗∗ Author of the initial concept<br />
Email addresses: xcmladenova@fch.vutbr.cz<br />
(D. Mladenova), zhivkov@fch.vutbr.cz (I. Zhivkov),<br />
ouzzane@fch.vutbr.cz (I. Ouzzane), vala@fch.vutbr.cz<br />
(M. Vala), dbudurova@polymer.bas.bg (D. Budurova),<br />
weiter@fch.vutbr.cz (M. Weiter)<br />
inorganic semiconductors, which require high processing<br />
temperatures, conjugated polymers are synthesized<br />
at near room-temperature conditions. Their solubility<br />
could be significantly improved to deposit wet-process<br />
films under atmospheric pressure. As a result, large-area<br />
films could be obtained by relatively inexpensive (compared<br />
to vacuum-based methods) coating technologies –<br />
spin and dip coating, spray and Langmuir Blodgett (LB)<br />
deposition, ink-jet printing, electrophoretic deposition,<br />
etc.<br />
The spin-coating (SC) technique is a widely used<br />
method to prepare high quality polymer films for electronic<br />
devices. However, this is one of the most uneconomical<br />
methods, because most of the solution dropped<br />
on the substrate is blown away during the spinning.[3]<br />
Electrophoretic deposition (EPD) allows the preparation<br />
of films of several hundred nanometers from a<br />
low suspension concentration.[4] The particle size in the<br />
EPD suspension can be controlled by changing the ratio<br />
Preprint submitted to Surface and Coatings Technology March 12, 2012
of solvent and non-solvent.[5] Deposition from a suspension<br />
with a high non-solvent concentration results<br />
in a rougher film structure. This effect seems to improve<br />
the performance of gas sensor devices[6], as the<br />
film porosity may enable the fast absorption/desorption<br />
of dopants and target molecules. In contrast, a deposition<br />
from a suspension with a high solvent concentration<br />
results in flat and uniform films, applicable as<br />
active layers in polymer electroluminescent devices or<br />
solar cells.[7]<br />
EPD enables quick patterned deposition, because the<br />
covered area can be specified by the electrification of<br />
selected electrodes.[8]<br />
The application of the EPD method in the development<br />
of thin layer based organic devices needs further<br />
investigation of the mechanism, kinetics of the electrophoretic<br />
process, and structure and properties of the<br />
films.<br />
This paper presents a comparative study of the properties<br />
of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-<br />
1,4-phenylene vinylene] (MDMO-PPV) films deposited<br />
electrophoretically and cast by SC. The study seeks<br />
potential applications for these films in solar energy<br />
conversion technology and other branches of organic<br />
electronics.<br />
2. Material and methods<br />
A stock solution (concentration of 0.165 g.l -1 )<br />
was prepared by dissolving 3.3 mg MDMO-PPV<br />
(Sigma-Aldrich, catalogue number: 546461) in 20 ml<br />
toluene (Sigma-Aldrich Toluene, catalogue number:<br />
PLC22C11X), then heating for 5 min at 60 ◦ C, followed<br />
by ultrasonic treatment for 10 min. A homogeneous solution<br />
without clearly visible undissolved particles was<br />
obtained.<br />
An EPD suspension with MDMO-PPV concentration<br />
of 0.0033 g.l -1 and toluene/acetonitrile ratio of 50%<br />
(Sigma-Aldrich Acetonitrile, catalogue number: 34851)<br />
was prepared. For a 20 ml suspension with a concentration<br />
of 0.0033 g.l -1 and toluene/acetonitrile ratio<br />
of 50%, 0.4 ml from the stock solution was diluted<br />
in 9.6 ml pure toluene, then 10 ml of acetonitrile<br />
was added. The suspension obtained clearly changes<br />
color[1], which could be related to the formation of (micro)<br />
nano-sized solid particles. The prepared suspension<br />
was immediately used for film deposition or measurement<br />
to prevent further particle coagulation.<br />
A solution (0% acetonitrile) with the same MDMO-<br />
PPV concentration of 0.0033 g.l -1 was prepared for optical<br />
absorption measurements. The optical absorption<br />
2<br />
spectra of the prepared suspension and solution were<br />
measured by a Varian Cary 50 UV-Vis dual beam spectrophotometer.<br />
The measurement was carried out in a<br />
quartz cuvette with an optical path of 10 mm. The baseline<br />
was taken from the cuvette, filled with pure toluene<br />
or toluene/acetonitrile mixture in a 1:1 ratio (50% acetonitrile).<br />
Then the spectra in the 650-270 nm range were<br />
measured.<br />
Figure 1: Electrophoretic cell with two ITO plate electrodes fixed at a<br />
distance of 3 mm<br />
An electrophoretic cell with two ITO plate electrodes<br />
fixed at a distance of 3 mm (Figure 1) was developed,<br />
simplifying a previously published construction.[3] The<br />
parallel electrode arrangement was adjusted by a proper<br />
spacer. The modified construction allows the removal of<br />
the spacer before immersing the cell in the suspension,<br />
reducing the dielectric induction and homogenizing the<br />
electric field between the electrodes.<br />
The electrical parameters (voltage and current) of<br />
the EPD process were controlled by the Keithley 2410<br />
SourceMeter (voltage range of 0 ÷1100 V and current<br />
measurement from 10pA to 1A).<br />
After applying voltage between the electrodes, the<br />
current continuously decreases. A deposition on the<br />
positive electrode starts when the current reaches about<br />
80-40 µA. The film, about 300 nm thick, completely<br />
grows within 3-4 min.<br />
A solution with a concentration of 8.95 g.l -1 was prepared<br />
and SC was done on the KW-4A Spin Coater<br />
(Chemat Technology, Inc.). Spinning was applied at<br />
1000 rpm for 3 sec. and 2600 rpm for 60 sec. to obtain<br />
about 300 nm-thick films.
The film thickness was determined by the chromatic<br />
distance method on the MicroProf R⃝ FRT optical surface<br />
measuring system. Samples for thickness measurement<br />
were prepared by scratching the film and a subsequent<br />
vacuum deposition of about 100 nm Al to equalize the<br />
optical reflection from both scratched and unscratched<br />
areas.<br />
Surface morphology images of MDMO-PPV films<br />
were taken by the NTEGRA Prima AFM. Measurements<br />
were carried out in tapping (semicontacting)<br />
mode (frequency of 170 kHz, amplitude 80-100 nm at a<br />
scanning rate of 0.8 Hz).<br />
Samples for electrical measurements of<br />
25×15 mm were cut from commercial ITO-coated<br />
glass slides (Sigma-Aldrich 636916-25PAK). The<br />
sample preparation is schematically drawn in Figure 2.<br />
The top Al electrodes of 100 nm thickness were<br />
deposited in a vacuum; copper wires were then bonded<br />
to the electrodes by a silver paste (Dotite R⃝ Silver Paint<br />
D-550).<br />
Photoelectrical measurements were carried out in an<br />
oil-free vacuum of 2.2×10 -5 Pa with the Keithley 6517A<br />
Electrometer. Monochromatic light was produced by<br />
a LOT-Oriel halogen lamp LSH502 and a LOT-Oriel<br />
monochromator MSH101. Light power was controlled<br />
by an iris diaphragm and measured by the Gigahertz-<br />
Optik - X97 Irradiance Radiometer.<br />
Figure 2: Preparation of a sample for photoelectrical measurement:<br />
a) structuring of ITO anode; b) MDMO-PPV deposition (EPD considered<br />
here); c) deposition of top Al cathode; d) wire bonding by a<br />
silver paste<br />
Photoelectrical measurements start with spectral dependence<br />
of the photocurrent at zero applied voltage; I-<br />
V characteristics were then measured in both directions<br />
of the voltage scale in the dark and exposed to light.<br />
Finally, the dependence of the photocurrent on the incidental<br />
light power (irradiance) was measured. The sample<br />
was exposed to monochromatic light at a wavelength<br />
of 560 nm.<br />
3<br />
3. Results and Discussion<br />
3.1. UV-VIS spectra<br />
The optical absorption spectra of the solution and<br />
the 50% toluene/acetonitrile suspension with the same<br />
MDMO-PPV concentration of 0.0033 g.l-1 are presented<br />
in Figure 3. The solution spectrum (Figure 3,<br />
curve 2) consists of the characteristic MDMO-PPV absorption<br />
peak.[9] The peak is slightly broad and asymmetric<br />
due to the ”red” shift with respect to the main<br />
peak shoulder. This effect could be related to a slight<br />
interaction of the MDMO-PPV molecules.<br />
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Figure 3: Optical absorption spectra of MDMO-PPV suspension<br />
(curve 1) and solution (curve 2)<br />
In the suspension spectrum (Figure 3, curve 1),<br />
the ”red” shoulder strength increases, which leads to a<br />
greater broadening of the peak. This effect could be related<br />
to the appearance of a precipitated solid phase during<br />
the suspension formation, caused by the polar precipitating<br />
acetonitrile. It is known that for more crystalline<br />
organic materials (e. g. poly(3-octylthiophene)),<br />
such precipitation can even lead to a ”red” shift of the<br />
whole characteristic absorption peak.[10] The effect of<br />
the peak broadening could be used for a qualitative estimation<br />
of the suspension properties.<br />
3.2. Film thickness measurement and surface morphology<br />
characterization<br />
2D and 3D AFM images of EPD and SC films deposited<br />
from the suspension and the solution (concentrations<br />
of 0.0033 g.l -1 and 8.95 g.l -1 , respectively) are<br />
presented in Figure 4. In both cases, a film thickness<br />
of about 300 nm was determined by recording the profile<br />
of a scratched area with a chromatic distance measurement.<br />
AFM 2D images of EPD and SC films show<br />
relatively smooth surfaces. The SC method results in a<br />
�
film with finer surface morphology (Figure 4b), which<br />
is an expected result. The surface morphology of the<br />
films obtained by EPD (Figure 4a) are rougher. Grains<br />
of about 0.2-0.5 µm diameter observed on both film surfaces<br />
could be considered as dust particles.<br />
Figure 4: AFM images of a) EPD and b) SC films; 2D (up) and 3D<br />
(down) images; scanned area of 5×5 µm<br />
From the dust-free areas of the EPD and SC films, average<br />
roughness values of 2.00 nm and 1.90 nm were<br />
found, respectively. The root mean square was calculated<br />
at the same conditions as 2.47 nm and 2.77 nm,<br />
respectively.<br />
3.3. Electrical measurements<br />
Spectral dependences of the photocurrent, measured<br />
at zero applied voltage between the electrodes for structures<br />
with EPD and SC MDMO-PPV films are presented<br />
in Figure 5, curve 1, and Figure 5, curve 2,<br />
respectively.<br />
Comparing Figures 3 and 5, it could be concluded<br />
that generally the MDMO-PPV photocurrent spectrum<br />
is similar to the absorption spectrum. There is a clear<br />
characteristic peak in the photocurrent spectrum at 560<br />
nm. The peak position is ”red” shifted (about 50 nm)<br />
with respect to the maximum of the absorption spectrum.<br />
This effect of energy gap reduction could be related<br />
to the formation of a solid state. The photocurrent<br />
peak obtained from EPD film (Figure 5, curve<br />
1) is slightly broader than the peak obtained from SC<br />
film (Figure 5, curve 2). This result is similar to that<br />
obtained from absorption spectra measurement (Sec.<br />
3.1). The photocurrent measured from structures with<br />
SC films is 3 times higher than that obtained from EPD<br />
samples. I-V characteristics measured in the dark on the<br />
ITO|MDMO-PPV|Al structure with SC MDMO-PPV<br />
4<br />
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Figure 5: Spectral dependence of the photocurrent at zero applied<br />
voltage: EPD (curve 1) and SC (curve 2) films of about 300 nm thicknesses<br />
film of about 300 nm thickness are presented in Figure<br />
6, curve 2. The curves are nonlinear and symmetrical.<br />
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Figure 6: I-V characteristics measured in the dark on the ITO|MDMO-<br />
PPV|Al structures with EPD (curve 1) and SC (curve 2) MDMO-PPV<br />
films of similar thicknesses about 300 nm<br />
The same type of measurement on structures with<br />
EPD MDMO-PPV films of about 300 nm thickness is<br />
presented in Figure 6, curve 1. In the second case<br />
the shape of the characteristics is typical for a diode<br />
structure. To estimate the electrical parameters of the<br />
samples, the same data are plotted in a semilogarithmic<br />
scale (Figure 7) (negative values of the current are multiplied<br />
by -1).<br />
From the dark current measurements on a sample<br />
with EPD film (Figure 7a, curve 1), a diode contact<br />
barrier could be observed at 0.16 V. Overcoming this<br />
barrier changes the sign but does not lead to a considerable<br />
increase of the current. Therefore this effect more<br />
probably results from the influence of charged defects in<br />
the depletion region of the diode. A similar but smaller<br />
�
Figure 7: I-V characteristics measured in the dark (curve 1) and under illumination (curve 2) with monochromatic light (λ=560 nm, irradiance<br />
0.59 mW.cm -2 ) on ITO|MDMO-PPV|Al structures with a) EPD and b) SC MDMO-PPV film with about 300 nm thicknesses<br />
contact barrier was observed on some of samples with<br />
SC films. Therefore it is probably not only connected<br />
with the EPD process, but more probably related to the<br />
metal (oxide)/organic interface.<br />
Overcoming the second contact barrier at 0.6 V leads<br />
to an exponential increase of the current. Therefore, this<br />
value could be considered as the diode forward voltage<br />
drop (Ud). Applying reverse bias, the electrical current<br />
slightly grows. Comparing the I-V characteristics (Figure<br />
7a, curve 1) in forward and reverse directions (at a<br />
±1.1 V voltage), a rectification ratio of 1.6×10 4 is determined.<br />
It could be concluded from the dark current<br />
measurements that the ITO|MDMO-PPV|Al structures<br />
with EPD MDMO-PPV film exhibit a clear diode behavior.<br />
I-V characteristics were measured on the same sample<br />
under illumination (Figure 7a, curve 2) with monochromatic<br />
light (λ=560 nm, irradiance 0.59 mW.cm -2 ).<br />
For positive voltages applied to a sample with EPD film,<br />
the I-V characteristics measured are similar to those<br />
measured in the dark (Figure 7a, curve 1). For negative<br />
applied voltages, the photocurrent measured is more<br />
than 2 orders of magnitude higher than the dark current,<br />
which express a clear photovoltaic (PV) cell behaviour.<br />
I-V characteristics measured on samples with SC<br />
MDMO-PPV films are nonlinear and symmetrical (Figure<br />
7b).<br />
Processing the data for the EPD MDMO-PPV PV<br />
cell (Figure 7a, curve 2) determined short circuit current<br />
Jsc=8.13×10 -10 A.cm -2 and open circuit voltage<br />
Uoc=0.5 V. From the area confined by Jsc and Uoc,<br />
the dependence of the electrical power on the voltage<br />
is plotted on Figure 8, right Y axis. The maximum<br />
electrical power found were Pmax=7.56×10 -8 mW.cm -2<br />
5<br />
at voltage Ump=0.18 V and current Jmp=4.20×10-10 A.cm-2 .<br />
The power conversion efficiency η=1.3×10-5 % is calculated.<br />
The low value of the power conversion efficiency<br />
measured for organic semiconductors is not exceptional.<br />
A similar value of 4.2×10-4 % was reported<br />
in the literature for MEH-PPV/C60 donor-acceptor composite,<br />
where acceptor doping is used to improve the<br />
efficiency.[11]<br />
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�<br />
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�� ������ ��<br />
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Figure 8: Dependence of the power (right Y axis) and current (left Y<br />
axis) on the voltage applied, as calculated from Figure 7a, curve 2<br />
The power conversion efficiency could be considerably<br />
increased in some of the following ways: decreasing<br />
the film thickness; using donor-acceptor composite;<br />
and preparing multilayer structures to reduce the<br />
electrode/semiconductor contact barrier. Such investigations<br />
are beyond the scope of this work. It should<br />
be noted here that our previous work demonstrated the<br />
advantage of the EPD method for constructing a multilayer<br />
structure from films generally dissolved in the<br />
same solvent.[12] Similar multilayer structures could be
used further for decreasing the contact barrier.<br />
The dependences of the photocurrent on the incidental<br />
light power (irradiance) for EPD and SC MDMO-<br />
PPV films are plotted on Figure 9. In both cases, the<br />
photocurrent is proportional to Gγ , where G is the photogeneration<br />
rate and γ is the slope in the presented<br />
graphs. For the EPD and SC devices, a slope of 0.96<br />
and 1.03 is found, respectively. This result agrees with<br />
the literature data[13] and represents a monomolecular<br />
photogeneration mechanism.<br />
�<br />
�������������������������������� �� ������ �� �<br />
�� ��<br />
�� ���<br />
��<br />
��<br />
�� ��<br />
�����<br />
�����<br />
������������� �� ������� �� �<br />
Figure 9: Dependence of the photocurrent on the light intensity measured<br />
at a fixed wavelength of λ=560 nm on samples with EPD (curve<br />
1) and SC (curve 2) MDMO-PPV films<br />
It could be concluded that the EPD process of film<br />
preparation compared to the SC method does not change<br />
the photogeneration mechanism. The short circuit photocurrent<br />
measured from samples with SC films (Figure<br />
9, curve 2) follows the tendency observed in Figure 5<br />
and is about 1.5 times higher than EPD films (Figure<br />
9, curve 1). This result demonstrates the good photoconductive<br />
properties of the MDMO-PPV films, which<br />
could be observed at low contact barrier. It confirms that<br />
the key point for increasing the MDMO-PPV power efficiency<br />
is the optimization of the PV cell, thus increasing<br />
the Uoc. This could be achieved better with EPD<br />
(compared to SC) technique.<br />
4. Conclusion<br />
Absorption spectra of solution and suspension with<br />
the same MDMO-PPV concentration of 0.0033 g.l -1<br />
were measured. An observed broadening of the characteristic<br />
peak could be related to the formation of a solid<br />
phase in the suspension.<br />
Thin films of about 300 nm thicknesses were prepared<br />
by EPD from a suspension and SC from a solu-<br />
�<br />
�� �<br />
6<br />
tion with a concentration of 0.0033 g.l -1 and 8.95 g.l -1 ,<br />
respectively.<br />
The AFM surface morphology study shows that SC<br />
produces films with smooth and flat surfaces. The surfaces<br />
of the films obtained by EPD method are rougher.<br />
The fine film structure of the SC films does<br />
not result in better photovoltaic properties.<br />
ITO|MDMO-PPV|Al structure with EPD MDMO-<br />
PPV films behaves as photovoltaic cell; the same<br />
sample configuration with SC MDMO-PPV films acts<br />
more like a photoresistor. EPD and SC films exhibit the<br />
same mechanism of charge-carrier photogeneration.<br />
Acknowledgements<br />
The authors are grateful to Prof. Stanislav Nespurek,<br />
Institute of Macromolecular Chemistry, Academy of<br />
Sciences of the Czech Republic, for his support and for<br />
many clarifying and stimulating discussions.<br />
This work was supported by the South Moravian Region<br />
and the Seventh Framework Programme for Research<br />
and Development (grant SIGA 885), by the IGA<br />
Brno University of Technology via FCH/FEKT-S-11-2<br />
project, Grant Agency of the Czech Republic project<br />
No. P205/10/2280, project ”Centre for Materials Research<br />
at FCH BUT” No. CZ.1.05/2.1.00/01.0012 supported<br />
by ERDF and the National Fund of the Ministry<br />
of Education and Science, Bulgaria (grant DO 02-254).<br />
References<br />
[1] A. J. Heeger, Rev. Mod. Phys. 73 (2001) 681.<br />
[2] J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.<br />
Mackay, P. L. Burns, A. B. Holmes, Nature 347 (1990) 539.<br />
[3] K. Tada, M. Onoda, J. Phys. D: Appl. Phys. 42 (2009) 172001<br />
(5pp).<br />
[4] K. Tada, M. Onoda, Thin Solid Films 518 (2009) 711-713.<br />
[5] K. Tada, M. Onoda, Jpn. J. Appl. Phys. 42 (2003) L1279.<br />
[6] J. J. Miasik, A. Hooper, B. C. Tofield, J. Chem. Soc., Faraday<br />
Trans. 1 82 (1986) 1117.<br />
[7] T. Piok, S. Gemarith, C. Gadermaier, H. Plank, F. P. Wenzl, S.<br />
Patil, D. Neher, Adv. Mater. 15 (2003) 800.<br />
[8] K. Tada, M. Onoda, Synthetic Metals 152 (2005) 341-344.<br />
[9] P. van Hal, M. Wienk, J. Kroon, W. Verhees, L. Slooff, W. van<br />
Gennip, P. Jonkheijm, R. Nanesen, Adv. Mater. 15 (2003) 118.<br />
[10] Vu Quoc Trung, Dissertation: Electrophoretic deposition of<br />
semiconducting polymer metal/oxide nanocomposites and characterization<br />
of the resulting films, Dresden, 2006, p. 109.<br />
[11] Xiaoliang Mo, Toshiko Mizokuro, Hiroyuki Mochizuki, Nobutaka<br />
Tanigaki, Takashi Hiraga, Jpn. J. Appl. Phys. 44 no. 1B (2005)<br />
656-657.<br />
[12] D. Mladenova, D. Dimov, D. Karashanova, S. Boyadzhiev, T.<br />
Dobreva, D. Budurova, V. Sinigersky, I. Zhivkov, J. Optoelectron.<br />
Adv. Mater. 12 (2010) 1952-1956.<br />
[13] V. D. Mihailetchi, J. Wildeman, P. W. M. Blom, Phys. Rev. Lett.<br />
94 (2005) 126602.
17th International Summer School on Vacuum, Electron, and Ion Technologies (VEIT 2011) IOP Publishing<br />
Journal of Physics: Conference Series 356 (2012) 012040 doi:10.1088/1742-6596/356/1/012040<br />
Characterization of electrophoretic suspension for thin<br />
polymer film deposition<br />
D Mladenova 1,2,5 , M Weiter 1 , P Stepanek 3 , I Ouzzane 1 , M Vala 1 , V Sinigersky 4<br />
and I Zhivkov 1,2<br />
1 Centre for Materials Research, Faculty of Chemistry, Brno University of Technology,<br />
118 Purkynova, 612 00 Brno, The Czech Republic<br />
2 Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences,<br />
Acad. G. Bonchev Str. Bl. 101, 1113 Sofia, Bulgaria<br />
3 Department of Supramolecular Polymer Systems, Institute of Macromolecular<br />
Chemistry, Academy of Sciences of The Czech Republic,<br />
2 Heyrovskeho nam., 162 06 Praha 6, The Czech Republic<br />
4 Institute of Polymers, Bulgarian Academy of Sciences,<br />
Acad. G. Bonchev Str., Bl. 103A, 1113 Sofia, Bulgaria<br />
E-mail: dnl@clf.bas.bg<br />
Abstract. The optical absorption and fluorescence spectra of poly [2-methoxy-5-(3′,7′dimethyloctyloxy)-1,4-phenylenevinylene]<br />
toluene solutions and 50:50% toluene/acetonitrile<br />
suspensions show clearly distinguishable differences (e.g., peak broadening and shifting),<br />
which could be used for characterization of suspensions with different acetonitrile content. The<br />
dynamic light scattering (DLS) measurement of the suspensions prepared showed a particle<br />
size of 90 nm. Thin films with thicknesses of about 400 nm were prepared by electrophoretic<br />
deposition (EPD) and spin coating. As the films are very soft, a contactless optical profilometry<br />
techique based on chromatic aberration was used to measure their thickness. AFM imaging of<br />
spin coated and EPD films revealed film roughness of 20÷40 nm and 40÷80 nm, respectively.<br />
The EPD film roughness seems to be less than the suspension particle size obtained by DLS,<br />
probably due to the partial film dissolving by the toluene present in the suspension.<br />
1. Introduction<br />
Among the variety of the methods for “wet” polymer thin film deposition [1] (e. g. spin and dip<br />
coating, spray and Langmuir-Blodgett deposition, and ink-jet printing), the electrophoretic deposition<br />
(EPD) [2] has the advantages of using a diluted suspension for deposition of relatively thick films,<br />
ability for covering a large area and the unique property of separating the solidification stage<br />
(precipitation of a solid phase in the suspension) from the film formation stage, which takes place on<br />
the electrode.<br />
One of the main problems impeding the wide usage of EPD for thin polymer film preparation is the<br />
instability – the suspension particle size depends strongly on the precipitation conditions and tends to<br />
5 To whom any correspondence should be addressed.<br />
Published under licence by IOP Publishing Ltd<br />
1
17th International Summer School on Vacuum, Electron, and Ion Technologies (VEIT 2011) IOP Publishing<br />
Journal of Physics: Conference Series 356 (2012) 012040 doi:10.1088/1742-6596/356/1/012040<br />
grow with the time. This effect creates difficulties in controlling the EPD thin film structure and<br />
morphology.<br />
This study aims to establish methods for characterization of critical stages of the EPD process of<br />
deposition of thin poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV)<br />
films.<br />
2. Experimental details<br />
The EPD suspension with MDMO-PPV (Sigma-Aldrich, catalogue number 546461) concentration of<br />
0.0033 g l -1 and toluene/acetonitrile ratio of 50:50% (toluene 99.8%, acetonitrile 99.8%) was prepared<br />
and used immediately for measurement or deposition to prevent further particle coagulation.<br />
Dynamic light scattering (DLS) measurements of the suspension prepared were performed on a<br />
Malvern Zetasizer Nano ZS instrument equipped with a helium-neon laser; the scattering angle was<br />
173°. The data were processed taking into account the viscosity of the toluene/acetonitrile mixture<br />
with 50 % acetonitrile content [3].<br />
The EPD thin film was deposited on the positive electrode at a current of about 50÷70 µA.<br />
A solution with concentration of 8.95 g l -1 was prepared and spin coating at approx. 2500 rpm for<br />
60 s was carried out on a KW-4A Chemat Technology Inc. spin coater.<br />
The film thickness of the deposited MDMO-PPV films was determined by a MicroProf® FRT<br />
optical profilometer based on chromatic aberration. The method has the advantage of performing fast<br />
high-resolution contactless and non-destructive measurements, which is of paramount importance in<br />
our case of soft polymer films. The films were scratched by a sharpened tungsten wire then a thin<br />
(about 100 nm) Al film was deposited in vacuum to equalize the optical reflection from both scratched<br />
and unscratched areas.<br />
Surface morphology images of MDMO-PPV films were taken by NTEGRA Prima AFM. The<br />
measurements were carried out in semicontacting mode (frequency of 170 kHz, amplitude 80÷100 nm<br />
and scanning rate of 0.5 Hz).<br />
3. Results and discussion<br />
3.1. Suspension characterization<br />
3.1.1. Absorption spectra. Optical absorption spectra of a solution and a 50:50 % toluene/acetonitrile<br />
suspension with the same MDMO-PPV concentration of 0.0033 g l -1 are presented in figure 1. The<br />
solution spectrum (curve 1) consists of the characteristic MDMO-PPV absorption peak [4].<br />
In the suspension (curve 2) spectrum, a “red” shoulder appears which leads to a broadening of the<br />
peak. This effect could be related to the appearance of a precipitated solid phase during the suspension<br />
formation caused by the precipitating polar acetonitrile.<br />
Absorbance units<br />
0.3<br />
0.2<br />
0.1<br />
0% - 1<br />
50% - 2<br />
0.0<br />
350 400 450 500 550 600<br />
Wavelength, � [nm]<br />
Figure 1. Absorption spectra of a MDMO-<br />
PPV solution (curve 1) and a suspension<br />
with 50% acetonitrile content (curve 2).<br />
2<br />
Emission, [a.u.]<br />
7 0% -1<br />
50% -2<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
450 500 550 600 650 700 750 800<br />
Wavelength, � [nm]<br />
Figure 2. Fluorescence spectra of a MDMO-<br />
PPV solution (curve 1) and a suspension with<br />
50% acetonitrile content (curve 2).
17th International Summer School on Vacuum, Electron, and Ion Technologies (VEIT 2011) IOP Publishing<br />
Journal of Physics: Conference Series 356 (2012) 012040 doi:10.1088/1742-6596/356/1/012040<br />
3.1.2. Fluorescence spectra. More detailed information about the material under study can be obtained<br />
from the fluorescence spectra, as they present information from two processes – absorption and<br />
subsequent emission. In figure 2, fluorescence spectra of a MDMO-PPV solution (curve 1) and a<br />
suspension with 50 % acetonitrile (curve 2) are plotted. The maximum of the spectrum in the<br />
suspension is “red” shifted by about 30 nm. The spectrum shift reflects more precisely the formation<br />
of the solid phase.<br />
It could be concluded that effect of the peak<br />
0.8<br />
20<br />
broadening in the absorption spectra could be used<br />
15<br />
for a qualitative estimation of the suspension<br />
0.6<br />
10<br />
properties, while the peak shifts in the fluorescence<br />
5<br />
spectra yield more detailed information about the<br />
0.4<br />
0<br />
formation of a solid phase in the suspension.<br />
3.1.3. Dynamic light scattering. The size of the<br />
particles forming the solid-state phase in the<br />
toluene/acetonitrile suspension with 50 %<br />
acetonitrile content was estimated by DLS. The<br />
typical correlation function obtained (figure 3)<br />
allows a straightforward determination of the<br />
particle size. The inset in the figure presents the<br />
particle size distribution by the intensity obtained<br />
using inverse Laplace transformation. The data<br />
obtained indicate an average particle diameter of<br />
90 nm.<br />
3.2. Thin film characterization<br />
3.2.1. Optical profilometer film thickness<br />
measurement. Figure 4, a) and b) present scanned<br />
optical aberration images of a scratched<br />
MDMOPPV. The scratched area is clearly<br />
distinguished, which allows a satisfactory film<br />
thickness determination. A film thickness of about<br />
400 nm was determined by processing the data<br />
from a single profile line (figure 4, c). Optical-<br />
aberration 3D and 2D images can be used as a<br />
Correlation function<br />
a)<br />
b)<br />
c)<br />
0.2<br />
0.0<br />
10 0<br />
average size 90 nm<br />
10 1<br />
Intensity (%)<br />
10 1<br />
10 2<br />
Time (�s)<br />
10 2<br />
Size, d (nm)<br />
10 3<br />
Figure 3. DLS measured curves.<br />
10 3<br />
10 4<br />
Figure 4. Optical aberration scanning<br />
of a scratched MDMO-PPV film: a) – 3D,<br />
b) – 2D and c) – 1D images.<br />
preliminary film-surface morphology estimation.<br />
A film thickness of about 400 nm was measured by the same procedure for the spin-coated films.<br />
3.2.2. AFM imaging. 2D and 3D AFM images of EPD deposited films are shown in figure 5. The<br />
surface roughness observed is less than but comparable to the particle size as obtained by DLS.<br />
Despite the general assumption that the size of the particle in the suspension should be preserved after<br />
EPD of a film [5], partial dissolving of the particles deposited on the substrate by the solvent (50%<br />
toluene in the suspension) is possible, which decreases the film roughness. Thus, varying the<br />
toluene/acetonitrile ratio gives an opportunity to control the film roughness.<br />
For comparison, the AFM image of a spin-coated film with similar thickness is presented in<br />
figure 6. The picture shows a predominant film surface roughness of 20÷40 nm, which is smoother<br />
than the surface of the EPD film. The peaks observed with height approx. 50÷70 nm could be<br />
connected with the presence of undissolved or aggregated MDMO-PPV particles due to the relatively<br />
high solution concentration.<br />
3
17th International Summer School on Vacuum, Electron, and Ion Technologies (VEIT 2011) IOP Publishing<br />
Journal of Physics: Conference Series 356 (2012) 012040 doi:10.1088/1742-6596/356/1/012040<br />
Figure 5. AFM 2D and 3D images of an EPD<br />
film measured on a 5�5 µm scanned area.<br />
Figure 6. AFM 2D and 3D images of a spin<br />
coated film measured on 5�5 µm scanned area.<br />
Conclusions<br />
A combination of experimental methods was applied to the characterization of the different stages of<br />
the polymer electrophoretic deposition process. The methods could be used to control the suspension<br />
stability and the film structure and morphology, which are critical parameters during the EPD of thin<br />
polymer films for solar energy conversion purposes.<br />
Acknowledgments<br />
This work was supported by the South Moravian Region and the 7 th Framework Program for Research<br />
and Development (Grant SIGA 885), and by the Bulgarian National Science Fund at the Ministry of<br />
Education, Youth and Science (Grant DO 02-254).<br />
References<br />
[1] Tada K and Onoda M 2009 Molecular Crystals and Liquid Crystals 505 124-9<br />
[2] Boccaccini A R and Zhitomirsky I 2002 Current Opinion in Solid State and Materials Science<br />
6 251-60<br />
[3] Rltroulls G, Papadopoulos N and Jannakoudakls D 1986 J. Chem. Eng. Data 37 146-8<br />
[4] Quoc T V 2006 Electrophoretic deposition of semiconducting polymer metal/oxide<br />
nanocomposites and characterization of the resulting films (Dresden Technischen<br />
Universität Dresden Germany) p 109<br />
[5] Landfester K, Montenegro R, Scherf U, Gqnter R, Asawapirom C, Patil S, Neher D and<br />
Kietzke T 2002 Adv. Mater. 14 651<br />
4
Materials Science and Engineering B 165 (2009) 148–152<br />
Contents lists available at ScienceDirect<br />
Materials Science and Engineering B<br />
journal homepage: www.elsevier.com/locate/mseb<br />
Morphology and properties of thin films of diketopyrrolopyrrole derivatives<br />
Martin Weiter a , Ota Salyk a , Pavel Bednáˇr a , Martin Vala a ,Jiˇrí Navrátil a,∗ , Odlˇrich Zmeˇskal a ,<br />
Jan Vyňuchal b , Stanislav Luňák Jr. c<br />
a Brno University of Technology, Faculty of Chemistry, Purkynova 118, 612 00 Brno, Czech Republic<br />
b Research Institute of Organic Syntheses, Rybitví 296, 533 54 Rybitví, Czech Republic<br />
c Department of Technology of Organic Compounds, Faculty of Chemical Technology, University of Pardubice, Studentská 95, 530 09 Pardubice, Czech Republic<br />
article info<br />
Article history:<br />
Received 29 August 2008<br />
Received in revised form 4 September 2009<br />
Accepted 23 September 2009<br />
Keywords:<br />
Surface morphology<br />
Organic substances<br />
Chemical vapour deposition<br />
Electron micros<strong>copy</strong><br />
Thin films<br />
1. Introduction<br />
abstract<br />
Organic electronic devices based on different types of organic<br />
semiconductors such as polymers, oligomers, dendrimers, dyes<br />
and pigments are already entering the commercial world. Among<br />
them the small organic molecule thin films are the subject of<br />
intense research activity as they provide high quality thin films and<br />
nanostructures. This paper deals with derivatives of 3,6-diphenyl-<br />
2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4 diones, commonly referred<br />
as DPP, which constitute a promising class of small molecular semiconductors.<br />
These compounds have been the object of intensive<br />
research for pigment application since 1974 as their exhibit a variety<br />
of shades in the solid state and especially chemical, light and<br />
thermal stability [1,2]. DPP itself has a high molar absorption coefficient,<br />
as well as high quantum yield of fluorescence, therefore low<br />
molecular weight derivatives of DPP have been extensively studied<br />
on their optical and photophysical properties [3–5]. Potential<br />
application of DPP derivatives as a luminescent media in a polymer<br />
matrices [6–8], solid-state dye lasers [9], OLED devices [10]<br />
and organic field-effect transistors [11] was reported. Recently a<br />
new application as hydrogen sensing material utilizing variable<br />
conductivity of DPP pyridyl derivative occurred [12].<br />
Thin films of small molecular semiconductors are usually prepared<br />
by means of a variety of complex techniques including<br />
physical or chemical vapour deposition, organic molecular beam<br />
∗ Corresponding author. Tel.: +420 541149396; fax: +420 541211697.<br />
E-mail address: xcnavratil@fch.vutbr.cz (J. Navrátil).<br />
0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.mseb.2009.09.017<br />
Thin layers of five derivatives of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4 diones, which constitute<br />
a promising class of small molecular semiconductors, were investigated. The morphology of the<br />
thin layers prepared by both vacuum evaporation and spin casting were studied using Scanning Electron<br />
Micros<strong>copy</strong> and Atomic Force Micros<strong>copy</strong>. The relation between molecular structure and their thin films<br />
morphology was found. The electroluminescence behaviour of selected derivatives is also discussed with<br />
regard to morphology studies.<br />
© 2009 Elsevier B.V. All rights reserved.<br />
epitaxy or solution-based deposition techniques. The performance<br />
of small molecular organic devices has been shown to be highly<br />
sensitive to film morphology and processing conditions. Often, the<br />
solution deposited active layers of devices (e.g. spin cast films)<br />
exhibit a high portion of microcrystallites and aggregates whereas<br />
the vapour deposition techniques provide high quality crystalline<br />
films, characterized by improved charge transport properties compared<br />
with those of solution deposited films. The relationship<br />
between the organic thin film morphology and the device performance<br />
is nowadays subject of the research activity.<br />
In this study we investigated a group of five DPP derivatives<br />
depicted in Fig. 1. In order to increase solubility required for low<br />
cost solution-based casting, the basic DPP structure was modified<br />
by different alkyl substitutions on the nitrogen atoms. In addition,<br />
the electronic properties of one derivative (DPP VI) were altered by<br />
di-donor substitution.<br />
Our previous studies based on quantum chemical calculations<br />
showed that both symmetrically (DPP III, DPP V, DPP VI) and nonsymmetrically<br />
(DPP II, DPP IV) substituted derivatives have rotated<br />
phenyl groups, as opposed to the non-substituted basic structure,<br />
which is nearly perfectly planar. The substitution of alkyl<br />
chain leads to the rotation of the adjacent phenyl group. The symmetrically<br />
substituted derivatives have thus rotated both phenyls.<br />
The rotation was independent on the length of the alkyl chain.<br />
These rotations subsequently modified absorption and photoluminescence<br />
spectra [13]. The theoretical results were confirmed<br />
by experimental optical characterization of solutions of selected<br />
derivatives. They showed that the increasing phenyl torsion leads<br />
to reduction of the conjugation extend and subsequently to a slight
M. Weiter et al. / Materials Science and Engineering B 165 (2009) 148–152 149<br />
Fig. 1. Structure of basic DPP molecule and prepared derivatives and their temperatures of melting point.<br />
hypsochromic shift of the absorption and bathochromic shift of the<br />
photoluminescence spectra as predicted from the calculations. The<br />
vibrational structure was less pronounced with increasing phenyl<br />
torsion and larger Stokes shift was observed. Simultaneously, the<br />
molar absorptivity decreased as the deformation increased. On the<br />
other hand, the measured fluorescence quantum yields were modified<br />
only slightly [13]. These properties, together with chemical,<br />
light and thermal stability predestine them as potential candidates<br />
for different optoelectronic applications. Therefore the film forming<br />
and electroluminescence properties of these materials were investigated<br />
and the question of relation between the chemical structure<br />
and morphology of prepared thin films was addressed.<br />
2. Experimental<br />
The DPP derivatives were synthesized and analyzed to confirm<br />
the molecule structure by A Bruker AMX 360 NMR spectrometer,<br />
ion trap mass spectrometer MSD TRAP XCT equipped with<br />
APCI, EA 1108 FISONS instrument for elemental analysis and<br />
Fourier transform infrared spectrometer (see [13] for details).<br />
Thin films were prepared by spin coating and by vacuum evaporation<br />
method. Low resistivity silicon substrates were used<br />
for morphology studies, whereas indium tin oxide (ITO) coated<br />
low alkali Corning glass was used for electro-optical and quartz<br />
glass for optical characterization. Thin layers spin-casted from<br />
chloroform–toluene solution (7:3) were typically 100–200 nm<br />
thick as measured by elipsometry. The vacuum deposition of<br />
200 nm thick layers was carried out at a pressure of 1 × 10 −4 Pa<br />
with deposition rate from 0.2 to 0.5 nm/s. The substrate temperature<br />
was maintained constant during the deposition process, being<br />
in the range 250–420 K. The annealing of the films was carried<br />
out in vacuum at various temperatures with regard to melting<br />
point of annealed substances. The morphology of the samples<br />
was investigated by Scanning Electron Micros<strong>copy</strong> (SEM) and by<br />
Atomic Force Micros<strong>copy</strong> (AFM). Multilayered sandwiched structures<br />
consisting of hole transport layer, emissive layer and electron<br />
transport layer were prepared for electroluminescence characterization.<br />
ITO covered glass substrates were used as transparent<br />
anodes. Subsequently, a hole-transporting layer of poly(3,4ethylenedioxythiophene):poly(styrenesulfonic<br />
acid) (PEDOT:PSS)<br />
and active electroluminescence layer of DPP derivative were spincasted.<br />
The structure was completed by vacuum deposition of<br />
electron-transporting layer of aluminium tris(8-hydroxyquinoline)<br />
(Alq 3) covered by vacuum deposited 100 nm thick aluminium top<br />
electrode.<br />
Fig. 2. Comparison of the structure of the thin layers of the basic DPP material DPP I<br />
prepared at different substrate temperatures as probed by SEM: (a) 255 K, (b) 290 K,<br />
(c) 373 K and (d) 423 K.
150 M. Weiter et al. / Materials Science and Engineering B 165 (2009) 148–152<br />
3. Results and discussion<br />
3.1. Morphology<br />
Thin layers prepared by both spin casting and vacuum depositions<br />
were polycrystalline but the shapes and dimensions of<br />
crystallites were found to be dependent on the structure of the<br />
materials and temperature of the substrate during deposition. The<br />
effect of substrate temperature T S during vacuum evaporation on<br />
film morphology of the basic DPP structure (DPP I) which exhibits<br />
highest melting point is illustrated in Fig. 2. At lower substrate<br />
temperatures (T S = 255 K and T S = 290 K) the dominating forms are<br />
oval crystallites forming island structure, whereas at higher T S<br />
(T S = 373 K and T S = 423 K) chaotically oriented ribbon-like crystallites<br />
of length increasing up to 1 �m prevail.<br />
The formation of larger clusters with increasing T S can be<br />
described as a thermally activated process in the coarse of which<br />
a particle impinged onto the substrate surface can move laterally<br />
unless it loses its kinetic energy and finds the energy potential<br />
well. It is usually after joining with other particles and this particle<br />
– molecules – cluster is the nucleus of future columnar or<br />
island structure. The high deposition temperature also significantly<br />
decreases the adhesion of deposited layers. It is caused by subsequent<br />
shrinkage during cooling, which is more pronounced for<br />
organic layers than for substrates and it resulted in peeling observable<br />
especially at T S = 423 K. The free crystallites endings in space<br />
give high reflection in SEM due to the charging and therefore they<br />
appear as bright spots.<br />
The studies made with different substrates show that the morphology<br />
is practically independent on the nature of the substrate.<br />
The key impact on the morphology of thin layers was found to be<br />
Fig. 3. Comparison of the structure of the thin films evaporated at room temperature<br />
as probed by SEM: (a) basic DPP molecule DPP I, (b) symmetrically substituted DPP<br />
III, (c) monosubstituted DPP IV, and (d) functionalized symmetrically substituted<br />
DPP VI.<br />
based on the substitution of central DPP unit by alkyl side chains<br />
with different lengths. Seeing that the influence of alkyl chain<br />
length is minor, the crucial factor is the type of substitution, which<br />
can be unsymmetrical (one substituted alkyl chain) in the case of<br />
Fig. 4. Surfaces of the thin films evaporated at room temperature as studied by AFM: (a) basic DPP molecule DPP I, (b) symmetrically substituted DPP III, (c) monosubstituted<br />
DPP IV, and (d) functionalized symmetrically substituted DPP VI.
Fig. 5. Current–voltage characteristics and total electroluminescence intensity of<br />
derivative DPP VI and the device structure as an insert.<br />
derivatives DPP II, DPP IV or symmetrical (two substituted alkyl<br />
chains) in case of derivatives DPP III, DPP V and DPP VI. A comparison<br />
of the structure of evaporated thin films probed by SEM<br />
is depicted in Fig. 3, the surfaces of the layers studied by AFM are<br />
shown in Fig. 4.<br />
The figures reveal that the symmetrically substituted derivatives<br />
form planar large crystallites with sizes increasing up to 1 �m,<br />
whereas the asymmetrically substituted derivatives form highly<br />
rough fiber crystallites, their lengths being up to 1 �m. It was found<br />
that the crystal size is slightly dependent on the alkyl chain length;<br />
in particular the derivative DPP II monosubstituted by butyl (C 4H 9)<br />
side chain forms smaller fiber crystallites in comparison with their<br />
analog DPP IV substituted by heptyl (C7H 15). This effect is worse<br />
distinguishable at planar large crystallites formed by symmetrically<br />
substituted derivatives DPP III and DPP V. Spontaneous crystallization<br />
of these materials after deposition was observed, but should<br />
be inhibited by annealing which also stabilizes the interfaces. The<br />
morphology of the samples prepared by spin casting was also studied,<br />
but no significant differences were recognized.<br />
The observed behaviour can be explained according to the<br />
molecular structure. It is worth to note, that the basic DPP molecule<br />
is almost planar and its intermolecular energy was calculated [14]<br />
deriving from �–� stacking forces (42%), hydrogen bonds (21%),<br />
electrostatic attraction and other minor contribution. Substitution<br />
of the alkyl group led to the rotation of the adjacent phenyl group<br />
and therefore not only caused loss of planarity and effective conjugation<br />
but also interfered with the intermolecular �–� stacking<br />
of the aryl rings. Introduction of the second alkyl also rotates the<br />
second phenyl which intensifies the above phenomena. This is<br />
exemplified as the decrease of melting point starting from 580 K for<br />
unsubstituted DPP DPP I, through 523 K and 484 K for monosubstituted<br />
DPP II and DPP IV, down to 393 K and 383 K for symmetrically<br />
substituted DPP III and DPP V.<br />
3.2. Electroluminescence<br />
On the basis of the findings described above symmetrically<br />
substituted derivatives DPP III, DPP V and DPP VI resulted as<br />
suitable for electroluminescence (EL) characterization. The typical<br />
current–voltage (I–V) characteristic together with electroluminescence<br />
(EL) intensity of DPP VI derivative is depicted in Fig. 5 in<br />
double logarithmic scale. It shows that the turn-on voltage for this<br />
diode is ∼3 V. At this voltage the previously ohmic nature of the<br />
I-V characteristic changes to the space-charge-limited, where the<br />
current flow is bulk-limited. The low value of the turn-on voltage<br />
implies that reasonable charge balancing was achieved due to<br />
M. Weiter et al. / Materials Science and Engineering B 165 (2009) 148–152 151<br />
Fig. 6. Normalized electroluminescence spectra of symmetrically substituted<br />
derivatives.<br />
the barrier reducing pre-contact layers (PEDOT:PSS and Alq3). This<br />
allowed us to measure also the spectrally resolved electroluminescence<br />
of the selected derivatives, see Fig. 6. The spectral position of<br />
the EL coincides with the position of fluorescence of the respective<br />
thin films.<br />
The normalized electroluminescence spectra show that the electroluminescence<br />
depends on the substitution used rather than on<br />
the length of the alkyl chains used, which is in accordance with the<br />
quantum chemical calculations and optical spectros<strong>copy</strong> measurement<br />
[1]. The substitution of piperidine groups on phenyls led to the<br />
large bathochromic shift of the absorbance and photoluminescence<br />
spectra and thus also to the observed EL. The EL efficiency is low,<br />
as only basic unoptimized OLED structure was used in this device<br />
structure ITO/PEDOT:PSS (60 nm)/U29 (150 nm)/Alq3 (40 nm)/Al<br />
(200 nm) the thickness was obtained by elipsometry. However, the<br />
EL response depends on many material parameters and the optimization<br />
of multilayered structure is necessary.<br />
4. Conclusions<br />
The morphology of thin films of DPP derivatives prepared by<br />
both evaporation and spin coating methods were studied by SEM<br />
and AFM. It was found that the crucial factor affecting morphology<br />
is type of substitution of basic DPP by alkyl chains, which can<br />
be unsymmetrical or symmetrical. The symmetrically substituted<br />
derivatives form planar large crystallites, whereas the asymmetrical<br />
ones form highly rough fiber crystallites. The symmetrically<br />
substituted derivatives showed a significant electroluminescent<br />
behaviour, nevertheless the optimization of multilayered structure<br />
is necessary to achieve high electroluminescence efficiency.<br />
Acknowledgment<br />
This work was supported by the Ministry of Industry and Trade<br />
of the Czech Republic by project FT-TA3/048 and by the Grant<br />
Agency of Academy of Science via project A401770601.<br />
References<br />
[1] O. Wallquist, Diketopyrrolopyrrole (DPP) pigments, in: High Performance Pigments,<br />
Wiley–VCH publishers, Weinheim, 2002.<br />
[2] M. Herbst, K. Hunger, Industrial Organic Pigments, Wiley–VCH publishers,<br />
Weinheim, 1993.<br />
[3] M. Fukuda, K. Kodama, H. Yamamoto, K. Mito, Dyes Pigments 53 (2002) 67–72.<br />
[4] J. Mizuguchi, A.C. Rochat, J. Imag. Sci. 32 (1988) 135.<br />
[5] J. Mizuguchi, J. Phys. Chem. A 104 (2000) 1817.<br />
[6] G. Lange, B. Tieke, Macromol. Chem. Phys. 200 (1999) 106–112.<br />
[7] T. Beyerlein, B. Tieke, Macromol. Rapid Commun. 21 (2000) 182–189.
152 M. Weiter et al. / Materials Science and Engineering B 165 (2009) 148–152<br />
[8] M. Smet, B. Metten, W. Dehaen, Tetrahedron Lett. 42 (2001) 6527–6530.<br />
[9] M. Fukuda, K. Kodama, H. Yamamoto, K. Mito, Dyes Pigments 63 (2004)<br />
115–125.<br />
[10] Patents WO 2004/090046, US 2005/0008892.<br />
[11] H. Yanagisawa, J. Mizuguchi, S. Aramakil, Y. Sakai, Jpn. J. Appl. Phys. 47/6 (2008)<br />
4728–4731.<br />
[12] H. Takahashi, J. Mizuguchi, J. Appl. Phys. 034908 (2006) 100.<br />
[13] M. Vala, M. Weiter, J. Vyňuchal, P. Toman, S. Luňák Jr., J. Fluoresc. (2008),<br />
doi:10.1007/s10895-008-0370-x.<br />
[14] D. Thetford, J. Cherryman, A.P. Chorlton, R. Docherty, Dyes Pigments 63 (2004)<br />
259–276.
Special Issue: Research Article<br />
Received: 10 January 2008, Accepted: 25 May 2008, Published online in Wiley InterScience: 18 November 2008<br />
(www.interscience.wiley.com) DOI: 10.1002/pat.1260<br />
Model of the influence of energetic disorder<br />
on inter-chain charge carrier mobility in<br />
poly[2-methoxy-5-(2 0 -ethylhexyloxy)p-phenylene<br />
vinylene]<br />
Petr Toman a *, Stanislav Nesˇpu˚rek a,b , Martin Weiter b , Martin Vala b ,<br />
Juliusz Sworakowski c , Wojciech Bartkowiak c and Miroslav Mensˇík a<br />
The theoretical model of the inter-chain charge carrier mobility in poly[2-methoxy-5-(2(-ethylhexyloxy)-p-phenylene<br />
vinylene] (MEH–PPV) doped with polar additive is put forward. The polymer chain states of a charge carrier were<br />
calculated by means of diagonalization of a tight-binding Hamiltonian, which includes disorder in both the local<br />
energies and transfer integrals. Consequently, the inter-chain charge carrier transport is taking place on a spatially<br />
and energetically disordered medium. Because it is believed that the additive does not significantly influence the<br />
polymer supramolecular structure, the polymer conformations were simplified as much as possible. On the other<br />
hand, the energetic disorder is rigorously described. The transfer rates between the polymer chains were determined<br />
using the quasi-classical Marcus theory. The model considered the following steps of the charge carrier transport: the<br />
charge carrier hops to a given polymer chain. Then, the charge carrier thermalizes to the Boltzmann distribution over<br />
all its possible states on this chain. After that, the charge carrier hops to any possible state on one of the four nearest<br />
neighboring chains. The results showed that the inter-chain charge carrier mobility is very strongly dependent on the<br />
degree of the energetic disorder. If the energetic disorder is doubled from 0.09 to 0.18 eV, the mobility decreases by<br />
two or three orders of magnitude. Copyright ß 2008 John Wiley & Sons, Ltd.<br />
Keywords: charge transport; conducting polymers; photochromism; Monte Carlo simulation; quantum chemistry<br />
INTRODUCTION<br />
Recently, poly[2-methoxy-5-(20-ethylhexyloxy)-p-phenylene vinylene]<br />
(MEH–PPV) was intensively studied because it exhibits<br />
properties that are interesting for many applications in material<br />
science. [1–3]<br />
It has attracted considerable attention due to its<br />
semiconductivity, photosensitivity, sensing properties, electroluminescence,<br />
etc. The numerous applications, including sensors<br />
for different gases, solar cells, field-effect transistors, electroluminescent<br />
diodes, etc., were mentioned. [4–6] For the efficient<br />
functioning of many of these devices, charge carrier transport<br />
plays an important role. [7] The charge carrier mobility must be as<br />
high as possible. Thus, the studies of transport properties<br />
represent an important point of the research.<br />
Charge carrier transport in 3D polymeric materials consists of<br />
charge moving through the chain (molecular wire) and of<br />
inter-chain hopping. [8] The transport states of the both features<br />
are usually characterized by geometrical and energetic disorder;<br />
thus, the charge moving is influenced by hopping mechanism<br />
through the intra-chain states (tail states) and by inter-molecular<br />
hops. The energy distribution of the transport states is strongly<br />
influenced by the dispersion of polarization energy. Note that<br />
intra-chain hopping states are usually shallower. It was found that<br />
the mobility value can be influenced by doping. [9] Some highly<br />
polar additives decrease the value of charge carrier mobility in<br />
Polym. Adv. Technol. 2009, 20 263–267 Copyright ß 2008 John Wiley & Sons, Ltd.<br />
the doped polymer. The occurrence of polar species in the<br />
polymer material results in the broadening of both types of the<br />
energy distribution. In this paper, we describe the background<br />
and energy distribution of the transport hopping states and the<br />
influence of the dipolar additives on charge carrier transport.<br />
* Correspondence to: P. Toman, Institute of Macromolecular Chemistry, Academy<br />
of Sciences of the Czech Republic, v.v.i., Máchova St. 7, 120 00 Praha 2,<br />
Czech Republic.<br />
E-mail: toman@imc.cas.cz<br />
a P. Toman, S. Nesˇpu˚rek, M. Mensˇík<br />
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech<br />
Republic, v.v.i., Heyrovsky´ Sq. 2, 162 06 Prague 6, Czech Republic<br />
b S. Nesˇpu˚rek, M. Weiter, M. Vala<br />
Faculty of Chemistry, Brno University of Technology, Purkyňova 118, 612 00<br />
Brno, Czech Republic<br />
c J. Sworakowski, W. Bartkowiak<br />
Institute of Physical and Theoretical Chemistry, Wroclaw University of<br />
Technology, Wyb. Wyspiańskiego 27, 50-370 Wroclaw, Poland<br />
Contract/grant sponsor: Czech Science Foundation; contract/grant number:<br />
203/06/0285.<br />
Contract/grant sponsor: Wroclaw University of Technology.<br />
263
264<br />
INTRA-CHAIN STATES AND CHARGE<br />
TRANSPORT THROUGH THE<br />
MOLECULAR WIRE<br />
The polymer chain states can be calculated by means of a<br />
method similar to the modeling of the on-chain charge carrier<br />
mobility presented in our previous paper. [10] The polymer chain is<br />
modeled by a sequence of N sites corresponding to the repeat<br />
units (alternating phenylenes and vinylenes). The charge carrier<br />
motion on such a chain can be described by the tight-binding<br />
approximation Hamiltonian:<br />
H ¼ XN<br />
n¼1<br />
where a n and a þ n<br />
"na þ n an bn;nþ1 a þ nþ1 an þ a þ n anþ1 (1)<br />
are annihilation and creation operators of a<br />
charge carrier at an nth site, en the energy of a charge carrier<br />
localized at this site, and bn,n þ 1 is the transfer integral between<br />
the sites n and n þ 1. Both quantities en and bn,n þ 1 are influenced<br />
by the random structure of the polymer chain and its<br />
surrounding.<br />
Using this Hamiltonian (1) with the molecular parameters en<br />
and b n,n þ 1 and the hole (MEH–PPV is the hole transporting<br />
material) wave function<br />
jcðtÞi ¼ XN<br />
cnðtÞjni (2)<br />
n¼1<br />
where jni is a state located at nth site; the time-dependent<br />
Schrödinger equation can be written in the form<br />
HjcðtÞi ¼ ih @<br />
@t cðtÞ j i (3)<br />
This equation can be solved either by means of the direct<br />
numerical integration [10] or using stationary states jwni, which are<br />
solutions of the time-independent Schrödinger equation<br />
H j’ ni<br />
¼ En j’ ni<br />
(4)<br />
The solution of eqn (3) is then given by the relation<br />
jcðtÞi ¼ XN<br />
n¼1<br />
kne iEnt<br />
h j’ ni<br />
(5)<br />
in which the time-independent coefficients kn can be found from<br />
the initial conditions c 0(t ¼ 0) ¼ 1 and c i(t ¼ 0) ¼ 0 for any i 6¼ 0.<br />
While both approaches, i.e. direct numerical integration of eqn<br />
(3) and solution using stationary states, yield identical charge<br />
carrier wave function jc(t)i, the latter also provides the spectrum<br />
of the energy eigenvalues En and eigenstates jwni for charge<br />
carrier.<br />
For the case of polar additive, the transfer integrals b n,nþ1 and<br />
the energetic disorder of en were calculated according to the<br />
model described in Reference 10. Because the mutual interactions<br />
of the additive molecules were neglected, the standard<br />
deviation s(e n) of the e n distribution was proportional to the<br />
dipole moment m of the additive and to the square root of the<br />
additive concentration Hc. The energetic disorder can be easily<br />
quantified by the standard deviation of the e n distribution, which<br />
is broadened in this case. However, there are also some other<br />
characteristics, like site-to-site correlation, that are important for<br />
Figure 1. Diagram of the polymer valence band. The value of the<br />
energetic disorder is indicated by the standard deviation s(e). Curve<br />
s(e) ¼ 0 corresponds to the as prepared polymer and curves s(e) ¼ 0.18<br />
and 0.36 eV to the polymer doped with the additives with different dipole<br />
moments. If the additive concentration is 0.4 nm 1 , these values correspond<br />
to the additive dipole moments 6 and 12 D, respectively.<br />
obtaining correct results. For typical values of m ¼ 12 D and<br />
c ¼ 1 10 4 A˚ 3 , the standard deviation s(en) ¼ 0.18 eV. Note that<br />
this model of the energetic disorder does not take into account<br />
the polarization effects of the reaction field of the dipoles.<br />
Figure 1 shows a typical diagram of the valence band of the as<br />
prepared and doped polymer. An increasing energetic disorder<br />
leads to the broadening of the originally sharp valence band edge<br />
and formation of the tail states in the gap. These states, owing to<br />
their relatively low density and consequently a weak connectivity,<br />
behave as hole traps. Furthermore, the valence states in a doped<br />
polymer are less delocalized along the polymer chain than the<br />
states in the as prepared polymer (as shown in Fig. 2).<br />
THEORETICAL MODEL OF THE<br />
INTER-CHAIN HOPPING<br />
P. TOMAN ET AL.<br />
The description of the inter-chain charge carrier motion in<br />
conjugated polymers is difficult due to the absence of any<br />
Figure 2. Delocalization of the charge carrier chain states on the polymer<br />
chain. The graphs on the left describe the as prepared polymer and<br />
the graphs on the right the doped polymers. Top graphs show the HOMOs<br />
and bottom graphs show the orbitals from the middle of the upper<br />
branch of the valence band.<br />
www.interscience.wiley.com/journal/pat Copyright ß 2008 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 263–267
INTER-CHAIN CHARGE TRANSPORT IN DOPED MEH–PPV<br />
well-ordered crystal structure typical for molecular crystals or<br />
inorganic semiconductors. Conjugated polymer chains are<br />
twisted and kinked and possess a rather high ratio of chemical<br />
defects. The structural disorder of a polymer chain is accompanied<br />
by an energetic disorder, i.e. some kind of more or less<br />
random distribution of the energies of the energetic states of the<br />
chain, in which the charge carrier can be found. The disordered<br />
chain structure results in a disordered supramolecular structure<br />
of the material. Thus, the inter-chain charge carrier transport is<br />
realized in spatially and energetically disordered systems.<br />
Moreover, such a system is dynamic and undergoes relaxation<br />
related to the charge carrier motion, which is called a polaron<br />
formation. Hence, it is hardly possible to describe the charge<br />
carrier transport completely from the first principles. The aim of<br />
the theoretical model presented in this paper is to describe<br />
charge carrier mobility changes related to the polar additive.<br />
Quantum chemical calculations showed that the polar additive<br />
molecules considerably influence the on-chain electrostatic<br />
potentials. Consequently, the energies and shape of the polymer<br />
chain states are modified. It is assumed that this reaction does not<br />
significantly alter the scale of the spatial supramolecular disorder.<br />
For these reasons, the supramolecular structure was simplified as<br />
much as possible.<br />
The simplified three-dimensional polymer structure can be<br />
schematically described by an array of equidistantly placed<br />
parallel polymer chains of a given length N (as shown in Fig. 3). It<br />
should be noted that the length N is chosen rather as a typical<br />
length of a conjugated segment without chemical or structural<br />
defects (several hundreds of repeat units) than the length of the<br />
real polymer chain. Thus, the continuous disorder model of the<br />
torsional disorder is combined with a rod-like model of the chain<br />
defects that completely stops the on-chain transport. The<br />
additive molecules are randomly placed between these chains<br />
without further specifications of their positions and orientations.<br />
Each polymer chain is characterized by a set of its eigenstates, in<br />
which a charge carrier can be found. No correlation between<br />
chains is assumed. Only the transitions between four nearest<br />
neighbor chains are taken into account. The corresponding<br />
transfer rates of hopping from the chain A are denoted in Fig. 3 by<br />
the letters L i, R i, D i, and U i.<br />
It should be remarked that the number M of the chains in each<br />
direction should be large enough (up to several hundreds) in<br />
order to neglect the influence of the finite size of the model<br />
system. Since the total number of the chains M 2 was taken to be<br />
very large, some periodicity was introduced. Only 400 independent<br />
chains were calculated and in this way an ‘‘elementary cell’’<br />
Figure 3. Simplified polymer structure for inter-chain charge carrier<br />
mobility modeling.<br />
20 20 chain was created. This ‘‘elementary cell’’ was then<br />
periodically repeated in both directions in order to get the whole<br />
considered model system of M 2 chains.<br />
The inter-chain transfer is slow in comparison with the charge<br />
carrier thermalization, which occurs typically in times of several<br />
picoseconds. Thus, it is possible to expect full charge carrier<br />
thermalization to the Boltzmann distribution over all states of the<br />
given chain between two subsequent inter-chain hops. Note that<br />
under usual experimental conditions, it is almost impossible that<br />
there is more than one charge carrier on one chain segment.<br />
Therefore, if there is a charge carrier on the given chain A, the<br />
probability of occupation of the state i at temperature T is<br />
piðEiÞ ¼<br />
expð Ei=kTÞ<br />
ZT ð Þ<br />
where Ei is the energy of state i, k the Boltzmann constant, and<br />
Z(T) is the partition function over all states of the chain A.<br />
The rate for charge transfer between an initial state i with the<br />
energy Ei on the chain A and a state j with the energy Ej on an<br />
adjacent chain B can be evaluated using Marcus equation<br />
vi!j ¼ J2 rffiffiffiffiffiffiffiffi<br />
ij p Ei Ej l<br />
exp<br />
h lkT<br />
2<br />
!<br />
(7)<br />
4lkT<br />
where l is the reorganization energy and Jij is the effective charge<br />
transfer integral. The use of the quasi-classical Marcus concept [11]<br />
instead of the quantum mechanical one is justified by the fact<br />
that the inter-chain transfer integrals Jij are much smaller than the<br />
reorganization energy l. Consequently, the charge carrier<br />
residing on a given chain loses any quantum coherence due<br />
to its interaction with the phonon reservoir before jumping to<br />
another chain.<br />
The reorganization energy can be in principle calculated using<br />
standard quantum chemical methods. Knowing the expansion<br />
coefficients ci,a of each chain state i, it is possible to calculate the<br />
transfer integrals Jij between states i and j located on two<br />
different chains<br />
Jij ¼ X<br />
(8)<br />
a;b<br />
ci;acj;bJabdab<br />
where a and b are repeat units of chains i and j, respectively, J ab<br />
the transfer integral between the corresponding repeat units of<br />
adjacent chains (as shown in Fig. 4), and dab is Kronecker delta.<br />
While the expansion coefficients c i,a are strongly dependent on<br />
the chain energetic disorder, the transfer integrals Jab can be<br />
taken constant, since no significant influence of the photochromic<br />
reaction on the spatial supramolecular disorder is assumed.<br />
The value of J ab can be estimated from the energy splitting<br />
between HOMOs of two benzene molecules located at the typical<br />
interchain distance ( 10 A˚). Since we believe that the spatial<br />
Figure 4. Detail of two neighboring polymer chains A and B. Letters a<br />
and b denote repeat units, i and j are polymer chain states.<br />
Polym. Adv. Technol. 2009, 20 263–267 Copyright ß 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/pat<br />
(6)<br />
265
266<br />
supramolecular disorder does not play a significant role,<br />
distribution of Jab was taken as a standard normal distribution<br />
multiplied by a typical value of s(Jab) ¼ 10 4 eV. Note that this is<br />
just a multiplication factor that can be used to scale the<br />
calculated absolute values.<br />
From the charge transfer rates ni ! j between two states and the<br />
thermalized occupation probability pi of the initial state the<br />
charge transfer rates n A ! B between two adjacent chains A and B<br />
can be easily calculated<br />
nA!B ¼ X<br />
i 2 Aj2 B<br />
piðEiÞni!j whereas summation goes through all states of the respective<br />
chain.<br />
Thus, our model assumes the following steps of the inter-chain<br />
charge carrier transport:<br />
charge carrier moving to any possible state on the chain A,<br />
charge carrier thermalization over all its possible states on the<br />
chain A,<br />
charge carrier hops to any possible state on one of the four<br />
nearest neighboring chains; the transfer rate is given by the<br />
Marcus eqn (7).<br />
Determining the n A ! B values makes possible to construct the<br />
master equation describing the inter-chain charge carrier motion<br />
dPAðtÞ<br />
dt<br />
(9)<br />
X<br />
¼ vA!BPBðtÞ (10)<br />
B<br />
where PA(t) is the probability of finding the given charge carrier<br />
on the chain A. Att¼0 is the charge carrier localized on a single<br />
chain, i.e. P0(t ¼ 0) ¼ 1 and all other PA equal to zero. The master<br />
equation can be solved either by direct numerical integration or<br />
by means of finding the eigenstates of the nA ! B matrix. The<br />
choice between these two approaches should be done with<br />
regard to the available computer resources.<br />
Once the time evolution of PA(t) is known, then the meansquare<br />
displacement D 2 (t) of the charge carrier in the direction<br />
perpendicular to that of the chain segments can be defined as<br />
D 2 ðtÞ ¼ X<br />
i 2 Ad2PAðtÞ (11)<br />
A<br />
with d being inter-chain distance and iA the dimensionless projection<br />
of the distance of the chains ‘‘A’’ and ‘‘0’’ to the direction of<br />
the electric field. To achieve numerical stability, this quantity was<br />
averaged over 100 different Monte Carlo realizations of the<br />
disordered polymer chains. It was found that the convergence in<br />
this case is much faster than that of convergence of the<br />
analogous quantity during the intra-chain transport calculations.<br />
The quantity D 2 (t) is related to the frequency-dependent<br />
charge carrier mobility at the zero field limit by means of<br />
well-known Kubo formula [12]<br />
mv ð Þ ¼ ev2<br />
2kT Re<br />
Z1 D 2 2<br />
3<br />
4 ðtÞ expð ivtÞdt5<br />
(12)<br />
0<br />
in which e is the elementary charge and v ¼ 2pf is the radian<br />
frequency of the external field.<br />
It should be pointed out that this equation comes from the<br />
fluctuation dissipation theorem stating that the linear response of<br />
a system in the thermodynamic equilibrium to a small external<br />
perturbation is the same as its response to a spontaneous<br />
fluctuation. Otherwise, the time evolution of D 2 (t) would describe<br />
the charge carrier thermalization showing a rapid increase at<br />
the initial stage, which would be followed by a normal diffusive<br />
motion, when the thermal equilibrium is achieved and the system<br />
starts fluctuating. For specific Monte Carlo data, it is rather<br />
difficult to predict the time necessary for the system thermalization.<br />
For this reason, to get correct results also for high<br />
frequencies, it is necessary to pay special attention to the way of<br />
the selection of the chain ‘‘0,’’ on which the charge carrier is<br />
localized at t ¼ 0 during each Monte Carlo run. Therefore, each<br />
master equation solution was preceded by a determination of the<br />
thermodynamic equilibrium distribution Pcell A of a charge carrier in<br />
an isolated ‘‘elementary cell’’ consisted of 20 20 polymer chains.<br />
The chain ‘‘0’’ was then randomly selected with the weights equal<br />
to the calculated equilibrium probabilities Pcell A . With regard to the<br />
Monte Carlo averaging, this procedure ensures that the model<br />
system is in the thermodynamic equilibrium from the start of the<br />
time evolution.<br />
RESULTS AND DISCUSSION<br />
P. TOMAN ET AL.<br />
The inter-chain hole mobility in MEH–PPV doped by a polar<br />
additive was modeled by the above-described model with the<br />
following parameters: The size of the array of the polymer chains<br />
(Fig. 3) was M ¼ 601 chains. Each of these chains consisted of<br />
N ¼ 501 repeat units (phenylenes and vinylenes). The temperature<br />
Figure 5. The dimensionless mean-square displacements D 2 (t)/d 2 as a<br />
function of time calculated for s(en) ¼ 0 and 0.18 eV.<br />
www.interscience.wiley.com/journal/pat Copyright ß 2008 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 263–267
INTER-CHAIN CHARGE TRANSPORT IN DOPED MEH–PPV<br />
Figure 6. Inter-chain hole mobility in MEH–PPV calculated for different<br />
degrees of the energetic disorder s(e n) using reorganization energy<br />
(a) l ¼ 0.1 eV, (b) l ¼ 0.4 eV.<br />
was T ¼ 293 K. The estimated inter-chain distance d ¼ 1 nm. The<br />
reorganization energy in MEH–PPV was estimated to be between<br />
0.1 and 0.4 eV on the basis of the B3LYP calculations of the<br />
oligomers. These values are consistent with the values<br />
determined by Prins et al. [13] for similar phenylene–vinylene<br />
derivatives.<br />
The dimensionless mean-square displacements D 2 (t)/d 2 calculated<br />
for polymers with different degree of the energetic disorder<br />
s(en) from zero to 0.18 eV are almost linear functions of time t,<br />
which is an evidence of the diffusive hole motion. However, for<br />
higher values of s(e n) there is higher slope of this dependence at<br />
short times (as shown in Fig. 5). It could be related to an imperfect<br />
thermal equilibrium at the beginning of the time evolution due to<br />
small number of Monte Carlo realization of the disorder and<br />
subsequent thermalization of the system. Nevertheless, we do<br />
not have a sure-footed explanation of this phenomenon.<br />
The frequency-dependent inter-chain hole mobility calculated<br />
according to eqn (12) for reorganization energies l ¼ 0.1 and<br />
0.4 eV is shown in Fig. 6. While the mobility in the polymer with a<br />
low degree of the energetic disorder is independent of the<br />
frequency, for higher degree of s(e n) there is a certain increase of<br />
the mobility with the frequency, which is related to the<br />
above-mentioned change of the slope in the D 2 (t)/d 2 plot.<br />
However, for all frequencies, there is a very strong dependence of<br />
the mobility on s(en). At lower frequencies, the mobility decreases<br />
by two or three orders of magnitude, if the width of the energetic<br />
distribution is doubled from 0.09 to 0.18 eV. Such a change of<br />
s(en) can be achieved by the change of the additive dipole<br />
moment. It should be remarked that our model of the energetic<br />
disorder does not involve the influence of the reaction field of the<br />
dipoles. Inclusion of this effect may decrease the energetic<br />
disorder up to 50%. Nevertheless, it is possible to increase the<br />
additive concentration up to about 20%wt and achieve the same<br />
effect.<br />
Acknowledgements<br />
The computer time at the MetaCenter (Prague and Brno) and at<br />
the Institute of Physics of the ASCR, v. v. i. (project Luna) is<br />
gratefully acknowledged.<br />
REFERENCES<br />
[1] (Eds: T. A. Skotheim, R. L. Elsenbaumer, J. R. Reynolds), Handbook of<br />
Conducting Polymers, Marcel Dekker, New York, 1998.<br />
[2] X. Y. Deng, L. P. Zheng, C. H. Yang, Y. F. Li, G. Yu, Y. Cao, J. Phys. Chem. B<br />
2004, 108, 3451–3456.<br />
[3] A. Fujii, S. V. Frolov, Z. V. Vardeny, K. Yoshino, Jpn. J. Appl. Phys. Part 2<br />
Lett. 1998, 37, L740–L742.<br />
[4] H. Yamamoto, J. Wilkinson, J. P. Long, K. Bussman, J. A. Christodoulides,<br />
Z. H. Kafafi, Nano Lett. 2005, 5, 2485–2488.<br />
[5] L. Edman, J. Swensen, D. Moses, A. J. Heeger, Appl. Phys. Lett. 2004, 84,<br />
3744–3746.<br />
[6] F. C. Chen, Q. F. Xu, Y. Yang, Appl. Phys. Lett. 2004, 84, 3181–3183.<br />
[7] M. Jaiswal, R. Menon, Polym. Int. 2006, 55, 1371–1384.<br />
[8] F. S. Santos, R. M. Faria, A. R. de Andrade, G. C. Faria, C. A. Amorin,<br />
S. Mergulhao, Thin Solid Films 2007, 515, 8034–8039.<br />
[9] S. Nesˇpu˚rek, H. Valerián, A. Eckhardt, V. Herden, W. Schnabel, Polym.<br />
Adv. Technol. 2001, 12, 306–318.<br />
[10] P. Toman, S. Nesˇpu˚rek, M. Weiter, M. Vala, J. Sworakowski,<br />
W. Bartkowiak, M. Mensˇík, Polym. Adv. Technol. 2006, 17, 673–678.<br />
[11] V. May, O. Kühn, Charge and Energy Transfer Dynamics in Molecular<br />
Systems, 1st edn. Wiley, Berlin, 2000.<br />
[12] R. Kubo, J. Phys. Soc. Japan 1957, 12, 570–586.<br />
[13] P. Prins, K. Senthilkumar, F. C. Grozema, P. Jonkheijm, A. P. H. J.<br />
Schenning, E. W. Meijer, L. D. A. Siebbeles, J. Phys. Chem. B 2005,<br />
109, 18267–18274.<br />
Polym. Adv. Technol. 2009, 20 263–267 Copyright ß 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/pat<br />
267
Eur. Phys. J. Appl. Phys. 48, 10401 (2009)<br />
DOI: 10.1051/epjap/2009112<br />
Regular Article<br />
THE EUROPEAN<br />
PHYSICAL JOURNAL<br />
APPLIED PHYSICS<br />
Photoinduced reversible switching of charge carrier mobility<br />
in conjugated polymers<br />
M. Weiter 1 ,J.Navrátil 1,a ,M.Vala 1 ,andP.Toman 1,2<br />
1 Brno University of Technology, Faculty of Chemistry, Purkynova 118, 612 00 Brno, Czech Republic<br />
2 Institute of Macromolecular Chemistry, AS CR, v.v.i., Prague, Czech Republic<br />
1 Introduction<br />
Received: 29 August 2008 / Received in final form: 3 April 2009 / Accepted: 21 April 2009<br />
Published online: 26 June 2009 – c○ EDP Sciences<br />
Abstract. Photoinduced reversible switching of charge carrier mobility in conjugated polymers was studied<br />
by theoretical and experimental methods. The quantum chemical calculations showed that the presence<br />
of dipolar species in the vicinity of a polymer chain modifies the on-chain site energies and consequently<br />
increases the width of the distribution of hopping transport states. The influence of photoswitchable<br />
charge carrier traps on charge transport was evaluated by current-voltage measurement and by impedance<br />
spectros<strong>copy</strong> method. It was found that deep traps switchable by photochromic reaction may significantly<br />
control the transport of charge carriers, which is exemplified as a significant decrease of the current and<br />
increase of parallel resistance measured by impedance spectros<strong>copy</strong>.<br />
PACS. 73.61.-r Electrical properties of specific thin films – 82.35.-x Polymers: properties; reactions;<br />
polymerization<br />
As an emerging area in organic electronics, polymer memories<br />
and switches have become an active research topic<br />
in recent years [1]. This paper deals with the concept of<br />
the photochromic switching of charge carrier transport in<br />
polymers. As the conductivity mechanism in these materials,<br />
a variable-range hopping in a positionaly random<br />
and energetically disordered system of localized states is<br />
widely accepted [2,3]. Over the last decades, hopping in<br />
random systems was extensively studied. Among these<br />
studies, the approach based on so-called effective transport<br />
energy level was shown to be especially efficient [4,5].<br />
When the effective transport energy is established, the<br />
variable range hopping problem is virtually reduced to<br />
trap controlled transport model. According to this model,<br />
the transport of charge carriers in molecular solids is<br />
strongly influenced by the presence of centres capable of<br />
localizing charge carriers (traps). It was shown that deep<br />
traps may significantly affect the energy of the transport<br />
level and mobility of charge carriers [6] and thus control<br />
their transport.<br />
Investigated polymer switch is based on switching<br />
of charge carrier mobility by photochromic species distributed<br />
in polymer matrix. Photochromic reactions, in<br />
addition to the changes in electronic spectra, are also accompanied<br />
by variations in refractive index, dielectric con-<br />
a e-mail: xcnavratil@fch.vutbr.cz<br />
10401-p1<br />
stant, enthalpy, etc. [7]. Moreover, the reversible changes<br />
in physical or chemical properties of the photochromic<br />
species can be transferred to the microenvironment and<br />
supramolecular structure, and thus, can induce rich modifications<br />
in the surroundings. In a molecular solid build<br />
of non-polar polarizable units, e.g. polymer segments, containing<br />
a small amount of polar guest species, its dipole<br />
moment contributes to the field acting on surrounding<br />
molecules and modifies the local values of the polarization<br />
energy. This modification can cause a local decrease<br />
of the ionisation energy in an otherwise perfect crystal lattice,<br />
which represents a trap for hole. Thus, the presence<br />
of polar species may result in production of local states<br />
(charge traps) – in their vicinity; even thus they are not<br />
necessarily trapping sites themselves [8,9]. Reversible creation<br />
of such polar species can be obtained by e.g. suitable<br />
photochromic molecules. The induced change of electrostatic<br />
potential due to the charge-dipole interactions<br />
also shifts the site energies of individual polymer repeating<br />
units, and consequently the polymer transport levels<br />
are modified. Since the position and orientation of the<br />
additive with respect of the polymer chain are essentially<br />
random the effect results in broadening of the distribution<br />
of the transport states and consequently to the lowering of<br />
the charge carriers mobility. In the case of reversible formation<br />
and annihilation of such traps the electric charge<br />
transport can be even changed from space-charge limited<br />
to trap limited [10].
C<br />
H 3<br />
CH 3<br />
CH 3<br />
N O<br />
The European Physical Journal Applied Physics<br />
NO 2<br />
C<br />
H 3<br />
CH 3<br />
CH 3<br />
N O<br />
Fig. 1. Photochromic reaction of the spiropyran (left) to its metastable merocyanine form (right).<br />
The purpose of the present work is to examine by<br />
quantum chemistry modeling and experimental characterization<br />
the optical and electrical switching properties<br />
of the suggested switch. For the study the<br />
π-conjugated photoconductive polymers poly[2-methoxy-<br />
5-(2’-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV)<br />
doped by photochromic spiropyran 6-nitro-1’,3’,3’, -trimethylspiro[2H-1-benzopyran-2,2’-indoline]<br />
(SP), which can<br />
be converted to a higher dipole moment possessing form<br />
referred to as (photo)merocyanine (MR), were used.<br />
The photochromic reaction of the spiropyran (SP) into<br />
(photo)merocyanine is depicted in Figure 1.<br />
2 Theoretical modeling<br />
In principle, the polar additive may influence the hole<br />
transport in two ways. First, if the highest occupied molecular<br />
orbital (HOMO) of the additive molecule is above the<br />
HOMO of the polymer, the additive molecules serve as energy<br />
level (chemical) traps. Second, the additive molecule<br />
possesses a large dipole moment that modifies the surrounding<br />
electrostatic potential landscape due to chargedipole<br />
interactions (dipolar traps) [11,12].<br />
It was shown in our previous papers that SP/MR attached<br />
to the σ-conjugated poly[methyl(phenyl)silylene]<br />
creates chemical traps for holes [11,13]. However, the<br />
HOMO’s of the π-conjugated polymers lie generally higher<br />
than the ones of the σ-conjugated polymers. The quantum<br />
chemical calculations show, that the HOMO of MEH-PPV<br />
is at least about 1 eV above the HOMO’s of SP and MR.<br />
For this reason we expect, that the additive does not create<br />
energy level traps in the investigated system, but influences<br />
the hole transport through the charge-dipole interactions.<br />
The polymer chain is modeled by a sequence of N =<br />
4000 sites corresponding to the repeat units (alternating<br />
phenylenes and vinylenes). Each site n is described by the<br />
energy εn of a hole located at this site. The hole transport<br />
between the sites n and m is described by the transfer integral<br />
bn,m. Because of linear character of the MEH-PPV<br />
chain and the size of the repeat units only the transfer integrals<br />
between the neighboring repeat units are important<br />
and the other can be neglected (tight-binding approximation).<br />
A typical value of the transfer integrals bn,n+1 is<br />
about 1 eV, being significantly higher than the random<br />
10401-p2<br />
NO 2<br />
disorder in site energies εn. This fact justifies the description<br />
of the on-chain hole motion using delocalized states<br />
approach rather than as hopping between localized states.<br />
Thus, hole motion on such a chain can be described by the<br />
Hamiltonian<br />
H =<br />
N�<br />
n=1<br />
�<br />
εna + � +<br />
n an − bn,n+1 a n+1an + a + ��<br />
n an+1 , (1)<br />
where an and a + n are annihilation and creation operators<br />
of a hole at an nth site. Both quantities εn and bn,n+1 are<br />
influenced by the random structure of the polymer chain<br />
and its surrounding. The energy εn is essentially equal to<br />
the negative of the first ionization potential of the corresponding<br />
repeat unit. Grozema et al. [14] developed a<br />
continuous disorder type model of the transfer integral<br />
bn,n+1 distribution, showing that the hole mobility in a<br />
pure MEH-PPV is limited by the torsional disorder. We<br />
assume that the influence of the additives on the electronic<br />
coupling between the polymer repeat units is small<br />
in comparison with the electrostatic charge-dipole interaction<br />
modulating the site energies εn.<br />
Polar species in the polymer chain vicinity modify<br />
the on-chain electrostatic potential due to the chargedipole<br />
interactions between a hole moving on the chain<br />
and dipole moments of individual additive molecules dispersed<br />
in the polymer. It is easy to show that the sum of<br />
these electrostatic potential changes shifts the hole site energies<br />
εn by the value 〈HOMO| �<br />
Δφi|HOMO〉, where<br />
i<br />
Δφi are the changes of the electrostatic potential describing<br />
the charge-dipole interactions of a charge carrier localized<br />
at |HOMO〉 with all surrounding polar additive<br />
molecules. The change of the shape of the highest molecular<br />
orbital |HOMO〉 of the corresponding repeat unit induced<br />
by the additive is neglected (frozen orbital approximation)<br />
[11]. Since the positions and orientations of the<br />
additive molecules with respect to the polymer chain are<br />
essentially random, the effect results in broadening of the<br />
distribution of transport states. The most important parameter<br />
of this distribution (energetic disorder) is its halfwidth<br />
σ(εn).<br />
Recently, energetic disorder was numerically modeled<br />
by a dipolar lattice model [15]. This model considers a<br />
regular three-dimensional cubic lattice of sites being subject<br />
to periodic boundary conditions. A fraction of the<br />
sites is occupied by randomly oriented, immobile, and
M. Weiter et al.: Photoinduced switching of charge carrier mobility in conjugated polymers<br />
non-interacting dipoles. The electrostatic potential is then<br />
calculated for each site as a sum of the Coulombic potentials<br />
of all dipoles. This model was later analytically<br />
evaluated by Young [16]. He investigated the shape of the<br />
distribution and derived an equation for its half-width σ,<br />
being proportional to the dipole moment and the square<br />
root of the dipole concentration. Our model of the energetic<br />
disorder is a modification of this approach in order<br />
to take into account the linear shape of the polymer<br />
main chain, the size of its substituents, the absence of<br />
any well-ordered structure, and the fact, that the charge<br />
transport proceeds predominantly on the conjugated main<br />
chain. For this reason, the polymer chain was modeled as<br />
a line. Randomly oriented additive molecules, represented<br />
by point dipoles, were randomly placed in the vicinity of<br />
this line, but it was assumed that no additive molecule<br />
was placed at a distance shorter than 10 ˚A, which corresponds<br />
to the mean size of the polymer substituents and<br />
the size of the polar additive molecules. On the other hand,<br />
the influence of the additive molecules distant more than<br />
50 ˚A from the polymer chain was neglected. The additive<br />
molecules were placed also beyond the chain ends in order<br />
to ensure the homogeneity of the εn distribution along the<br />
whole chain. The concentration of the additive was taken<br />
to be c = 4 × 10 −4 ˚A −3 . For each center representing<br />
a repeat unit, the energetic disorder was calculated as a<br />
sum of Coulombic electrostatic potentials from all additive<br />
molecules. With regard to the value of the minimal<br />
distance of an additive molecule from the chain the size<br />
of the polymer repeat units was neglected. The energetic<br />
disorder of εn was calculated according to the abovedescribed<br />
model for several values of the dipole moment<br />
of the additive m. According to the central limit theorem<br />
[17], the resulting εn distribution is a Gaussian-type<br />
distribution. For a typical value of m =12Dthehalfwidth<br />
σ(εn) =0.37 eV. Because the mutual interaction of<br />
the additive molecules is neglected, the half-width σ(εn)is<br />
proportional to the dipole moment m of the additive and<br />
to the square root of the additive concentration √ c just<br />
as in the dipolar lattice model [15]. However, the value of<br />
σ(εn) calculated according to our model does not follow<br />
the relations derived by Young [16]. Furthermore it should<br />
be noted, that the εn values show a strong site-to-site<br />
correlation between up to about 10th nearest neighboring<br />
sites. The correlation coefficient between the nearest<br />
neighbor εn values is 0.97. This fact can be explained by<br />
the long-range character of the charge-dipole interactions<br />
and the size of the MEH-PPV substituents hindering from<br />
close contact between additive molecules and the main<br />
chain. It should be pointed out, that the site-to-site correlation<br />
significantly affects the transport, thus the direct<br />
numerical calculation of the εn values cannot be replaced<br />
by a simple generation of random numbers from a normal<br />
distribution.<br />
The dipole moments of SP and MR forms calculated by<br />
the Hartree-Fock method are 5.5 and 11.9 D, respectively.<br />
These values suggest that the SP → MR reaction results<br />
in approximately doubling the energetic disorder. Moreover,<br />
one should take into account that, while the value<br />
obtained for SP is close to reality, the dipole moment of<br />
Mobility [cm 2 /Vs]<br />
10401-p3<br />
1000<br />
100<br />
10<br />
1<br />
0,1<br />
0,01<br />
no additive (0 D)<br />
SP (6 D)<br />
MR (12 D)<br />
1E-3<br />
0,1 1 10 100 1000<br />
Frequency [GHz]<br />
Fig. 2. The calculated frequency-dependent mobility for different<br />
additive dipole moments. The additive concentration was<br />
c =4× 10 −4 ˚A −3 .<br />
MR is probably underestimated since the polar environment<br />
increases the zwitterionic character of MR. Bletz<br />
et al. [18] reported the dipole moment of MR measured<br />
in a polar environment to amount to 15÷20 D. Thus the<br />
real switching effect may be more effective than that estimated<br />
using the calculated values. The hole on-chain mobilities<br />
μ(ω), calculated for different values of the additive<br />
dipole moments, are shown in Figure 2. All curves exhibit<br />
a saturation of μ(ω) at low frequencies corresponding to<br />
the diffusive charge carrier motion in the long-time limit<br />
(t >25 ps), and a rapid increase for higher frequencies<br />
related to the fast initial hole delocalization (t
Current (A)<br />
10 -4<br />
10 -5<br />
10 -6<br />
10 -7<br />
10 -8<br />
10 -9<br />
0%<br />
2%<br />
5%<br />
10%<br />
2 4 6 8 10 12 14<br />
Voltage (V)<br />
Fig. 3. Current-voltage characteristics for various concentration<br />
of spiropyran after the photochromic conversion induced<br />
by high photon dose corresponding to the merocyanine absorbance<br />
saturation.<br />
3 Experimental<br />
Polymer device was manufactured as a sandwich cell with<br />
a dielectric layer of MEH-PPV containing 0–30% wt. of<br />
admixedSP.Theactivepolymer layer was spin casted<br />
from chloroform solution on transparent indium tin oxide<br />
(ITO) electrode covering part of the glass substrate. The<br />
thickness of the active layer was about 150 nm. The structure<br />
was completed by evaporation of aluminium top electrode<br />
typically 100 nm thick. Average electrode area that<br />
delimitates the active area of the device was 3 mm 2 .The<br />
photochromic reaction of spiropyrane was activated using<br />
a Nd:YAG pulse laser (1 μJ/pulse) with third harmonic<br />
generation at 355 nm in order to measure the irradiation<br />
of the sample precisely. The electric response was studied<br />
by measuring the current-voltage j(V ) characteristics of<br />
the samples in the dark with Keithley 6517A electrometer.<br />
Dielectric properties were studied using a Hewlett<br />
Packard 4192A impedance analyzer. The measurements<br />
were performed in vacuum cryostat at room temperature.<br />
4 Results and discussion<br />
The photoswitching of charge carrier mobility was studied<br />
by standard current-voltage j(V )measurement.The<br />
results for typical devices are shown as log-log plot in Figure<br />
3 where the variation of the current at high irradiation<br />
dose corresponding to the merocyanine concentration saturation<br />
for different spiropyrane concentration is shown.<br />
In organic thin film devices the current is typically contact<br />
limited in low field region, whereas at higher field region<br />
space-charge or charge-trap limited conductivity are commonly<br />
accepted [19]. The results show this behavior. At<br />
low forward-bias voltages below 7–10 V the increase of<br />
j with V is relatively small, whereas in the higher field<br />
region the slope of the dependence is more pronounced,<br />
The European Physical Journal Applied Physics<br />
10401-p4<br />
which is in accordance with Space-charge limited current<br />
(SCLC) theory. This theory proposes that the space<br />
charge which limits conduction is stored in the traps. In<br />
the case of energetically discrete trapping level, the SCL<br />
current can be expressed as<br />
jSCL = 9<br />
2 V<br />
εε0μθ , (2)<br />
8 L3 where, ε and ε0 is the relative permittivity and permittivity<br />
of vacuum, μ is the charge carrier mobility, V is the<br />
applied voltage, L is the electrode distance and θ is the<br />
ratio of free to total charge carriers.<br />
However, in cases of practical interest traps are usually<br />
distributed in energy. In that case traps will be filled from<br />
the bottom to the top of the distribution as applied electric<br />
field increases. This is equivalent to an upward-shift in<br />
the quasi-Fermi level with electric field. As a consequence,<br />
θ increases with electric field and the j(V ) characteristics<br />
becomes steeper. In terms of present work, the distribution<br />
of charge traps describes those induced by spiropyrane<br />
to merocyanine photochromic conversion. The presence<br />
of distribution of traps opens additional pathways to<br />
the relaxation of charge carriers towards steeper states. A<br />
zero order analytic description of the effect can be based<br />
on the Hoesterey and Letson formalism [20]. The latter is<br />
premised on the argument that the carrier mobility in a<br />
system with relative trap concentration c is the product<br />
of the mobility in the trap-free system μ0 multiplied by<br />
trapping factor:<br />
μ(c) =μ0<br />
� � ��−1 Et<br />
1+c exp , (3)<br />
kT<br />
where Et is the energy of trapping level, k is the<br />
Boltzmann constant and T is the temperature. Consequently,<br />
the current flowing through the sample with enhanced<br />
number of trapping states will be less than in sample<br />
without traps. Figure 3, which shows the decrease of<br />
the photocurrent with increasing concentration of spiropyrane<br />
at high irradiation dose, proves this notion. Complementary,<br />
the decrease of the current with increasing<br />
photon dose demonstrated by number of laser pulses was<br />
observed. As the merocyanine concentration increase exponentially<br />
with photon dose and tends to saturate at high<br />
irradiation, the decrease of the current limits by about<br />
two orders of magnitude. In both cases the decrease is<br />
fully reversible after thermal fading in the dark or irradiation<br />
with a red light. The essential message is that the<br />
current thorough the irradiated device is significantly lowered<br />
by the presence of polar charge traps caused by photochromic<br />
conversion of spiropyrane to merocyanine. In<br />
another words the presence of traps lowers the mobility of<br />
charge carriers as expected from theoretical calculations.<br />
To provide better insight into studied phenomena the<br />
real and complex part of the impedance ZRe and ZIm were<br />
recorded at test frequencies between 10 and 5 × 10 6 Hz.<br />
The measured data were analyzed in the form of Cole-Cole<br />
diagram (real part of Z vs. imaginary part of Z) wherein<br />
the frequency increases from right to left. The variations
ZIm (Ω)<br />
2,5x10 6<br />
2,0x10 6<br />
1,5x10 6<br />
1,0x10 6<br />
5,0x10 5<br />
0,0<br />
M. Weiter et al.: Photoinduced switching of charge carrier mobility in conjugated polymers<br />
0<br />
50<br />
100<br />
200<br />
400<br />
0 1x10 6<br />
2x10 6<br />
3x10 6<br />
Z Re (Ω)<br />
4x10 6<br />
5x10 6<br />
Fig. 4. Cole-Cole plot of the 20% mixture of the MEH-PPV:SP<br />
before (0 pulses) and after the photochromic conversion for four<br />
light doses expressed as a number of pulses (1 µJ/pulse).<br />
of ZIm with ZRe as a function of photon dose at constant<br />
bias of 5 V for the device with 20% of spiropyrane are<br />
showninFigure4.AlltheZIm versus ZRe dependency<br />
shows a single semicircle which increases in size with increasing<br />
photon dose. This single semicircle could be fitted<br />
very well to a parallel combination of bulk resistance Rp<br />
and capacitance Cp in series with a resistance Rs, which<br />
is probably caused by the Ohmic contact at hole injecting<br />
ITO/MEH-PPV interface. From Figure 4 the bulk parallel<br />
resistance can be evaluated as the virtual intercept<br />
point of each Cole-Cole plot with ZRe axis. Following this<br />
evaluation, Figure 4 clearly demonstrates the increasing<br />
parallel resistance of the sample with increasing photon<br />
dose, which is in accordance with previous results. The<br />
comparison of data obtained by impedance analysis and<br />
steady-state j(V ) characterization is depicted in Figure 5<br />
for samples doped by 20% of spiropyrane after irradiation.<br />
Herein the results are related to the absorption of<br />
the samples at 590 nm, which is proportional to merocyanine<br />
concentration as was described above. In this figure<br />
the left y-axis represents the relative increase of the parallel<br />
resistance obtained by impedance analysis, whereas the<br />
reverse value of the relative decrease of the current evaluated<br />
form the j(V ) characteristics is marked on right<br />
y-axis. It is shown that the dependencies which manifest<br />
the influence of merocyanine charge traps on the charge<br />
transport in MEH-PPV matrix are both almost linear.<br />
5 Conclusions<br />
The possibility of switching of charge carrier mobility<br />
was demonstrated. The quantum chemistry calculations<br />
showed that the presence of polar additive in the vicinity<br />
of polymeric chain modifies its transport levels. This increase<br />
in the energetic disorder eventuates in the decrease<br />
of the hole mobility. The change of the additive dipole<br />
moment from ca. 6 D to 12 D, which corresponds to the<br />
studied photochromic reaction, results in an almost five-<br />
Parallel resistance (Ω)<br />
10401-p5<br />
7x10 6<br />
6x10 6<br />
5x10 6<br />
4x10 6<br />
3x10 6<br />
2x10 6<br />
1x10 6<br />
0<br />
Parallel resistance<br />
Current change<br />
0.04 0.06 0.08 0.10 0.12<br />
Δ Absorbance<br />
Fig. 5. The dependence of ratio of parallel resistance after the<br />
photochromic conversion to the value before conversion on the<br />
change of absorbance (left y-axis) and the ratio of the current<br />
before the photochromic conversion and after for different light<br />
dose at 4.2 V of the 20% mixture of the MEH-PPV:SP.<br />
fold decrease of the on-chain mobility. The experimental<br />
behaviour of the system explored by means of currentvoltage<br />
characterization showed a significant decrease of<br />
the current thorough the sample after irradiation. The current<br />
decrease is proportional either to the concentration<br />
of spiropyrane in the sample or to the irradiation dose.<br />
The increase of parallel resistance of the sample with irradiation<br />
dose obtained by impedance analysis confirms this<br />
outcome. According to the trap controlled hopping model<br />
for the description of charge transport it was stated that<br />
the presence of new dipolar traps results in the decrease<br />
of the charge carrier mobility and thereby lowering the<br />
current as predicted by the theoretical calculations.<br />
This work was supported by project KAN401770651 from<br />
the Academy of Sciences of the Czech Republic, project GA<br />
203/06/0285 via the Czech Science Foundation and by project<br />
No. 0021630501 from Ministry of Education, Youth and Sport.<br />
A possibility of using computer time in MetaCenter is gratefully<br />
acknowledged.<br />
References<br />
1. G. Hadziioannou, G.G. Malliaras, Semiconducting polymers,<br />
Vols. 1 and 2 (Wiley-VCH, Weinheim, 2007)<br />
2. M. Pope, C.E. Swenberg, Electronic Processes in Organic<br />
Crystals and polymers, 2nd edn. (Oxford University Press,<br />
Oxford, 1999)<br />
3. H. Bässler, Phys. Stat. Sol. B 15, 175 (1993)<br />
4. V.I. Arkhipov, E.V. Emelianova, G.J. Adriaenssens, Phys.<br />
Rev. B 64, 125125 (2001)<br />
5. V.I. Arkhipov, P. Heremans, E.V. Emelianova, G.J.<br />
Adriaenssens,H.Bässler,J.Phys.:Condens.Matter42,<br />
9899 (2002)<br />
6. V.I. Arkhipov, J. Ryenaert, Y.D. Jin, P. Heremans, E.V.<br />
Emelianova, G.J. Adriaenssens, H. Bässler, Synt. Met.<br />
138, 209 (2003)<br />
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0<br />
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7. H. Durr, H. Bouas-Laurent (Eds.), Photochromism:<br />
molecules and systems (Elsevier, Amsterdam, 2003)<br />
8. S. Neˇspùrek, J. Sworakowski, Thin Solid Films 393, 168<br />
(2001)<br />
9. M. Weiter, M. Vala, S. Neˇspùrek, J. Sworakowski, O.<br />
Salyk, O. Zmeˇskal,Mol.Cryst.Liq.Cryst.430, 227 (2005)<br />
10. P. Anderson, N.D. Robinson, M. Berggren, Adv. Mater.<br />
17, 1798 (2005)<br />
11. P. Toman, W. Bartkowiak, S. Neˇspùrek, J. Sworakowski,<br />
R. Zale´sny, Chem. Phys. 316, 267 (2005)<br />
12. F.L.E. Jakobsson, X. Crispin, M. Berggren, Organ.<br />
Electron. 10, 95 (2009)<br />
13. P. Toman, S. Neˇspùrek, W. Bartkowiak, J. Sworakowski,<br />
J. Lumin. 112, 386 (2005)<br />
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www.edpsciences.org<br />
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14. F.C. Grozema, P.T. van Duijnen, Y.A. Berlin, M.A.<br />
Ratner, L.D.A. Siebbeles, J. Phys. Chem. B 106, 7791<br />
(2002)<br />
15. A. Dieckmann, H. Bässler, P.M. Borsenberger, J. Chem.<br />
Phys. 99, 8136 (1993)<br />
16. R.H. Young, Phil. Mag. B 72, 435 (1995)<br />
17. J.A. Rice, Mathematical Statistics and Data Analysis, 2nd<br />
edn. (Duxbury Press, Belmont, 1995), ISBN 0-534-20934-3<br />
18. M. Bletz, U. Pfeifer-Fukumura, U. Kolb, W. Baumann, J.<br />
Phys. Chem. A 106, 2232 (2002)<br />
19. P.W.M. Blom, M.J.M. deJong, J.J.M. Vleggaar, Appl.<br />
Phys. Lett. 68, 3308 (1996)<br />
20. D.C. Hoesterey, G.M. Letson, J. Phys. Chem. Solids 24,<br />
1609 (1963)
Fractal–cantorian geometry of space-time<br />
Oldrich Zmeskal *, Martin Vala, Martin Weiter, Pavla Stefkova<br />
Faculty of Chemistry, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic<br />
article info<br />
Article history:<br />
Accepted 25 March 2009<br />
* Corresponding author.<br />
E-mail address: smeskal@fch.vutbr.cz (O. Zmeskal).<br />
abstract<br />
0960-0779/$ - see front matter Ó 2009 Published by Elsevier Ltd.<br />
doi:10.1016/j.chaos.2009.03.106<br />
Chaos, Solitons and Fractals 42 (2009) 1878–1892<br />
Contents lists available at ScienceDirect<br />
Chaos, Solitons and Fractals<br />
journal homepage: www.elsevier.com/locate/chaos<br />
This contribution is concerned with the extension of fractal theory used for the description<br />
of elementary stationary physical fields (gravitational, electric fields, fields of weak and<br />
strong interactions) as well as stationary fields of other physical quantities (thermal and<br />
acoustic) defined in the authors’ previous contributions to space-time area. This theory,<br />
defined generally in E-dimensional Euclidean space, was applied for description of stationary<br />
effects in one-, two- and three-dimensional space, respectively ðr ¼ xi þ yj þ zk, where<br />
i, j, k are orthogonal unitary vectors of Euclidean space). The agreement of laws formulated<br />
in various science disciplines with presented theory was proven for Euclidean objects (e.g.<br />
Newton gravitation law, Coulomb law, Planck’s radiation law, and 1st Fick’s law). In addition,<br />
the presented theory enables extension of validity of given laws for objects having<br />
fractal character.<br />
In this contribution, another extension of fractal theory is presented in the area of socalled<br />
pseudo-Euclidean coordinates, where E-dimensional space consists of p Euclidean<br />
and q pseudo- Euclidean dimensions ðE ¼ p þ qÞ.<br />
Special case of this space is the space-time ðs ¼ xi þ yj þ zk þ ictlÞ, where number of<br />
Euclidean dimensions is p ¼ 3 and number of pseudo-Euclidean dimensions is q ¼ 1, which<br />
are only preserved (i is imaginary unit, c is speed of light and i, j, k, l are orthogonal unitary<br />
vectors of Minkowski space).<br />
Physical quantities of this four-dimensional orthogonal space are very often transformed<br />
into three-dimensional curved space by means of parametric formulation of quantities<br />
r 0 ðt 0 Þ¼x 0 ðt 0 Þi þ y 0 ðt 0 Þj þ z 0 ðt 0 Þk; ðx 0 ¼ bðx vtÞ; y 0 ¼ y; z 0 ¼ z, where t 0 ¼ bðt ðv=c 2 ÞxÞ and<br />
b ¼ð1 v 2 =c 2 Þ 1=2 , respectively). This forms the basis for the formulation of laws of special<br />
and general theory of relativity. The time dilatation and the length contraction or relativist<br />
transformation of the mass result from these transformations.<br />
In many other cases physical laws are formulated in reality in four-dimensional spacetime<br />
(i.e. by means of independent coordinates x, y, z, t). It is concerned with 2nd Fick’s<br />
law, 2nd Fourier’s law, Schrödinger’s equation, continuity equation, etc.<br />
However, it is possible to eliminate one coordinate (e.g. time t) from equations by implementation<br />
of suitable quantity independent of this coordinate (e.g. on time t) in special<br />
cases. It is the case of steady flows of physical quantities (e.g. steady state electric current,<br />
steady state heat flow). In this case it is possible to formulate physical laws formally like in<br />
stationary cases (i.e. in three-dimensional space with coordinates x, y, z but by means of<br />
dynamic physical quantities). In this way the fractal theory of so-called space charge limited<br />
currents (SCLC) was solved (give express citation).<br />
Ó 2009 Published by Elsevier Ltd.
1. Introduction<br />
Space-time is a geometrical model of universe that combines 3D space and 1D time into a single construct called the<br />
space-time continuum. Time plays in this continuum the role of the 4th dimension. According to the Euclidean space perception,<br />
our universe has three-dimensions of space, and one-dimension of time. By combining the two concepts into a single<br />
manifold, physicists are able to significantly simplify the form of most physical laws.<br />
The problem of the actual number of dimensions of our universe is still open, as some theories (such as the string theory)<br />
predict as many as 26. In these theories, however, all the additional dimensions are such that the universe measured along<br />
them is subatomic in size. As a result, even if the universe had many more dimensions, we would only perceive 4 of them [1].<br />
The general relativistic invariant line element in E-dimensional space reads, in terms of Einstein’s convention of summation<br />
on identical lower and upper indices [2]<br />
ds 2 ¼ g lmdx l dx m ; ðl; m ¼ 1; 2; ...; EÞ: ð1Þ<br />
The new concept of space-time brings with it a new concept of distance. Whereas distances are always positive in E-dimensional<br />
Euclidean spaces g ll ¼ 1 for l ¼ 1; 2; ...; E (Einstein notation has been used for the tensors) and equal to zero for any<br />
other situations (g lm ¼ 0 for l–mÞ, so thus<br />
dr 2 ¼ dx 2<br />
1<br />
þ ...þ dx2 E ; ð2Þ<br />
the distance between any two events in E-dimensional spaces with p Euclidean and q pseudo-Euclidean dimensions<br />
ðE ¼ p þ qÞ may be real, zero, or imaginary [3]<br />
ds 2 ¼ dx 2<br />
1 þ ...þ dx2 p<br />
dx 2<br />
pþ1 ... dx 2<br />
pþq : ð3Þ<br />
The signs would be all positive ðg ll ¼ 1; l ¼ 1; 2; ...; pÞ for a Euclidean coordinates, negative (g ll ¼ 1 for<br />
l ¼ p þ 1; p þ 2; ...; p þ qÞ for pseudo-Euclidean coordinates of space, and equal to zero for any other situations (g lm ¼ 0<br />
for l–m).<br />
There are three-dimensions connected with the Euclidean and one with pseudo-Euclidean coordinate in four-dimensional<br />
time-spaces. These spaces have line elements given by Eqs. (2) or (3), respectively<br />
ds 2 ¼ dx 2 þ dy 2 þ dz 2<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892 1879<br />
dðctÞ 2 ; ð4Þ<br />
where x, y, z are Cartesian coordinates of three-dimensional space and c is the speed of light in vacuum (or speed of any other<br />
physical quantity).<br />
Eq. (4) formulates dimension of elementary vector<br />
ds ¼ dxi þ dyj þ dzk þ dðictÞl ð5Þ<br />
with orthogonal unitary vectors i, j, k, l. For spherical arrangement, it is possible to integrate elementary vector (Eq. (5)) ina<br />
simplistic manner. For constant speed c, it is given space-time vector formulating point location in space-time with coordinates<br />
(x, y, z, ict) from the beginning of coordinate system<br />
s ¼ xi þ yj þ zk þ ictl: ð6Þ<br />
This situation is formulated for 2D – Euclidean space with coordinates (x, y) inFig. 1a and for 2D orthogonal space with one<br />
Euclidean and one pseudo-Euclidean coordinate for Fig. 1b. Both of these spaces are subspaces of 4th dimensional Minkow-<br />
y<br />
r (x , y )<br />
r = x i + y j<br />
x<br />
ct<br />
s = x i + ict l<br />
s (x , ct )<br />
Fig. 1. (a) 2D Euclidean spherical space ðp ¼ 2; q ¼ 0Þ and (b) 2D pseudo-Euclidean hyperbolical space ðp ¼ 1; q ¼ 1Þ.<br />
x
1880 O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892<br />
ski space. Pictures can be also understood as two views at 3D orthogonal time-space (x, y, ict) with two Euclidean and one<br />
pseudo-Euclidean coordinate.<br />
The space-time interval (line element) quantifies the new distance (in Cartesian coordinates x, y, z, t)<br />
s 2 ¼ x 2 þ y 2 þ z 2<br />
c 2 t 2 ; ð7Þ<br />
as the differences of the space and time coordinates of the two events are denoted by r and t, respectively, given as<br />
r 2 ¼ x 2 þ y 2 þ z 2 .<br />
Pairs of events in space-time may be classified into three distinct types based on ‘how far’ apart they are:<br />
time-like (sufficient time elapse, for there to be a cause-effect relationship between the two events; s 2 < 0).<br />
light-like (the space between the two events is exactly balanced by the time between the two events; s 2 ¼ 0).<br />
space-like (insufficient time elapse for there to be a cause-effect relation between the two events; s 2 > 0).<br />
Events with a negative space-time interval are in each other’s future or past, and the value of the interval defines the<br />
proper time measured by an observer travelling between them. Events with a space-time interval of zero are separated<br />
by the propagation of a light signal.<br />
Certain types of world-lines (called geodesics of the space-time), are the shortest paths between any two events, with<br />
distance being defined in terms of space-time intervals. The concept of geodesics becomes critical in general relativity, since<br />
geodesic motion may be thought of as ‘‘pure motion” (inertial motion) in space-time, this means, free from any external<br />
influences.<br />
The characteristic feature of this space-time is the Light Cone, a double-cone centred at each event in space-time. By the<br />
conventional choice of units used in relativity, the sides of the cone are sloped at 45 °. This corresponds to choose units<br />
where time is measured in seconds and distances in light-seconds. A light-second is the distance light travels in one second.<br />
The upper-cone (called the future light-cone) represents the future of a light-flash emitted at that event. The lower-cone<br />
(called the past light-cone) represents all directions from which light-flashes can be received at that event.<br />
The generalization of the Eq. (7) consists in change from spherical arrangement of the spatial coordinates to the ellipsoidal<br />
arrangement<br />
x<br />
x1<br />
2<br />
þ y<br />
y 1<br />
2<br />
þ z<br />
z1<br />
2<br />
t<br />
t1<br />
2<br />
¼ 1: ð8Þ<br />
This change of invariant vector notation s at Eq. (7) enables to generalize description also on the cases where the components<br />
of speed in direction of individual axes are different.<br />
For s ¼ x1 ¼ y 1 ¼ z1 ct1 the Eq. (8) describes the situation given at Fig. 1b, which represents limiting case (maximal<br />
speed of dilatation, i.e. the speed of light). The equation also describes cases when light (energy) is propagated with smaller<br />
speed, i.e. s > x1 ¼ y 1 ¼ z1 ¼ vt1 (e.g. for speed of light v in transparent medium with refractive index n ¼ c=v). In this case,<br />
the invariant vector is s ¼ nx1 ¼ ny 1 ¼ nz1 ct1.<br />
The last most common case that is possible to describe by means of the Eq. (7) is the case when the speed of energy (light)<br />
passing through the medium depends on the direction of dilatation (anisotropic medium, s ¼ nxx1 ¼ nyy 1 ¼ nzz1 ct1Þ. This<br />
situation is demonstrated for 2D – Euclidean space with coordinates (x, y)atFig. 2a, for 2D orthogonal space with one Euclidean<br />
and one pseudo-Euclidean coordinates at Fig. 2b. It is necessary to formulate the space-time vector s(x, y, z, t) (Eq. (6))<br />
more generally<br />
y<br />
r = n x x i + n y y j<br />
r (x , y )<br />
x<br />
vt<br />
s = n x x i + ict l<br />
l = x i + ivt l<br />
l (x , vt )<br />
Fig. 2. (a) 2D Euclidean spherical space ðp ¼ 2; q ¼ 0Þ for nx–ny and (b) 2D pseudo-Euclidean hyperbolical space ðp ¼ 1; q ¼ 1Þ for v < c (n > 1,<br />
respectively).<br />
x
s ¼ nxxi þ nyyj þ nzzk þðictÞl: ð9Þ<br />
Other generalization consists in shift of the centre of the spherical object (mass point, sources of energy, light) in point<br />
with coordinates ðx0; y 0; z0; ct0Þ<br />
x x0<br />
x1<br />
2<br />
þ y y 0<br />
y 1<br />
2<br />
þ<br />
z z0<br />
z1<br />
2<br />
t t0<br />
t1<br />
2<br />
¼ 1: ð10Þ<br />
In this case, the space-time vector will be equal for an observer being in time t ¼ 0 at the beginning of the coordinate<br />
system (see Eq. (9))<br />
s0 ¼ nxx0i þ nyy 0j þ nzz0k þ ict0l: ð11Þ<br />
The change of its position in regards to the initial position ðDs ¼ s s0Þ will be Ds ¼ nxx1 ¼ nyy 1 ¼ nzz1 ct1, respectively.<br />
It is possible to express space-time vector by relation<br />
s ¼ nxðx x0Þi þ nyðy y 0Þj þ nzðz z0Þk þ icðt t0Þl: ð12Þ<br />
The situation described is demonstrated for 2D orthogonal space with one Euclidean and one pseudo-Euclidean coordinate at<br />
Fig. 3. It describes event that began in the past and its consequences are observed ðt0 < 0Þ at the time of the measurement,<br />
the depicts the situation when Fig. 3b the measurement began before the beginning of the relation ðt0 > 0Þ. In both cases the<br />
event arises in the beginning of the coordinate system ðx0 ¼ 0Þ, for v < c (nx > 1, respectively).<br />
We receive by means of extension of the conclusion on 4D pseudo-Euclidean space (space-time): e.g. x0 ¼ y 0 ¼ z0 ¼ 0,<br />
s 2 ¼ðnxxÞ 2 þðnyyÞ 2 þðnzzÞ 2<br />
ðctÞ 2 þ 2ct0t ðct0Þ 2 : ð13Þ<br />
In this relation, the coordinates (x, y, z) are converted in ‘‘optical coordinates”. This impact on space coordinates where the<br />
movement is instigated by mean of the light speed in vacuum ðnxx; nyy; nzzÞ. Mathematically, it asserts to the transformation<br />
of an ellipsoid to a sphere.<br />
The time components of the space-time vector have the following meaning: the ct0 responds to the time necessary for the<br />
phenomenon arriving to the observer, the 2ct0t expresses the diffusivity of the process (with diffusivity a ¼ 4ct0Þ. This means<br />
the incipient part of the process (induction period of the process) and the last member (ct) represents the respective process.<br />
Considering Eq. (13) it becomes apparent, that for the short times where t < t0 the diffuse processes will initially be used and<br />
only later the processes without diffusion (harmonic).<br />
The El Naschie’s golden field theory [4] is based on the so called Golden mean value, which can be given by the<br />
1; 1; 2; 3; ...Fnþ1; ... which obey Fnþ1 ¼ Fn þ Fn 1[5]. The Golden ratio of two successive Fibonacci numbers is limited by<br />
the value<br />
Fnþ1<br />
lim ¼<br />
n!1 Fn<br />
1<br />
/ ¼ 1 þ /; ð1Þ<br />
pffiffiffi<br />
where Golden mean value is / ¼ð 5 1Þ=2 ¼ 0:6180339887. Eq. (4) implies that the Golden mean value can be calculated<br />
as<br />
p<br />
a<br />
ffiffiffi<br />
positive root of quadratic equation /ð1 þ /Þ ¼1. The negative root can be expressed as 1=/ ¼ ð1þ /Þ ¼<br />
ð 5 þ 1Þ=2 ¼ 1:6180339887.<br />
El Naschie have demonstrated that the Golden mean value is equal to the Hausdorff dimension d ð0Þ<br />
c of the involved one-<br />
dimensional (superscript is equal to zero) Cantor transfinite sets which states have a probability equal to one [6]. By setting<br />
¼ / we can find the intersection rule of sets to any dimension n as follows<br />
d ð0Þ<br />
c<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892 1881<br />
vt<br />
l = x i +iv (t −t 0) l<br />
x<br />
vt 0 l (x , vt )<br />
vt<br />
vt 0 l (x , vt )<br />
l = x i +iv (t −t 0) l<br />
Fig. 3. 2D pseudo-Euclidean hyperbolical space ðp ¼ 1; q ¼ 1Þ (a) for future time coordinates (future light-cone, t0 < 0) and (b) for past time coordinates<br />
(past light-cone, t0 > 0).<br />
x
1882 O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892<br />
d ðnþ1Þ<br />
c ¼ d ð0Þ<br />
c<br />
n<br />
¼ / n : ð2Þ<br />
The average value of e1 with infinitely many dimensions nf using golden mean weights (centre of probability) can be<br />
written as<br />
Dime ð1Þ<br />
D E<br />
H ¼hdci ¼ X nf !1<br />
n/<br />
0<br />
n / 1<br />
¼ ¼ 2<br />
ð1 /Þ / 3 ¼ 4 þ /3 : ð3Þ<br />
As follows form the work of Celerier and Notale [7], the velocity V denoted in general (in scale of relativity) can be obtained<br />
from a non-relativistic limit of the relativistic geodesics equation and not from the non-relativistic formalism, which involves<br />
symmetry breakings in a fractal 3D space. This velocity can be determined as a sum of two terms: a scale-independent, differentiable,‘‘classical”<br />
part and a power-law divergent, scale-dependent, non-differentiable ‘‘fractal” part [8]<br />
V ¼ v þ w ¼ v 1 þ a s ðDF 1Þ=DF ; ð4Þ<br />
Dt<br />
where DF is the fractal dimension of the path. Depending on the resolution at which it is considered, the transition scale s<br />
yields two distinct behaviours of the velocity. Since V v when Dt s and V w when Dt s. For the description of a<br />
quantum particle of mass m, the de Broglie scale can be identified with s of the system ðs ¼ h=EÞ and the fractal-like domain<br />
with the quantum one. It should be emphasized that, the geometry is actually fractal at all scales, even though the fractal<br />
contribution w becomes dominated by the classical one v at scales larger than the de Broglie scale.<br />
The scaled fluctuation a is described by a dimensionless stochastic variable which is normalized according to hai ¼0 and<br />
ha2i¼1. Because the DF ¼ 2 plays the role of a critical dimension [9], we will consider only the case of this fractal dimension<br />
2 here. The above description applies to any of the fractal geodesics. It follows that one of the geometric consequences of the<br />
fractal character of space is that there is infinity of fractal geodesics relating any couple of its points [9]. From Eq. (4) we can<br />
calculate coefficient<br />
a ¼ lnðw=aÞ<br />
lnðs=DtÞ ¼ DF 1<br />
; ð5Þ<br />
DF<br />
which describes the division of relativistic and non relativistic part of velocity.<br />
The last Eq. (5) points that the coefficient of space-time fractal structure relates to generally describing structure with<br />
Hausdorff dimension d ð0Þ<br />
c defined for e1 space (3) [10–12].<br />
2. Field, potential and other quantities of conservative fractal structures<br />
Most of natural objects in space-time can be considered as fractal objects [13]. These objects can be analysed separately<br />
(e.g. in 3D space as stationary objects and/or in 1D time as signals) or in complex form. Fractals can be described using the<br />
fractal measure (K) and fractal dimension (D). Using the elementary cell, the fractal measure defines (in practice) the magnitude<br />
of the convergence of space (or space-time); while the fractal dimension describes the trend of the change of coverage<br />
as a function of size of the measuring cell (its radius can be larger or smaller than this radius of the elementary cell).<br />
Fractal dimension can vary between two limits:<br />
for D ¼ 0, the change of structure of fractal as the function of size of measure will be maximum (in the real world, e.g. the<br />
idealized objects mass point or point charge represent such situation),<br />
for D ¼ E, where E is the Euclidean dimension, the change of fractal structure will not depend on the change of measure (in<br />
the real world, e.g. the idealized object rigid body or homogeneity charged body represents this situation).<br />
In our previous papers [14,15], the density of fractal physical quantity qðrÞ, (e.g. mass density) in E-dimensional Euclidean<br />
space En ðE ¼ nÞ was defined<br />
qðrÞ ¼ Km0<br />
; ð14Þ<br />
rE D<br />
where NðrÞ ¼Km0 is the count of coverings of radius r by elementary quantity m0, K is a fractal measure and D a fractal<br />
dimension.<br />
It is possible to define intensity of the physical field E by application of Gauss–Ostrogradsky theorem<br />
divE ¼ kGNqðrÞ; ð15Þ<br />
and for radial field with the help of equation<br />
D dEr<br />
D E þ 1 dr ¼ kGNqðrÞ; ð16Þ<br />
where constant GN is Newton’s gravitational constant and k has various meaning for individual configurations of the fields<br />
(for various fractal dimensions, see 0).
Intensity of gravitational field E (gravitational acceleration) and potential V are inter related by the equation<br />
E ¼ gradV; ð17Þ<br />
so both quantities are connected with density of fractal quantity qðrÞ by means of relation<br />
DV ¼ div gradV ¼ divE ¼ kGNqðrÞ; ð18Þ<br />
where D is the Laplace operator. This equation can be rewritten for radial distribution of fractal quantity qðrÞ in the form<br />
D d<br />
D E þ 1<br />
2 V r<br />
dr 2 ¼ kGNqðrÞ: ð19Þ<br />
From the density of quantity qðrÞ we can determine radial field intensity Er and corresponding potential V r of physical<br />
(e.g. gravitational) field<br />
Er ¼ kGN<br />
D<br />
Km0<br />
r E D 1 ; V r ¼<br />
kGN Km0<br />
DðD E þ 2Þ rE D 2 ð20Þ<br />
on dimension r of elementary cell.<br />
Table 1 gives the notable fractal dimensions in E-dimensional Euclidean space. Constants s0, r0 and q 0 are linear, surface<br />
and volume mass density of fractal structure, whereas m0 is the elementary quantity.<br />
By the integration of mass density over volume dV we can derive the mass of fractal matter in the space of volume<br />
V ¼ rE Z<br />
MðrÞ ¼<br />
V<br />
qdV ¼ E<br />
D Km0r D : ð21Þ<br />
For mass point ðD ¼ 0Þ, the mass will be constant, which reflects the independence of the dimensions of limiting space. On<br />
the other hand for homogenous body ðD ¼ EÞ, this mass will enhance with Eth power of dimension of limiting space (for<br />
E ¼ 3 with the third power).<br />
3. Quantities of fractal structures in space-time (of gravitational field)<br />
3.1. Length contraction and time dilation<br />
It is also possible to apply quantities defined in previous chapter for E-dimensional Euclidean space on space-time. In this<br />
case, the vector r will be created ðE 1Þ with spatial coordinates ðp ¼ 1; 2; 3Þ and with one time coordinate ðq ¼ 1Þ. Dimension<br />
of this vector (space-time vector)<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
r ¼ c2t 2<br />
q<br />
; ð22Þ<br />
r 2 T<br />
where rT is size of spatial vector rT;t is time coordinate and c is speed of the light in vacuum, which is invariant in regard to<br />
the choice of inertial relative system.<br />
By means of simple modification of Eq. (22) it is possible to deduce relation for transformation of coordinates of the vector<br />
rT radial fractal ðE 1Þ-dimensional space to E-dimensional space-time vector r<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
rðvÞ ¼ict 1 r2 T =ðctÞ2<br />
q<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
¼ r0 1 v 2 =c2 q<br />
; ð23Þ<br />
where v ¼ rT=t ¼ const: is the speed of movement of fractal structure in the space and r0 ¼ ict ¼ const: ði ¼ ffiffiffiffiffiffiffi p<br />
1Þ<br />
is its<br />
dimension (radius) in the case that fractal structure does not move with regard to the relative system.<br />
It is also possible using Eq. (23) to define the relation for transformation of time interval from the equation (on the<br />
assumption, that r ¼ const:).<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
r ¼ ict0 ¼ ict 1 v 2 =c2 q<br />
; ð24Þ<br />
where t0 is the time of the process duration with regard to relative system at rest.<br />
Table 1<br />
Notable fractal dimensions in E-dimensional Euclidean space.<br />
D Independent quantity Kind of field Volume for E ¼ 3 kGNm 0<br />
Generally For E ¼ 3<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892 1883<br />
0 0 M0 ¼ const: Mass point V ¼ 4pr 3 =3 GNm0<br />
E 2 1 V r ¼ const: Equipotential field V ¼ 2pr 3 2GNs 0<br />
E 1 2 Er ¼ const: Homogenous intensity of field V ¼ 8r 3 4pGNr 0<br />
E 3 q ¼ const: Homogenous density of field 4pGNq 0
1884 O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892<br />
By means of simple arrangement we receive<br />
t0<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
1 v2 =c2 tðvÞ ¼p<br />
: ð25Þ<br />
Eqs. (23) and (25) are known as relations for length contraction and time dilation in a special theory of relativity.<br />
3.2. Density of fractal quantity<br />
It is also possible to modify other quantities defined in the previous chapter in the same way. Density of fractal quantity is<br />
possible to be expressed in E-dimensional space-time (for t ¼ const:) for example with the relation<br />
q0 ð1 v2 =c2Þ Km0<br />
qðvÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
ðr2 T c2t 2 q ¼ q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; ð26Þ<br />
E D<br />
E D<br />
Þ<br />
where q 0 ¼ Km0=ðictÞ E D is the density of fractal structure in rest with regard to relative system. For speeds much smaller<br />
than the speed of light it is possible to use approximate relation following from the neglecting of members of higher orders<br />
during the series development<br />
E D<br />
qðvÞ ffiq0 1 þ<br />
2<br />
v2 c2 ffi q E D<br />
0 exp<br />
2<br />
v 2<br />
c 2 : ð27Þ<br />
From the Eq. (26) it follows, e.g. for movement of mass point ðD ¼ 0Þ in the direction of axis xðE ¼ 2; p ¼ 1; q ¼ 1Þ where one<br />
coordinate is time and the other represents axis that the relation for density of body can be expressed as<br />
qðvÞ ¼<br />
q0 1 v 2 ; ð28Þ<br />
=c2 which is known from the special theory of relativity.<br />
However, it is also possible to understand Eq. (26) more generally. During the expansion of mass in E-dimensional space,<br />
the density will increase in relation to the space properties (characterized by fractal dimension D). The dominant changes of<br />
the density occur for energy expansion from the mass point ðD ¼ 0Þ. The density will not be changed with the speed for<br />
homogeneously filled space ðE ¼ DÞ.<br />
The dependences of expansion density of fractal quantity at Eq. (26) from surface source (E ¼ 4, D ¼ 2, linear arrangement),<br />
line source (E ¼ 4, D ¼ 1, cylindrical arrangement) and point source (E ¼ 4, D ¼ 0, spherical arrangements) are plotted<br />
in Fig. 5. The results for surface point source are in correlation with results for movement of mass point in 1D space (2D<br />
space-time, respectively), ðE ¼ 2; D ¼ 0Þ, see Fig. 4.<br />
For small speeds of movement (relativistic compression), it is possible to implement simplifying approximation: linear or<br />
exponentional function, Eq. (27), respectively. It follows from the analysis of the dependences, that already for speeds smaller<br />
than half of the speed of light ðv < c=2Þ the mistake between correct and the approximated values is less than 10%.<br />
It is also possible to express the dependence of the intensity and potential of fractal field on the speed of dilation and<br />
contraction of the space (with constant speed)<br />
ð1<br />
Er0<br />
v2 =c2 E<br />
Þ<br />
D 1<br />
ð1<br />
V r0<br />
v2 =c2 E<br />
Þ<br />
D 2<br />
ErðvÞ ¼q<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; V rðvÞ ¼q<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; ð29Þ<br />
where Er0 ¼ E r0 =ðictÞ E D 1 , V r0 ¼ V r0 =ðictÞ E D 2 , ðE r0 ¼ GNKm0=D; V r0 ¼ E r0 =ðE D 2ÞÞ and v ¼ rT=t ¼ const:<br />
For conditions discussed for density of fractal structure ðE ¼ 2; D ¼ 0Þ, thus corresponds to the movement of mass point<br />
(body) in direction of axis x and the relations will have form<br />
Er0<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
1 v 2 =c2 ErðvÞ ¼p<br />
; V r ¼ V r0 ¼ const: ð30Þ<br />
Fig. 4. Space-time geometry in 4th dimensional space ðE ¼ 4Þ with three Euclidean ðp ¼ 3Þ and one pseudo-Euclidean dimension ðq ¼ 1Þ: (a) plane-parallel,<br />
(b) cylindrical, (c) spherical and their fractal dimensions D. The arrows mark direction of the movement (expansion) of the mass.
From the first equation, it follows that the intensity of gravitational field (gravitational acceleration) grows up with second<br />
root. The second equation tells us that the field is equipotential in the whole space (the dimension of speed of fractal structure<br />
is constant in 1D space).<br />
These relations will also have the same form in 4D space for geometrical shapes (places) defined in the plane<br />
yzðE ¼ 4; D ¼ 2Þ moving in direction of axis x with constant speed v.<br />
3.3. Mass in theory of relativity<br />
From Eq. (26), it follows that the density of fractal quantity depends on the difference of Euclidean and fractal theory.<br />
Protruding from this assertion is that for various arrangements (see Table 2) the density of fractal quantity can be described<br />
with the same dependence (for E D ¼ const:).<br />
The relativistic change of the mass will depend on the direction of change of the density (movement of the mass) in contrast<br />
to the Eq. (21) where the symmetrical change of the density in E-dimensional space was assumed.<br />
If this change is only in one direction (plan-parallel arrangement) the volume is directly proportional to the length of the<br />
object in this direction) V r.<br />
The volume will be directly square power proportional to the length of the object V r2 , in two directions, it means in<br />
the plane (cylindrical arrangement).<br />
The volume will be directly third power proportional to the length of the object V r3 in three directions, it means in 3 D<br />
space (spherical arrangement).<br />
Assuming movement of the mass point in direction of x axis ðE ¼ 2; D ¼ 0Þ only one coordinate of the volume ðEV ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
¼ 1Þ<br />
will be changed, the volume will be V ¼ V 0 1 v 2 =c2 p<br />
, where V 0 ¼ ict and the mass M(r) is approximately possible to be<br />
expressed by means of the Eq. (28) in the form<br />
M0<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
1 v 2 =c2 MðvÞ qðrÞV ¼ p ; ð31Þ<br />
where M0 ¼ q 0 V 0 is also a known relation from the special theory of relativity. Using Eqs. (21) and (26) a more general rel-<br />
ativistic relation for the mass can be derived<br />
Z<br />
MðvÞ ¼ qdV<br />
M0<br />
¼ q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; ð32Þ<br />
V<br />
ρ (a.u.)<br />
10<br />
5<br />
0<br />
0<br />
ð1 v 2 =c2 E D 1<br />
Þ<br />
STR EXP CTR<br />
STR EXP CTR<br />
STR EXP CTR<br />
where M0 ¼ Km0=ðD E þ 1Þ=ðictÞ E D 1 . Based on the values listed in Table 2 (plan-parallel arrangement) we receive further<br />
known relation for the mass transformation for the theory of relativity.<br />
0.5<br />
v/c<br />
D = 0<br />
D = 1<br />
D = 2<br />
Fig. 5. The relativistic (STR), classical (CTR) and exponential (EXP) dependences of the density of fractal quantity on the speed (or expanding) of energy in<br />
4th dimensional space ðE ¼ 4Þ for planar ðD ¼ 2Þ, cylindrical ðD ¼ 1Þ and spherical ðD ¼ 0Þ case.<br />
Table 2<br />
Possible model arrangement of space-time.<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892 1885<br />
Arrangement Plan-parallel Cylindrical Spherical<br />
Space-time dimension (E) 4 3 2 4 3 4<br />
Euclidean dimension (p) 3 2 1 3 2 3<br />
Pseudo-Euclidean dimension (q) 1 1 1 1 1 1<br />
Fractal dimension (D) 2 1 0 1 0 0<br />
Volume dimension ðEV ¼ E D 1Þ 1 1 1 2 2 3<br />
1
1886 O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892<br />
Analogous results can be also derived for EV ¼ 2, when V ¼ V 0 ð1 v 2 =c 2 Þ a V 0 ¼ðictÞ 2 and for EV ¼ 3, when<br />
V ¼ V 0 ð1 v 2 =c 2 Þ 3=2 and V 0 ¼ðictÞ 3 , respectively. Equation can be generally written as<br />
M0<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
ð1 v 2 =c2Þ E D EV MðvÞ ¼q<br />
; ð33Þ<br />
where M0 ¼ Km0EV =ðD E þ EV Þ=ðictÞ E D E V .<br />
It is evident from Table 2 that this relation changes to the Eq. (31) for M0 ¼ Km0EV =ðictÞ all combinations of dimensions.<br />
Conclusion arising from this fact is that the relativistic mass transformation of physical quantity is not dependent on the<br />
nature of the mass (energy) movement.<br />
An interesting conclusion follows from these considerations: individual quantities can come into the transformation relations<br />
with various fractal and Euclidean dimension.<br />
3.4. Density of energy and energy in theory of relativity<br />
It is possible to express the density of energy in the E-dimensional space-time (for t ¼ const), e.g. by means of Eqs. (14)<br />
and (20) (Eqs. (26) and (29), respectively) as<br />
wðvÞ ¼qðvÞV rðvÞ ¼<br />
w0<br />
ð1 v 2 =c2 ; ð34Þ<br />
E D 1<br />
Þ<br />
where w0 ¼ GNK 2 m 2 0 =ðc2 t 2 Þ E D 1 is the density of energy of fractal structure provided its with regard to the relative system in<br />
rest.<br />
For speeds much less than that of light it is also possible alongside that of density, to use the relations that come from the<br />
neglecting of members of higher orders during the series development.<br />
wðvÞ ffiw0 1 þðE D 1Þ v2<br />
c2 ffi q 2<br />
0 exp ðE D 1Þv<br />
c2 : ð35Þ<br />
from Eq. (34) it follows thus for the movement of mass point (D=0) in direction of x axis (E = 2, one coordinate is time, the<br />
other represents axis x) that the equation for density of energy of fractal body becomes<br />
wðvÞ ¼<br />
w0<br />
1 v 2 : ð36Þ<br />
=c2 By integration of Eq. (36) over volume V ¼ V 0 ð1 v 2 =c 2 Þ E V =2 , where V 0 ¼ðictÞ E V we receive relativistic relation for<br />
energy<br />
Z<br />
EðvÞ ¼<br />
V<br />
E0<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
ð1 v 2 =c2Þ 2E 2D EV 2<br />
wdV ¼ q ; ð37Þ<br />
where E0 ¼ GNK 2 m 2 0 EV =ð2D 2E þ EV þ 2Þ=ðictÞ 2E 2D E V 2 .<br />
For special cases listed in Table 2 (e.g. for EV ¼ðE D 1Þ it is possible to simplify the equation<br />
E0<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
ð1 v 2 =c2Þ EV EðvÞ ¼q<br />
; ð38Þ<br />
where E0 ¼ GNK 2 m 2 0 =ðictÞE V ¼ GNK 2 m 2 0 =V 0 .<br />
Further modification (for plan-parallel arrangement, EV ¼ 1Þ of Eq. (38), we receive well known relation for transformation<br />
of energy in special theory of relativity<br />
E0<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
1 v2 =c2 EðvÞ ¼p<br />
; ð39Þ<br />
from which E0=M0 ¼ GNKm0 ¼ c 2 .<br />
4. Time dependences of fractal quantities (of gravitational and thermal field)<br />
4.1. Density of fractal quantity without diffusion<br />
In previous chapter, we equated time from the relation for density of fractal structure (14) defined in space-time (26).<br />
In this way, we received connection between densities of the structure in the system moving with speed v in regards to its<br />
density in the body. It is also possible to eliminate the position vector of dimensional coordinates rT in the same way.<br />
Vector r will be again defined ðE 1Þ by spatial coordinates ðp ¼ 1; 2; 3Þ and one time coordinate ðq ¼ 1Þ. It is possible to<br />
express the time change of the density of fractal structure in E- dimensional space-time (for r ¼ const:) likewise (26)
q 0<br />
Km0<br />
qðtÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
ðr2 T c2t 2 q ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
E D<br />
Þ ðt 2 =s2 q ; ð40Þ<br />
E D<br />
M 1Þ<br />
where q0 ¼ Km0=ðirTÞ E D is the density of fractal structure at time t ¼ 0s, i ¼ ffiffiffiffiffiffiffi p<br />
1 and sM ¼ rT=c is the time constant characterizing<br />
the change of the quantity in any given location of space.<br />
For times much longer than the dimension of the time constant (it means for the speeds v much smaller than speed of<br />
light c) it is possible to write simplified relation<br />
qðtÞ ffiq 0<br />
sM<br />
t<br />
E D<br />
: ð41Þ<br />
This relation is very often used like an approximation for description of time process of density (concentration of the<br />
charge, respectively) in homogenous electric field ðD ¼ E 1Þ.<br />
However, realistic approximation of Eq. (40) comes from the series development with neglecting of members of higher<br />
orders (PWR), by means of exponentional function (EXP), respectively<br />
qðtÞ ffiq 0<br />
sM<br />
t<br />
E D<br />
E D<br />
1 þ<br />
2<br />
s 2 M<br />
t 2<br />
ffi q 0<br />
sM<br />
t<br />
E D<br />
exp<br />
E D<br />
2<br />
s 2 M<br />
t 2 : ð42Þ<br />
These approximations have their own foundation for very long times. From the analysis of dependences it follows (see Fig. 6)<br />
that the error between correct and approximated values is less than 10% only for values twice longer than the time constant.<br />
It is not possible to use the mentioned approximation for the longer times ðt > 2sMÞ, because the density has generally a<br />
complex character<br />
q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi : ð43Þ<br />
qðtÞ ¼ð 1Þ ðE DÞ=2 q0 ð1 t 2 =s2 D<br />
MÞE For times much shorter than the dimension of time constant, it is possible to use again simplified relations following from<br />
the series development with neglecting of components of higher orders (PWR) or more precisely by means of exponential<br />
function (EXP)<br />
qðvÞ ffiq0ð 1Þ ðE DÞ=2 E D<br />
1 þ<br />
2<br />
t 2<br />
s 2 M<br />
4.2. Density of fractal quantity with diffusion<br />
ffi q 0 ð 1Þ ðE DÞ=2 exp<br />
E D<br />
2<br />
t 2<br />
s 2 M<br />
: ð44Þ<br />
If we suppose that the effect occurred in other time than the measurement was initiated (later or sooner, see Eqs. (10) or<br />
(12)), the spatial vector can be expressed by the relation r 2 ¼ r 2 T c 2 ðt t0Þ 2 , where r 2 T ¼ x2 þ y 2 þ z 2 is the radius of fractal<br />
space and t0 is delay of the time response (so-called induction period). The density of fractal structure will be<br />
Km0<br />
qðtÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
r2 T c2ðt t0Þ 2<br />
Km0<br />
r<br />
h i<br />
¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
E D<br />
r2 0 þ 4a0t c2 q ;<br />
2 E D<br />
t<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
ð45Þ<br />
where r0 ¼ r2 T c2t 2<br />
q<br />
0 is the quantity of initial space-time vector and a0 ¼ c2t0=2 is the maximum value of diffusivity (for<br />
D ¼ E). By simple modification of Eq. (45) we obtain<br />
ρ (a.u.)<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892 1887<br />
COR EXP PWR<br />
COR EXP PWR<br />
COR EXP PWR<br />
D = 0<br />
D = 1<br />
D = 2<br />
0<br />
0 1 2 3<br />
t/ τ M<br />
Fig. 6. Correct time dependences of fractal quantity density (COR), and their exponential (EXP) and power (PWR) approximations for t sM and for t sM<br />
in 4th dimensional space ðE ¼ 4Þ for planar ðD ¼ 2Þ, cylindrical ðD ¼ 1Þ and spherical ðD ¼ 0Þ case.
1888 O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892<br />
Km0<br />
qðtÞ ¼<br />
ð4a0tÞ ðE DÞ=2 1 þ r2 0<br />
4a0t þ c2t 4a0<br />
ðD EÞ=2<br />
: ð46Þ<br />
It is also possible to divert this relation by means of simple modifications (see Appendices A and B) to the exponential<br />
function<br />
Km0<br />
qðtÞ ¼<br />
exp<br />
ðE DÞ=2<br />
ð4a0tÞ<br />
E D<br />
2<br />
r 2 0<br />
4a0t<br />
: ð47Þ<br />
If we compare Eqs. (47) and (62) we can deduce that the conversion is possible only under following condition c 2 ¼ 2r 2 0 .<br />
These corrections are often used in models applying the exponential functions. It is also possible to express time dependences<br />
of intensity and potential of fractal field in given place space (for rT ¼ const:) in the same way<br />
ErðtÞ ¼<br />
V rðtÞ ¼<br />
Er0 exp<br />
ðE D 1Þ=2<br />
ð4a0tÞ<br />
V r0<br />
exp<br />
ðE D 2Þ=2<br />
ð4a0tÞ<br />
E D 1<br />
2<br />
E D 2<br />
2<br />
r 2 0<br />
4a0t<br />
r 2 0<br />
4a0t<br />
; ð48Þ<br />
; ð49Þ<br />
where a0 ¼ c 2 t0=2 is the maximum value of diffusivity (for D ¼ E), E r0 ¼ kGNKm0=D and V r0 ¼ E r0 =ðE D 2Þ, similarly to<br />
Eq. (29).<br />
4.3. Thermal field of fractal structures<br />
For thermal field, the potential expression of the temperature is given in Eq. (48). In the case of thermal field for the<br />
dependence of temperature on radius using, we can write [16,17]<br />
TrðrÞ ¼ hc<br />
k<br />
D Eþ2<br />
Kr<br />
: ð50Þ<br />
DðD E þ 2Þ<br />
If the vector r in the Eq. (50) is expressed like r 2 ¼ r 2 T c 2 ðt t0Þ 2 , where t0 is the time response delay (so-called induction<br />
period), it is possible (alike by decomposition of the density) to re-write it to the relation<br />
TrðtÞ ¼ hc<br />
k<br />
ðD<br />
Kð4a0tÞ<br />
Eþ2Þ=2<br />
DðD E 2Þ<br />
1 þ r2 0<br />
4a0t þ c2 t<br />
4a0<br />
ðD Eþ2Þ=2<br />
: ð51Þ<br />
If the heat diffuses by significantly smaller speed ðr2 T a0t, small distances or long times), then the terms in parenthesis can<br />
be viewed as significant in the expansion of exponential function ð1 x e xÞ and we can write<br />
TrðtÞ ¼ hc<br />
k<br />
ðD Eþ2Þ=2<br />
Kð4a0tÞ<br />
DðE D 2Þ exp<br />
E D 2<br />
2<br />
r 2 0<br />
4a0t<br />
If we substitute for the thermal diffusivity a ¼ 2a0=ðE D 2Þ, where a is the coefficient of thermal diffusivity of the body<br />
with fractal dimension D, we obtain<br />
TrðtÞ ¼ Khc<br />
k<br />
½2ðED2ÞatŠ DðE D 2Þ<br />
ðD Eþ2Þ=2<br />
exp<br />
r 2 0<br />
4at<br />
For the total heat transferred to the body from the heat source<br />
QðtÞ ¼ 2hc<br />
k<br />
we can write<br />
ð52Þ<br />
: ð53Þ<br />
Kt<br />
D½ðE D 2Þ=2pŠ ðD EÞ=2 ð54Þ<br />
QðtÞ<br />
TrðtÞ ¼<br />
exp<br />
ðE DÞ=2<br />
cpqð4patÞ<br />
r 2 0<br />
4at<br />
: ð55Þ<br />
From the last relation it is apparent, that the time dependence of the temperature will depend on the experimental arrangement.<br />
This applies to the values of Euclidean and fractal dimensions and also on properties of excitation source; see Eq. (54).<br />
Using the pulse transient method, the delivered heat will be constant, and by the step wise method, the delivered heat<br />
will change linearly with time. Generally speaking, the actuating quantities (e.g. heat) can take different fractal dimensions<br />
in the equation.<br />
Eq. (55) is applicable in [18–20] for fractal dimensions D ¼ 0; 1; 2 and topological dimension E ¼ 3; see Fig. 7.<br />
The typical time responses of temperature for the rectangle (Dirac) pulse of different input power of heat with various<br />
pulse widths are presented in Fig. 8, [16].
The heat power and width of the pulse were changed to find the optimal measurement conditions and subsequently to<br />
determine the reliable data of the thermal diffusivity, specific heat and thermal conductivity of the studied material.<br />
5. Conclusion<br />
Fig. 7. Heat flow geometry for (a) plane-parallel, (b) cylindrical and (c) spherical coordinates Euclidean space.<br />
ΔT (°C)<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892 1889<br />
1.2<br />
0.6<br />
0.0<br />
0<br />
Fig. 8. Temperature responses of the sample measured by the pulse transient method for different heat powers and various pulse widths.<br />
Two important applications of the fractal theory are discussed in this contribution. Both of them originate from the theory<br />
of space-time.<br />
The first deals with the formulation of laws of the special theory of relativity for fractal structures in E-dimensional spacetime<br />
with ðE 1Þ-dimensional coordinates and with one time coordinate. The obtained results are in accordance with the<br />
special theory of relativity of the mass point moving in 4D space-time. However, it is also possible to extend these results<br />
to objects that embody fractal structure both in macro world (space objects) and in micro world (elementary particles).<br />
By extending to the more-dimensional space-time ðE > 4Þ it is also possible to formulate laws of general theory of relativity<br />
with the help of the described theory.<br />
Hypothetically, it is possible to apply the mentioned mathematical apparatus also to space-times with more time coordinates.<br />
Other important conclusion being also in accordance with the special theory of relativity consists in an idea that<br />
various physical quantities can come into the resulting equations with various fractal dimension [22–24]. Example for the<br />
calculation of mass of ðm ¼ qVÞ the density has Euclidean dimension Eq ¼ 3, but the volume has only EV ¼ 1.<br />
The second presented application is focused on problems of transient measurement of various physical quantities having<br />
fractal character. In this case the theory is generally formulated in E-dimensional space. It follows from the analysis described<br />
in the contribution that the expression of time dependences by means of combination of exponentional and power<br />
functions (e.g. by formulation of Planck radiation law) is equivalent to the expression by means of power functions defining<br />
fractal physical quantities in space-time as discussed in this contribution. The article also outlines the conditions enabling<br />
the series development of these functions to the power range under which errors are negligible (number of development<br />
members, coefficients of power and exponential function). If we balance the result from power dependence describing characters<br />
of fractal fields, we can discuss suitable conditions under which it is possible to approximate power range by means of<br />
the functions used in classic theories. We believe that it is possible to eliminate the possible deviations of the experiments<br />
and models just by consistent using of proposed fractal theories.<br />
In practice the transient measurement can be realized for various experiment arrangements of the experiment (planar,<br />
cylindrical, and spherical). In this case, the responses depend not only on experimental arrangement but also on the properties<br />
of actuating signal. Example for calculation of specific heat by means of pulsed transient, the method of the temperature<br />
with dimension depends on experiment arrangement (for spherical arrangement, it means from punctual temperature<br />
source ET ¼ 3) comes into the relation cp Q=T, heat with Euclidean dimension EQ ¼ 1.<br />
400<br />
t (s)<br />
15 100<br />
20 200<br />
23 800<br />
28 700<br />
40 000<br />
J m –2<br />
J m –2<br />
J m –2<br />
J m –2<br />
J m –2
1890 O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892<br />
Acknowledgements<br />
This work was supported by Project KAN401770651 from The Academy of Sciences of the Czech Republic and by Project<br />
and by Grant FT-TA3/048 from the Ministry of Industry and Trade of the Czech Republic.<br />
Appendix A. Series development of exponential function on power – in denominator<br />
The conversion between fractal expression of time dependences of physical quantities (i.e. by means of power range) and<br />
their classical expression (by means of exponential function, of power, and of exponential function, respectively) it is possible<br />
to realize with the help of exponential function series development into the power range. Exponential function<br />
f ðxÞ ¼f0x m expð nxÞ ð56Þ<br />
can be represented by Planck radiation law<br />
2phm 3<br />
wðmÞ ¼<br />
c3 ; ð57Þ<br />
½expðhm=kTÞ 1Š<br />
which means the dependence of density of energy (spectral radiance) on the frequency of the thermal radiation. The variable<br />
m is frequency of thermal radiation, coefficient m is its exponent ðm ¼ 3Þ and coefficient n is dependent on the temperature<br />
n ¼ h=kT (for high frequencies and low temperatures when the constant ‘‘1” can be negligible). Function (56) has maximum<br />
for xm ¼ m=n, that constitutes in the relation for the density of energy maximal spectral radiance for frequency mm ¼ mkT=h,<br />
more exactly defined with using Lambert’s function u ¼ m þ Wð me m Þ for frequency mm ¼ ukT=h, respectively. It will be<br />
mm 2:821439kT=h for m ¼ 3 [16].<br />
The perverted values of the variables can be possibly used to transpose into an exponential function in a similar way as<br />
Eq. (56) into the form<br />
f ðxÞ ¼f0x m expð n=xÞ: ð58Þ<br />
This equation can express, e.g. Planck radiation law in the form<br />
2phc<br />
wðkÞ ¼<br />
k 5 ½expðhc=kkTÞ 1Š<br />
This means the dependence of the density of energy (spectral radiance) on wavelength of the thermal radiation. In this<br />
case, the variable x is the wavelength of temperature radiation, coefficient m is its exponent ðm ¼ 3Þ and coefficient n is<br />
dependent on temperature n ¼ hc=kT (for low wavelengths and low temperatures, when the constant ‘‘1” can be negligible).<br />
Function (58) has maximum for xm ¼ n=m, that represents maximal spectral radiance for the wave length km ¼ mhc=kT in<br />
the relation for density of energy, resp. more exactly with using Lambert’s function u ¼ m þ Wð me m Þ for the wave length<br />
km ¼ uhc=kT. It will be km 4:965114hc=kT for m ¼ 5, respectively<br />
Eq. (58) can also express temperature dependence on the time acquired using pulsed transient method [16–20]<br />
Q<br />
TðtÞ ¼<br />
cpqð4patÞ m exp r 2<br />
T =4at : ð60Þ<br />
where Q is the total heat transferred to a body, rT is the distance of heat source from temperature detector, cp is the specific<br />
heat capacity at constant pressure, q is the mass density and a is the coefficient of thermal diffusivity. Coefficient m represents<br />
an exponent dependent on an experiment configuration (e.g. m ¼ 3=2 for planar arrangement) and coefficient<br />
n ¼ r 2 T =4a.<br />
By the development of exponential functions (see Eqs. (56), or(58), respectively) to the power series we get following<br />
relations<br />
or<br />
f0x m<br />
f ðxÞ ¼f0x m expð nxÞ; f ðxÞ ¼<br />
ð1 þ x þ x2 =2 þ ...Þ n ; ð61Þ<br />
f0x m<br />
f ðxÞ ¼f0x m expð n=xÞ; f ðxÞ ¼<br />
ð1 þ 1=x þ 1=2x2 þ ...Þ n : ð62Þ<br />
These equations lead already for the first three members of the development to power series to functions having two extremes.<br />
For the first case it is possible to express the location of these extremes by relations<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
ðm nÞ ðm nÞ<br />
xm ¼ 2 q<br />
þ 4mðn m=2Þ<br />
: ð63Þ<br />
2ðn m=2Þ<br />
Positions of these two extremes will be approximately equal for m n<br />
ð59Þ
xm ¼<br />
ðm nÞ ðm nÞ<br />
: ð64Þ<br />
2n<br />
The first of which is identical with the position of the extreme of the exponential function xm ¼ m=n, (see Eq. (56)). For<br />
Wien’s displacement law [21] it means, that for xm ¼ mmm=m0, where m0 ¼ kT=h is minimal frequency of Einstein–Bose distribution<br />
function (see Eq. (58)) that the following condition must be satisfied: 1 < hmm=kT < hm=kT.<br />
In the same manner, the Wien’s displacement law is formulated with the help of wave length (see Eq. (59)) which must<br />
satisfy the following condition: k0=hc > km=hc > k=hc for xm ¼ mkm=k0, where k0 ¼ hc=kT is minimal wave length of Einstein–<br />
Bose distribution function (see Eq. (59)).<br />
And finally for the maximum of the temperature responses (see Eq. (60)) an expression of the exponential function is possible<br />
to be obtained by means of power range for xm ¼ mam=a0, where a0 ¼ hc=kT is the coefficient of thermal diffusivity provided<br />
that 4a0=r 2 T > 4am=r 2 T > 4a=r2 T .<br />
These dependences are compared in Fig. 9. The deviations between both models are dependent on the m/n ratio. For<br />
m=n 1 the deviations are negligible already by application of a polynomial approximation by means of second order.<br />
Appendix B. Series development of an exponential function to the power function – in numerator<br />
By the development of exponential functions into the power series (see Eqs. (56), or(58), respectively) we receive the<br />
following equations<br />
or<br />
f (x)<br />
0.3<br />
0.2<br />
0.1<br />
0<br />
0 0.2 0.4 0.6 0.8 1<br />
0.0000<br />
0 5 10 15 20<br />
x<br />
x<br />
f ðxÞ ¼f0x m expð nxÞ; f ðxÞ ¼f0x m ð1 x þ x 2 =2 ...Þ n ; ð65Þ<br />
þf ðxÞ ¼f0x m expð n=xÞ; f ðxÞ ¼f0x m ð1 1=x þ 1=2x 2<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892 1891<br />
EXP<br />
PWR<br />
Fig. 9. (a) Development of an exponential function (56) into the power series (61) for m ¼ 3, n ¼ 18 (model of Planck radiation law in the form given by Eq.<br />
(57)), (b) development of an exponential function (58) into the power series (62) for m ¼ 5, n ¼ 30 (model of Planck radiation law in the form given by Eq.<br />
(59)).<br />
...Þ n : ð66Þ<br />
These equations lead already for the first three members of the development to the power series to functions having two<br />
extremes. For the first case it is possible to express the position of these extremes by means of relations<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
ðm þ nÞ ðm þ nÞ<br />
xm ¼ 2 q<br />
þ 4mðn þ m=2Þ<br />
: ð67Þ<br />
2ðn þ m=2Þ<br />
Positions of these two extremes will be approximately equal for m n<br />
ðm þ nÞ ðm nÞ<br />
xm ¼ : ð68Þ<br />
2n<br />
The first is identical with position of extreme of exponential function xm ¼ m=n, (see Eq. (56)). For Wien’s displacement law<br />
[21] it means, that for xm ¼ mmm=m0, where m0 ¼ kT=h is minimal frequency of Einstein-Bose distribution function (see Eq.<br />
(57)) has to satisfy the following condition: 1 < hmm=kT < hm=kT.<br />
Likewise, Wien’s displacement law can be formulated by means of wave length (see Eq. (59)) which must satisfy the following<br />
condition: k0=hc > km=hc > k=hc for xm ¼ mkm=k0, where k0 ¼ hc=kT is minimal wave length of Einstein–Bose division<br />
(see Eq. (59)).<br />
And finally for the maximum of the temperature response (see Eq. (60)) an expression of exponential function is possible<br />
to obtain by means of power for xm ¼ mam=a0, where a0 ¼ hc=kT is the coefficient of thermal diffusivity provided that<br />
4a0=r2 T > 4am=r2 T > 4a=r2 T .<br />
f (x)<br />
0.0010<br />
0.0008<br />
0.0006<br />
0.0004<br />
0.0002<br />
EXP<br />
PWR
1892 O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 1878–1892<br />
f (x)<br />
These dependences are compared in Fig. 10. The deviations between both models are also dependent on the m/n ratio as<br />
in the first case. For m=n 1 the deviations are negligible already by application of a polynomial approximation by means of<br />
second order.<br />
References<br />
0.3<br />
0.2<br />
0.1<br />
EXP<br />
PWR<br />
0<br />
0.0000<br />
0 0.2 0.4<br />
x<br />
0.6 0.8 1 0 5 10<br />
x<br />
15 20<br />
Fig. 10. (a) Development of an exponential function (56) into power series at Eq. (65) for m ¼ 3, n ¼ 18 (model of Planck radiation law in the form given by<br />
Eq. (57)), (b) development of exponential function given by Eq. (58) into power series (66) for m ¼ 5, n ¼ 30 (model of Planck radiation law in the form given<br />
by Eq. (59)).<br />
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f (x)<br />
0.0010<br />
0.0008<br />
0.0006<br />
0.0004<br />
0.0002<br />
EXP<br />
PWR
Notes to ‘‘An irreducibly simple derivation of the Hausdorff dimension<br />
of spacetime” by M.S. El Naschie<br />
Oldrich Zmeskal *, Martin Weiter, Martin Vala<br />
Faculty of Chemistry, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic<br />
article info<br />
Article history:<br />
Accepted 20 January 2009<br />
Communicated by: Prof. G. Iovane<br />
1. Random Hausdorff dimension<br />
abstract<br />
Some new details of calculation of random Hausdorff dimension, not only for spacetime,<br />
but for any other topological dimension are presented in this note. In the second part, there<br />
are some first iteration of Hausdorff dimensions (based on the Cantor discontinuum or<br />
Sierpinsi triangle) and their limited values.<br />
Ó 2009 Elsevier Ltd. All rights reserved.<br />
The terms for calculation of random Hausdorff dimension of space-time described in contribution [1] can be generally<br />
calculated by iteration equation<br />
Dnþ1 ¼ D1 þ a<br />
ð1Þ<br />
Dn<br />
where D 1 = 4 and a = 1.<br />
We suppose that the Eq. (1) converges to value D1 for n ? 1. Then we get its limited value by solving of the quadratic<br />
equation<br />
from which<br />
D 2<br />
1 D1D1 a ¼ 0; ð2Þ<br />
D1ð1;2Þ ¼ D1<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
D 2<br />
q<br />
1 þ 4a<br />
: ð3Þ<br />
2<br />
We will get really the fractal dimension value D 1 = 4.2361...for D 1 = 4 and a = 1 from this equation. The fractional part,<br />
namely 0.2361... is easily shown to be exactly / 3 where<br />
/ ¼<br />
pffiffiffi<br />
5 1 =2: ð4Þ<br />
Any other values of constant D 1 and a for other powers of number / (Golden mean value) are set in the Table 1. These<br />
values can be determined by the iteration calculation<br />
D1;n ¼ D1;n 1 þ D1;n 2; an ¼ an 1 ð5Þ<br />
for a1 =1,D1,1 = 1 or with using of index nan =( 1) n . Constants D1 and a are representing the coefficients for conversion of / n<br />
to / n . The limited values of interaction (1) or calculated from (3) or (4) are at the last column.<br />
* Corresponding author.<br />
E-mail address: zmeskal@fch.vutbr.cz (O. Zmeskal).<br />
0960-0779/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.<br />
doi:10.1016/j.chaos.2009.01.022<br />
Chaos, Solitons and Fractals 42 (2009) 532–533<br />
Contents lists available at ScienceDirect<br />
Chaos, Solitons and Fractals<br />
journal homepage: www.elsevier.com/locate/chaos
Table 1<br />
Coefficients of n-order random Hausdorff dimension.<br />
n an D1,n D1 ¼ D1;n þ a<br />
D1<br />
0 1 2 / 0 =2 / 0<br />
1 1 1 / 1 =1+/ 1<br />
2 1 3 / 2 =3 / 2<br />
3 1 4 / 3 =4+/ 3<br />
4 1 7 / 4 =5 / 4<br />
5 1 11 / 5 =11+/ 5<br />
6 1 18 / 6 =18 / 6<br />
7 1 29 / 7 =29+/ 7<br />
8 1 47 / 8 =47 / 8<br />
9 1 76 / 9 =76+/ 9<br />
10 1 123 / 10 = 123 / 10<br />
n ( 1) n<br />
D1,n 1 + D1,n 2 / n = D1,n +( 1) n / n<br />
Table 2<br />
Values of n-order Hausdorff dimension for different parameters m and r.<br />
m n=1 n =2 n =3 n =4<br />
2. Some rational values of Hausdorff dimensions<br />
Calculated values of Hausdorff dimensions for integer parts repeating number m and selected rational ratio measure change<br />
r near to values random Hausdorff dimensions from Table 1 (D = ln m/ln r) are presented in Table 2. The basic Hausdorff fractal<br />
dimension were selected as Cantor discontinuum (m =2,r =3,D 1 = ln 2/ln 3) and more precision values by the term<br />
ln r<br />
D ¼<br />
lnðmrÞ ¼<br />
1<br />
: ð6Þ<br />
1 þ ln m= ln r<br />
For the second and the third iteration, it is D2 = ln3/ln(2 3) and D3 = ln6/ln (3 6).<br />
The general iteration equation for n = 1 is then equal to<br />
Dkþ1 ¼ 1<br />
:<br />
1 þ Dk and for n =2to<br />
ð7Þ<br />
1<br />
Dkþ1 ¼<br />
2<br />
Dk<br />
:<br />
Dk<br />
ð8Þ<br />
The limited value of this iteration is the golden mean value as is evident from the first column of Table 2. In a similar way,<br />
we can set the first and higher iteration of n-order random Hausdorff dimensions. The limited values are (as is evident from<br />
Table 2, second row), equal to n-th power of golden mean value (/ n ).<br />
3. Conclusion<br />
It is evident that the limit values of Hausdorff dimension derivative from Cantor discontinuum are equal to the n-th power<br />
of golden mean value. These results are the same as the result of random Hausdorff dimension. The differences between the<br />
experimental coefficients m and their ideal value (first column m r) are caused by the differences between iteration value and<br />
n power golden mean value.<br />
Acknowledgement<br />
This work was supported by project KAN401770651 from The Academy of Sciences of the Czech Republic.<br />
Reference<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 42 (2009) 532–533 533<br />
0.6180 r 0.3820 r 0.2361 r 0.1459<br />
2 3 0.6309 6 0.3869 19 0.2354 116 0.1459<br />
3 6 0.6132 18 0.3801 105 0.2361 1864 0.1459<br />
6 18 0.6199 109 0.3819 1978 0.2361 21554068 0.1459<br />
18 107 0.6186 1934 0.3820 207692 0.2361 401574147 0.1459<br />
108 1951 0.6180 210655 0.3820 410885916 0.2361<br />
1944 209518 0.6180 407303919 0.3820<br />
209952 408668420 0.6180<br />
408146688<br />
[1] El Naschie MS. An irreducibly simple derivation of the Hausdorff dimension of spacetime. Chaos, Solitons & Fractals 2009;41(4):1902–4.<br />
/ n<br />
1.0000...<br />
1.6180...<br />
2.6180...<br />
4.2361...<br />
6.8541...<br />
11.0902...<br />
17.9443...<br />
29.0344...<br />
46.9787...<br />
76.0132...<br />
122.9919...
J Fluoresc (2008) 18:1181–1186<br />
DOI 10.1007/s10895-008-0370-x<br />
ORIGINAL PAPER<br />
Comparative Studies of Diphenyl-Diketo-Pyrrolopyrrole<br />
Derivatives for Electroluminescence Applications<br />
Martin Vala & Martin Weiter & Jan Vyňuchal &<br />
Petr Toman & Stanislav Luňák Jr.<br />
Received: 31 October 2007 /Accepted: 14 March 2008 /Published online: 22 May 2008<br />
# Springer Science + Business Media, LLC 2008<br />
Abstract Four different derivatives of diphenyl-diketopyrrolopyrrole<br />
(DPP) with alkyl side groups were synthesized<br />
to increase their solubility. Quantum chemical<br />
calculations revealed that the substitution influenced molecular<br />
geometry and subsequently modified absorption and<br />
photoluminescence spectra. The theoretical results were<br />
confirmed by experimental characterization. With increasing<br />
phenyl torsion the vibrational structure was less<br />
pronounced and larger Stokes shift was observed. Simultaneously,<br />
the molar absorption coefficient decreased as the<br />
deformation increased. On the other hand, the measured<br />
fluorescence quantum yields were modified only slightly.<br />
This indicates the possibility to prepare soluble derivatives<br />
without loss of quantum yields and to use these materials<br />
for construction of efficient and stable electroluminescent<br />
devices. Furthermore, the electroluminescence of the thin<br />
M. Vala (*) : M. Weiter<br />
Faculty of Chemistry, Brno University of Technology,<br />
Purkynova 118,<br />
Brno 612 00, Czech Republic<br />
e-mail: vala@fch.vutbr.cz<br />
J. Vyňuchal<br />
Research Institute of Organic Syntheses,<br />
Rybitvi 296,<br />
Rybitvi 533 54, Czech Republic<br />
P. Toman<br />
Institute of Macromolecular Chemistry AS CR, v. v. i.,<br />
Heyrovsky Sq. 2,<br />
162 06 Prague 6, Czech Republic<br />
S. Luňák Jr.<br />
Department of Technology of Organic Compounds,<br />
Faculty of Chemical Technology, University of Pardubice,<br />
Studentská 95,<br />
Pardubice 530 09, Czech Republic<br />
layer devices based on the soluble low molecular DPPs<br />
were characterized and discussed.<br />
Keywords Diphenyl-diketo-pyrrolopyrrole .<br />
Organic light emitting diodes . OLED . Organic electronics .<br />
Electroluminescence<br />
Introduction<br />
Nowadays, we can notice the strong effort to produce<br />
organic electroluminescent devices (OLED) that can be used<br />
as a completely new generation of lamps and full colour flat<br />
panel displays. The potential advantages of these devices are<br />
high efficiency, low driving voltage, versatility in its<br />
application (flexibility, transparency), low weight, relatively<br />
cheap production, environmentally friendly materials, etc.<br />
There is a big deal of discussion whether low molecular<br />
materials with high stability or polymeric ones with excellent<br />
film forming properties and therefore offering low-cost<br />
solution-based processing are more advantageous [1].<br />
Nowadays, the most efficient devices used in display<br />
prototypes utilise combination of the materials providing<br />
specific functionality, e.g. hole or electron transport,<br />
reduction of injection barrier for holes and electrons, and<br />
of course the exciton formation and efficient radiative<br />
recombination. The materials chosen to build the device<br />
have to fulfil not only those requirements mentioned, but<br />
also their IP (ionisation potential) and EA (electron affinity)<br />
have to be aligned to allow effective charge migration [2].<br />
As can be seen, only one advantageous attribute, e.g. high<br />
fluorescence quantum yield, is not enough to build efficient<br />
electroluminescent device. It is therefore common to<br />
functionalise promising material in such way, which does<br />
not affect the intrinsic desired properties.
1182 J Fluoresc (2008) 18:1181–1186<br />
In this study we investigated group of several derivatives<br />
of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4<br />
dione, also known as DPP, see Fig. 1. DPP itself has a<br />
high quantum yield of fluorescence (ΦF), as well as a high<br />
molar absorption coefficient. Although it has been already<br />
reported that some derivatives are potentially suitable for<br />
electro-optical applications [3], these materials are mainly<br />
used as high performance pigments and are valued for their<br />
fatigue resistance. Several derivatives have been synthesised<br />
and studied to increase solubility of the DPP in organic<br />
solvents and thus offer cheap solution-based deposition<br />
techniques. The article discuss the influence of the change<br />
of the structure on the electronic spectra, fluorescence<br />
quantum yields, position of HOMO and LUMO orbitals<br />
necessary for efficient charge transfer, etc. In addition to<br />
experimental optical characterization, quantum chemical<br />
calculations were employed to determine these parameters<br />
and to find links between structure and functionality.<br />
Experimental<br />
The synthesised materials were analysed to confirm the<br />
molecule structure by A Bruker AMX 360 NMR spectrometer,<br />
ion trap mass spectrometer MSD TRAP XCT equipped<br />
with APCI, EA 1108 FISONS instrument for elemental<br />
analysis and Fourier transform infrared spectrometer. The<br />
referred ultraviolet-visible spectros<strong>copy</strong> (UV-VIS) absorption<br />
spectra were recorded in dimethylsufoxide (DMSO).<br />
The molar absorption coefficients were calculated from the<br />
dependence of absorption on concentration A=f(c) measured<br />
in 5 cm quartz cuvette (c
J Fluoresc (2008) 18:1181–1186 1183<br />
solubility of the sample. MS analysis, M=288: positive-ion<br />
MS: m/z 289 [M+H] + , 100%. Elemental analysis: calculated:<br />
C (74.99), H (4.20), N (9.72); found: C (74.99), H<br />
(4.20), N (9.72).<br />
Synthesis of DPP2 and DPP3<br />
11.6 g (0.04 mol) of the intermediate DPP1 and 120 g of<br />
dried dimethylformamide (DMF) were charged into the<br />
evaporative flask. The suspension was shortly stirred up.<br />
Subsequently, 16.2 g of 30% methanolic solution of sodium<br />
methylate was added. The fine slurry of sodium salt of<br />
DPP1 was stirred for 20 min, methanol was distilled out<br />
under the vacuum (t
1184 J Fluoresc (2008) 18:1181–1186<br />
ular conformations of the selected DPP derivatives were<br />
determined by means of the Hartree-Fock (HF) and B3LYP<br />
methods at 6–31G* level. The molecules were considered<br />
to be isolated, geometry was fully optimized with no<br />
molecular symmetry taken a priori into account. B3LYP is a<br />
hybrid HF/density functional theory (DFT) method, which<br />
combines Becke’s three-parameter exchange functional [6]<br />
with the Lee–Yang–Paar correlation functional [7]. It was<br />
successfully used for the calculations of the conformations<br />
of the different π-conjugated systems [8–10]. At the same<br />
time, the computer requirements of this method are<br />
comparable rather with the HF method than other “correlated”<br />
methods. However, the B3LYP method has also<br />
some disadvantages. B3LYP method, like other DFT-type<br />
methods, overestimates the electron delocalization and the<br />
degree of conjugation. The second drawback is that<br />
standard DFT-type methods cannot be used in combination<br />
with the configuration interaction (CI) method for the<br />
calculation of the relaxed excited state conformations. For<br />
this reason, molecular conformations were also calculated<br />
using HF method. For all the studied molecules we found<br />
only usual differences between HF and B3LYP calculated<br />
conformations, that are connected with the inclusion of the<br />
correlation energy in the B3LYP method.<br />
The optimized conformations show that some of the<br />
substituents influence the molecular geometry of the central<br />
DPP unit. The most important conformational parameters<br />
are the torsion angles α and β of the phenyls (see Fig. 1).<br />
While the unsubstituted DPP molecule is planar α=β=0°,<br />
derivatives denoted as DPP3 and DPP5 possess significantly<br />
rotated phenyl groups (see Table 1); in DPP2 and DPP4<br />
only the phenyl next to the substituted nitrogen atom is<br />
rotated. Phenyl group rotation significantly reduces the<br />
charge transfer integral between phenyls and the central<br />
DPP unit. Consequently the effective extent of conjugation<br />
is decreased and many electronic properties like absorption<br />
and luminescence are modified.<br />
Absorption and luminescence<br />
First excited states S1 of the studied molecules were<br />
calculated using ab initio configuration interaction method<br />
using single-excitation (CIS method) at the geometry<br />
optimized by the HF method. Absorption excitation<br />
energies ES0!S1<br />
are shown in Table 1. The ab initio CIS<br />
method enables to determine the trends in the excited state<br />
properties connected with the conformation change of the<br />
central DPP unit. However, the calculated absolute values<br />
of the excitation energies poorly reproduce the experimental<br />
results.<br />
The full UV–VIS absorption spectra of the studied<br />
molecules were calculated using time-dependent B3LYP<br />
(TD-B3LYP) method at the B3LYP optimized geometry.<br />
Time-dependent density functional methods recently became<br />
an effective and rather accurate tool for single point<br />
calculations of electronic excitations in various, namely<br />
conjugated, molecular systems [11–13], but they are not<br />
suitable for the excited state conformation optimization<br />
necessary for luminescence spectra calculations. They<br />
rather well reproduce the experimental peak positions, but<br />
obviously without a strong vibrational structure of the first<br />
transition.<br />
To obtain information about luminescence transitions,<br />
relaxed conformations of the S1 state were optimized by<br />
means of ab initio CIS method. During the S1 state<br />
relaxation the bond length alternation in the central DPP<br />
unit is reversed and the phenyl character becomes partly<br />
quinoidal. Finding relaxed conformations enabled determination<br />
of the first luminescence peak energy E lum, Stokes<br />
shift ΔEStokes, and deformation energy Edef of the relaxed<br />
excited state (see Table 1). It was found that the excitation<br />
energies ES0!S1 of the derivatives with the rotated phenyl<br />
groups exhibit a hypsochromic shift strongly correlated<br />
with the values of the torsion angles α and β. Similarly also<br />
ΔEStokes and Edef are significantly influenced by the phenyl<br />
Table 1 Phenyl torsion angles calculated by the HF and B3LYP methods, lowest absorption energy ES0!S1 , first luminescence peak E lum, Stokes<br />
shift ΔEStokes, and deformation energy Edef of the relaxed excited state<br />
Derivative B3LYP method HF method ES0!S1 E lum ΔE Stokes E def<br />
α [°] β [°] α [°] β [°] [eV] [eV] [eV] [eV]<br />
DPP1 0.0 0.0 0.0 0.0 2.837 2.427 0.410 0.339<br />
DPP2 36.0 6.7 46.9 16.9 2.935 2.424 0.511 0.437<br />
DPP3 36.5 36.5 46.1 46.1 3.012 2.441 0.571 0.482<br />
DPP4 36.7 7.7 46.9 16.9 2.933 2.422 0.511 0.437<br />
DPP5 35.9 35.9 46.0 46.0 3.010 2.438 0.571 0.482<br />
For the absorption and luminescence calculations, the ground state conformations were optimized by means of the HF method and the relaxed<br />
excited state conformations were calculated using the ab initio CIS method
J Fluoresc (2008) 18:1181–1186 1185<br />
rotation. The calculated first luminescence peak changes<br />
only slightly.<br />
Optical and optoelectrical characterisation<br />
The substitution of central DPP unit by alkyl side chains<br />
resulted in hypsochromic shift of the absorption (see<br />
Fig. 2a). This shift increases with the number of alkyl<br />
groups but is not affected by the length of the alkyls used.<br />
Furthermore, the vibration modes present in the spectrum of<br />
the non-substituted molecules were reduced. This tendency<br />
is further amplified as the substitution introduces second<br />
alkyl group. The spectral shift is accompanied with<br />
decrease of molar absorption coefficient (see Table 2).<br />
The fluorescence spectra of the synthesized derivatives<br />
(Fig. 2b) shows bathochromic shift with respect to the<br />
primary DPP1. The increasing Stokes shift is also accom-<br />
(a)<br />
A/A Max<br />
A/A Max<br />
(b)<br />
FL/FL Max<br />
FL/FL Max<br />
1.0<br />
0.5<br />
0.0<br />
1.0<br />
0.5<br />
0.0<br />
1.0<br />
0.5<br />
0.0<br />
1.0<br />
0.5<br />
DPP1<br />
DPP4<br />
DPP5<br />
DPP1<br />
DPP2<br />
DPP3<br />
300 400 500 600<br />
wavelength (nm)<br />
DPP1<br />
DPP4<br />
DPP5<br />
DPP1<br />
DPP2<br />
DPP3<br />
0.0<br />
450 500 550 600 650 700<br />
wavelength (nm)<br />
Fig. 2 Absorption a and fluorescence b spectra measured in<br />
dimethylsulfoxide. The spectra were normalized to the longer<br />
wavelength band. Clear hypsochromic shift in absorption and bathochromic<br />
shift in fluorescence (FL) spectra accompanied with the lost<br />
of vibronic structure can be observed as the substitution takes place<br />
Table 2 Experimental values for molar absorption coefficient ɛ,<br />
position of absorption l AbsMax and luminescence l FLMax peak<br />
maximums, Stokes shift Δl Stokes and fluorescence quantum yield Φ F<br />
(ΔΦ F
1186 J Fluoresc (2008) 18:1181–1186<br />
technique. It has to be mentioned that the DPP1 is not<br />
soluble enough for spin casting technique and was prepared<br />
by thermal vacuum evaporation. The mono substituted<br />
derivatives prepared by spin casting showed tendency to<br />
form highly rough (tens of nanometer) and up to more than<br />
5 μm long curved “lying grass” like structures. The<br />
symmetrically substituted derivatives formed large crystals<br />
up to tens of μm long tightly packed together. This filmforming<br />
properties influence the electric response and<br />
therefore the electroluminescence behaviour to a considerable<br />
extent.<br />
Conclusions<br />
Basic structure and four derivatives of diphenyl-diketopyrrolopyrrole<br />
were synthesized and compared from the<br />
point of view of their potential use as light emitting layer for<br />
organic electroluminescence device. It was found that the<br />
electronic properties strongly depend on the planarity of<br />
the molecules, which was decreased by the substitution. The<br />
measurements showed that the original high fluorescence<br />
quantum yield reminds nearly the same. This indicates the<br />
possibility to prepare derivatives soluble in solvents suitable<br />
for cheap large-scale production without loss of fluorescence<br />
quantum yields and therefore to use these materials for<br />
construction of efficient electroluminescent devices. In order<br />
to get the best performance it is necessary to optimise the<br />
OLED structure for each derivative.<br />
Acknowledgement The research was supported by the Ministry of<br />
Industry and Trade of the Czech Republic via Tandem project No. FT-<br />
TA3/048 and by the Grant Agency of the Academy of Sciences of the<br />
Czech Republic via project A401770601.<br />
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<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
Conductivity of natural and modified DNA measured by scanning tunneling<br />
micros<strong>copy</strong>. The effect of sequence, charge and stacking<br />
Irena Kratochvílová a, ⁎, Karel Král a , Martin Bunček b , Alena Víšková b , Stanislav Nešpůrek c , Anna Kochalska c ,<br />
Tatiana Todorciuc c , Martin Weiter d , Bohdan Schneider e<br />
a Institute of Physics, ASCR, v.v.i., Na Slovance 2, CZ-182 21 Prague, Czech Republic<br />
b GENERI BIOTECH s.r.o., Machkova 587, CZ-500 11 Hradec Kralové, Czech Republic<br />
c Institute of Macromolecular Chemistry ASCR, v.v.i., Heyrovského nám. 2, CZ-162 06 Prague 6, Czech Republic<br />
d Faculty of Chemistry, Brno University of Technology, Purkyňova 118, CZ-612 00 Brno, Czech Republic<br />
e Institute of Biotechnology AS CR, v.v.i., Vídeňská 1083, CZ-142 20 Prague, Czech Republic<br />
article info<br />
Article history:<br />
Received 1 June 2008<br />
Received in revised form 12 August 2008<br />
Accepted 16 August 2008<br />
Available online 28 August 2008<br />
Keywords:<br />
DNA conductivity<br />
Charge transport in molecular systems<br />
STM<br />
Electronic properties of biomolecules<br />
1. Introduction<br />
abstract<br />
The DNA molecule, containing the genetic code of all living species,<br />
has recently become a center of great attention on the part of chemists<br />
and physicists, one of the reasons being DNA's potential use in<br />
nanoelectronic devices [1–4], both as a template for assembling<br />
nanocircuits and as an active element of such circuits. Charge<br />
migration along DNA molecules has attracted scientific interest for<br />
more than fifty years. The reports on the possible high rates of charge<br />
transfer between the donors and acceptors through DNA, obtained<br />
from solution chemistry experiments [5], have triggered a series of<br />
direct electrical transport measurements on bundles and networks,<br />
because a truly conducting form of DNA would have a major impact on<br />
the developments in nanotechnologies.<br />
Extended electronic states of DNA could play an important role in<br />
biology, e.g. through the processes of DNA-damage sensing or<br />
repairing via long-range charge transfer [6,7]. The prevailing DNA<br />
architecture, the double helix, has well stacked, nearly parallel bases<br />
with overlapping π-electron systems. Such π-electron systems may be<br />
⁎ Corresponding author.<br />
E-mail addresses: krat@fzu.cz (I. Kratochvílová), kral@fzu.cz (K. Král),<br />
martin.buncek@generi-biotech.com (M. Bunček), nespurek@imc.cas.cz (S. Nešpůrek),<br />
weiter@fch.vutbr.cz (M. Weiter), bohdan@img.cas.cz (B. Schneider).<br />
0301-4622/$ – see front matter © 2008 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.bpc.2008.08.005<br />
Biophysical Chemistry 138 (2008) 3–10<br />
Contents lists available at ScienceDirect<br />
Biophysical Chemistry<br />
journal homepage: http://www.elsevier.com/locate/biophyschem<br />
The conductivity of DNA covalently bonded to a gold surface was studied by means of the STM technique. Various<br />
single- and double-stranded 32-nucleotide-long DNA sequences were measured under ambient conditions so as<br />
to provide a better understanding of the complex process of charge-carrier transport in natural as well as<br />
chemically modified DNA molecules. The investigations focused on the role of several features of DNA structure,<br />
namely the role of the negative charge at the backbone phosphate group and the related complex effects of<br />
counterions, and of the stacking interactions between the bases in Watson–Crick and other types of base pairs.<br />
The measurements have indicated that the best conductor is DNA in its biologically most relevant doublestranded<br />
form with Watson–Crick base pairs and charged phosphates equilibrated with counterions and<br />
water. All the studied modifications, including DNA with non-Watson–Crick base pairs, the abasic form, and<br />
especially the form with phosphate charges eliminated by chemical modifications, lower the conductivity of<br />
natural DNA.<br />
© 2008 Elsevier B.V. All rights reserved.<br />
good candidates for long-distance and one-dimensional (linear)<br />
charge transport [8]. Several authors have indicated that DNA<br />
conducts electric charge via the hopping mechanism [9–11], but the<br />
electronic properties of DNA remain controversial. Charge-transfer<br />
reactions and conductivity measurements exhibit a large variety of<br />
possible electronic behavior, ranging from Anderson and band-gap<br />
insulators to effective molecular wires and induced superconductors.<br />
Indeed, understanding the conductance of a complicated polyelectrolyte<br />
aperiodic system is in itself a major scientific problem [12–16].<br />
Historically, two basic experimental approaches have been<br />
pursued to investigate conduction through molecules: (i) contacting<br />
isolated single molecules or molecules in thin films, and (ii) studying<br />
transport in thick films and devices, such as organic thin-film<br />
transistors or light-emitting diodes [17,18]. The optimal experimental<br />
setup is to position isolated molecules between two electrical<br />
contacts, but this is very difficult to implement and especially difficult<br />
to verify. Working with the ordered arrays of parallel π-conjugated<br />
molecules, where in principle the binding to the substrate can be<br />
precisely controlled, offers a possibility to measure the individual<br />
molecules by making use of the scanning–tunneling-microscope<br />
(STM) tip. STM experiments have already been proven as a very<br />
suitable tool for investigating ordered monolayer films, making it<br />
possible to contact one or more molecules and obtain images of the<br />
structural characteristics of the film [19,20]. The experiments have
also been used to investigate room-temperature electronic properties<br />
of twelve base-pair d(GC) 12-d(GC) 12 DNA molecules attached to the<br />
gold surface by a thiol link [21]. The STM/STS experiments offer a novel<br />
way of probing the electronic properties of biomolecules on surfaces<br />
on the atomic level.<br />
In this work, we have used the STM technique in order to study the<br />
conductivity of several oligonucleotide sequences in single- and<br />
double-stranded forms under ambient conditions. The purpose of<br />
these investigations was to understand the complex processes of<br />
charge transport through DNA molecules and to dissect the role of the<br />
sugar-phosphate backbone, and of its charged phosphate group in<br />
particular, as well as of base stacking in the same strand and across<br />
strands. To this end, some of the studied oligonucleotide sequences<br />
were designed to form non-Watson–Crick base pairs, with the aim of<br />
making it possible to estimate the role of these mutation-causing base<br />
pairs. The role of the bases and of their stacking interactions in DNA<br />
conductivity was also studied at a more fundamental level by<br />
removing two or three nitrogenous bases at the central steps of a<br />
few sequences forming modified abasic nucleotides. Other sequences<br />
contain chemically modified phosphate groups, which eliminate their<br />
charge (Fig. 1). Such modifications make it possible to see the roles of<br />
the charged backbone and of the alkali-metal and alkaline-earthmetal<br />
counterions on conductivity.<br />
2. Experimental<br />
2.1. Material<br />
STM experiments were performed on 32-mer oligonucleotides of<br />
various sequences, some of which had been chemically modified. For<br />
further reference, all the measured oligonucleotides, including their<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
4 I. Kratochvílová et al. / Biophysical Chemistry 138 (2008) 3–10<br />
Fig. 1. The chemical scheme of the studied nucleotides: (a) a natural nucleotide; b) a<br />
part of an abasic oligonucleotide with base removed; c) oligonucleotide with its<br />
phosphate neutralized by a p-isopropoxy group.<br />
labels, are listed in Table 1. The sequences were designed to investigate<br />
the differences: i) between the single-stranded (ss) and doublestranded<br />
(ds) DNA of C/G-rich composition, ii) between the singlestranded<br />
(ss) and double-stranded (ds) DNA of A/T-rich composition,<br />
iii) between dsDNA with only canonical Watson–Crick and containing<br />
non-Watson–Crick (“mismatched”) base pairs, and iv) between DNA<br />
with sequences composed of only natural (non-modified) nucleotides<br />
and chemically modified nucleotides. Further, two types of chemical<br />
modifications were studied: i) the elimination of nitrogenous bases,<br />
thus creating abasic nucleotides, in order to be able to gauge the<br />
influence of the stacked bases on the DNA conductivity, and ii) the<br />
replacement of the charged phosphate group with its neutral<br />
surrogate, which makes it possible to estimate the role of the<br />
phosphate charge on the DNA conductivity. The investigated DNA<br />
samples (Table 1) were measured in pairs selected in such a way that<br />
only one parameter would vary in a pair (see Fig. 1 for the chemical<br />
structures of the nucleotides). For instance, the phosphate charge<br />
present in the standard oligonucleotide (Fig. 1, label A) is eliminated in<br />
DNA E of the identical sequence, the sequences differ between A and B,<br />
or between C and its abasic mutant D.<br />
The sequences were selected so that they would not form such<br />
special structural features as loops or complex tertiary structures.<br />
Guanosine nucleotides were evenly distributed as every fourth base in<br />
the mixed sequences. The pure G/C (Table 1, ss: A and ds: 1) and A/T<br />
(Table 1, B and 2) oligonucleotides were designed for the evaluation of<br />
the effect which the guanine base might have on the overall DNA<br />
conductivity. The C/G, T/A and the mixed C/G/T/A sequences were<br />
prepared as single-stranded and double-stranded with their respective<br />
complementary sequences in order to evaluate the effect of<br />
single- versus double-stranded DNA forms. So as to ascertain the<br />
influence of the heterocyclic nitrogenous bases on the overall charge<br />
transport in DNA, we compared the conductivity of the mixed C/G/T/A<br />
sequence (Table 1, C — MIX) containing all four bases in equal<br />
proportion with guanine evenly distributed on the one hand with a<br />
chain with the same sequences except for three removed bases in the<br />
middle (Positions 15–17,Table 1,D— MIX_S) on the other. These abasic<br />
sequences produce a gap with three base pairs missing in the<br />
respective ds sample (Table 1, ds 3 and 4). Oligo d(T) where two bases<br />
in the middle were substituted by either two guanines or by abasic<br />
spacer, making two base gaps (Fig.1b), were also synthesized. To study<br />
the impact of charge in the DNA backbone on the charge transport, all<br />
the above sequences were also synthesized with a chemically<br />
modified phosphate group, which eliminates the phosphate charge.<br />
Thus the sequences were prepared as standard phosphodiesters<br />
(Fig. 1a), rendering a negative charge on each phosphate, and also as<br />
neutral but still polar p-isopropoxy derivatives (Fig. 1c, Table 1, E and<br />
NGC). How non-canonical (non-W–C) base pairs affect the conductivity<br />
was measured on DNA chains modified by introducing sequences<br />
with two non-W–C (“mismatched”) pairs (Table 1, ds 6 and 7).<br />
All the sequences were analyzed by MALDI-TOF mass spectros<strong>copy</strong><br />
in order to prove their identity and evaluate their purity. The<br />
sequences with neutralized phosphates charges (p-isopropoxy modified)<br />
showed a small or undetectable number of Na + /K + adduct peaks,<br />
and having compared them to MALDI-TOF measured concentration of<br />
Na + /K + in charged nucleotides, we concluded that the sequences with<br />
uncharged phosphates may contain no or only a very small number of<br />
Na + /K + counterions associated with the oligonucleotide.<br />
Oligonucleotides were synthesized in GENERI BIOTECH, s.r.o., on an<br />
ABI394 synthesizer using standard phosphoramidite chemistry. The<br />
standard base phosphoramidites, 5′-thiol modifier C6 and dSpacer CE<br />
phosphoramidite were purchased from Glen Research, USA, and used<br />
according to the manufacturer's recommendations. The p-isopropoxy<br />
derivatives of base amidites (deoxy adenosine (n-bz) p-isopropoxy<br />
phosphoramidite, deoxy Cytidine (n-bz) p-isopropoxy phosphoramidite,<br />
deoxy guanosine (n-ibu) p-isopropoxy phosphoramidite and<br />
thymidine p-isopropoxy phosphoramidite) were purchased from
Table 1<br />
The investigated DNA sequences<br />
ChemGenes Corporation, USA, and also used according to the<br />
manufacturer's recommendations. All oligonucleotide sequences<br />
were reverse-phase chromatography purified. The 5′-thiol-modified<br />
oligonucleotides were purified according to the manufacturer recommendations,<br />
aliquoted immediately after the purification and kept<br />
under argon atmosphere before use to prevent oxidative dimerization<br />
of the sulfhydryl groups. All the synthesized oligonucleotide sequences<br />
were analyzed by HPLC and MALDI-TOF for quality control. According<br />
to the MALDI-TOF analysis, the purity of all sequences was N98%.<br />
All the chemicals were purchased from Sigma Aldrich, USA. The<br />
blocking solution of 1mM 2-mercaptoethanol in MilliQ water (BS), a<br />
hybridization buffer of 100mM Tris.HCl/100mM NaCl in MilliQ water<br />
(HB) and a washing buffer of 100mM Tris–HCl/300mM NaCl in MilliQ<br />
water (WB) were prepared by dissolving the respective amounts of<br />
chemicals in pure MilliQ water, free of DNA/RNA and nucleases<br />
(Millipore Inc., USA). The 5′-thiol-modified oligonucleotides were<br />
dissolved in MilliQ water to 50μM concentration prior to use. 5μl of<br />
50μM solution of each 5′-thiol-modified oligonucleotide were spotted<br />
onto the gold support, forming a drop of ca 3mm in diameter. The<br />
spotted gold support slides were kept closed in a cartridge with<br />
humidified atmosphere created by water-filled basins. This cartridge<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
I. Kratochvílová et al. / Biophysical Chemistry 138 (2008) 3–10<br />
Code Name Sequence Backbone modification<br />
A G/C Au–S-CGCCGCCGCCGCCGCCGCCGCCGCCGCCGCCG-3′ NO<br />
B A/T Au–S-ATAATAATAATAATAATAATAATAATAATAAT-3′ NO<br />
C MIX Au–S-GTTAGCACGATAGTCCGATAGTCAGTCAGTCC-3′ NO<br />
D MIX_dS Au–S-GTTAGCACGATAGTxxxATAGTCAGTCAGTCC-3′ NO<br />
E NGC Au–S-CGCCGCCGCCGCCGCCGCCGCCGCCGCCGCCG-3′ Uncharged p-isopropoxy<br />
Code Sequence Backbone modification<br />
1 Au–S-CGCCGCCGCCGCCGCCGCCGCCGCCGCCGCCG-3′<br />
||||||||||||||||||||||||||||||||<br />
3′-GCGGCGGCGGCGGCGGCGGCGGCGGCGGCGGC-5′<br />
NO<br />
2 Au–S-ATAATAATAATAATAATAATAATAATAATAAT-3′<br />
||||||||||||||||||||||||||||||||<br />
3′-TATTATTATTATTATTATTATTATTATTATTA-5′<br />
NO<br />
3 Au–S-GTTAGCACGATAGTCCGATAGTCAGTCAGTCC-3′<br />
||||||||||||||||||||||||||||||||<br />
3′-CAATCGTGCTATCAGGCTATCAGTCAGTCAGG-5′<br />
NO<br />
4 Au–S-GTTAGCACGATAGTxxxATAGTCAGTCAGTCC-3′ NO<br />
|||||||||||||| |||||||||||||||<br />
5<br />
3′-CAATCGTGCTATCAxxxTATCAGTCAGTCAGG-5′<br />
Au–S-TTTTTTTTTTTTTTTGGTTTTTTTTTTTTTTT-3′<br />
||||||||||||||||||||||||||||||||<br />
3′-AAAAAAAAAAAAAAACCAAAAAAAAAAAAAAA-5′<br />
NO<br />
6 Au–S-TTTTTTTTTTTTTTTGGTTTTTTTTTTTTTTT-3′<br />
|||||||||||||||‡‡|||||||||||||||<br />
3′-AAAAAAAAAAAAAAAGGAAAAAAAAAAAAAAA-5′<br />
NO<br />
7 Au–S-TTTTTTTTTTTTTTTCCTTTTTTTTTTTTTTT-3′<br />
|||||||||||||||‡‡|||||||||||||||<br />
3′-AAAAAAAAAAAAAAACCAAAAAAAAAAAAAAA-5′<br />
NO<br />
8 Au–S-TTTTTTTTTTTTTTTCCTTTTTTTTTTTTTTT-3′<br />
||||||||||||||||||||||||||||||||<br />
3′-AAAAAAAAAAAAAAAGGAAAAAAAAAAAAAAA-5′<br />
NO<br />
9 Au–S-TTTTTTTTTTTTTTTxxTTTTTTTTTTTTTTT-3′<br />
||||||||||||||| |||||||||||||||||<br />
3′-AAAAAAAAAAAAAAAxxAAAAAAAAAAAAAAA-5′<br />
NO<br />
10 Au–S-CGCCGCCGCCGCCGCCGCCGCCGCCGCCGCCG-3′<br />
||||||||||||||||||||||||||||||||<br />
3′-GCGGCGGCGGCGGCGGCGGCGGCGGCGGCGGC-5′<br />
Uncharged p-isopropoxy<br />
11 Au–S-ATAATAATAATAATAATAATAATAATAATAAT-3′<br />
||||||||||||||||||||||||||||||||<br />
3′-TATTATTATTATTATTATTATTATTATTATTA-5′<br />
Uncharged p-isopropoxy<br />
12 Au–S-GTTAGCACGATAGTCCGATAGTCAGTCAGTCC-3′<br />
||||||||||||||||||||||||||||||||<br />
3′-CAATCGTGCTATCAGGCTATCAGTCAGTCAGG-5′<br />
Uncharged p-isopropoxy<br />
Single-stranded samples are labeled with letters A–E and named, double-stranded DNA are numbered 1–12. G, A, C, T are the symbols that designate the deoxynucleotides with<br />
either unmodified backbone (NO “backbone modification”) or the phosphate charge neutralized by an isopropoxy group (Fig. 1), whereas the symbols “x” indicate abasic nucleotides.<br />
Strands chemically attached to the gold plate start with Au–S. For double-stranded samples, “|” labels the Watson–Crick pair while “‡“ indicates a non-Watson–Crick pair.<br />
was incubated at 50°C for 16h, after which the gold support was<br />
washed twice with MilliQ water, and 5μl of 1mM 2-mercaptoethanol<br />
were spotted onto the same places as the oligonucleotides. The 2mercaptoethanol<br />
(2-ME) was used to enhance the accessibility of the<br />
immobilized probes to the complementary target sequences. The thiol<br />
group of 2-ME rapidly displaced the weaker absorptive contacts<br />
between the DNA nucleotides and the substrate, leaving the probes<br />
tethered primarily through the thiol end groups. The spots were “selfaligned”,<br />
because the gold support is hydrophobic and any water<br />
solution put onto the oligonucleotide-modified gold support concentrates<br />
in the oligonucleotide-containing spots. The slides were<br />
incubated in the same humidified cartridge at 40°C for 2h, after<br />
which the slides were washed 5 times with MilliQ water and dried<br />
under high-purity nitrogen (9.6). The gold-support slides with singlestranded<br />
oligonucleotide spots were kept in a closed box until use. The<br />
gold-support slides with the oligonucleotide spots planned to be<br />
analyzed as double-stranded were hybridized with the corresponding<br />
complementary oligonucleotides. The complementary oligonucleotides<br />
were incubated at 1μM concentration in HB at 80°C for 10min.<br />
Immediately after the incubation, 5μl of the corresponding complementary<br />
oligonucleotides were dropped onto the particular spots and<br />
5
incubated at 40°C for 1h, after which the spots were washed twice by<br />
WB and finally by MilliQ water. Subsequently, the slides were dried<br />
under a stream of nitrogen. The whole process was photographed for<br />
the easier alignment of the STM tip onto the oligonucleotide spots. All<br />
the slides were kept in a closed box in a cool and dry place before use.<br />
DNA molecules were attached to the gold surface through a sulfur–<br />
gold linkage (Fig. 2) [22]. The ssDNA with the 5′-thiol-modifier were<br />
used to produce a thiol group at the 5′-terminus of a synthetic<br />
oligonucleotide. Recent neutron reflective studies have indicated that<br />
ssDNA oligonucleotides form a compact layer on bare gold. Prior to the<br />
formation of thiolated-ssDNA, self-assembled 40nm thin Au (99.99%<br />
purity) monolayers were vacuum-evaporated on single crystalline<br />
silicon substrate b100N wafers, which were cut into 1cm × 1cm pieces.<br />
Subsequently, the substrates were blown using pure nitrogen and<br />
exposed to a solution of thiolated oligonucleotide probes in a sodium<br />
chloride buffer.<br />
2.2. Measurements<br />
The STM measurements were performed with the NTEGRA Prima<br />
NT MDT system under ambient conditions. Both topographic and<br />
spectroscopic data were obtained using freshly cut Pt/Ir tips. With<br />
STM, for a set point of small voltage ~ 10mV and relatively large<br />
current ~ 0.5nA and thus quite a small tip-sample distance, we<br />
observed the topography of the samples. The topographic images<br />
showed a considerable difference between the bare gold substrates<br />
and the samples with DNA molecules. In the case of molecular layers,<br />
specific surface patterns were observed. The occurrence of these<br />
patterns was attributed to an etching of the bottom gold layers arising<br />
from the reaction of the thiol-groups with the gold atoms, forming<br />
dissolvable complexes [23,24].<br />
The DNA layers thickness have been investigated using variable<br />
angle spectroscopic ellipsometry (VASE, J.A.Woollam & co.) working in<br />
rotating analyzer mode. The measurements were carried out in the<br />
spectral range 300–1100nm at three angles of incidence 65, 70 and<br />
75°. The structural model was constructed to extract the optical<br />
functions of the film from the ellipsometry measurements. Structural<br />
model consists of: Si substrate/Au layer/DNA layer. The Si-substrate<br />
was considered as a semi-infinite medium. The optical constants of Si<br />
and Au materials were taken from [25]. The thicknesses of both layers<br />
and the optical constants of DNA layer were determined using a direct<br />
fitting procedure applied to the experimental ellipsometric data. The<br />
DNA molecules were ordered in similar geometric structures with the<br />
nearest-neighbor spacing being 1–2nm. The thickness of the DNA<br />
layers, determined by ellipsometry was 8.5–9.0nm. For 32-nucleotidelong<br />
DNA molecules, this length corresponds to the average distance<br />
between the neighboring bases of about 0.3nm, which corresponds to<br />
the base–base stacking distance intermediate between the B- and A-<br />
DNA forms. The DNA double strand is not likely to be completely<br />
straight. The mean inclination of a double helix from the fixed vertical<br />
line at the length of 32 base pairs is expected to be about 25° as can be<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
6 I. Kratochvílová et al. / Biophysical Chemistry 138 (2008) 3–10<br />
Fig. 2. The scheme of the attachment of the DNA 32-mers to the gold surface.<br />
Table 2<br />
A comparison of the conductivity of DNA samples measured on the same gold plate in one experiment<br />
estimated from the generally accepted persistent length P of a dsDNA<br />
of “random sequence”, P ~ 150 base pairs. A confrontation of the<br />
ellipsometry measurement and the persistent length of a DNA double<br />
strand leads to a conclusion that DNA molecules are oriented<br />
approximately perpendicularly to the Au substrate.<br />
In all cases, we qualitatively compared conductivity of two types of<br />
oligonucleotides. In our experimental setting the tip–sample distance<br />
was necessary and not easy to control. Firstly, we worked in constant<br />
height mode in very small area on both sides of the compared<br />
oligonucleotides type border. Secondly, beside current-voltage (I(V))<br />
characteristics we also measured I(h) curves (I — current, h — distance<br />
between the tip and the surface) at the same points. I(h) curves have<br />
the advantage of being normalized and in [26] new approach for<br />
reading sequence of DNA molecule via tunnel-current decay has been<br />
demonstrated. The I(h) decay consists of two regions: the area of very<br />
short-range chemical interactions between the end of the tip and the<br />
terminate of the molecule plus the area of tunneling through the<br />
barrier between the tip and sample. The current initially decays slowly<br />
and then more rapidly. The I(h) curves are standardly fitted with two<br />
exponentials:<br />
i ¼ i0expð−β1hÞ; 0 b h b hc<br />
i ¼ ihc ð Þexpð−β2ðh−hcÞÞ; hc b h:<br />
Value of β 2 has been found in [26] as corresponding to decay<br />
constant of tunneling current. Thus, it can be supposed that at the<br />
breakpoint (h c) the interactions typical for tip extremely close to the<br />
substrate are broken and the tunnel current becomes dominant. The<br />
same position of the breakpoint and decay constants (β 2, β 1) means<br />
the same tip-surface interaction conditions. Working with different<br />
set points we can arrange the position of the tip in the proper<br />
interaction territory so that the distance between the tip and the DNA<br />
terminate is under control. Comparing histograms of the decay<br />
constants and I(h) curves breakpoints (hc) measured on various DNA<br />
samples we controlled position of the tip above the compared DNA<br />
modifications.<br />
In our experiments, we have measured current passing through<br />
the molecules so close to the tip that their contribution is at set voltage<br />
higher than the noise. For tip very close to the molecules (defined by<br />
the set point value) and very low voltages the number of the<br />
molecules contributing to the total current is as small as possible.<br />
Current–voltage characteristics were recorded for various feedback<br />
voltage and current set points, i.e. for different initial tip–sample<br />
distances. In all the experiments, the STM tip acted as the electrical<br />
contact on the “top” side of the assembled monolayer of the DNA<br />
molecules, whereas the supporting gold substrate acted as the other<br />
“bottom” contact. The distance between the top of the molecules and<br />
the STM tip was for all the set points 2.5–4Å. The I(V) and I(h) curves<br />
were measured at three different set points: 0.1nA, 0.1V; 0.2nA, 0.1V;<br />
0.5nA, 0.1V, corresponding to three different sample–tip separations,<br />
using the same strategy for all the samples. The experimental<br />
conditions were controlled using I(h) decay analysis and thus we<br />
were able to distinguish between the area of tunneling current and<br />
current passing through chemically interacted systems. The experimental<br />
setup (tip very close to the surface) allows us to expect that the<br />
current flows mainly through the molecule nearest to the tip without<br />
significant lateral intermolecular contribution to conduction. Two<br />
hundred consecutive I(V) sweeps in both voltage directions were<br />
DNA samples measured in one experiment 1/2 A/1 3/4 5/9 8/9 1/10 2/11 8/7 5/6 3/12<br />
DNA samples with higher conductivity<br />
The codes of DNA samples are listed in Table 1.<br />
1 1 3 5 8 1 2 8 5 3
Fig. 3. The typical I(V) curves of DNA GC 32-base double strand (DS GC, see Table 1,ds1)<br />
and DNA AT 32-base double strand (DS AT, see Table 1, ds 2). Set point 0.1 nA, 0.1 V,<br />
measured in one voltage direction.<br />
taken on each sample (Table 1). Final results presented in this paper<br />
are based on the average of a set from the collected I(V) curves. Only<br />
those curves which were not affected much by drift of the STM were<br />
included in the statistics.<br />
All the results taken into the consideration showed the same<br />
trends, i.e. the symmetry for both biases, linearity and super-linearity<br />
for lower and higher voltages, respectively. Due to large statistics we<br />
can (from our measurements) set the quantitative proportion of<br />
different DNA systems conductivities assuming approximately equal<br />
number of molecular densities.<br />
3. Results and discussion<br />
The results of the measurements are summarized in Table 2 and<br />
Figs. 3–7. The conductivity measurements for each considered<br />
structural and chemical feature and their possible explanations are<br />
discussed below. Noise, in the form of height fluctuations, was higher<br />
in the case of the topographic images of films than in the case of bare<br />
Fig. 4. The typical I(V) curves of DNA GC 32-base single strand (SS GC, see Table 1, SSA)<br />
and DNA GC 32-base double strand (DS GC, see Table 1, ds 1). Set point 0.1 nA, 0.1 V,<br />
measured in one voltage direction.<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
I. Kratochvílová et al. / Biophysical Chemistry 138 (2008) 3–10<br />
Fig. 5. I(V) curves of double-helical DNA 32-mers with mixed G, A, C, T sequences<br />
(Table 1, ds sample 3) and double-helical DNA 32-mers with mixed G, A, C, T sequences<br />
with three bases missing in the middle (MIX_DS, see Table 1, ds 4). Set point 0.1 nA,<br />
0.1 V, measured in both voltage directions.<br />
gold. All the organic molecules that have been studied by STM until<br />
now show symmetry of the (I(V)) characteristics. The symmetry of the<br />
I(V) characteristics has been explained on a simple model [27].<br />
The conductivity of all 32-nucleotide-long DNA molecules including<br />
the sulfur spacer was comparable in the STM experiments. We<br />
measured the conductivity of the DNA molecules using a scanning<br />
tunneling microscope under ambient conditions, i.e. in a humid air<br />
(40% humidity) environment and room temperature. Xu et al. [21],<br />
who measured DNA samples using ultrahigh vacuum scanning<br />
spectros<strong>copy</strong>, concluded that DNA was a wide-band-gap insulator.<br />
From this comparison, we can see that the absence or at least<br />
substantially lower content of water in the ultrahigh vacuum plays a<br />
significant role in the charge pathway. In agreement with what Enders<br />
et al. pointed out in their summary [28], we have concluded that water<br />
molecules supporting the molecular structure improve the conditions<br />
for the electrical current passing through the whole DNA molecule.<br />
Fig. 6. I(V) curves of double-helical DNA 32-mers with G-C bases without chargebearing<br />
groups on the phosphates – DS NGC (see Table 1, ds 10) and standard doublehelical<br />
DNA 32-mers with G-C bases – DS GC (see Table 1, ds 1). Set point 0.1 nA, 0.1 V,<br />
measured in both voltage directions.<br />
7
3.1. G/C chain vs. A/T chain<br />
The Guanine/Cytosine-rich and Adenine/Thymine-rich oligonucleotides<br />
(Table 1, ds 1 and 2) were selected for the evaluation of the<br />
guanine effect on the overall DNA conductivity.<br />
We found that a double-helical sample with G/C bases is a better<br />
conductor than the double strand made of A/T bases (Fig. 3, Table 2).<br />
The guanine base is known to be easily oxidized, generating a charge<br />
carrier (hole). Once charges, and especially holes, are created in the<br />
uniform DNA chain, the hopping charge transport can apparently<br />
occur between discrete guanine sites or delocalized (e.g. polaron)<br />
domains [29]. Furthermore, the stacking of the adjacent base pairs is<br />
also likely to affect the conductivity via the π-electron overlap. The G–<br />
C pairs with more compact stacking compared to that in the A–T pairs<br />
have higher conductivity.<br />
3.2. Double-stranded vs. single-stranded DNA<br />
Typical I(V) curves of both ss and ds GC sequences (A and 1 in<br />
Table 1) are shown in Fig. 4. The conductivity of the double-stranded<br />
form is higher than that of the single strand when complementary<br />
sequences are compared, which is in agreement with the theoretical<br />
model of the same system [30]. Single strands can be considered much<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
8 I. Kratochvílová et al. / Biophysical Chemistry 138 (2008) 3–10<br />
Fig. 7. Typical I(V) curves of DNA molecules containing 4 non-Watson-Crickbase pairs in<br />
the middle (TGG vs. AGG and TCC vs. ACC, see Table 1, ds 6 and ds 7, respectively) and<br />
standard dsDNA (TGG vs. ACC and TCC vs. AGG, see Table 1, ds 5 and ds 8, respectively).<br />
Set point 0.1 nA, 0.1 V, measured in both voltage directions.<br />
less structurally regular than the double helices because of their local<br />
and especially long-range deviations from the regular helical<br />
arrangement. Reduced stacking interactions then decrease the<br />
potential overlap of their π-electron systems and overall conductivity.<br />
Also the first solvation shell, known to play a crucial role in the<br />
structural integrity of the double-helical DNA [31,32], is likely to be<br />
much less ordered in ss than in dsDNA. Double-stranded DNA has a<br />
more regular structure including the first solvation shell, and its longrange<br />
periodicity decreases the dispersion of the polarization energy<br />
and makes the distribution of hopping states narrower. Considering<br />
these effects, the charge-carrier mobility, and thus conductivity,<br />
increases in more regular dsDNA systems.<br />
3.3. Base pairing — Watson–Crick vs. “mismatched” pairs<br />
Non-Watson–Crick or “mismatched” base pairs are the type of DNA<br />
damage where two non-complementary bases are paired, e.g. adenine<br />
with cytosine or thymine with guanine. The role of the non-W–C base<br />
pairs and their different stacking in DNA conductivity was estimated<br />
by comparing the conductivity of two dsDNA, the canonical 32nucleotide-long<br />
double-stranded DNA molecules (Table 1, ds 5 and 8)<br />
with analogical systems containing two non-W–C pairs in the middle<br />
(Table 1, sequences 6 and 7). The unequivocal conclusion is that the<br />
molecules containing non-W–C pairs are less conductive than the<br />
DNA double strands containing only W–C pairs (Fig. 7, Table 2). The<br />
mismatched molecules are expected to be under mechanical stress,<br />
and the disrupted regular periodicity of the stacked W–C pairs and<br />
their possibly worse stacking interaction can lead to a decrease in the<br />
electron transport efficiency [33].<br />
3.4. Abasic nucleotides — structure change and interrupted stacking<br />
The role of base stacking in DNA conductivity was also estimated<br />
by comparing the conductivity of two double-stranded sequences<br />
with a mixed G/A/C/T sequence, one containing all bases in W–C<br />
pairs (number 3 in Table 1) and the other with the same sequences<br />
except for three bases in the middle, at positions 15–17, which were<br />
substituted with an abasic nucleotide spacer (number 4 in Table 1).<br />
The measurements consistently showed that samples with the abasic<br />
spacer had a lower conductivity than the standard DNA sample. The<br />
I(V) curves obtained for double-helical DNA samples 3 and 4 are<br />
compared in Fig. 5. In this case, the conductivity of the sample with<br />
the gap in the middle of the sequence was much lower than the<br />
conductivity of a standard DNA chain. The same result – a lower<br />
conductivity of an abasic dsDNA sample when compared with an<br />
intact dsDNA – was obtained in the case of the ds AT chain with two<br />
bases missing in the middle (sample 9).<br />
There are no examples of atomic-resolution structures of DNA<br />
molecules with two or even three base pairs replaced by an abasic<br />
nucleotide. However, structures missing just one base in one of the two<br />
strands in a double-helical construct may be significantly deformed, as<br />
for example in the crystal structure of human APE1 bound to abasic<br />
DNA [34]. Therefore, it is not possible simply to dissect the effect of the<br />
overall ds structure and quality of the stacking; both effects, less<br />
regular double-helical structure and sub-optimal stacking interactions,<br />
are likely to cause the lower conductivity of the abasic samples. The<br />
DNA backbone constitutes a quasi-one-dimensional periodic system,<br />
which is essentially independent of the base pair sequence and could<br />
allow for extended Bloch states. Nevertheless, conductivity through<br />
the backbone seems to be low because of the insulating sugar groups<br />
separating the phosphate groups from each other.<br />
3.5. Phosphate charge<br />
To study the influence of the DNA negative charge on its<br />
conductivity, we synthesized DNA chains with phosphates modified
y isopropoxy groups, thus eliminating phosphate negative charges<br />
(p-isopropoxy-modified sequences 10, 11, 12 in Table 1). The<br />
measurements undertaken for the pairs of double-stranded samples<br />
1–10, 2–11, and 3–12 (Table 1) demonstrated that in all three cases the<br />
p-isopropoxy-modified DNA sequences had a lower conductivity than<br />
their counterparts of natural DNA (Table 2, Fig. 6).<br />
In the polar aqueous environment, DNA forms a double helix with<br />
the hydrophobic base planes shielded from the aqueous solvent by the<br />
base-pair stacking in its core region with the charged phosphate fully<br />
exposed to the solvent. The phosphate negative charges are compensated<br />
by partially condensed counterions, typically mono- and bivalent<br />
alkali- and alkaline-earth metals such as Na + and Mg 2+ . The metals are<br />
fully integrated into the solvation shell, which is partially ordered [35–<br />
37]. On the basis of a MALDI-TOF analysis, the presence of Na + and K +<br />
cations in the p-isopropoxy-modified sequences is highly unlikely, and<br />
these metal cations probably do not play any role in the conductivity of<br />
sequences 10 and 11. In natural sequences with the presence of Na + or<br />
Mg 2+ counterions, the conductivity is increased partly due to the Na + or<br />
Mg 2+ states being localized in the large π–π⁎ energy gap. In the case of<br />
sodium, we have small activation gaps (of a few kT) between the water<br />
and sodium states, which could lead to hopping conductivity between<br />
the Na + centered states. For Mg 2+ , the occupied water-state energies are<br />
not only close to the Mg 2+ levels but also very close to the unoccupied π⁎<br />
states, leading to the possibility of electron doping of DNA by water or<br />
Mg 2+ . Like in the case of the abasic dsDNA, the lower conductivity of<br />
uncharged dsDNA is caused, besides the electronic effects, also by the<br />
less regular structure of the uncharged dsDNA.<br />
The shape of the I(V) curves can in general be interpreted as<br />
follows: The current passes through the molecules and also tunnels<br />
through the tip–molecule area. For low voltages, ohmic behavior was<br />
observed due to the Boltzmann distribution of the charge carriers and<br />
the constant position of the Fermi level. The higher the voltage is the<br />
higher is the current passing through each molecule, resulting in the<br />
nonlinear effect of the charge–carrier injection (the shift of the DNA<br />
Fermi level to the electronic tail states and their occupation).<br />
Therefore, based on our topographic data, we assume that the<br />
differences in the conductivity of different samples come from the<br />
properties of the individual molecules, not from the specific properties<br />
of the molecular monolayers.<br />
4. Conclusions<br />
The STM technique was used to study the conductivity of various<br />
DNA sequences in single- and double-stranded forms with one strand<br />
always covalently bonded to the gold surface. The purpose of our<br />
investigations was to provide more insight into the complex process of<br />
the charge–carrier transport in DNA molecules — the role of the sugarphosphate<br />
backbone, the role of the counterions complexed around<br />
the negatively charged phosphate oxygens, and the effect of the<br />
interactions between the bases and their pairs.<br />
The double-helical sample with the G/C bases is a better conductor<br />
than the double-stranded sequence containing only A and T nucleotides.<br />
Easy oxidation of the guanine base makes it possible to generate<br />
the charge carriers (holes). The charges, especially holes, created on<br />
the uniform DNA chain can move by the hopping charge transport<br />
through the discrete guanine sites.<br />
The conductivity of the double-stranded form is greater than that<br />
of the single-stranded one when complementary sequences are<br />
compared. Single strands can be considered much less structurally<br />
regular than the double helices. Regular structures could form better<br />
long-range ordering, which decreases the dispersion of the polarization<br />
energy and makes the distribution of hopping states narrower.<br />
Under these conditions, the charge–carrier mobility, and thus<br />
conductivity, in regular systems increases.<br />
We found that the conductivity of the samples with abasic spacers<br />
was lower than that of the natural dsDNA samples because of the lack<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
I. Kratochvílová et al. / Biophysical Chemistry 138 (2008) 3–10<br />
of stacking interactions and likely also because of the structural<br />
irregularities and lost periodicity of the double strand.<br />
The DNA chains without charge-bearing phosphate groups were<br />
less conductive than natural dsDNA. The reasons are likely to be both<br />
electronic (the fact that neutral DNA lacks counterions with their effect<br />
on lowering the energy gaps between nucleotides) and structural<br />
(neutral DNA is likely to be distorted, irregular and hence more<br />
insulating than the natural dsDNA) [38].<br />
Theoretical computations show that the orbitals occupied by the<br />
π-electrons contribute strongly to the electronic states, which arises<br />
from the electronic coupling between the neighboring bases [39,40].<br />
Unlike the π-electrons, which can form extended states, water<br />
molecules and counterions create localized states of electrons,<br />
resulting in the hopping mechanism of conduction, with the inclusion<br />
of the electron-phonon interactions. The DNA molecule contains<br />
electronic states, which have an extended character and in which the<br />
electrons easily carry a ballistic current, and localized states of<br />
electrons, mediating the hopping mechanism of conduction. These<br />
two rather complementary pictures of conduction, π-electron overlap<br />
and hopping through energetically near states, may fit together.<br />
Based on our measurements, we can conclude that the best<br />
conductor is the natural double-stranded DNA form with its bases<br />
forming Watson–Crick pairs and surrounded by counterions and water<br />
molecules. Since electronic states are strongly connected with molecular<br />
structure and DNA structure changes with base sequences, type and<br />
concentration of counterions, and relative humidity, the electronic<br />
states are expected to depend on all of the following parameters:<br />
1. change of the regular structure;<br />
2. water (as a possible ion conductor) amount;<br />
3. counterion presence; in the case of DNA containing sodium, Na +<br />
can mediate hopping conductivity between the Na + -centered<br />
states; for DNA containing magnesium, there is a possibility of<br />
electron doping of DNA by water or Mg 2+ states.<br />
Acknowledgements<br />
This work was supported by the institutional funds to Institute of<br />
Physics AS CR, v.v.i. (AV0Z10100520), Grant Agency of the Academy of<br />
Sciences of the Czech Republic (Grants No. KAN 200100 801, KAN 401<br />
770 651, KAN 400 720 701), the Ministry of Education Youth and Sports<br />
of the Czech Republic COST OC 137, COST 1041/2006-32 the Czech<br />
Science Foundation (Grant No. 203/08/1594, 202/07/0643), by the<br />
European Commission through the Human Potential Programme<br />
(Marie-Curie RTN BIMORE, Grant No. MRTN-CT-2006-035859) and by<br />
institutional funds to Institute of Biotechnology AS CR (AV0Z50520701).<br />
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DOI: 10.2478/s11534-008-0072-7<br />
Central European Journal of Physics<br />
Scanning tunneling spectrosopy study of DNA<br />
conductivity<br />
Research Article<br />
Irena Kratochvílová 1∗ , Karel Král 1 , Martin Bunček 2 , Stanislav Nešp o<br />
urek 3 , Tatiana Todorciuc 3 , Martin<br />
Weiter 4 , Jiˇrí Navrátil 4 , Bohdan Schneider 5 , Jiˇrí Pavluch 6<br />
1 Institute of Physics, AS-CR, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic<br />
2 GENERI BIOTECH s.r.o., Machkova 587, 500 11 Hradec Kralové, Czech Republic<br />
3 Institute of Macromolecular Chemistry, AS-CR, v.v.i., Heyrovský sq. 2, 16206 Prague, Czech Republic<br />
4 Faculty of Chemistry, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic<br />
5 Institute of Organic Chemistry and Biochemistry, AS-CR, v.v.i., Fleming sq. 2, 166 10 Czech Republic<br />
6 Faculty of Mathematics and Physics, Charles University in Prague, V Holešovièkách 2, 180 00 Prague, Czech Republic<br />
Received 16 November 2007; accepted 21 February 2008<br />
Abstract: We used STM to study the conductivity of 32 nucleotide long DNA molecules chemically attached to a gold<br />
surface. Two oligonucleotides containing all four base types namely G, A, C, T, one single stranded and<br />
one double helical, all showed conductance data significantly higher than DNA containing only T and A that<br />
were either single stranded d(T32) or double helical d(T32).d(A32) in confirmation. Within each sequence<br />
group, the conductivity of the double helical form was always higher than that of the single strand. We<br />
discuss the impact of structure, particular base stacking and affinity to the phase transition.<br />
PACS (2008): 87.14.gk, 81.07.-b, 81.07.Nb, 81.40.Rs<br />
Keywords: molecular electronics • DNA • scanning tunneling micros<strong>copy</strong> • conductivity • charge carrier transport<br />
© Versita Warsaw and Springer-Verlag Berlin Heidelberg.<br />
DNA is an important and promising molecule, not only due<br />
to its genetic function, but also as a molecular scaffold<br />
for nanotechnology [1]. The double helical DNA architecture,<br />
is well stacked consisting of near parallel bases<br />
stacked with their π-electron systems overlapping. Such<br />
π-electron systems may be good candidates for long distance<br />
and one-dimensional (linear) charge transport. In-<br />
∗ E-mail: krat@fzu.cz<br />
Author <strong>copy</strong><br />
vestigations of DNA conductivity and its physical origin<br />
have significant implications towards the study of DNA<br />
damage and repair in biological systems, application of<br />
DNA in electronic nano-devices, and DNA-based electrochemical<br />
biosensors. The electronic transport of various<br />
types of organic molecules operating through πconjugated<br />
systems, including DNA, has been the subject<br />
of several recent studies, both theoretical and experimental<br />
[2–5]. Despite significant achievements, results on<br />
DNA conductivity published by different research groups<br />
present often conflicting and controversial explanations of
Irena Kratochvílová et al.<br />
the DNA-mediated charge transport [6–8].<br />
Two basic experimental approaches have been pursued to<br />
investigate the conduction through molecules: (i) contacting<br />
isolated single molecules or molecules in thin<br />
films, (ii) studying transport in thick films and devices,<br />
such as organic thin-film transistors or light emitting<br />
diodes. The optimal experimental setup is to position<br />
isolated molecules between two electrical contacts but it<br />
is very difficult to realize and especially difficult to verify.<br />
Working with ordered arrays of parallel π-conjugated<br />
molecules where in principle, the coupling to the substrate<br />
can be precisely known, offer the possibility to address the<br />
individual molecule by making use of the scanning tunneling<br />
microscope (STM) tip. STM experiments have already<br />
been proven as well suitable tools for the investigation of<br />
ordered monolayer films allowing electrical contacting of<br />
one or a few molecules and to obtain images of the structural<br />
characteristics of the film [9, 10].<br />
In this work, we used the STM to study the conductivity of<br />
different DNA sequences in single- and double-stranded<br />
forms covalently bonded to a gold surface under ambient<br />
conditions (i.e. air, room temperature). It is known that<br />
water molecules surrounding DNA strongly participate in<br />
DNA structure holding. Working under vacuum, the water<br />
molecules are expelled and the DNA structure is reserved<br />
mainly by the effect of inner shell water molecules and<br />
this results in drastic changes.<br />
Our DNA molecules were short and very precisely prepared<br />
as self-assembled 32-mer oligodeoxynucleotides<br />
making monolayers and were immobilized by attaching to<br />
a gold surface through the formation of disulfide bridges<br />
via the C6 spacer at the DNA 5’-end as shown in Fig.<br />
1. The 40 nm thin gold layer was prepared by evaporation<br />
of pure gold (99.99% purity) on a silicon wafer<br />
which was cut into 1 cm×1 cm pieces. Synthesized were<br />
two types of oligonucleotides, two 32-mers with uniform<br />
sequences, d(32T), d(32A), and two 32-mers with non-selfcomplementary<br />
sequences containing all four nucleotides,<br />
G, A, C, and T in equimolar ratio:<br />
5’-d(CAGCAGGGTCAGTCAATCTATCGTCCGTAATG)-3’<br />
(labeled further as d(GACT)), and the complementary sequence<br />
5’-d(CATTACGGACGATAGATTGACTGACCCTGCTG)-3’<br />
The spacer was incorporated to the 5’-end of d(32T) and<br />
d(GACT) oligonucleotides. These two molecules were<br />
subsequently chemically attached to the gold surface in<br />
their single stranded forms Complexation with their complementary<br />
strands, resulted in chemically intact DNA<br />
molecules, via double helice formation. All DNA samples<br />
were prepared as triethylammonium salt (TEA); bulky TEA<br />
counterion with low mobility was selected to reduce ionic<br />
conductance of the samples.<br />
Figure 1. Scheme of attachment of the DNA 32-mers to the gold<br />
surface.<br />
The STM measurements were performed with NTEGRA<br />
Prima NT MDT system using freshly cut Pt/Ir tips. Both<br />
topographic and spectroscopic data were obtained. With<br />
STM, for very small voltages of ∼10 mV and relatively<br />
large current of ∼ 0.5 nA, in view of the very small tipsample<br />
distances, the topography of the samples were observed.Topographic<br />
images showed a considerable difference<br />
between the bare gold substrates and DNA sample<br />
molecules . In the case of molecular layers, specific surface<br />
patterns were observed. The occurrence of these patterns<br />
has been attributed to etching of the bottom gold layers<br />
arising from the reaction of thiol-groups with gold atoms,<br />
forming dissolvable complexes [11, 12]. All the molecules<br />
were ordered in similar geometric structures with the nearest<br />
neighbour spacing of 1-2 nm. By ellipsometry, in which<br />
the thickness of all the measured molecular layers was established<br />
as 8.5-9 nm, we have confirmed that the 32 nucleotide<br />
long DNA molecules are oriented approximately<br />
perpendicular to the substrate.<br />
I(V ) spectra were taken for different feedback voltage and<br />
current setpoints, i.e., for different initial sample-tip distances.<br />
In all experiments, the STM tip act as the electrical<br />
contacts on the “top” side of the assembled monolayer<br />
of the DNA molecules, the supporting gold substrate acts<br />
as the other (bottom) contact. The distance between the<br />
top of the molecules and the STM tip for all the set points<br />
was 2.5-4 Å. In view of the experimental setup, it was predicted<br />
that the flow of current occurs mainly through the<br />
molecule nearest to the tip without significant lateral intermolecular<br />
contribution to conduction.<br />
Noise, in the form of height fluctuations, was larger in the<br />
case of topographic images of films than in the case of the<br />
bare gold. All the organic molecules that have been studied<br />
by STM till now showed symmetrical current-voltage<br />
(I − V ) characteristics. The symmetry of the I − V characteristics<br />
has been explained in a simple model [13].<br />
I(V ) curves were measured at three different set points:<br />
0.1 nA, 0.1 V; 0.2 nA, 0.1 V; 0.5 nA, 0.1 V using the same<br />
strategy for all four DNA sequences. Six hundred consecutive<br />
I(V ) sweeps in both voltage directions (from -<br />
1.5 V to +1.5 V and from +1.5 V to -1.5 V) were taken<br />
on each sequence. We compared I(V ) spectra taken for<br />
different molecular systems for the same substrate under<br />
the same conditions (set point at the gold terrace). Final<br />
Author <strong>copy</strong><br />
423
424<br />
results presented in this paper are based on the average<br />
of the curve data set for the subsets collected. Only<br />
those curves I(V ) were included in the averaging in the<br />
analysis that were not affected by appreciable drift of the<br />
STM. All the results taken into consideration showed the<br />
same trends. Typical examples of I(V ) curves measured<br />
at set point 0.1 nA; 0.1 V are shown in Figs. 2, 3 and 4.<br />
In our experiments we have measured current tunneling<br />
through the tip-molecule area for the molecules so close<br />
to the tip that their contribution is set at voltage levels<br />
above the noise. For tips very close to the molecules<br />
(defined by the set point value) at very low voltages the<br />
number of molecules contributing to the total current is<br />
small. As we could not estimate exactly the number of<br />
the molecules, we proceeded to to compare conductances<br />
of different DNA sequences for both single- and doublestranded<br />
forms. The term conductivity is widely used in<br />
equivalent experiments (STM/AFM measured current vs.<br />
voltage curves on bundles of molecules) that we have decided<br />
to keep its common meaning.<br />
The shape of the I(V ) curves can be interpreted as follows:<br />
current passes through the molecule and also tunnels<br />
through the tip-molecule area. For low volatges the<br />
ohmic behaviour was observed due to the Botzmann distribution<br />
of the charge carriers and constant position of the<br />
Fermi level. As voltage is increasing the current passing<br />
through each molecule is higher – nonlinear effect<br />
of charge carrier injection takes place (shift of the DNA<br />
Fermi level to the electronic tale states and their occupation).<br />
Also the number of the molecules contributing to<br />
the total current is rapidly increasing so the total current<br />
is rising up very sharply at higher voltages.<br />
Comparison of conductivity of two single stranded sequences,<br />
d(T32) and d(GACT), and two double stranded,<br />
d(T32).d(A32) and d(GACT).d(CTGA), can be summarized<br />
as follows. The largest difference was found between conductivity<br />
of single-stranded form of oligonucleotides containing<br />
only T and the double-stranded form of mixed G, A,<br />
C, T sequences. Both samples with the “mixed” sequence,<br />
either in single- or double-stranded form, showed larger<br />
conductivity than either sample containing the d(T32) sequence<br />
(Figs. 2, 3). We observed the double helical sample<br />
with the mixed G, A, C, T sequences to be the best<br />
conductor from our DNA sample set (Fig. 4). Conductivity<br />
of the double-stranded form is larger than that of the<br />
single-strand when complementary sequences are compared,<br />
which is in agreement with theoretical model of<br />
the same system [7].<br />
Based on our topographic data, we assume that the main<br />
reason for differences in sampleconductivity arise from the<br />
properties of the individual molecules, not as a function of<br />
the molecular monolayers.<br />
Scanning tunneling spectrosopy study of DNA conductivity<br />
Figure 2. Typical I(V ) curve of DNA 32-mer d(T32):d(A32) double<br />
helix (DS TA) and d(T32) single-stranded form (SS TA).<br />
Set point 0.1 nA, 0.1 V, measured in both voltage directions.<br />
Author <strong>copy</strong><br />
Figure 3. Typical I(V ) curves of DNA 32-mer with mixed G, A, C, T<br />
sequences: Double helical d(GACT).d(CTGA) (DS MIX)<br />
and the single-stranded d(GACT) form (SS MIX). Set point<br />
0.1 nA, 0.1 V, measured in both voltage directions.<br />
Two channels can significantly contribute to the charge<br />
transport along the DNA double helix; they include electronic<br />
conduction along the base pair sequences and ionic<br />
conduction associated with the counterions.<br />
In this work, DNA molecules were synthesized as TEA<br />
salts to minimize the concentration of small, and therefore<br />
highly mobile, cations as sodium or potassium but ionic<br />
conduction cannot be ruled out due to water and solvated
Irena Kratochvílová et al.<br />
Figure 4. Comparison of I(V ) curves of double helical DNA 32-mers<br />
with mixed G, A, C, T sequences d(GACT).d(CTGA) (DS<br />
MIX) and d(T32):d(A32) (DS TA). Set point 0.1 nA, 0.1 V,<br />
measured in both voltage directions.<br />
ions used to dissolve and subsequently deposited on the<br />
DNA molecules. However, an ionic conduction mechanism<br />
would not require just presence the presence of mobile<br />
ions, such as in a liquid or a molten phase, but also their<br />
macroscopic reservoir to maintain the observed stable current<br />
flow for as long as the voltage was applied. Neither of<br />
the two conditions were completely satisfied in our experiments.<br />
Thus, in our case, the ionic conduction mechanism<br />
should not be a strong source of the DNA conductivity.<br />
On the other hand, any molecule or ion associated with<br />
DNA via ionic or covalent bonding might in principle affect<br />
the DNA electronic structure, the π-electron distribution<br />
of the stacked bases with their energetic states,<br />
and hence its electrical conductivity. Ions can act in the<br />
molecular system like dopants-supplying the system by<br />
charge carrier – electron or hole. The electronic interactions<br />
between the π-electron systems of the bases may<br />
generate a molecular energy band with electronic states<br />
delocalized over the entire length of the molecule. We assume<br />
the impurity concentrations from sample to sample<br />
to be approximately the same, therefore we did not expect<br />
the impurity concentration to be a source of differences in<br />
the conductivity between samples.<br />
The question arises, what can be the main origin of the different<br />
conductivity of our molecular samples? Besides the<br />
charge-carrier concentration, the explanation for the measured<br />
conductivity differences in the DNA samples may<br />
relate to the the specific electronic structure, molecular<br />
chains structural differences and electron-phonon interactions.<br />
Single strands of both investigated sequences can<br />
be considered to be much less structurally regular than the<br />
double helices because of their local and especially longscale<br />
deviations from the regular helical arrangement [14].<br />
Reduced stacking interactions can decrease the potential<br />
overlap of the π-electron systems. Furthermore, a more<br />
regular structure may promote better long range ordering<br />
resulting in a decrease in the dispersion of the polarization<br />
energy and narrowing in the distribution of hopping<br />
states. Under these conditions, the charge carrier mobility,<br />
and thus conductivity, in regular systems increases.<br />
Based on our experimental results we propose that electrical<br />
conductivity in DNA strands is higher for the welldefined<br />
right-handed double helical forms due to their<br />
smaller structural deviations resulting in larger π-electron<br />
potential overlap and smaller dispersion of the polarization<br />
energy (Figs. 2, 3).<br />
Guanine base is the most easily oxidized. This allows<br />
the generation of a charge carrier (hole). Once charges,<br />
and especially holes, are created on the uniform DNA<br />
chain, the hopping charge transport phenomena can occur<br />
among discrete guanine sites or delocalized (e.g., polaron)<br />
domains [12]. Furthermore, the stacking distance of<br />
the adjacent base pairs also affects the π-electron overlap.<br />
The crystal structure analysis of oligonucleotides<br />
indicates that the axial rise of residue is 2.88 Å for<br />
poly(dG).poly(dC) and 3.22 Å for poly(dA).poly(dT) [15].<br />
The more compact the base stacking is, the more favorable<br />
the charge transport is expected to be. Charge transport<br />
enhancement in tighter stacked compact samples is<br />
in agreement with our results showing higher conductivity<br />
of double helical mixed d(GACT).d(CTGA) sequences than<br />
d(T32).d(A32) samples (Fig. 4).<br />
Based on our theoretical expectations and experimental<br />
data, we can say that the d(GACT).d(CTGA) is the better<br />
conductor of all the DNA samples investigated due to the<br />
regular structural double helical form with compact base<br />
(G-C) stacking.<br />
Author <strong>copy</strong><br />
Obviously, the DNA conductivity is a very complex problem<br />
and requires further attention. Detailed understanding<br />
of the conduction mechanism remains a challenge and,<br />
once achieved, it will undoubtedly provide a better insight<br />
into the biological, chemical and physical properties of<br />
DNA molecules.<br />
Acknowledgements<br />
This work was supported by the Grant Agency of the<br />
Academy of Sciences of the Czech Republic (grants<br />
KAN400720701, KAN401770651, KAN 200100801 and<br />
COST OC 137) and by institutional project AVOZ<br />
10100520. The authors thank the scientists participating<br />
in BIMORE (Bio-inspired Molecular Optoelectronics)<br />
425
426<br />
project for valuable discussions.<br />
References<br />
[1] B. Rezek, D. Shin, T. Nakamura, C.E Nebel, JACS<br />
Com. 844, 128 (2006)<br />
[2] D.T. Odom, E.A. Dill, J.K. Barton, Chem. Biol. 7, 475<br />
(2000)<br />
[3] H. Cohen, C. Nogues, R. Naaman, D. Porath, P. Natl.<br />
Acad. Sci. USA 102, 11589 (2005)<br />
[4] H. Cohen et al., Faraday Discuss. 131, 367 (2006)<br />
[5] D. Ullien, H. Cohen, D. Porath, Nanotechnology 18,<br />
424015 (2007)<br />
[6] R.G. Enders, D.L. Cox, R.R.P. Singh, Rev. Mod. Phys.<br />
Scanning tunneling spectrosopy study of DNA conductivity<br />
76, 195 (2004)<br />
[7] E.B. Starikov et al., Eur. Phys. J. E 18, 437 (2005)<br />
[8] M. Taniguchi , T. Kawai, Physica E 33, 1 (2006)<br />
[9] A.I. Onipko et al., Phys. Rev. B 61, 11118 (2000)<br />
[10] J.J.W.M. Rosnik, M.A. Blauw, L.J. Geerlings, E. van der<br />
Drift, S. Radelaar, Phys. Rev. B 62, 10459 (2000)<br />
[11] C. Schoenenberger , J.A.M. Sondag-Huethorst, J. Jorritsma,<br />
L.G.J. Fokkink, Langmuir 10, 611 (1994)<br />
[12] J.J.W.M Rosink et al., Opt. Mater. 9, 416 (1998)<br />
[13] S. Datta et al., Phys. Rev. Lett. 79, 2530 (1997)<br />
[14] S. Neidle, Nucleic acid structure and recognition (Oxford<br />
University Press, 2002)<br />
[15] L. Cai, H. Tabata, T. Kawai, Appl. Phys. Let. 77, 3105<br />
(2000)<br />
Author <strong>copy</strong>
Macromol. Symp. 2008, 268, 125–128 DOI: 10.1002/masy.200850826 125<br />
Light-Induced Change of Charge Carrier Mobility in<br />
Semiconducting Polymers<br />
Martin Vala,* 1 Martin Weiter, 1 Oldrˇich Zmesˇkal, 1 Stanislav Nesˇpu˚rek, 1,2<br />
Petr Toman 2<br />
Summary: Light-driven devices based on reversible change of carrier mobility in<br />
semiconducting polymers were investigated. The mobility was altered using a<br />
photochromic spiropyran capable of a reversible change of permanent dipole<br />
moment and ionization potential. While the latter attribute may result in formation<br />
of chemical traps and is more important for matrices with similar ionization potential<br />
such as PVK, the former phenomenon results in formation of polar traps and is more<br />
pronounced in the case of lower-band-gap materials.<br />
Keywords: charge transport; conducting polymers; poly(phenylenevinylene);<br />
poly(vinylcarbazole); supramolecular structures<br />
Introduction<br />
This contribution deals with the concept of<br />
a current switch built from polymer materials<br />
doped by photochromic compounds<br />
capable of reversible switching between<br />
ON and OFF states by changing the charge<br />
carrier mobility in the semiconducting<br />
polymers. [1,2] The colour changes due to<br />
the photochromic transformations are also<br />
accompanied by changes of other chemical<br />
and physical properties, such as emission<br />
spectra, refractive index, dielectric permittivity<br />
and enthalpy. These modifications are<br />
intrinsic in photochromic phenomena and<br />
thus offer wider possibilities for practical<br />
applications of photochromic compounds.<br />
6-nitro substituted spiropyrans, which can<br />
change their dipole moment from ca. 6 D to<br />
12 20 D due to a photochromic reaction,<br />
[3–5] can be mentioned as examples.<br />
The changes in physical or chemical properties<br />
transferred to the microenvironment or<br />
supramolecular structure further induce<br />
1 Faculty of Chemistry, Brno University of Technology,<br />
Purkyňova 118, 612 00 Brno, Czech Republic<br />
Fax: þ420 541 149 398;<br />
E-mail: vala@fch.vutbr.cz<br />
2 Institute of Macromolecular Chemistry, v.v.i., Academy<br />
of Sciences of the Czech Republic, Heyrovsky´<br />
Sq. 2, 162 06 Prague, Czech Republic<br />
Copyright ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim<br />
modification in the surrounding environment.<br />
It has been shown that dopants added to<br />
a surrounding charge-transporting material<br />
can produce: (i) chemical traps based on the<br />
formation of local centres by the dopant<br />
with ionization energy lower and/or electron<br />
affinity higher than in the virgin<br />
material (for further discussion, see e.g.<br />
ref. [6] and references therein), and (ii) dipolar<br />
traps created by dopant with high dipole<br />
moments. Dipolar traps are based on<br />
modification of local values of polarization<br />
energy (leading to modification of ionisation<br />
energy and/or electron affinity) and<br />
broadening of the hopping-states distribution<br />
due to the charge-dipole interactions in<br />
the environment. [7] Other types of traps,<br />
such as traps produced by structure defects<br />
will not be discussed in this paper. Based on<br />
the calculations performed on model systems,<br />
a concept of optoelectrical switch was<br />
put forward. The proposed device consists<br />
of bistable polar species capable of switching<br />
between low-dipole and high-dipole<br />
states. In consequence, the created polar<br />
traps modify the effective mobility of<br />
charge carriers in a reversible and controlled<br />
manner. [1,2,8,9] This contribution<br />
extends the research to another semiconducting<br />
polymer poly(vinylcarbazole).
126<br />
Macromol. Symp. 2008, 268, 125–128<br />
Since the structure of the polymer differs<br />
from those previously reported, a different<br />
switching mechanism is expected.<br />
Experimental Part<br />
All the samples were prepared in the form of<br />
sandwich cell of the diode type. The samples<br />
were fabricated on a transparent ITO<br />
(indium tin oxide) electrode using the spincoating<br />
technique. As a polymer matrix,<br />
poly(vinylcarbazole) (PVK) was used.<br />
As the active photochromic unit 1 0 ,3 0 -<br />
dihydro-1 0 ,3 0 ,3 0 -trimethyl-6-nitrospiro[2H-<br />
1-benzopyran-2,2 0 -[2H]-indole] (SP) was<br />
used. All the chemicals were supplied by<br />
Aldrich and were used as recieved. The<br />
preparation conditions of the active materials<br />
were optimised to obtain typically<br />
150 nm thick layers. The structures were<br />
finished by vacuum evaporation of 100 nm<br />
thick top aluminium electrode. The average<br />
device area was 3 mm 2 . The current-voltage<br />
characteristics were measured with a Keithley<br />
6517A electrometer. All electric measurements<br />
were performed at room temperature<br />
under vacuum. Absorption and<br />
photoluminescence spectroscopies were<br />
used to monitor the photochromic reaction.<br />
For optical measurements, thin layers of the<br />
same thickness were spin-coated on quartz<br />
glass substrates. A xenon arc lamp (450 W)<br />
with selected wavelength (360 20 nm) and<br />
IR filter were used to switch the photochromic<br />
moiety from the stable to metastable<br />
state.<br />
Results and Discussion<br />
The photochromic behaviour of spiropyrans<br />
was first reported in 1952. [10] Since that time<br />
it has been intensively studied for their use<br />
as a binary element for computer memories<br />
(for further information, see [3] ). Absorption<br />
of UV light by the spiropyran results in a<br />
cleavage of the oxygen-spiro carbon bond<br />
leading to the formation of the coloured<br />
open-form (lmax ¼ 600 nm) isomer having a<br />
high dipole moment as opposed to the closed<br />
form, which is colourless and possess a lower<br />
dipole moment. The open form is often<br />
called (photo)merocyanine (MC) because it<br />
is similar to merocyanine dyes. The MC<br />
form reverts thermally to the closed form,<br />
which manifests itself as bleaching of the<br />
samples. The reverse reaction can be<br />
accelerated by absorption of visible light.<br />
The reaction scheme and the absorbance<br />
change during the reaction is depicted in<br />
Figure 1 and 2. It can be seen that, even<br />
before the photochromic conversion, some<br />
MC forms are present as the system is in its<br />
thermodynamic equilibrium (note the logarithmic<br />
scale on the y axis). The change of<br />
the absorbance is only neat. The electric<br />
properties, however, were modified dramatically.<br />
Figure 2 shows the photoswitching of the<br />
electric current of the PVK:SP samples with<br />
various concentrations of the photochromic<br />
SP. It can be seen that addition of a higher<br />
amount of SP already led to a decrease in<br />
current before the photochromic conversion.<br />
The photochromic reaction caused a<br />
further reversible decrease in all cases. This<br />
behaviour differs from our previous observations<br />
where we investigated the influence<br />
Figure 1.<br />
The absorbance spectra of the PVK:SP 5% (by weight)<br />
before (&) and after the photochromic reaction (*).<br />
The reverse reaction can be accelerated by illumination<br />
with VIS light (longer wavelength absorption<br />
band of MC).<br />
Copyright ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de
Macromol. Symp. 2008, 268, 125–128 127<br />
Figure 2.<br />
Current-voltage characteristics of the PVK:SP blends measured before (open symbols) and after (full symbols)<br />
the photochromic reaction. Circles represent 2.5%, squares 5% and triangles 10% (by weight) mixtures. The<br />
inset shows the photochromic reaction: under illumination of UV light the spiropyran (SP) converts to the<br />
metastable (photo)merocyanine (MC) form.<br />
of SP on a series of poly(phenylenevinylene)<br />
(PPV) derivatives. [4,9] The addition of<br />
SP molecules up to 30% (by wt.) into the<br />
PPV matrix does not lead to such decrease.<br />
The carbazole units in the polymer act as<br />
individual molecules [11] and the electronic<br />
coupling is weaker compared with the PPV<br />
polymers. The addition of dopant (SP) led<br />
to a more pronounced disorder and thus<br />
lowered charge carrier mobility in comparison<br />
with the PPV-based systems where few<br />
percent of dopant hardly influenced the<br />
transport. This results in a lowering of the<br />
electric current after addition of SP.<br />
Another remarkable difference is in the<br />
behaviour after photochromic conversion.<br />
In the case of PPV-based system, the yield<br />
of the photochromic reaction was poor [12]<br />
and also the reversibility of the current<br />
switching was unsatisfactory. [4] These drawbacks<br />
are improved in the system based on<br />
the PVK matrix.<br />
Figure 2 shows a drop of the current and a<br />
change of the shape of the current-voltage<br />
characteristics after photochromic reaction.<br />
The shape can be interpreted in terms of the<br />
space-charge-limitedcurrent(SCLC)theory.<br />
When the applied electric field is increased,<br />
the traps are filled, which is equivalent to a<br />
shift of the quasi-Fermi level with electric<br />
field. This causes the occupancy of electronic<br />
states to change and enables the scanning of<br />
the distribution of energy from the current<br />
changes. In terms of the present work, the<br />
distribution of charge traps describes those<br />
induced by the spiropyran–merocyanine<br />
photochromic conversion. At a certain<br />
voltage the Fermi level crosses over the trap<br />
level and the current is no more influenced by<br />
this trapping level, approaching the values<br />
similar to those before the photochromic<br />
conversion (for further discussion, see [13] ).<br />
In our previous papers the current<br />
switching was explained as a reversible<br />
decrease in hole mobility due to the<br />
increased trap concentration caused by<br />
the presence of an additive with permanent<br />
dipole moment forming polar traps. [4]<br />
Using the Hoesterey-Letson formalism, [14]<br />
it has been shown that the carrier mobility<br />
with relative trap concentration c is the<br />
product of mobility of the trap-free system<br />
m0 and trapping factor:<br />
mðcÞ ¼m 0 1 þ c exp Et<br />
kT<br />
1<br />
; (1)<br />
where Et is the energy of trapping level, k is<br />
the Boltzmann constant and T is the<br />
temperature. It follows that the current<br />
flowing through the sample is reduced when<br />
the concentration of traps is increased.<br />
Table 1 shows calculated ionization<br />
potentials (IP) and electron affinities (EA)<br />
reported in literature for the SP and<br />
Copyright ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de
128<br />
Macromol. Symp. 2008, 268, 125–128<br />
Table 1.<br />
Energy levels of the compounds.<br />
EA [eV] IP [eV] Ref.<br />
SP – closed 1.10 8.79 [15]<br />
SP – opened (MC) 1.76 7.09 [16]<br />
PVK 1.50 7.60 [11]<br />
MEH-PPV 1.87 4.57 [17]<br />
polymers considered in this study. It should<br />
be noted that these IP and EA values are for<br />
isolated molecules. Therefore they differ<br />
from the IP and EA values of the corresponding<br />
solid films because of the large<br />
solid-state polarization corrections. Since<br />
there are no data on SP values of EA and IP<br />
in the solid state, we will use these gas phase<br />
values to explain the observations.<br />
After the photochromic conversion, the<br />
SP molecules change the electronic properties<br />
and the IP gain value higher than PVK<br />
matrix. This new energetic level inside the<br />
PVK band-gap can acts as a trapping level<br />
for holes. The photoproduct (MC) also<br />
possesses a higher dipole moment and,<br />
therefore, also polar traps are produced.<br />
The concentration of SP dopant can thus<br />
drop to several percent compared with<br />
several tenths of percent for the PPV-based<br />
systems to achieve current switching withing<br />
two orders of magnitude.<br />
The situation in PPV-based systems is<br />
different. Since the calculated IP of MEH-<br />
PPV is higher than that of the MC form, it is<br />
unlikely that the photochromic conversion<br />
produces a new trapping level in the MEH-<br />
PPV band-gap. Even for rough estimation<br />
of the gas-phase IP from the widely used<br />
solid-state value (ca. 5.35 eV [18] ) considering<br />
polarization effect ca. 1.5 eV, we<br />
obtain a higher value than the IP of MC.<br />
Therefore, the formation of chemical traps<br />
is less likely to occur and only polar traps<br />
are present.<br />
Conclusions<br />
Charge mobility switching in semiconducting<br />
polymers using a photochromic spiropyran<br />
was demonstrated. It has been<br />
shown that the PVK:SP-based devices<br />
allow high performance with a ten-timeslower<br />
concentration of the active units<br />
compared with the PPV-based devices<br />
where the switching process is rather<br />
inefficient. However, the actual switching<br />
mechanism differs since also the formation<br />
of chemical traps is likely to occur as<br />
opposed to the PPV-based devices where<br />
the position of IP does not allow such<br />
behaviour.<br />
Acknowledgements: The presented research has<br />
been supported by the Academy of Sciences of<br />
the Czech Republic (project KAN 401770651).<br />
[1] S. Nesˇpu˚rek, J. Sworakowski, Thin Solid Films 2001,<br />
393, 168.<br />
[2] S. Nesˇpu˚rek, P. Toman, J. Sworakowski, Thin Solid<br />
Films 2003, 438–439, 268.<br />
[3] H. Dürr, H. B. Laurent, Photochromism: Molecules<br />
and Systems, 2 nd ed., Elsevier Science B.V., Amsterdam<br />
2003.<br />
[4] M. Weiter, M. Vala, O. Zmesˇkal, S. Nesˇpu˚rek, P.<br />
Toman, Macromol. Symp. 2007, 247, 318.<br />
[5] M. Bletz, U. Pfeifer-Fukumura, U. Kolb, W.<br />
Baumann, J. Phys. Chem A 2002, 106, 2232.<br />
[6] J. Sworakowski, K. Janus, S. Nesˇpu˚rek, M. Vala, IEEE<br />
Trans. Dielectr. Eetrl. Insul. 2006, 13, 1001.<br />
[7] J. Sworakowski, IEEE Trans. Diel. Elet. Insul. 2000, 7,<br />
531.<br />
[8] S. Nesˇpu˚rek, J. Sworakowski, C. Cambellas, G. Wang,<br />
M. Weiter, Appl. Surf. Sci. 2004, 234, 395.<br />
[9] M. Weiter, M. Vala, O. Salyk, O. Zmesˇkal, S.<br />
Nesˇpu˚rek, J. Sworakowski, Mol. Cryst. Liq. Cryst.<br />
2005, 430, 227.<br />
[10] E. Fischer, Y. Hirshberg, J. Chem. Soc. 1952, 4522.<br />
[11] M. Pope, C. E. Swenbrg, Electronic Processes in<br />
Organic Crystals and Polymers, 2 nd ed., Oxford University<br />
Press, Oxford 1999.<br />
[12] M. Vala, M. Weiter, G. Rajtrová, S. Nesˇpu˚rek, P.<br />
Toman, J. Sworakowski, Nonlinear Opt. Quantum Opt.<br />
accepted.<br />
[13] to be published.<br />
[14] D. C. Hoesterey, G. M. Letson, J. Phys. Solids 1963,<br />
24, 1069.<br />
[15] M. Yurtsever, B. Ustamehmetoglu, A. S. Sarac, A.<br />
Mannschreck, Int. J. Quantum Chem. 1999, 75, 111.<br />
[16] Y. Kawanishi, K. Seiki, T. Tamaki, M. Sakuragi, Y.<br />
Suzuki, J. Photochem. Photobiol. A: Chem. 1997, 109,<br />
237.<br />
[17] Y. Li, Y. Sun, Y. Li, F. Ma, Comput. Mater. Sci. 2007,<br />
39, 575.<br />
[18] I. H. Campbell, T. W. Hagler, D. L. Smith, J. P.<br />
Ferraris, Phys. Rev. Lett. 1996, 76, 1900.<br />
Copyright ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
Nonlinear Optics and Quantum Optics, Vol. X, pp. 01–11<br />
Reprints available directly from the publisher<br />
Photo<strong>copy</strong>ing permitted by license only<br />
○2007 c Old City Publishing, Inc.<br />
Published by license under the OCP Science imprint,<br />
a member of the Old City Publishing Group<br />
Photochromic Properties of Spiropyran in<br />
Polymeric π–Conjugated Matrices<br />
MARTIN VALA (a) ,MARTIN WEITER (a) ,GABRIELA RAJTROVÁ (a) ,<br />
STANISLAV NEˇSP?REK (a,b) ,PETR TOMAN (b) , AND<br />
JULIUSZUSZ SWORAKOWSKI (c)<br />
(a) Faculty of Chemistry, Brno University of Technology,<br />
Purkyòova 118, 612 00, Brno, Czech Republic<br />
(b) Institute of Macromolecular Chemistry,<br />
Academy of Sciences of the Czech Republic, Heyrovsk´y Sq. 2,<br />
162 06 Prague 6, Czech Republic<br />
(c) Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology,<br />
Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland<br />
The contribution reports the results of studies of the photochromic reaction<br />
of 6-nitro-1 ′ ,3 ′ ,3 ′ -trimethylspiro[2H-1-benzopyran-2,2 ′ -indoline] (SP) in<br />
poly(methyl methacrylate) and in πconjugated photoconductive polymer<br />
matrices based on poly( p-phenylene vinylene) derivatives. The polymer<br />
matrix strongly influences the reaction rate, lifetime of coloured species<br />
and the value and distribution of activation energies of the bleaching<br />
process. It was also found that the activation energy depends on the<br />
intensity of light used for photochromic conversion and the exposure<br />
time. Photochromic reaction is influenced by energy transfer processes<br />
between the conjugated chain and photochromic additive.<br />
Key words: Spiropyran, photochromism, decolouration, kinetics, molecular switch<br />
INTRODUCTION<br />
Photochromic molecules have received much attention over the past decades<br />
for their potential applications in optoelectronic devices, like switches and<br />
memories. Among photochromic dyes, spiro-compounds play an important<br />
role because of high dipole moments of metastable coloured forms [1, 2].<br />
This feature was used in the design of a special type of optoelectrical<br />
switch [3, 4], based on modulation of “on-chain” charge carrier mobility<br />
by a light-induced reversible change of dipole moments. Spiropyrans (SP)<br />
consist of two heterocyclic moieties linked together by a common spiro<br />
1
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
2 MARTIN VALA ET AL.<br />
carbon atom. The two parts of the molecule are oriented in two orthogonal<br />
planes. The absorption of UV light results in a cleavage of the oxygen–spiro<br />
carbon atom bond leading to the formation of a coloured open form isomer<br />
possessing high dipole moment as opposed to the closed form, which is<br />
colourless. The open form is often called photomerocyanine (MC) because<br />
it is similar to merocyanine dye. The MC form reverts thermally to the<br />
closed form which manifests itself in bleaching of the samples.<br />
In this paper we investigate the photochromism of 6-nitro-1 ′ ,3 ′ ,3 ′ -<br />
trimethylspiro[2H -1-benzopyran-2, 2 ′ -indoline] (SP) incorporated in<br />
poly(methyl methacrylate) and in three derivatives of poly(pphenylenevinylene)<br />
(PPV). Absorption and luminescence spectra and<br />
the kinetics of their changes on the course of the thermally driven<br />
photochromic reaction are examined. The kinetic results are investigated in<br />
the framework of the distribution model and the matrix relaxation model.<br />
EXPERIMENT<br />
Three types of π-conjugated polymer matrices were used:<br />
poly[2-methoxy-5-(2 ′ -ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV),<br />
poly[2-methoxy-5-(3 ′ ,7 ′ -dimethyloctyloxy)-1, 4-phenylenevinylene] (MD<br />
MO-PPV) and poly[(p-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexy<br />
loxy)-p-phenylenevinylene)] P(MEHPV-alt-PV).<br />
In order to study photochromism in a optically transparent polymeric<br />
medium poly(methyl methacrylate) (PMMA) was used. All the chemicals<br />
were used as supplied by Aldrich, with no further purification. The samples<br />
were prepared as thin films by spin-coating method from chloroform/toluene<br />
(1:1) solutions of the polymers. For optical measurements quartz substrates<br />
were used. The substrates were thoroughly cleaned using ultrasonic stirring<br />
in liquid detergent solution, de-ionised water, acetone and chloroform. The<br />
thickness of the layers was typically between 80 and 120 nm. The reaction<br />
converting SP into merocyanine (MC) form was activated using discharge<br />
lamp HBO 200 with a (360 ± 20) nm band filter.<br />
RESULTS AND DISCUSSION<br />
Absorption spectra<br />
The absorption spectra of the PMMA/SP sample prior to and after illumination<br />
with UV light are shown in Figure 1. Upon illumination, a new absorption<br />
band appears in the 500 – 650 nm region, and the spectrum also changes in<br />
350–450 nm region. The former wavelength region consists of a main peak<br />
at 583 nm and a shoulder at 545 nm. Such spectrum is often explained
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
PHOTOCHROMIC PROPERTIES OF SPIROPYRAN IN POLYMERIC π–CONJUGATED MATRICES 3<br />
FIGURE 1<br />
The change in absorption of spiropyrane after the photochromic conversion in PMMA/SP<br />
samples. The inset depicts the structures of spiropyrane (SP) and merocyanine (MC) forms.<br />
in terms of aggregation behaviour of MC in solutions used for the thin<br />
film preparation [1, 5]. According to the model, the monomer absorbance<br />
(545 nm) is accompanied with aggregate absorbance at longer wavelengths<br />
(583 nm).<br />
Figure 2 shows absorption spectra of three PPV derivatives used in<br />
the experiments reported in this paper. The positions of the lowest-energy<br />
absorption bands depend on the nature of the side groups. No vibronic<br />
FIGURE 2<br />
Absorption spectra of three PPV derivatives. The absorbances have been normalised to the<br />
lowest energy bands.
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
4 MARTIN VALA ET AL.<br />
FIGURE 3<br />
Differential spectra of the SP/MC photochromic system in three derivatives of PPV. The<br />
samples were exposed to the same UV radiation dose.<br />
structure was observed. The second (weaker) absorption band at about<br />
330 nm is assigned to the π → π* transitions of π–electrons delocalised<br />
along the polymer main chain [6]. The positions of the bands of the PPV<br />
derivatives coincide with those of the SP/MC system, hence the spectra of<br />
the photochromic system dispersed in the polymer matrix are quite complex<br />
superpositions of bands originating from excitations of both components.<br />
It is therefore more instructive to use differential spectra depicting changes<br />
of the absorbance due to the photochromic reaction; a comparison of such<br />
spectra is shown in Figure 3. The spectra in Figure 3 were determined after<br />
the samples had been exposed to the same dose of the UV radiation. Two<br />
features can be noticed: (i) a difference between the positions of the maxima<br />
of the low-energy bands, and (ii) a marked difference in the intensities<br />
of the bands. While the former feature can be rationalised as due to the<br />
solvatochromic effect [7] associated with different polarities of the polymer<br />
matrices (playing in this case the role of solvents), the source of the latter<br />
one is unclear and should probable be sought in a competition between<br />
the absorption of the photochromic system and the polymer matrices. Inner<br />
filter effect can also play a significant role. The absorption of the polymers<br />
at 365 nm used for the SP → MC conversion correlate with intensity of<br />
the change. Creation of thinner layers led to higher changes but due to the<br />
low values of measured absorbance also to high measurement errors. As a<br />
result, the kinetics of the photochromic reaction could be investigated with<br />
a satisfactory accuracy only in the MEH-PPV/SP and PMMA/SP systems.
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
PHOTOCHROMIC PROPERTIES OF SPIROPYRAN IN POLYMERIC π–CONJUGATED MATRICES 5<br />
FIGURE 4<br />
Normalised PL spectrum of the PMMA/MC sample after irradiation (15 and 120 s respectively)<br />
by UV light (discharge lamp HBO 200 with a (360±20) nm band filter).<br />
Photoluminescence<br />
The stable form of spiropyran (SP) does not show any luminescence<br />
contrary to its MC form. The photoluminescence (PL) band of thin film of<br />
PMMA/MC is shown in Figure 4. The PL spectrum is broad and featureless<br />
peaking at approximately 660 nm. Samples with a high SP → gC conversion<br />
repeatedly exhibited a slight narrowing of the PL band (cf. Figure 4).<br />
No noticeable vibrational progression was observed. Photoluminescence<br />
excitation (PLE) experiment showed two components corresponding to<br />
those observed in the absorption spectrum.<br />
The situation is different for the PPV/SP samples. The polymers show<br />
their own luminescence and creation of MC form via the photochromic<br />
reaction therefore could result in the appearance of new PL bands. However,<br />
MC possesses a high dipole moment [2] which generally causes a quenching<br />
of luminescence. Such behaviour was indeed observed in all three PPV<br />
matrices. The quenching was found to be strong even in the case when<br />
the SP → MC conversion was very weak (sometimes not observable in<br />
absorption spectra). After the reverse (MC → SP) reaction was completed,<br />
the polymer PL was recovered, the reversibility of the process was, however,<br />
unsatisfactory because of a strong photodegradation of the polymer matrices.<br />
Interestingly, the PL band of MC excited through its own absorption<br />
could not be found in the samples. This effect is not fully clear yet<br />
and its understanding needs further studies. Instead, the PL of MC could<br />
be triggered through the polymer excitation. This means that the energy
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
6 MARTIN VALA ET AL.<br />
FIGURE 5<br />
Normalised absorbance and luminescence spectra of P(MEHPV-alt-PV) with dispersed<br />
spiropyran.<br />
absorbed by polymer (donor) is transferred to MC molecules (acceptor).<br />
According to the Forster-Dexter theory the dipole-dipole energy transfer<br />
probability is, among other, related to the overlap of donor emission and<br />
acceptor absorption. For all samples the polymer PL completely overlap<br />
the MC absorption, compare Figure 3 and Figure 5.<br />
Determination of kinetic parameters of the photochromic reaction<br />
Many models have been proposed so far to accurately describe the<br />
decolouration kinetics behaviour in solid matrices (see, e.g., [1] and<br />
references therein). It is usually stressed that physical and steric effects<br />
restrict the internal rotation needed for isomerization and therefore can<br />
play a significant role. The photochromic molecules are surrounded by<br />
microheterogenous matrix and therefore experience statistically different<br />
environment. This results in a distribution of activation energies Ea (assumed<br />
to be Gaussian), which control the reaction rates. Such processes can be<br />
described by so-called ‘stretched exponential’ function [8]<br />
n R(t) = n R(0) exp � −(t/τ) α� , (1)<br />
where α is a parameter describing the deviation from a purely exponential<br />
decay (α
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
PHOTOCHROMIC PROPERTIES OF SPIROPYRAN IN POLYMERIC π–CONJUGATED MATRICES 7<br />
completely ‘frozen’ medium during the decay. Based on a similar physical<br />
model (a statistical distribution of activation energies and/or pre-exponential<br />
factors of reacting molecules in an otherwise frozen matrix) is the method<br />
put forward in [9, 10] which additionally allows for estimation of the width<br />
of the distribution.<br />
The other extreme case is based on the assumption that the modification<br />
of the first-order rate constant is caused by the relaxation of the surrounding<br />
matrix from a nonequilibrium state immediately after the probe reorientation<br />
into relaxed state (therefore referred to as the relaxation model) in a<br />
comparable time scale as the probe decay kinetics [11]. The initial value<br />
of the first order rate constant k0 is thus modified by the matrix relaxation<br />
into its final (relaxed) value k∞ and the rate of the change is governed by<br />
a matrix mean relaxation time τm. The dependence of probe concentration<br />
decay in time is given by the relation [11]<br />
n R(t) = n R(0) exp � −k∞t − [k0 − k∞]τm[1 − exp(−t/τm)] � . (2)<br />
The kinetics of the decays (i.e., of the thermally driven MC → SP reaction)<br />
can be determined by following changes of the absorbance at a fixed<br />
wavelength.<br />
The experimental decays were nonexponential in the whole measured<br />
temperature region, being dependent on the dose of UV radiation to which<br />
the samples had been exposed. Figure 6 shows two decays for different<br />
irradiation times at the same temperature for MEH-PPV/SP sample. Different<br />
doses of delivered light convert different fraction of SP molecules into MC<br />
but this should not affect the rate of the thermal MC → SP reaction which<br />
is a first order process. However, by analysis of our samples by the two<br />
methods considered, such dependence was found. A possible explanation<br />
is a formation of other MC isomers at longer exposure time as it follows<br />
from the spectroscopic studies [12]. Assuming a statistical distribution of<br />
microenvironments in a frozen matrix (resulting in a Gaussian distribution<br />
of activation energies), one obtains the following result: longer times of<br />
exposition of UV radiation resulted in higher values of the activation energy<br />
and in broader distributions σ of the activation energies. Moreover, the<br />
latter parameter was found to decrease with increasing temperature (see<br />
Figure 7). Similar conclusions may be drawn if the relaxation model is<br />
employed to interpret our data (Figure 8). Exposure to higher irradiation<br />
(longer conversion times) led to lower values of k0, k∞ and km = 1/τm<br />
and consequently to higher Ea. All the samples analysed showed a similar<br />
behaviour, the exact values of the parameters slightly differing between the<br />
samples but exhibiting the same trends. The Arrhenius parameters obtained<br />
from absorbance decays are listed in Table 1.
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
8 MARTIN VALA ET AL.<br />
FIGURE 6<br />
Normalised absorbance decay of MC measured at 30 o C in MEH-PPV for two different<br />
irradiation times (15s – (a), 120 s – (b)). The dotted lines represent the slope of k∞, (see<br />
Eq. (2)).
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
PHOTOCHROMIC PROPERTIES OF SPIROPYRAN IN POLYMERIC π–CONJUGATED MATRICES 9<br />
FIGURE 7<br />
Top: Arrhenius plot for k(E0) of the MC form decay obtained for different irradiation<br />
times. The Ea slightly increases with exposure time; see Table 1 for the values. Bottom:<br />
Distribution of the Ea at different temperatures parametric in irradiation time.
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
10 MARTIN VALA ET AL.<br />
FIGURE 8<br />
Arrhenius plot for the kinetics parameters of Eq. (2) (• k0, ◦ k∞, �1/τm) of the MC<br />
decays after 15 s (top) and 120 s (bottom) irradiation by UV light.<br />
CONCLUSION<br />
The experiments reported in this contribution seem to demonstrate that<br />
the SP ↔ MC reaction is a complex process occurring probably via<br />
intermediate states, their relative importance depending on the conversion.<br />
One of plausible explanation may be sought in assuming a co-existence of<br />
several forms of MC which may interconvert during the reaction [2, 13].<br />
TABLE 1<br />
Kinetic parameters for the decay of the MC form of SP in PMMA/SP and MEH-PPV/SP<br />
samples for different irradiation dose.<br />
conversion E0 Ea(k0) Ea(k∞) Ea(τ m)<br />
time (s) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol)<br />
PMMA/SP 15 127 ± 6 92 ± 10 128 ± 11 69 ± 10<br />
30 131 ± 4 105 ± 6 133 ± 10 93 ± 22<br />
60 132 ± 4 112 ± 3 137 ± 14 95 ± 9<br />
120 130 ± 7 117 ± 8 149 ± 28 99 ± 19<br />
MEH-PPV/SP 15 67 ± 40 – – –<br />
30 137 ± 15 111 ± 5 128 ± 11 88 ± 14<br />
60 169 ± 11 – – –<br />
120 142 ± 20 159 ± 6 179 ± 10 168 ± 15
nloqo_E1_vala Nonlinear Optics and Quantum Optics December 11, 2006 17:38<br />
PHOTOCHROMIC PROPERTIES OF SPIROPYRAN IN POLYMERIC π–CONJUGATED MATRICES 11<br />
ACKNOWLEDGMENTS<br />
This work was supported by the grant 3446/2005/G1 from the Development<br />
Fund of Universities, by the project No. 203/03/133 from the Czech Science<br />
Foundation, by project No. 0021630501 from the Ministry of Education,<br />
Youth and Sports, by the Academy of Sciences of the Czech Republic<br />
(Project T400500402 in the program “Information Society”), and by the<br />
Wroclaw University of Technology.<br />
REFERENCES<br />
[1] Such, G., Evans, R. A., Yee, L. H., and Davis T. P. (2003). Factors influencing<br />
photochromism of spiro-compound within polymeric matrices. Journal of Macromolecular<br />
Science, Part C-Polymer Reviews, 43, 547–579.<br />
[2] Toman, P., Bartkowiak, W., Neˇspu¸rek, S., Sworakowski, J., Zale´sny, R. (2005).<br />
Quantum-chemical insight into the design of molecular optoelectrical switch. Chem.<br />
Phys., 316, 267–278.<br />
[3] Neˇspu¸rek, S., Sworakowski, J., Combellas, C., Wang, G., and Weiter, M. (2004). A<br />
molecular device based on light controlled charge carrier mobility. Appl. Surf. Sci.,<br />
234, 395–402.<br />
[4] Neˇspu¸rek, S., and Sworakowski, J. (2001). Molecular current modulator consisting<br />
of conjugated polymer chain with chemically attached photoactive side groups. Thin<br />
Solid Films, 393, 168–176.<br />
[5] Uznanski, P., (2000). From spontaneously formed aggregates to J-aggregates of<br />
photochromic spiropyran. Synthetic Metals, 109, 281–285.<br />
[6] Yang, L. G., Yhang, Q. H., Peng, W., Huang, T. C., Yeng, L. C., Gu, P. F.,<br />
and Liu, X. (2005). Concentration dependence of photoluminescence properties in<br />
poly[(2-methoxy,5-octoxy)1,4-phenzlenevinylene] thin films. Journal of Luminescence,<br />
114, 31–38.<br />
[7] Reichardt, Ch. (1988). Solvents and Solvent Effects in Organic Chemistry, VCH,<br />
Weinheim.<br />
[8] Richert, R., and Bassler, H. (1985). Merocyanine ↔ spiropyran transformation in a<br />
polymer matrix: An example of a dispersive chemical reaction. Chem. Phys. Lett.,<br />
116, 302–306.<br />
[9] Sworakowski, J., and Neˇspu¸rek, S. (1998). A straightforward method of analysis of<br />
first-order processes with distributed parameters. Chem. Phys. Letters, 298, 21–26.<br />
[10] Janus, K., and Sworakowski, J. (2004). Analysis of first-order reactions with distributed<br />
parameters. Struct. Chem. 15, 461–468.<br />
[11] Levitus, M., and Aramendía, P. F. (1999). Photochromism and thermochromism of<br />
phenantrospirooxazine in poly(alkylmehacrylates). J. Phys. Chem. B, 103, 1864–1870.<br />
[12] To be published.<br />
[13] Toman, P., Neˇspu¸rek, S., Weiter, M., Vala, M., Sworakowski, J., Bartkowiak, W., and<br />
Menˇsík, M. (2005). Photoswitching in Polymers with Photochromic Dipolar Species.<br />
This volume.
318<br />
Macromol. Symp. 2007, 247, 318–325 DOI: 10.1002/masy.200750136<br />
A Molecular Photosensor Based on Photoswitching<br />
of Charge Carrier Mobility<br />
Martin Weiter,* 1 Martin Vala, 1 Oldrˇch Zmesˇkal, 1 Stanislav Nesˇpu˚rek, 1,2 Petr Toman 2<br />
Summary: Polymer photoelectronic device based on interaction between<br />
p-conjugated polymer matrices and photochromic molecules was fabricated. The<br />
theoretical and experimental studies proved that the photochromic reaction in<br />
studied devices should eventuate in changes of optical and electrical properties<br />
of polymers such as luminescence and conductivity. The quantum chemical calculations<br />
showed that the presence of dipolar species in the vicinity of a polymer chain<br />
modifies the on-chain site energies and consequently increases the width of the<br />
distribution of hopping transport states. Optical switching was studied using<br />
standard absorption and photoluminescence spectros<strong>copy</strong>. A strong photoluminescence<br />
quenching after the photochromic conversion caused by radiative energy<br />
transfer was observed. The influence of photoswitchable charge carrier traps on<br />
charge transport were evaluated by Space Charge Limited Current (SCLC) method. It<br />
was shown that deep traps may significantly affect the energy of the transport level,<br />
and thus modulate the transport of charge carriers.<br />
Keywords: charge transport; conjugated polymers; quantum chemistry<br />
Introduction<br />
Polymer materials are poised as never<br />
before to transform the world of circuit<br />
and display technology. Nowadays, various<br />
polymers and polymer composites are used<br />
in xerography and laser printers, electroluminescent<br />
diodes and flat displays, the<br />
functional polymers are applied even in the<br />
logical circuits, which give rise to a new<br />
branch – ‘Plastic Logic’. Multistable molecular<br />
systems attract much attention<br />
because of their potential use in optical<br />
and electro-optic devices such as broadband<br />
optical modulators, holographic and<br />
image forming media, rectifiers, sensors<br />
and switching devices. [1–3] New polymer<br />
1 Faculty of Chemistry, Brno University of Technology,<br />
Purkyňova 118, 612 00, Brno, Czech Republic<br />
Fax: þ420 541 211 697<br />
E-mail: weiter@fch.vutbr.cz<br />
2 Institute of Macromolecular Chemistry, Academy of<br />
Sciences of the Czech Republic, Heyrovsky´ Sq. 2,<br />
162 06 Prague 6, Czech Republic<br />
Copyright ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim<br />
materials are able not only to substitute the<br />
expensive crystalline semiconductors in<br />
these devices, but their specific properties<br />
originate the essentially new devices and<br />
technologies.<br />
Due to their electronic structure,<br />
p-conjugated polymers are one of the most<br />
utilized materials in organic optoelectronic<br />
devices. The essential property which<br />
comes out from conjugation is that the<br />
p-electrons are much more mobile than the<br />
s-electrons; they can jump from site to site<br />
between carbon atoms with a low potential<br />
energy barrier as compared to the ionization<br />
potential. The p-electron system has<br />
all the essential electronic features of<br />
organic materials: light absorption and<br />
emission, charge generation and transport.<br />
As the conductivity mechanism in these<br />
materials, a variable-range hopping in a<br />
positionaly random and energetically disordered<br />
system of localized states is widely<br />
accepted. [4,5] Over the last decades, hopping<br />
in random systems was extensively
Macromol. Symp. 2007, 247, 318–325 319<br />
studied. Among these studies, the approach<br />
based on so-called effective transport<br />
energy level was shown to be especially<br />
efficient. [6,7] When the effective transport<br />
energy is established, the variable range<br />
hopping problem is virtually reduced to<br />
trap controlled transport model. According<br />
to this model, the transport of charge<br />
carriers in molecular solids is strongly<br />
influenced by the presence of centres<br />
capable of localizing charge carriers (traps).<br />
It was shown that deep traps may significantly<br />
affect the energy of the transport<br />
level and mobility of charge carriers [8] and<br />
thus control their transport.<br />
The molecular photosensor suggested in<br />
this paper is based on the control of charge<br />
carrier transport in p-conjugated polymers<br />
by photochromic additive. In a photochromic<br />
molecule the absorption of a light<br />
quantum leads to reversible conformational<br />
changes and the process follows displacements<br />
of the optical absorption bands.<br />
Examples of such photochromic molecules<br />
utilized in optical and optoelectronic<br />
switches are spiropyran and spirooxazine<br />
derivatives. In these materials a photochromic<br />
transformation between low and<br />
high energy state is accompanied by change<br />
of conjugation lengths resulting in control<br />
of the electronic structure along the<br />
molecule. The photochromic conversion<br />
can also results in the change of the dipole<br />
moment of the molecule, which consequently<br />
act as charge trap. This feature has<br />
been used in the design of a special type of<br />
optoelectrical sensor.<br />
In our earlier papers [9–11] the concept of<br />
an electroactive molecular modulator was<br />
put forward. The molecular sensor should<br />
be based on the electronic function of a<br />
polymer matrix which contains photochromic<br />
groups able to modulate the transport<br />
of charge carriers. Guest molecules will in<br />
general have different energy levels from<br />
the host. In particular, if ionization energies<br />
and electron affinities are suitably different,<br />
charge carrier traps can be formed. A<br />
special type of traps may occur if guest<br />
molecules possess a permanent dipole<br />
moment. The dipole moment contributes<br />
to the field acting on surrounding molecules<br />
and shifts polymer transport levels. These<br />
dipolar traps are formed on neighbouring<br />
host molecules, even though the impurity<br />
itself does not necessarily form a chemical<br />
trap. For practical sensing applications, one<br />
can easy monitor either the steady-state or<br />
alternating conductivity of thin polymeric<br />
films prepared by low-cost manufacturing<br />
by common printing techniques. [12]<br />
In<br />
principle, the spectral and kinetic parameters<br />
of the sensor should be tuned by the<br />
selection of polymeric matrix and/or<br />
photochromic additive. The purpose of<br />
the present work is to examine by quantum<br />
chemistry modeling and experimental<br />
characterization the optical and electrical<br />
switching properties of the suggested<br />
sensor. For the study the p-conjugated<br />
photoconductive polymers poly[2-methoxy-<br />
5-(2 0 -ethylhexyloxy)-p-phenylenevinylene]<br />
(MEH-PPV) and poly[(p-phenylenevinylene)alt-(2-methoxy-5-(2<br />
0 -ethylhexyloxy)- p-phenylenevinylene)]<br />
P(MEHPV-alt-PV) doped by<br />
photochromic spiropyran 6-nitro- 1 0 ,3 0 ,3 0 ,trimethylspiro[2H-1-benzopyran-<br />
2,2 0 -indoline]<br />
(SP), which can be converted to a<br />
higher dipole moment possessing form<br />
referred to as (photo)merocyanine (MC),<br />
were used.<br />
Theoretical Modeling<br />
The presence of polar additives in the<br />
vicinity of a MEH-PPV chain modifies the<br />
polymer transport levels due to the charge–dipole<br />
interactions. MEH-PPV is a hole<br />
transporting material, hence the energy " n<br />
of a charge carrier located on an n-th<br />
repeating unit (phenylene or vinylene) is<br />
essentially equal to the negative of its first<br />
ionization potential In. For a doped polymer,<br />
the value of In is shifted by the sum of<br />
the electrostatic potentials describing the<br />
charge–dipole interactions of this charge<br />
carrier with all surrounding polar additive<br />
molecules. Since the positions and orientations<br />
of the additive with respect to the<br />
polymer chain are essentially random, this<br />
effect leads to an increase of the width s(")<br />
Copyright ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de
320<br />
Macromol. Symp. 2007, 247, 318–325<br />
of distribution of hopping transport states<br />
(energetic disorder). If point dipoles are<br />
assumed, s(") is proportional to the additive<br />
dipole moment m, i.e. sMC(")/<br />
sSP(") ¼ mMC/mSP. The dipole moments of<br />
SP and MC forms calculated by the<br />
Hartree-Fock method are 5.5 and 11.9 D,<br />
respectively. While the value obtained for<br />
SP is close to reality, the dipole moment of<br />
MC is probably underestimated since the<br />
polar environment increases the zwitterionic<br />
character of MC. Bletz et al. [13]<br />
reported the dipole moment of MC measured<br />
in a polar environment to amount to<br />
15 20 D. For these reasons, one can<br />
expect two- or three-fold increase of the<br />
energetic disorder during the SP ! MC<br />
reaction.<br />
The influence of energetic disorder on<br />
the hole mobility was calculated by means<br />
of a tight-binding approximation model.<br />
The polymer chain is modeled by a<br />
sequence of N ¼ 4000 sites corresponding<br />
to the repeating units (phenylenes and<br />
vinylenes). The hole motion on such a chain<br />
can be described by the Hamiltonian<br />
H ¼ XN<br />
n¼1<br />
½"na þ n an bn;nþ1ða þ<br />
nþ1 an þ a þ n anþ1ÞŠ;<br />
where an and a þ n<br />
(1)<br />
are annihilation and<br />
creation operators of a hole at an n-th site,<br />
"n is the energy of a charge carrier localized<br />
at this site, and bn,n þ 1 is the transfer integral<br />
between the sites n and n þ 1. Both<br />
quantities " n and b n,n þ 1 are influenced by<br />
the random structure of the polymer chain<br />
and its surrounding. For transfer integrals<br />
bn,n þ 1, the random distribution suggested<br />
by Grozema et al. [14] was used. In order to<br />
get the "n distribution, randomly oriented<br />
additive molecules, modeled as point<br />
dipoles, were randomly placed in the<br />
vicinity of the chain. It was assumed that<br />
no additive molecule was placed at a<br />
distance shorter than 10A ˚ from the chain.<br />
This value was estimated from the chemical<br />
structures of the studied molecules. The<br />
concentration of the additive was taken to<br />
be c ¼ 4 10 4 A ˚ 3 . For each repeating<br />
unit, "n was calculated as a sum of In and<br />
Coulombic electrostatic potentials from all<br />
additive molecules.<br />
Using the Hamiltonian (1) with the<br />
molecular parameters "n and bn,n þ 1 and<br />
the hole wave function |c(t)h taken in the<br />
form of a linear combination of the states<br />
located at the individual sites, the timedependent<br />
Schrödinger equation was<br />
numerically integrated. Getting the wave<br />
function in the site representation, it is easy<br />
to calculate the frequency dependent<br />
intramolecular (‘‘on-chain’’) charge carrier<br />
mobility m(v) by the well-known Kubo<br />
formula [15]<br />
mðvÞ ¼ ev2<br />
2kBT Re<br />
2<br />
4<br />
Z 1<br />
0<br />
3<br />
D 2 ðtÞ expð ivtÞdt5;<br />
(2)<br />
where e is the elementary charge, kB is the<br />
Boltzmann constant, T is temperature,<br />
v ¼ 2pf is frequency of the external field,<br />
and D 2 (t) is the mean-square displacement<br />
of the hole defined by the relation<br />
D 2 ðtÞ ¼hcðtÞjn 2 jcðtÞil 2 ; (3)<br />
where l ¼ 3.35 A˚ is the inter-unit distance.<br />
The charge carrier mobility m(v) calculated<br />
for the undoped polymer chain and<br />
the chains doped by additives with different<br />
dipole moments up to 12 D are shown in<br />
Figure 1. For m > 12 D the mobility m(v) is<br />
so small that it is difficult to achieve the<br />
numerical stability. The presented data<br />
were obtained by averaging over 3000<br />
realizations of the transfer integral and<br />
energetic disorder. The results show monotonic<br />
decrease of m(v) with increasing m in<br />
the whole frequency range. The change of<br />
the additive dipole moment from ca. 6 D to<br />
12 D (corresponding to the calculated<br />
change of the dipole moment during the<br />
studied photochromic reaction) should<br />
result in an almost five-fold decrease of<br />
the on-chain mobility. The effect would be<br />
even more pronounced if the experimental<br />
value of the MC dipole moment ( 18 D,<br />
see ref. [13] ) is considered.<br />
Although our simple model is neglecting<br />
several other effects like dynamical fluctua-<br />
Copyright ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de
Macromol. Symp. 2007, 247, 318–325 321<br />
tions of disorder or polaron formation that<br />
influence the mobility values, we believe<br />
the calculated mobility ratio is essentially<br />
correct, since the photochromic mobility<br />
switching arises from the change of the<br />
levels of the static disorder.<br />
Experimental Part<br />
µ [cm 2 /Vs]<br />
100<br />
Polymer devices were manufactured as a<br />
sandwich cell with a dielectric multilayer.<br />
Samples consisted of transparent indium tin<br />
oxide (ITO) electrode on a glass substrate<br />
on to which the 15 nm thin layer of poly(2,3dihydrothieno-1,4-dioxin)<br />
(PEDOT) was<br />
spin cast from water solution to decrease<br />
the injection barrier for holes. Then, the<br />
active polymer layer, typically 150 nm<br />
thick, was spin coated from chloroform<br />
solutions of P(MEHPV-alt-PV) with<br />
5 30% wt. of spiropyran. To decrease<br />
the contact injection barrier for electrons a<br />
thin 10 nm layer of Alq3 (8-hydroxyquinoline,<br />
aluminium salt) was vacuum evaporated<br />
and the structure was completed by<br />
evaporation of aluminium top electrode<br />
100 nm thick. Average device area was<br />
3mm 2 .<br />
The photochromic reaction of SP was<br />
activated using a mercury discharge lamp<br />
HBO 200 with band filter (360 20) nm.<br />
Optical switching was studied using standard<br />
absorption and photoluminescence<br />
10<br />
1<br />
0.1<br />
0.01<br />
0.001<br />
m = 0 D<br />
m = 3 D<br />
m = 6 D<br />
m = 9 D<br />
m = 12 D<br />
0.1 1 10 100<br />
frequency [GHz]<br />
Figure 1.<br />
The charge carrier mobility m calculated for different additive dipole moments m. For frequencies lower than ca.<br />
0.5 GHz mobility is almost frequency-independent because of the diffusive charge carrier motion.<br />
spectros<strong>copy</strong> (PL). The electric response<br />
was studied by measuring the current-voltage<br />
(I-V) characteristics of the samples<br />
in the dark with Keithley 6517A<br />
electrometer.<br />
Results and Discussion<br />
The photochromic behaviour of spiropyran<br />
derivatives has been investigated by many<br />
researchers. [16–18] Under irradiation of an<br />
appropriate light energy, the spiropyran<br />
exhibits photochromism as shown in<br />
Figure 2. The photochromic reaction is<br />
accompanied by a charge redistribution<br />
resulting in a significant increase of the<br />
dipole moment of the molecule.<br />
The studied spiropyran (SP) is stable in<br />
its colourless closed ring isomeric form,<br />
while UV irradiation produces a metastable<br />
open ring isomer (photo)merocyanine<br />
(MC) absorbing at 550–600 nm. The maximum<br />
of the absorption band of P(MEHP-<br />
V-alt-PV) is situated at 455 nm, the addition<br />
of SP increases the absorption of the<br />
sample in the UV region. Since the neat SP<br />
do not absorb in the VIS region, the absorbance<br />
of the doped samples is not affected<br />
by the presence of the SP molecules and<br />
can be seen in the top part of the Figure 3.<br />
The basic absorption spectra of the 30%<br />
mixtures and their relative changes after<br />
1 minute illumination at 360 nm (conversion<br />
Copyright ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de
322<br />
Macromol. Symp. 2007, 247, 318–325<br />
C<br />
H 3<br />
CH 3<br />
CH 3<br />
N O<br />
Figure 2.<br />
Photochromism of the spiropyran molecule.<br />
NO2<br />
C<br />
H 3<br />
CH 3<br />
CH 3<br />
N O<br />
Figure 3.<br />
Top: Normalised absorbance of P(MEHPV-alt-PV) with dispersed spiropyran (full line) and its relative change<br />
(scattered) after 1 minute illumination at 360 nm. New absorption band coming from the (photo)merocyanine<br />
(MC) can be observed. Bottom: Normalised photoluminescence (PL) of the P(MEHPV-alt-PV):SP mixture (full line).<br />
The scattered line represents a change in the spectrum after the MC formation. Decrease in the photoluminescence<br />
of the polymer accompanied by the formation of new band belonging to the MC can be observed.<br />
The excitation wavelength used was 455 nm. The energy was therefore absorbed by the P(MEHPV-alt-PV) and<br />
subsequently transferred to the MC molecules.<br />
Copyright ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de<br />
NO2
of SP to MC form) are also depicted in<br />
Figure 3. The photoinduced colour change<br />
of SP is caused by the extension of<br />
conjugation in the MC isomer compared<br />
to the orthogonal structure of the SP<br />
form. [19] Annealing in the dark or irradiation<br />
with a He-Ne laser gradually restores<br />
the original spectrum, the change being<br />
reversible.<br />
Bottom graph of the Figure 3 shows the<br />
normalized photoluminescence (PL) spectra<br />
of P(MEHPV-alt-PV):SP mixtures and<br />
their relative changes after 1 minute<br />
illumination at 360 nm. The stable form<br />
of spiropyran (SP) does not show any<br />
luminescence contrary to its MC form. The<br />
polymer shows its own luminescence<br />
(Figure 3 bottom) and creation of MC<br />
form via the photochromic reaction therefore<br />
could result in the appearance of new<br />
PL bands. However, MC possesses a high<br />
dipole moment that generally causes<br />
quenching of luminescence. The quenching<br />
was found to be strong even in the case<br />
when the SP ! MC conversion was very<br />
weak (sometimes not observable in absorption<br />
spectra, typically for
324<br />
Macromol. Symp. 2007, 247, 318–325<br />
Figure 4.<br />
Top: the current-voltage characteristics of the ITO/<br />
PEDOT/MEH-PPV:SP/Al (30% by wt.) sample before<br />
(&) and after ( ) the photochromic conversion of<br />
spiropyran molecules by irradiation of UV light.<br />
Decrease in the current flowing accompanied by<br />
the change of the shape can be clearly seen. Bottom:<br />
The normalised distribution of energy states on the<br />
shift of the Fermi level centred around the common<br />
trapping level DEF ¼ EF Et. The arrows depict the<br />
decrease of the density of traps introduced by the<br />
spiropyran molecules and the creation of new trapping<br />
level due to the formation of metastable (photo)merocyanine.<br />
See text for further discussion.<br />
mð2m 1Þðm 1ÞŠ reflects the influence of<br />
the second derivative of the I(V) on the total<br />
charge carrier concentration nsL. The h(E)<br />
function is called density of states (DOS)<br />
and can be obtained after the deconvolution<br />
of the integral in (4). In the case of<br />
gradual changes in the density of states one<br />
can assume that dnsL/dEF h(E). Since this<br />
condition is fulfilled we will use this<br />
simplification in further discussion.<br />
Figure 4 shows the influence of the<br />
spiropyran photochromic conversion on the<br />
current flowing. The top of the figure shows<br />
the experimental I(V) characteristic in the<br />
bilogarithmic scale. It can be seen that the<br />
current is decreased and also that the shape<br />
of the dependence has changed. In the first<br />
region (0–4 V) the slope of the two<br />
characteristics approaches identically to<br />
m ¼ 2, suggesting that the currents are<br />
governed by the presence of one shallow<br />
trapping level with concentration Nt at the<br />
position Et. Above this region the slope<br />
increases differently according to their<br />
respective density of energy states.<br />
The bottom of the Figure 4 reveals the<br />
distribution of the energetic levels normalised<br />
to the maximum of the most populated<br />
trapping level h(E)/N t common to both of<br />
the systems on the shift of the Fermi level<br />
DEF. It has to be noted that the thermodynamic<br />
Fermi level after the spiropyran<br />
conversion was shifted by about 0.02 eV<br />
towards the LUMO orbital. The figure uses<br />
recalculated energy axis centred on the<br />
common trapping level DEF ¼ EF Et,<br />
which is the most populated. It can be<br />
clearly seen that the minor trapping level<br />
situated at 0.06 eV under the reference E t<br />
was reduced after the photochromic conversion.<br />
On the other hand new level<br />
appeared at about 0.02 eV. Since this effect<br />
is reversible, we attribute this switching to<br />
the creation of the metastable (photo)merocyanine<br />
and extinction of spiropyran<br />
molecules, respectively.<br />
Conclusions<br />
The new molecular photosensor based on<br />
photoswitching of charge carrier mobility<br />
was demonstrated. The quantum chemistry<br />
calculations showed that the presence of<br />
polar additive in the vicinity of polymeric<br />
chain modifies its transport levels. This<br />
increase in the energetic disorder eventuates<br />
in the decrease of the hole mobility.<br />
The change of the additive dipole moment<br />
Copyright ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de
Macromol. Symp. 2007, 247, 318–325 325<br />
from ca. 6 D to 12 D, which corresponds to<br />
the studied photochromic reaction, results<br />
in an almost five-fold decrease of the<br />
on-chain mobility. The experimental behaviour<br />
of the system explored by means of<br />
SCLC method showed a change of the<br />
density of states in the bandgap of the<br />
polymer. Reversible creation of new trapping<br />
level during the photochromic conversion<br />
was observed. According to the trap<br />
controlled hopping model for the description<br />
of charge transport, the presence of<br />
new trapping level results in the decrease of<br />
the charge carrier mobility as predicted by<br />
the theoretical calculations.<br />
Furthermore, the presence of polar<br />
centres produced by photochromic conversion<br />
of the spiropyran molecules led to a<br />
quenching of the polymer photoluminescence<br />
and in the case of high concentration<br />
of SP (30% wt.) in a P(MEHPV-alt-PV),<br />
radiative energy transfer was observed. This<br />
is in agreement with our previous work, [20]<br />
where the switching was demonstrated as a<br />
drop of polymer photoconductivity.<br />
Acknowledgements: This work was supported by<br />
the projects 203/03/D133 and 203/06/0285 from<br />
the Czech Science Foundation.<br />
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395.<br />
[12] Y. Yoshioka, G. E. Jabbour, Synth Met. 2006,<br />
accepted.<br />
[13] M. Bletz, U. Pfeifer-Fukumura, U. Kolb, W.<br />
Baumann, J. Phys. Chem. A 2002, 106, 2232.<br />
[14] F. C. Grozema, P. T. van Duijnen, Y. A. Berlin, M. A.<br />
Ratner, L. D. A. Siebbeles, J. Phys. Chem. B 2002, 106,<br />
7791.<br />
[15] R. Kubo, J. Phys. Soc. Japan 1957, 12, 570.<br />
[16] E. Berman, R. E. Fox, F. D. Thomson, J. Am. Chem.<br />
Soc. 1959, 81, 5605.<br />
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[18] X. Li, J. Li, Y Wang, T Matsuura, J Meng,<br />
J. Photochem. Photobiol. 2004, 161, 201.<br />
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O. Salyk, O. Zmesˇkal, Mol. Cryst. Liq. Cryst. 2005, 430,<br />
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Copyright ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ms-journal.de
Polymer optical sensor based on photochromic switching of charge<br />
carrier mobility<br />
Martin Weiter a , Martin Vala ,a , Oldřich Zmeškal a , Jiří Navrátil a , Petr Toman b , Stanislav Nešpůrek a,b , ,<br />
a Faculty of Chemistry, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech<br />
republic;<br />
b Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic,Heyrovský Sq.<br />
2, 162 06 Prague 6, Czech Republic<br />
ABSTRACT<br />
Polymer photoelectronic device based on interaction between π-conjugated polymer matrices and photochromic<br />
molecules was fabricated. The theoretical and experimental studies proved that the photochromic reaction in studied<br />
devices should eventuate in changes of optical and electrical properties of polymers such as luminescence and<br />
conductivity. The quantum chemical calculations showed that the presence of dipolar species in the vicinity of a polymer<br />
chain modifies the on-chain site energies and consequently increases the width of the distribution of hopping transport<br />
states. Optical switching was studied using standard absorption and photoluminescence spectros<strong>copy</strong>. A strong<br />
photoluminescence quenching after the photochromic conversion caused by radiative energy transfer was observed. The<br />
influence of photoswitchable charge carrier traps on charge transport was evaluated by current-voltage measurement and<br />
by Impedance spectros<strong>copy</strong> method. It was shown that deep traps may significantly affect energy of the transport level,<br />
and thus control the transport of charge carriers. Based on these findings, polymer optical sensor was proposed.<br />
Keywords: optical sensor, organic semiconductors, charge transport, photochromic switching<br />
1 INTRODUCTION<br />
1.1 Organic semiconductors for optical sensing applications<br />
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors,<br />
e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide and others. Nowadays, after more than 15 years<br />
of academic and industrial research worldwide, the class of organic materials, conjugated polymers and organic<br />
molecular systems, has reached a very high level of outstanding material properties and the potential for different<br />
industrial applications is now emerging. Various polymers and molecular materials are used in organic light-emitting<br />
diodes (OLED) and flat organic displays equipped with organic field-effect transistors, some other foreseen applications<br />
including optical sensors are very close to industrial applications. Printing technology is currently enabling the<br />
production of these high-efficiency organic devices. Given the need for very low-cost circuits for everything from smart<br />
cards carrying <strong>personal</strong> information, to building entry cards, to inventory control, it is reasonable to assume that within<br />
10 years, the square footage of organic circuitry might exceed that of silicon electronics, though one expects that silicon<br />
transistors would still vastly outnumber and outperform those fabricated from organic materials [1].<br />
Polymers, oligomers, dendrimers, dyes, pigments, liquid crystals, organo-mineral hybrid materials, all organic<br />
semiconductors share in common part of their electronic structure. It is based on conjugated π electrons. By definition, a<br />
conjugated system is made of an alternation between single and double bonds. The essential property which comes out<br />
from conjugation is that the π electrons are much more mobile than the σ electrons; they can jump from site to site<br />
between carbon atoms with a low potential energy barrier as compared to the ionisation potential. As π-orbitals overlap<br />
is weaker than σ-orbitals overlap, the energy spacing (band gap) between bounding and antibounding molecular orbitals<br />
is larger for the π–π ∗ difference than for the σ –σ ∗ one. One can thus, in a first approach, limit the band study to the π–π ∗<br />
molecular orbitals. Those are respectively the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest<br />
Unoccupied Molecular Orbital), in terms of molecular physics. There are also the usual valance (VB) and conduction<br />
bands (CB) terms used in semiconductor physics, respectively.<br />
Optical Sensing Technology and Applications, edited by<br />
Francesco Baldini, Jiri Homola, Robert A. Lieberman, Miroslav Miler,<br />
Proc. of SPIE Vol. 6585, 658519, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.722907<br />
Proc. of SPIE Vol. 6585 658519-1
The π electron system has all the essential electronic features of organic semiconductors: light absorption and emission,<br />
charge generation and transport. In addition, most conjugated polymers have semiconductor band gaps of 1.5–3.5 eV,<br />
which means that they are ideal for optoelectronic devices such as OLEDs, light sensors and photovoltaic panels. When<br />
electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, a photon can be<br />
absorbed to produce an excited molecular state. In organic thin-film photoconductive materials, the generated molecular<br />
state is generally believed to be an exciton, i.e., an electron-hole pair in a bound state, which is transported as a quasiparticle.<br />
Due to the low dielectric constant in organics the Coulomb interaction between the electron in the LUMO and<br />
the hole in the HOMO is strong and the exciton is of the Frenkel type. The binding energy of an optical excitation, Eexc,<br />
is a key parameter for the understanding of the opto-electronic properties of organic solids [2, 3]. It determines the<br />
energy of the dissociation of an exciton and the reverse process, that is, the recombination of an electron–hole pair<br />
yielding an excitation, which can decay either radiatively or non-radiatively. If Eexc, is large, photogeneration of charge<br />
carriers is an endothermic, i.e. inefficient process. Obviously, in a photovoltaic device, one would like Eexc, to be as small<br />
as possible, whereas in a light emitting diode (LED) it is the opposite.<br />
In order to produce a photocurrent, the exciton ought to dissociate into a pair of free charges. The separated charges then<br />
need to travel to the respective device electrodes, holes to the anode and electrodes to the cathode to provide voltage and<br />
be available for injection into an external circuit. The transport of charges is affected by recombination during the<br />
journey to the electrodes - particularly if the same material serves as transport medium for both electrons and holes.<br />
Also, interaction with atoms or other charges may slow down the transport speed and thereby limit the current.<br />
Therefore, a carrier mobility is significant property of organic semiconductors. Mobility expresses the easiness with<br />
which a charge carrier can move through a conducting material responding an electric field. Recent photosensitive<br />
devices can be distributed into three classes. Solar cells, also called photovoltaic (PV) devices, are a type of<br />
photosensitive optoelectronic devices that are specifically used to generate electrical power. Photoconductor cells consist<br />
of active layer and signal detection circuitry, which monitors the resistance of the device to detect changes due to the<br />
light absorption. Another type of photosensitive optoelectronic device is a photodetector. In operation, a photodetector is<br />
used in conjunction with a current detecting circuit, which measures the current generated when the photodetector is<br />
exposed to an electromagnetic radiation and may have an applied bias voltage. As a general rule, a photovoltaic cell<br />
provides power to a circuit, device or equipment, but does not provide a signal or current to control detection circuitry, or<br />
the output of information from the detection circuitry. In contrast, a photodetector or photoconductor provides a signal or<br />
current to control detection circuitry, or the output of information from the detection circuitry but does not provide power<br />
to the circuitry, device or equipment.<br />
1.2 Concept of molecular optical sensor<br />
The concept of the optical polymer sensor presented in this paper differs from the principles described above. It is based<br />
on the photochromic control of charge carrier transport in polymers. As the conductivity mechanism in these materials, a<br />
variable-range hopping in a positionaly random and energetically disordered system of localized states is widely<br />
accepted [4, 5]. Over the last decades, hopping in random systems was extensively studied. Among these studies, the<br />
approach based on so-called effective transport energy level was shown to be especially efficient [6, 7]. When the<br />
effective transport energy is established, the variable range hopping problem is virtually reduced to trap controlled<br />
transport model. According to this model, the transport of charge carriers in molecular solids is strongly influenced by<br />
the presence of centres capable of localizing charge carriers (traps). It was shown that deep traps may significantly affect<br />
the energy of the transport level and mobility of charge carriers [8] and thus control their transport.<br />
Proposed polymer optical sensor is based on switching of charge carrier mobility by photochromic species distributed in<br />
polymer matrix. Photochromic reactions, in addition to the changes in electronic spectra, are also accompanied by<br />
variations in refractive index, dielectric constant, enthalpy etc [9]. Moreover, the reversible changes in physical or<br />
chemical properties of the photochromic species can be transferred to the microenvironment and supramolecular<br />
structure, and thus, can induce rich modifications in the surroundings. In a molecular solid build of non-polar polarizable<br />
units, e.g. polymer segments, containing a small amount of polar guest species, its dipole moment contributes to the field<br />
acting on surrounding molecules and modifies the local values of the polarization energy. This modification can cause a<br />
local decrease of the ionisation energy in an otherwise perfect crystal lattice, which represents a trap for hole. Thus, the<br />
presence of polar species may result in production of local states (charge traps) - in their vicinity; even thus they are not<br />
necessarily trapping sites themselves [10], [11]. Reversible creation of such polar species can be obtained by e.g. suitable<br />
photochromic molecules. The induced change of electrostatic potential due to the charge-dipole interactions also shifts<br />
the site energies of individual polymer repeating units, and consequently the polymer transport levels are modified. Since<br />
Proc. of SPIE Vol. 6585 658519-2
the position and orientation of the additive with respect of the polymer chain are essentially random the effect results in<br />
broadening of the distribution of the transport states and consequently to the lowering of the charge carriers mobility. In<br />
the case of reversible formation and annihilation of such traps the electric charge transport can be even changed from<br />
space-charge limited to trap limited [12].<br />
All of the effects mentioned above affect reduction of charge carrier mobility and thereby lowering of the current<br />
flowing through the device. The reversible transformation of the optical signal into electrical one can be thus achieved<br />
and optoelectronic sensor can be constructed. The purpose of the present work is to examine by quantum chemistry<br />
modeling and experimental characterization the optical and electrical switching properties of the suggested sensor. For<br />
the study the π-conjugated photoconductive polymers poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene]<br />
(MEH-PPV) doped by photochromic spiropyran 6-nitro-1',3',3',-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (SP),<br />
which can be converted to a higher dipole moment possessing form referred to as (photo)merocyanine (MC), were used.<br />
2 THEORETICAL MODELING<br />
The polymer chain is modeled by a sequence of N=4000 sites corresponding to the repeat units (alternating phenylenes<br />
and vinylenes). Each site n is described by the energy εn of a hole located at this site. The hole transport between the sites<br />
n and m is described by the transfer integral bn,m. Because of linear character of the MEH–PPV chain and the size of the<br />
repeat units only the transfer integrals between the neighboring repeat units are important and the other can be neglected<br />
(tight-binding approximation). Thus, hole motion on such a chain can be described by the Hamiltonian<br />
N<br />
∑<br />
n=<br />
1<br />
H = ε<br />
+<br />
+<br />
+<br />
[ a a − b ( a a + a a ) ],<br />
n<br />
n<br />
n<br />
n,<br />
n+<br />
1<br />
n+<br />
1 n<br />
where an and an + are annihilation and creation operators of a hole at an n-th site. Both quantities εn and bn,n+1 are<br />
influenced by the random structure of the polymer chain and its surrounding. The energy εn is essentially equal to the<br />
negative of the first ionization potential of the corresponding repeat unit. Grozema et al. [14] developed a continuous<br />
disorder type model of the transfer integral bn,n+1 distribution, showing that the hole mobility in a pure MEH–PPV is<br />
limited by the torsional disorder. We assume that the influence of the additives on the electronic coupling between the<br />
polymer repeat units is small in comparison with the electrostatic charge–dipole interaction modulating the site energies<br />
εn. Hence, we have used in our model the same distribution of the transfer integrals bn,n+1 as Grozema et al. also for the<br />
description of the hole transport on MEH–PPV chain influenced by polar additives.<br />
Polar species in the polymer chain vicinity modify the on-chain electrostatic potential due to the charge–dipole<br />
interactions between a hole moving on the chain and dipole moments of individual additive molecules dispersed in the<br />
polymer (see Fig. 1). It is easy to show that the sum of these electrostatic potential changes shifts the hole site energies εn<br />
by the value < ∑ ∆ φi|HOMO<br />
> , where ∆φi are the changes of the electrostatic potential describing the charge–<br />
HOMO| i<br />
dipole interactions of a charge carrier localized at |HOMO> with all surrounding polar additive molecules. The change<br />
of the shape of the highest molecular orbital |HOMO> of the corresponding repeat unit induced by the additive is<br />
neglected (frozen orbital approximation) [15]. Since the positions and orientations of the additive molecules with respect<br />
to the polymer chain are essentially random, the effect results in broadening of the distribution of transport states. The<br />
most important parameter of this distribution is its half-width σ(εn).<br />
In order to model the charge–dipole interactions, randomly oriented additive molecules, represented by point dipoles,<br />
were randomly placed in the vicinity of the chain. It was assumed that no additive molecule was placed at a distance<br />
shorter than 10 Å from the chain (this value was estimated from the chemical structures of MEH-PPV and SP/MR). On<br />
the other hand, the influence of the additive molecules distant more than 50 Å from the polymer chain was neglected.<br />
The additive molecules were placed also beyond the chain ends in order to ensure the homogeneity of the εn distribution<br />
along the whole chain. The concentration of the additive was taken to be c = 4 × 10 -4 Å -3 . For each center representing a<br />
repeat unit, the energetic disorder was calculated as a sum of Coulombic electrostatic potentials from all additive<br />
molecules. With regard to the value of the minimal distance of an additive molecule from the chain the size of the<br />
polymer repeat units was neglected. The energetic disorder of εn was calculated according to the above-described model<br />
for several values of the dipole moment of the additive m. The resulting εn distribution is a Gaussian-type distribution<br />
with a high site to site εn correlation. For a typical value of m = 12 D the half-width σ(εn) = 0.37 eV. If the mutual<br />
interaction of the additive molecules is neglected, the half-width σ(εn) is proportional to the dipole moment m of the<br />
n<br />
n+<br />
1<br />
Proc. of SPIE Vol. 6585 658519-3<br />
(1)
additive and to the square root of the additive concentration c . The εn values show a strong correlation between up to<br />
about 10 th nearest neighboring sites. The correlation coefficient between the nearest neighbor εn values is 0.97. This fact<br />
can be explained by the long-range character of the charge–dipole interactions and the size of the MEH–PPV<br />
substituents hindering from close contact between additive molecules and the main chain.<br />
∆φ [eV]<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
-2 -1 0<br />
Repeat unit<br />
1 2<br />
Fig. 1. The change of the on-chain electrostatic potential of a MEH–PPV chain caused by one individual additive molecule<br />
(MR) in a particular position.<br />
The dipole moments of SP and MR forms calculated by the Hartree-Fock method are 5.5 and 11.9 D, respectively. These<br />
values suggest that the SP → MR reaction results in approximately doubling the energetic disorder. Moreover, one<br />
should take into account that, while the value obtained for SP is close to reality, the dipole moment of MR is probably<br />
underestimated since the polar environment increases the zwitterionic character of MR. Bletz et al. [16] reported the<br />
dipole moment of MR measured in a polar environment to amount to 15÷20 D. Thus the real switching effect may be<br />
more effective than that estimated using the calculated values.<br />
Using the Hamiltonian (1) with the calculated molecular parameters εn and βn,n+1 and the hole wave function |ψ(t)〉 taken<br />
in the site representation<br />
ψ<br />
( t)<br />
c(<br />
t)<br />
n ,<br />
= ∑<br />
n=<br />
1<br />
where |n> is the state located on n-th site, the time-dependent Schrödinger equation<br />
∂<br />
ih ψ () t = H ψ () t<br />
(3)<br />
∂t<br />
Proc. of SPIE Vol. 6585 658519-4<br />
(2)
was numerically integrated. At t = 0, the hole was assumed localized on a single unit in the middle of the chain.<br />
Obtaining the wave function, one may calculate the mean-square displacement ∆ 2 (t) of the charge carrier defined by the<br />
relation<br />
2<br />
2<br />
() t = ψ ( t)<br />
n ψ ( t)<br />
d<br />
2<br />
∆ , (4)<br />
where d = 3.35 Å is the inter-unit distance. The calculated data were averaged over 3000 Monte Carlo realizations of the<br />
Hamiltonian (1) in order to achieve numerical stability. According to fluctuation–dissipation theorem, this quantity is<br />
related to the frequency dependent intramolecular hole mobility µ(ω) by the Kubo formula [17]<br />
2 ∞<br />
− eω<br />
⎡<br />
⎤<br />
µ , (5)<br />
2<br />
( ω ) = Re⎢∫<br />
∆ ( t)<br />
exp( −iωt)<br />
dt⎥<br />
2k<br />
BT<br />
⎣ 0<br />
⎦<br />
where e is the elementary charge, kB is the Boltzmann constant, and T is temperature.<br />
The hole on-chain mobilities µ(ω), calculated for different values of the additive dipole moments, are shown in Fig. 2.<br />
All curves exhibit a saturation of µ(ω) at low frequencies corresponding to the diffusive charge carrier motion in the<br />
long-time limit (t > 25 ps), and a rapid increase for higher frequencies related to the fast initial hole delocalization<br />
(t < 10 ps). This increase is mainly induced by the effective reduction of the number of disordered sites visited by a<br />
charge carrier during its oscillations induced by the electric filed. In both regions, the mobility decreases with the<br />
increasing additive dipole moment. It follows from Fig. 2 that the change of the additive dipole moment from ca. 6 D to<br />
12 D (corresponding to the calculated change of the dipole moment during the photochromic reaction SP ↔ MR) should<br />
result in an about five-fold decrease of the on-chain mobility. The effect would be even more pronounced if the<br />
experimental value of the MR dipole moment (≈18 D [16]) is considered.<br />
mobility [cm 2 /Vs]<br />
1000<br />
100<br />
10<br />
1<br />
0.1<br />
0.01<br />
no additive (0 D)<br />
SP (6 D)<br />
MR (12 D)<br />
0.001<br />
0.1 1 10 100 1000<br />
frequency [GHz]<br />
Fig. 2. The calculated frequency-dependent mobility for different additive dipole moments. The additive concentration was<br />
c = 4 × 10 -4 Å -3 .<br />
It should be noted that the results may be influenced by several other effects not taken into account by our model, namely<br />
dynamical fluctuations of both types of disorder and polaron formation. While the former effect results in a thermally<br />
activated diffusive charge carrier motion, the latter one increases charge localization. We assumed an ideal chemical<br />
structure, but the presence of polymerization defects decreases the on-chain mobility. The effect of the substituents was<br />
considered only in the modeling of the energetic disorder arising from the charge – additive dipole interaction. However,<br />
the presence of substituents has some influence also on the torsional and energetic disorder of undoped polymer chains.<br />
These phenomena will alter the mobility values but they should not significantly alter the mobility ratio since the<br />
photochromic mobility switching arises from the change of the levels of the static disorder.<br />
Proc. of SPIE Vol. 6585 658519-5
3 EXPERIMENTAL<br />
Polymer devices were manufactured as a sandwich cell with a dielectric layer of MEH-PPV (poly[2-methoxy-5-(2'ethylhexyloxy)-p-phenylenevinylene])<br />
containing 0-30% wt. of admixed spiropyrane (6-nitro-1',3',3',-trimethylspiro[2H-<br />
1-benzopyran-2,2'-indoline]). The device and material structure is depicted in Fig. 3 and Fig. 4, respectively. The active<br />
polymer layer was spin coated from chloroform solutions on transparent indium tin oxide (ITO) electrode covering part<br />
of the glass substrate. The thickness of the active layer was about 150 nm. The structure was completed by evaporation<br />
of aluminium top electrode typically 100 nm thick. Average electrode area that delimitate the active area of the device<br />
was 3 mm 2 . The same solutions of active materials were spin cast on quartz substrates for optical measurements. The<br />
photochromic reaction of spiropyrane was activated using a Nd:YAG laser with third harmonic generation at 355 nm in<br />
order to measure the irradiation of the sample precisely. Optical switching was studied using standard absorption and<br />
photoluminescence spectros<strong>copy</strong> (PL). The electric response was studied by measuring the current-voltage j(V)<br />
characteristics of the samples in the dark with Keithley 6517A electrometer. Dielectric properties were studied using a<br />
Hewlett Packard 4192A impedance analyzer. The measurements were performed in vacuum cryostat at room<br />
temperature.<br />
+<br />
External<br />
Circuit<br />
Al<br />
Organic Film<br />
ITO<br />
Glass Substrate<br />
Incident Light<br />
Fig. 3. Schematic structure the devices used for characterization.<br />
4 RESULTS AND DISCUSSION<br />
4.1 Optical switchning<br />
Under irradiation of an appropriate light energy, the spiropyran exhibits photochromism as shown in Fig. 4. The<br />
photochromic reaction is accompanied by a charge redistribution resulting in a significant increase of the dipole moment<br />
of the molecule. The studied spiropyran (SP) is stable in its colourless closed ring isomeric form, while UV irradiation<br />
produces a metastable open ring isomer merocyanine (MC) absorbing at 550-600 nm. The maximum of absorption band<br />
of MEH-PPV is situated at 445 nm, the addition of SP increases the absorption of the sample in the UV region as shown<br />
in Fig.4. After irradiation with UV light the spectra exhibit a significant absorption band at 590 nm caused by the colored<br />
merocyanine form. Annealing in dark or irradiation with a red light gradually restores the original spectrum, the change<br />
is fully reversible.<br />
A strong photoluminescence (PL) quenching after the photochromic conversion caused by radiative energy transfer was<br />
observed as figured in Fig. 5. The stable form of spiropyran does not show any luminescence contrary to its MC form.<br />
MEH-PPV show their own luminescence and creation of MC form via the photochromic reaction therefore could result<br />
in the appearance of new PL bands. However, MC possesses a high dipole moment which generally causes a quenching<br />
of luminescence. The quenching was found to be strong even in the case when the SP → MC conversion was very weak,<br />
even not observable in absorption spectra. Furthermore the PL of MC could be triggered through the polymer excitation.<br />
This means that the energy absorbed by polymer (donor) is transferred to MC molecules (acceptor). According to the<br />
Forster-Dexter theory the dipole-dipole energy transfer probability is, among other, related to the overlap of donor<br />
emission and acceptor absorption. After the reverse (MC → SP) reaction was completed, the polymer PL was recovered,<br />
the reversibility of the process was, however, influenced by photodegradation of the polymer matrix.<br />
Proc. of SPIE Vol. 6585 658519-6<br />
-<br />
External<br />
Circuit
Absorbance (a.u.)<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
spiropyran (SP) merocyanine (MC)<br />
MEH-PPV+SP<br />
MEH-PPV+MC<br />
MEH-PPV<br />
) CH3 n<br />
0.0<br />
200 300 400 500 600 700<br />
C<br />
H 3<br />
wavelength (nm)<br />
Fig. 4. The photochromic reaction of the spiropyran (SP) into (photo)merocyanine (MC) manifests itself as color change of<br />
the system. The dotted line shows the UV-VIS absorbance spectrum of MEH-PPV polymeric matrix. Full line<br />
represents the absorbance spectrum of MEH-PPV:SP mixture. The spectrum after the photochromic conversion (dashed<br />
line) shows distinct appearance of a band peaking at 590 nm. The insets show the chemical formulas and the<br />
photochromic reaction of the materials used. The R 1 stands for methyl and the R 2 for 2-ethylhexyl.<br />
photoluminiscence (arb. u.)<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
(<br />
OR 1<br />
OR 2<br />
MEH-PPV<br />
450 500 550 600 650 700 750<br />
wavelength (nm)<br />
MEH-PPV:SP<br />
MEH-PPV:MC<br />
Fig. 5. The full line represents photoluminescence spectrum of the MEH-PPV:SP thin layer. The spectrum after the<br />
photochromic conversion changes as can bee seen from the dotted line.<br />
Proc. of SPIE Vol. 6585 658519-7
As was described above, the operation of the proposed sensor is based on incidence of merocyanine, which influence the<br />
charge transport in MEH-PPV matrix. Therefore the samples with different concentration of spiropyrane were prepared.<br />
For precise study of the sensitivity, the samples were irradiated by Nd:YAG laser pulses at 355 nm, each laser pulse<br />
represents a dose of 1.5·10 14 photons. The changes of the merocyanine concentration were determined after irradiation<br />
following the absorbance at λmax=590 nm. The situation is depicted in Fig. 6 for samples with 10%, 20% and 30%<br />
concentration of SP. The results show that the absorbance is proportional to SP/MC concentration and exponentially<br />
follows the irradiation dose, which is demonstrated by linear slope of the dependency in logarithmic scale of x-axes. At<br />
high irradiation dose, the concentration of merocyanine tends to saturate, as demonstrated in the inset of the Figure.<br />
Since each one MC molecule poses potential charge trap, the sensitivity of the sensor thus can be designated by its<br />
concentration. Thus, the optical and electronic properties of studied sensor can be related. The relationship between<br />
optical and electrical properties of proposed sensor will be described hereafter.<br />
∆ Absorbance<br />
0.20<br />
0.15<br />
0.10<br />
0.05<br />
0.00<br />
∆ Absorbance<br />
0,15<br />
0,10<br />
0,05<br />
10 %<br />
20 %<br />
30 %<br />
0 100 200 300 400 500<br />
Number of pulses<br />
100<br />
Number of pulses<br />
10%<br />
20%<br />
30%<br />
Fig. 6. The dependence of the MEH-PPV:SP samples absorption monitored at 590 nm on the light dose represented by<br />
number of NdYAG laser pulses at 355 nm. Each pulse represents dose of 1.5·10 14 photons.<br />
4.2 Current-voltage measurements<br />
The photoswitching of charge carrier mobility was studied by standard current-voltage j(V) measurement. The results for<br />
typical devices are shown as log-log plot in Fig. 7 and Fig. 8. The variation of the current with increasing irradiation dose<br />
is depicted in Fig. 7for sample doped by 20% of spiropyrane, whereas the variation of the current at the irradiation dose<br />
corresponding to the merocyanine concentration saturation for different spiropyrane concentration is shown in Fig. 8. In<br />
organic thin film devices the current is typically contact limited in low field region, whereas at higher field region spacecharge<br />
or charge-trap limited conductivity are commonly accepted [18]. The results show this behavior. At low forwardbias<br />
voltages below 7-10 V the increase of j with V is relatively small, whereas in the higher field region the slope of the<br />
dependence is more pronounced, which is in accordance with Space-charge limited current (SCLC) theory. This theory<br />
proposes that the space charge which limits conduction is stored in the traps. In the case of energetically discrete trapping<br />
level, the SCL current can be expressed as<br />
j SCL<br />
2<br />
9 V<br />
= εε 0µθ<br />
,<br />
(7)<br />
3<br />
8 L<br />
where , ε and ε0 is the relative permitivity and permitivity of vacuum, µ is the charge carrier mobility, V is the applied<br />
voltage, L is the electrode distance and θ is the ratio of free to total charge carriers.<br />
Proc. of SPIE Vol. 6585 658519-8
Current (A)<br />
10 -4<br />
10 -5<br />
10 -6<br />
10 -7<br />
10 -8<br />
10 -9<br />
0%<br />
2%<br />
5%<br />
10%<br />
2 4 6 8 10 12 14<br />
Voltage (V)<br />
Fig. 7. Current-voltage- characteristics for various concentration of spiropyran after the photochromic conversion induced<br />
by high photon dose corresponding to the merocyanine absorbance saturation.<br />
Current (A)<br />
10 -4<br />
10 -6<br />
10 -8<br />
∆ Absorbance<br />
0,14<br />
0,12<br />
0,10<br />
0,08<br />
0,06<br />
0,04<br />
0,02<br />
0,00<br />
0 100 200 300 400 500<br />
Number of pulses<br />
10<br />
1 10<br />
-10<br />
Voltage (V)<br />
Fig. 8. Current-voltage characteristics of the 20 % mixture of the MEH-PPV:SP for different light dose. The dose chosen<br />
can be seen in the inset depicting the absorbance change. The selected numbers of pulses are highlighted.<br />
Proc. of SPIE Vol. 6585 658519-9<br />
0<br />
50<br />
100<br />
200<br />
400
However, in cases of practical interest traps are usually distributed in energy. In that case traps will be filled from the<br />
bottom to the top of the distribution as applied electric field increases. This is equivalent to an upward-shift in the quasi-<br />
Fermi level with electric field. As a consequence, θ increases with electric field and the j(V) characteristics becomes<br />
steeper. In terms of present work, the distribution of charge traps describes those induced by spiropyrane to merocyanine<br />
photochromic conversion. The presence of distribution of traps opens additional pathways to the relaxation of charge<br />
carriers towards steeper states. A zero order analytic description of the effect can be based on the Hoesterey and Letson<br />
formalism [19]. The latter is premised on the argument that the carrier mobility in a system with relative trap<br />
concentration c is the product of the mobility in the trap-free system µ0 multiplied by trapping factor:<br />
⎡ ⎛ Et<br />
⎞⎤<br />
µ ( c)<br />
= µ 0 ⎢1<br />
+ c exp⎜<br />
⎟⎥<br />
,<br />
(8)<br />
⎣ ⎝ kT ⎠⎦<br />
where Et is the energy of trapping level, k is the Boltzmann constant and T is the temperature. Consequently, the current<br />
flowing in a sample with enhanced number of trap states will be less than in sample without traps.<br />
Figures 7 and 8 prove this notion. Figure 7 shows the decrease of the current with increasing photon dose demonstrated<br />
by number of laser pulses. As the merocyanine concentration increase exponentially with photon dose and tends to<br />
saturate at high irradiation (see Fig. 6), the decrease of the current limits by about two orders of magnitude.<br />
Complementary, Fig. 8 shows the decrease of the photocurrent with increasing concentration of spiropyrane at the<br />
irradiation dose corresponding to the merocyanine absorbance saturation (see Fig. 6. for details). In both cases the<br />
decrease is fully reversible after thermal fading in the dark or irradiation with a red light. The essential message of<br />
Figures 7 and 8 is that the current thorough the irradiated device is significantly lowered by the presence of polar charge<br />
traps caused by photochromic conversion of spiropyrane to merocyanine. In another words the presence of traps lowers<br />
the mobility of charge carriers as expected from theoretical calculations.<br />
4.3 Impedance analysis<br />
To provide better insight into studied phenomena the real and complex part of the impedance ZRe and ZIm were recorded<br />
at test frequencies between 10 and 5·10 6 Hz. The measured data were analyzed in the form of Cole-Cole diagram (Real<br />
part of Z vs. imaginary part of Z) wherein the frequency increases from right to left. The variations of ZIm with ZRe as a<br />
function of photon dose at constant bias of 5 V for the device with 20% of spiropyrane are shown in Fig. 9.<br />
Z Im (Ω)<br />
2x10 6<br />
1x10 6<br />
0<br />
0<br />
50<br />
100<br />
200<br />
400<br />
0 1x10 6<br />
2x10 6<br />
Z Re (Ω)<br />
Fig. 9. Cole-cole plot of the 20 % mixture of the MEH-PPV:SP before (0 pulses) and after the photochromic conversion for<br />
four light doses (50, 100, 200, 400 laser pulses).<br />
−1<br />
3x10 6<br />
Proc. of SPIE Vol. 6585 658519-10<br />
4x10 6<br />
5x10 6
All the ZIm versus ZRe dependency shows a single semicircle which increases in size with increasing photon dose. This<br />
single semicircle could be fitted very well to a parallel combination of bulk resistance Rp and capacitance Cp in series<br />
with a resistance Rs, which is probably caused by the Ohmic contact at hole injecting ITO/MEH-PPV interface. From<br />
Fig. 9 the bulk parallel resistance can be evaluated as the virtual intercept point of each Cole-Cole plot with ZRe axis.<br />
Following this evaluation, Fig. 9 clearly demonstrates the increasing parallel resistance of the sample with increasing<br />
photon dose, which is in accordance with previous results. The comparison of data obtained by impedance analysis and<br />
steady-state j(V) characterization is depicted in Fig. 10 for samples doped by 20%.of spiropyrane after irradiation. Herein<br />
the results are related to the absorption of the samples at 590 nm, which is proportional to merocyanine concentration as<br />
was described above. In this figure the left y-axis represents the relative increase of the parallel resistance obtained by<br />
impedance analysis, whereas the reverse value of the relative decrease of the current evaluated form the j(V)<br />
characteristics is marked on right y-axis. It is shown that the dependencies which manifest the influence of merocyanine<br />
charge traps on the charge transport in MEH-PPV matrix are both almost linear.<br />
R i /R 0<br />
7<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
paralel resistance change<br />
current change<br />
0,04 0,06 0,08 0,10 0,12<br />
∆ Absorbance<br />
Fig. 10. The dependence of ratio of parallel resistance after the photochromic conversion to the value before conversion on<br />
the change of absorbance (left y-axis) and the ratio of the current before the photochromic conversion and after for<br />
different light dose (50, 100, 200, 400 laser pulses) at 4.2 V of the 20% mixture of the MEH-PPV:SP.<br />
5 CONCLUSIONS<br />
The new molecular photosensor based on photoswitching of charge carrier mobility was demonstrated. The quantum<br />
chemistry calculations showed that the presence of polar additive in the vicinity of polymeric chain modifies its transport<br />
levels. This increase in the energetic disorder eventuates in the decrease of the hole mobility. The change of the additive<br />
dipole moment from ca. 6 D to 12 D, which corresponds to the studied photochromic reaction, results in an almost fivefold<br />
decrease of the on-chain mobility. The experimental behaviour of the system explored by means of current-voltage<br />
characterization showed a significant decrease of the current thorough the sample after irradiation. The current decrease<br />
is proportional either to the concentration of spiropyrane in the sample or to the irradiation dose. The increase of parallel<br />
resistance of the sample with irradiation dose obtained by impedance analysis confirms this outcome. According to the<br />
trap controlled hopping model for the description of charge transport it was stated that the presence of new trapping level<br />
results in the decrease of the charge carrier mobility and thereby lowering the current as predicted by the theoretical<br />
calculations.<br />
Proc. of SPIE Vol. 6585 658519-11<br />
18<br />
16<br />
14<br />
12<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
I 0 /I i
6 ACKNOWLEDGMENTS<br />
This work was supported by project KAN401770651 from The Academy of Sciences of the Czech Republic, project<br />
203/06/0285 from the Czech Science Foundation, and by project No. 0021630501 from Ministry of Education, Youth<br />
and Sports. A possibility of using computer time in MetaCenter (Prague and Brno) is gratefully acknowledged.<br />
REFERENCES<br />
1. P. Harrop, R. Das, Organic Electronics: Forecasts, Players & Opportunities 2005-2025, IDtechEx, New<br />
York, 2005.<br />
2. A. J. Heeger, in: N. S. Sariciffci, (Ed.), Primary Photoexcitations in Conjugated Polymers: Molecular Excitons<br />
Versus Semiconductor Band Model, World Scientific, Singapore, 1997.<br />
3. H. Baessler, in: N.S. Sariciftci (Ed.), Primary Photoexcitations in Conjugated Polymers: Molecular Excitons<br />
Versus Semiconductor Band Model, World Scientific, Singapore, 1997.<br />
4. M. Pope, C. E. Swenberg, Electronic Processes in Organic Crystals and polymers, 2nd ed., Oxford University<br />
Press, Oxford, 1999.<br />
5. H. Bässler, Phys. Stat. Solidi B 15,175 (1993).<br />
6. V. I. Arkhipov, E. V. Emelianova, G. J. Adriaenssens, Phys. Rev. B 64, 125125, (2001).<br />
7. V. I. Arkhipov, P. Heremans, E. V. Emelianova, G. J. Adriaenssens, H. Bässler, J. Phys.: Condens matter 42, 9899<br />
(2002).<br />
8. V. I. Arkhipov, J. Ryenaert, Y. D. Jin, P. Heremans, E. V. Emelianova, G. J. Adriaenssens, H. Bässler, Synt. Met.<br />
138, 209, (2003).<br />
9. H. Durr, H. Bouas-Laurent, (eds.) Photochromism: molecules and systems, Elsevier, Amsterdam, 2003.<br />
10. S. Nešpůrek, J Sworakowski, Thin Solid Films 393, 168 (2001).<br />
11. M. Weiter, M. Vala, S. Nešpůrek, J. Sworakowski, O. Salyk, O. Zmeškal, Mol. Cryst. Liq. Cryst. 430, 227 (2005).<br />
12. P. Anderson, N. D. Robinson, M. Berggren, Adv. Matter. 17, 1798 (2005).<br />
13. T. Tsuioka, H. Kondo, Appl. Phys. Lett. 83, 937 (2003).<br />
14. F. C. Grozema, P. T. van Duijnen, Y. A. Berlin, M. A. Ratner, L. D. A., Siebbeles, J. Phys. Chem. B 106, 7791,<br />
(2002)<br />
15. P. Toman, W. Bartkowiak, S. Nešpůrek, J. Sworakowski, R. Zaleśny, Chem. Phys. 316, 267 (2005).<br />
16. M. Bletz, U. Pfeifer-Fukumura, U. Kolb, W. Baumann, J. Phys. Chem. A 106, 2232, (2002).<br />
17. R. Kubo, J. Phys. Soc. Japan 12, 570 (1957).<br />
18. P. W. M. Blom, M. J. M. deJong, J. J. M. Vleggaar, Apl. Phys. Lett. 68, 3308 (1996).<br />
19. D. C. Hoesterey, G. M . Letson, J. Phys. Solids 24, 1609 (1963).<br />
Proc. of SPIE Vol. 6585 658519-12
Space-charge-limited currents: An E-infinity<br />
Cantorian approach<br />
Oldrich Zmeskal a, *, Stanislav Nespurek a,b , Martin Weiter a<br />
a Faculty of Chemistry, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic<br />
b Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic<br />
Abstract<br />
Accepted 31 March 2006<br />
The theory of space-charge-limited currents for trap-free insulator, insulator with single trap level and exponential<br />
trap distribution in energy is presented using fractal analysis. It is shown that independent of the electrode configuration<br />
it is possible to write a general equation for current–voltage characteristic changing only the parameter of fractal dimension.<br />
On the basis of cylindrical electrode configuration the expression for the current–voltage dependence on the surface<br />
gap sample was derived.<br />
Ó 2006 Elsevier Ltd. All rights reserved.<br />
1. Introduction<br />
Chaos, Solitons and Fractals 34 (2007) 143–158<br />
The knowledge of parameters of local states existing in all real dielectrics and wide band-gap semiconductors, e.g.,<br />
their densities, energetic and spatial distributions and cross-section, is a crucial requirement in characterization of the<br />
transport and storage of charge carriers in low-conductivity materials. Various methods [1–10] have been employed to<br />
study the localization (‘‘trapping’’) of charge carriers and the parameters of traps. Until the mid-seventies, the determination<br />
of parameters of local states was considered to be largely of pure academic interest. The number of papers dealing<br />
with the problem has increased rapidly due to the growing demand for reliable methods for characterization of local<br />
states in amorphous and polycrystalline semiconductors (cf., e.g., Ref. [11] and references therein). In practice, it is<br />
impossible to determine all trapping parameters from measurements using a simple method. Thus a complex characterization<br />
requires that several methods be employed independently. Among the methods commonly used to study<br />
the parameters of local states, the technique of space-charge-limited (SCL) currents occupies a prominent place. Both<br />
steady-state and transient SCL currents have been studied by several authors for over 30 years (cf., e.g., Refs. [1,3] and<br />
literature therein).<br />
The forms of the equations describing SCL currents strongly depend on the electrode configuration. Many authors<br />
presented different equations for SCL currents in different devices with various electrode shapes. At present, there is a<br />
strong interest in the construction of thin film (TF) field-effect transistors (FET), where the surface (gap) electrode configuration<br />
is important, and solar cells are based on the sandwich type structures. The spectroscopic character of SCL<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
* Corresponding author. Tel.: +420 541 149 406; fax: +420 541 211 697.<br />
E-mail address: zmeskal@fch.vutbr.cz (O. Zmeskal).<br />
0960-0779/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.<br />
doi:10.1016/j.chaos.2006.04.006<br />
www.elsevier.com/locate/chaos
144 O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158<br />
current–voltage characteristics [12–21] allows determination of the electronic structure of local states and therefore to<br />
get some details concerning charge carrier transport. However, for the surface electrode configuration (FET) the theory<br />
of SCL currents is not fully developed. Stöckmann [22] first derived a simplified equation for surface SCL currents; later<br />
on this equation was modified with more precision for three different electrode configurations [24]. As the theory of SCL<br />
currents and their thermomodulated (TM) spectroscopic version were developed for sandwich sample configuration, it<br />
would be useful to find transformations of these relations for the cases of surface samples, i.e. FET structures, and generally<br />
also for other different shapes of electrodes. In this paper an attempt is made to find a general SCL current equation<br />
independent of the electrode configuration using E-Infinity theory [25–27].<br />
2. Ohmic current<br />
2.1. Classical approach<br />
Let us consider a planar sample of a homogeneous wide band-gap semiconductor or insulator devoid of any local<br />
states. According to the commonly accepted band model, the concentration of thermally generated electrons can be<br />
described within the Boltzmann statistics (cf. Fig. 1)<br />
nf0 ¼ N c exp Ec0 EF0<br />
: ð1Þ<br />
kT<br />
Here, Nc is the effective density of states in the band (density of extended states), Ec0 is the energy of the band edge in<br />
an isolated sample and EF0 is the energy of the ‘‘thermodynamic’’ Fermi level; both energies are measured with respect<br />
to an external reference level (vacuum level, in this case), k is the Boltzmann constant and T is temperature. Note that at<br />
equilibrium, nf0, Ec0 and EF0 are all constant within the sample. The electric current should therefore obey the wellknown<br />
Ohm’s law<br />
I X ¼ elnf0F LA ¼ jXA; ð2Þ<br />
where e = 1.602 · 10 19 C is the unit (elementary) charge, l is the charge carrier mobility, FL is the electric field<br />
strength, A is the sample area and jX = elnf0FL is the current density. Generally, the electric field strength strongly depends<br />
on electrode configuration, shape and sample dimensions. Thus, for the special cases of the sample configuration<br />
the term for electric field must be modified. To write one expression for the electric field of samples of various geometries,<br />
it seems to be of advantage to use a fractal analysis.<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
Fig. 1. Energy diagram of a molecular solid: A g is the electron affinity of the molecule, A s is the electron affinity of the solid and P e is<br />
the polarisation energy of the solid by electron; Ig is the ionisation energy of the molecule, Ic is the ionisation energy of the solid and Ph<br />
is the polarisation energy of the solid by hole; other symbols are explained in the text.
Eq. (2) presents a general current equation for three electrode configurations given in Fig. 2, where A is the area of<br />
the anode. Thus, for the case<br />
(A) plane-parallel geometry (cross-section area is A = Z · W, where Z and W are sizes of rectangle electrode), the<br />
current is I = ZWj,<br />
(B) cylindrical geometry (A =2pLW, where W is the length and L is the diameter of the cylinder – electrode distance),<br />
the current is I =2pLWj,<br />
(C) spherical geometry (A =4pL 2 ) the current is I =4pL 2 j.<br />
The electric field must be determined independently for each case.<br />
2.2. Fractal approach<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158 145<br />
Fig. 2. Current flow geometry: (A) plane-parallel, (B) cylindrical, (C) spherical and their fractal dimensions D for (a) Ohmic regime,<br />
(b) monoenergetic space charge regime, (c) real space charge regime with exponential distribution of localized states in energy,<br />
l is the distribution parameter. See text for details.<br />
The density distribution of a fractal physical quantity q(r) based on fractals [28] and Elnaschie’s E-Infinity Cantorian<br />
space time theory [26] can be written in the form [29,30]<br />
qðrÞ ¼Kr D E ¼ NðrÞ<br />
; ð3Þ<br />
rE where E is the Euclidean dimension, N(r) is the count of coverings of radius r by an elementary quantity, K is the fractal<br />
measure and D is the fractal dimension. Thus, for the electric charge density one can write<br />
qeðrÞ ¼eqðrÞ ¼eKr D E ; ð4Þ<br />
where K is the number of elementary electric charge in unit volume (r = 1). Using Eq. (4) we can calculate the overall<br />
charge Q(r) as<br />
Z<br />
Z<br />
QðrÞ ¼ qe dV ¼ eKr<br />
V<br />
r<br />
D E dðr E Þ¼eK ErD<br />
; ð5Þ<br />
D<br />
where dV * =d(r E ) is an elementary volume in E-dimensional space.<br />
The mean value of the density of the electric charge characterizes the change of the electric field over size r. Thus, we<br />
will be using the Gauss–Ostrogradsky law<br />
divF ¼ qeðrÞ ; ð6Þ<br />
e0er<br />
where e0er is the permittivity of the material (e0 = 8.854 · 10 12 Fm 1 is the electric permittivity of vacuum).<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
The vector F(r) defines the intensity of the electric field using the scalar qe(r). The radial component of the intensity<br />
Fr (when the field is point-symmetric) can be written as<br />
D<br />
D dF r<br />
E þ 1 dr ¼ qeðrÞ :<br />
e0er<br />
ð7Þ<br />
The electric field F and the potential V are related by the expression<br />
F ¼ gradV ; ð8Þ
146 O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158<br />
Table 1<br />
Parameters of fractal structures for various marginal conditions<br />
Q(r)= qe(r)= Fr(r)= Vr(r)= eK ErD<br />
E D<br />
D eKr<br />
eKr D<br />
eK[E(D E +2)r D<br />
K * = K/(e 0e r).<br />
so that both quantities can be determined by the density qe(r) distribution<br />
DV ¼ divF ¼ qeðrÞ ; ð9Þ<br />
e0er<br />
where D is the Laplace operator. This equation can be rewritten for the radial distribution of the density of physical field<br />
qe(r) as<br />
D<br />
D d<br />
E þ 1<br />
2 V r<br />
dr2 ¼ qeðrÞ :<br />
e0er<br />
ð10Þ<br />
From the density qe(r) we can determine the radial electric field strength Fr and the corresponding potential Vr in the<br />
forms<br />
F r ¼ eK D Eþ1 r<br />
e0er D ; V r ¼ eK D r<br />
e0er DðD<br />
Eþ2<br />
:<br />
E þ 2Þ<br />
ð11Þ<br />
The applied voltage is represented by the quantity e/e0er. From Eq. (11) follows the relation<br />
F r ¼ ðD E þ 2ÞV r=r: ð12Þ<br />
Table 1 summarizes the parameters of fractal structures for various marginal conditions, Table 2 gives fractal dimensions<br />
in E-dimensional space under various electrode configurations (Euclidean dimensions). The fractal dimensions for<br />
various electrode configurations are summarized in Fig. 2. Thus, for our planar sample and Ohmic current, one finds<br />
E = 1 (one-dimensional line of force multiplied by sample area) and D = E<br />
regard to the zero potential point can be written as<br />
1 = 0. The lateral electric field (12) with<br />
F L ¼ðV0 V LÞ=L ¼ U L=L; ð13Þ<br />
where L is the electrode distance and UL is the applied voltage.<br />
3. Space-charge-limited currents (SCLC)<br />
3.1. Classical approach<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
It should be realized that the situation described above is seldom met in real insulators and wide band-gap semiconductors<br />
where the concentration of thermally generated carriers (nf0) is small. In many cases, most carriers come from<br />
external sources, i.e., they are injected from suitable contacts or are, e.g., generated by photons of suitable energy. The<br />
efficient injection of charge carriers needs an ‘‘injecting’’ contact, i.e., a contact which allows increasing the bulk carrier<br />
concentration (n f(x)>n f0). This contact, the so-called ‘‘Ohmic’’ contact, does not limit the current density in the sample.<br />
This condition is normally formulated by setting<br />
eK<br />
E DrD eK E eK<br />
eK[D(D E + 2)]r<br />
D E<br />
rD Eþ1<br />
D eK<br />
rD Eþ1<br />
E eK<br />
eK * (D E +2)r<br />
D E+1<br />
rD Eþ2<br />
D Eþ2<br />
rD Eþ2<br />
EðD Eþ2Þ<br />
eK * D E+2<br />
r<br />
Table 2<br />
Notable fractal dimension in E-dimensional space<br />
D E =3<br />
0 D =0 Q = const. Point charge (quantity)<br />
E 2 D =1 Vr = const. Equipotential field (of fractal structure)<br />
E 1 D =2 Er = const. Homogenous intensity of field (of fractal structure)<br />
E D =3 qe = const. Homogenous density of charge (fractal structure)
nfð0Þ !1 and; consequently; F ð0Þ ¼0: ð14Þ<br />
In this case the contact can act as a reservoir of carriers. With Ohmic contact, the current–voltage relationship<br />
should be superlinear; its course depends on many factors as will be shown below. Suppose that the sample is contacted<br />
with a planar electrode. The metal is characterized by its work function / e – energy difference between the<br />
Fermi level of the electrons in the metal EFM and the vacuum level (Fig. 3). In the sample, the Fermi level EF0 is<br />
located within the energy band-gap (except for the degenerate condition of extrinsic semiconductors) and the work<br />
function / s is defined similarly as in the metal. When an electric contact is formed between the electrode and the<br />
insulator, electron transfer by diffusion takes place and, as a rule, a contact potential barrier is formed, D/ =<br />
/s /e. The thermal equilibrium is achieved when the energy of the Fermi level (measured again with respect to<br />
an external reference level) is constant over the whole system. This in turn means that the concentration of carriers<br />
in sample (and, consequently, the energy difference between the band edge and the Fermi level) should change. Of<br />
particular interest is the situation where either /e /s or /e /s. In the former case, the concentration of holes at<br />
the interface is much greater than in the sample bulk, hence a reservoir of excess holes is formed near the interface.<br />
In the latter case, shown schematically in Fig. 3b, a space charge of excess electrons is built up in the interface region<br />
(space-charge region) which may now serve as a reservoir of injected electrons (in subsequent discussion, we shall<br />
limit ourselves to the case of electron injection only). It should be noted that the energy of the band edge and hence<br />
the concentration of free carriers are now position-dependent (the positive direction pointing towards increasing carrier<br />
energy, i.e. towards the band-gap in the case of electron injection; the vacuum level is taken as zero reference<br />
level). Thus, for free charge carrier concentration, we can write (cf. Eq. (1))<br />
nfðxÞ ¼N c exp EcðxÞ EFðxÞ<br />
kT<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158 147<br />
: ð15Þ<br />
Let us consider now a system consisting of a wide band-gap sample and two electrodes, one of them being able to<br />
supply the sample with any number of carriers (injecting contact) and the other just serving as an exit electrode (extracting<br />
contact). If the system is suitably biased so as to facilitate the transport of injected carriers, the resulting current is<br />
controlled by the space charge built up in the vicinity of the injecting contact.<br />
Since the injected space charge density decreases with increasing distance from x = 0 and reaches nearly nf0 in the<br />
bulk of a sufficiently thick sample at x = z 0 (see Fig. 4), the internal field created by this accumulated space charge<br />
therefore decreases with increasing distance. On applying an external voltage UL = U1, the internal field at x = z1 is<br />
equal and opposite to the applied field, i.e., the effective field equals zero. The point where dw/dx = 0, where w stands<br />
for electrostatic potential, is called the ‘‘virtual injecting electrode‘‘. Under equilibrium conditions, the negative<br />
potential gradient at x < z1 tends to send back to the contact all the electrons that represent the excess over the<br />
SCL current permitted by the insulator (semiconductor). Thus, at x = z1 we can assume that the electrons are<br />
released without initial velocity. When the applied voltage is increased (U L = U 2,U 3,...), this field balances a higher<br />
internal field at x = z2,z3,... This also implies that the electron density of the virtual electrode at U2 is higher than<br />
that at U1, which can support a higher SCL current. The higher the applied field, the closer is the virtual cathode to<br />
the contact [31].<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
Fig. 3. The electrode – weakly conducting sample system before contact (a) and after contact (b), at thermal equilibrium. I c stands for<br />
the solid-state ionization energy, Ac for the electron affinity of a molecule in the solid-state sample; meanings of other symbols have<br />
been explained in the text. Local levels often present in samples have not been shown for the sake of simplicity.
148 O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158<br />
Fig. 4. Energy diagram for an Ohmic contact of the system electrode – weakly conducting sample – electrode. (0) without bias; (1)–(3)<br />
with bias. The curves show the influence of the applied voltage on the width of the accumulated region w, and the height of the barrier<br />
wmax. /e and /s are the work functions of the electrode and sample, respectively. Ac is the electron affinity of a molecule in the solid<br />
state sample. In this case /e < /s.<br />
The problem of SCL current may be mathematically treated by solving simultaneously the set of equations:<br />
(i) the current equation<br />
j ¼ el nfðxÞF ðxÞ<br />
kT dnfðxÞ<br />
e dx<br />
; ð16Þ<br />
(ii) the Poisson equation<br />
dF ðxÞ<br />
dx ¼<br />
e<br />
½nsðxÞ ns0Š: ð17Þ<br />
e0er<br />
In the above equations, ns(x)=nt(x)+nf(x) and ns0 = nt0 + nf0 are the total density of charge carriers in the sample<br />
(density of free and trapped carriers) after the voltage is applied and at the thermodynamic equilibrium, respectively.<br />
Let us now briefly discuss some particular solutions of the SCL current–voltage characteristics. We shall assume the<br />
injection level to be sufficiently high; thus, the concentration of thermally generated carriers will be negligibly small<br />
compared with that of injected ones. We shall also neglect the contribution of the diffusion current (second term in<br />
Eq. (16)); the other simplification is justified [32] in homogeneous samples for biasing voltages exceeding kT/e.<br />
In this case, ns0 =0,ns = nf, and from Eqs. (16) and (17) one obtains<br />
F ðxÞ ¼<br />
2j<br />
le0er<br />
1=2<br />
x 1=2 : ð18Þ<br />
Since F(x) = dw/dx and UL = w0 wL, where w0 and wL are virtual electrostatic potentials on cathode and anode,<br />
respectively, one finally arrives at<br />
j ¼ 9<br />
8 le0er<br />
U 2<br />
L<br />
: 3<br />
L<br />
This basic equation, describing the current–voltage characteristic in insulator, is often called Child’s law.<br />
ð19Þ<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
3.1.1. Profile of the physical parameters over the sample thickness<br />
Let us now calculate the spatial profiles of the electric field, concentration of free carriers and the position of the<br />
quasi-Fermi level in the sample. Inserting (19) into (18), one gets<br />
F ðxÞ ¼ 3<br />
2<br />
U L<br />
L<br />
x<br />
L<br />
1=2<br />
and, with (17), one obtains<br />
ð20Þ
nfðxÞ ¼ 3e0er<br />
4e<br />
U L<br />
L 2<br />
x<br />
L<br />
1=2<br />
: ð21Þ<br />
Finally, inserting (21) into (15), one arrives at<br />
EFðxÞ ¼EcðxÞþkT ln 3e0er<br />
4N ce<br />
U L<br />
L 2<br />
x<br />
L<br />
1=2<br />
: ð22Þ<br />
From the above equations which are particular cases of a general solution, the following conclusions can be drawn:<br />
(i) At the collector (x = L), the electric field and the concentration of carriers differ from their average values (U L/L<br />
and e0erUL/eL 2 ) by factors 3/2 and 3/4, respectively.<br />
(ii) As the injection level increases (i.e. on increasing the voltage), the position of the quasi-Fermi level shifts from its<br />
‘‘thermodynamic’’ value towards the band.<br />
From Eq. (20), for x = L follows FL = (3/2)(UL/L). This expression can also be written using fractal parameters. It is<br />
very useful to introduce the parameter c = d(lnUL)/d(lnj), a reciprocal value of the derivative of the current–voltage<br />
(j U) characteristic in the double-logarithmic coordinates (note that c values are related to the energetic structure<br />
of electronic states [12,18]). The parameter c can be written using the fractal dimension D. For trap-free case, simple<br />
monoenergetic level and exponential distribution of traps c are constants in the space-charge-limited region; thus we<br />
have the fractals with constant dimension.<br />
3.2. Fractal approach<br />
From the theory of semiconductors [18,19,22] and using Eqs. (12) and (13) it follows for the electric field that<br />
F L ¼ð2 EcÞU L=L: ð23Þ<br />
Thus, using the parameters of the fractal structures and comparing Eqs. (12) and (23) the parameter c can be written as<br />
c =(E D)/E. The space charge in the semiconductor is formed when fractal dimension D 2h0,Ei, i.e. coefficient c,<br />
changes in the interval c 2h0,1i. The parameter c can be generally defined by the Fisher’s scaling [23] which connects<br />
the anomalous dimension g = E D = cE and dimension of corresponding random walk of charge m = D.<br />
Using Eqs. (4) and (11) we can define the energy density<br />
w ¼ q eV r ¼ ½DðD E þ 2ÞŠ<br />
and the energy flow<br />
i ¼ q eF r ¼ q2 e r<br />
e0erF 2<br />
2<br />
e0erV r<br />
r<br />
; ð24Þ<br />
r ¼ ¼ðD<br />
De0er r<br />
E þ 2Þw: ð25Þ<br />
The energy density can be rewritten in the form<br />
Eð1<br />
w ¼ ensLU L ¼<br />
2<br />
cÞð2 EcÞe0erU L<br />
L 2 : ð26Þ<br />
Energy flow then immediately correlates with the SCL current density<br />
j ¼ li ¼ lqeF L ¼ le0erEð1 cÞð2 EcÞ 2 U 2<br />
L<br />
L<br />
3 ; ð27Þ<br />
where qe = enfL is the density of free charge. In this equation the term<br />
f E ¼ mð2 gÞ 2 ¼ Eð1 cÞð2 EcÞ 2<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
can be specified as multiplication term (cf. 9/8 in Eq. (19)). This multiplication factor can be written using Fisher’s scaling<br />
[23] by the anomalous dimension g = cE and geometry of random walk of charge m = D = E(1 c).<br />
3.2.1. Profile of free carrier concentration over the sample thickness<br />
From Eqs. (16) and (17) one can obtain a nonlinear integral equation for the charge carrier density<br />
nfðxÞ<br />
Z L<br />
0<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158 149<br />
nfðx 0 Þdx 0 ¼ je0er<br />
e2 : ð29Þ<br />
l<br />
ð28Þ
150 O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158<br />
Introducing dimensionless variables [24]<br />
mfðnÞ ¼<br />
e2lL je0er<br />
1=2<br />
nfðnÞ; ð30Þ<br />
where n = x/L one can rewrite Eq. (29) in the form<br />
mfðnÞ<br />
Z 1<br />
0<br />
mfðn 0 Þdn 0 ¼ 1: ð31Þ<br />
From Eq. (16) (without diffusion current) and (30), the current density can be written in the form [24]<br />
Z 1<br />
U<br />
j ¼ le0er<br />
2<br />
L<br />
L 3<br />
dn<br />
0<br />
0<br />
mfðn 0 2<br />
: ð32Þ<br />
Þ<br />
Comparing Eqs. (27) and (32) one can get the dependence of the dimensionless concentration on the slope of the<br />
current–voltage characteristic<br />
Z 1<br />
2<br />
Eð1 cÞð2 EcÞ 2 ¼<br />
0<br />
dn 0<br />
mfðn 0 Þ<br />
ð33Þ<br />
and from Eqs. (26), (27) and (30), the equivalent equation for dimensionless concentration of the electric charge<br />
Z 1<br />
dn<br />
mfðnÞ<br />
0<br />
0<br />
mfðn 0 1<br />
¼ ;<br />
Þ 2 Ec<br />
where mfðnÞ ¼<br />
ð34Þ<br />
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p<br />
Eð1 cÞ.<br />
For one-dimensional case, E = 1, for SCL condition, c = 1/2, the term E(1 c)(2 Ec) 2<br />
in Eq. (27) is equal 9/8, which is in agreement with Eq. (19). The term (2 Ec) in Eq. (23) is equal 3/2, which is in<br />
agreement with Eq. (20) and the term E(1 c)(2 Ec) in Eq. (26) is equal 3/4, which is in agreement with Eq. (21).<br />
Comparing the last terms one can get the nonlinear integral equation from which one can get the dependence mf(n) Z 1<br />
dn<br />
0<br />
0<br />
mfðn 0 Þ ¼<br />
1<br />
mfðnÞ 2 E þ m2 f ðnÞ<br />
ð35Þ<br />
½ Š;<br />
where E = 1 for one-dimensional transport of charge carriers (bulk regime).<br />
Solution of this equation can be written in the form [24]<br />
mfðnÞ ¼bn a ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p<br />
Eð1 cÞn<br />
Ec 1 ; ð36Þ<br />
where coefficient b ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p<br />
Eð1 cÞ characterizes the relative concentration of the charge at the extracting contact (n =1)<br />
and coefficient a = Ec 1 characterizes the profile of the free charge concentration across the sample. The solution is<br />
valid for Eqs. (32)–(34) and for Child’s law (E =1,c = 1/2), Eq. (31), where mf(n)=(2n) 1/2 , see Eq. (21).<br />
The fractal dimensions for SCLC regime for various electrode configurations are presented in Fig. 2b.<br />
4. Space-charge-limited currents: realistic case<br />
4.1. Monoenergetic traps<br />
In real samples one should expect the local states to influence the concentration of free carriers. Consequently, the<br />
set of equations (16) and (17) can be solved if the dependence between the densities of free and localized carriers is<br />
known. If one considers a steady-state situation, these two parameters are coupled via the position of the quasisteady-state<br />
Fermi level, the free carriers density being given by Eq. (15), and the trapped carrier density by the<br />
Fermi–Dirac statistics<br />
ntðxÞ ¼N tf ½EFðxÞ EtŠ; ð37Þ<br />
where<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
1<br />
f ½EFðxÞ EtŠ ¼ 1 þ exp EFðxÞ Et<br />
ð38Þ<br />
kT<br />
is the Fermi–Dirac function and Ht is the concentration of local states. It is convenient to introduce a free-to-total ratio<br />
parameter H (H 6 1)
H ¼ nfðxÞ<br />
nsðxÞ ¼<br />
nfðxÞ<br />
: ð39Þ<br />
nfðxÞþntðxÞ<br />
Combining Eqs. (37)–(39) one obtains<br />
1 ntðxÞ<br />
¼ 1 þ<br />
H nfðxÞ<br />
¼ 1 þ<br />
N c exp<br />
N t<br />
EcðxÞ EFðxÞ<br />
1 þ exp kT EFðxÞ<br />
h i: ð40Þ<br />
Et<br />
kT<br />
Let us note that the SCL current–voltage characteristic depends on the position of the quasi-Fermi level near the collecting<br />
electrode EF(L) with respect to energies of the distribution of local states [1]. For the sample with local states<br />
situated above the quasi-Fermi level, E F(L)>E t, all traps (for electrons) are shallow, and it follows from Eq. (40) that<br />
the parameter H is practically independent of the position of EF and, consequently, of the voltage applied to the sample<br />
(Et > Ec)<br />
H ¼ N c<br />
exp<br />
N t<br />
Et Ec<br />
kT<br />
< 1: ð41Þ<br />
Upon solving Eqs. (16) and (17), one finally obtains (the diffusion current being neglected)<br />
j ¼ 9<br />
2<br />
U L le0erH : 3 8 L<br />
ð42Þ<br />
Whilst the current is still proportional to the square of the applied voltage, it is, however, smaller than Child’s current<br />
(Eq. (19)). It should be stressed that H, although independent of voltage in the voltage range under consideration, does<br />
depend on the concentration of local states Nt [33]. The results of the fractal dimensions in the SCLC regime for various<br />
electrode configurations and monoenergetic states are presented in Fig. 2b.<br />
For H 1, the sample should behave as a trap-free insulator, and should conform to Child’s law (Eq. (19)). The<br />
voltage at which filling of all traps is achieved is usually called ‘‘trap-filled-limit voltage’’ (UTFL, here c ! 0), and<br />
can be shown [34,35] to amount to<br />
U TFL ¼ eN tL 2<br />
;<br />
2e0er<br />
where Nt is the overall concentration of local states.<br />
ð43Þ<br />
4.1.1. Profile of free carrier concentration over the sample thickness<br />
From Eqs. (16), (17) and (39) one can obtain (analogously to (29)) a nonlinear integral equation for the charge carrier<br />
density<br />
je0er<br />
e 2 l<br />
¼ nfðxÞ<br />
Z L<br />
0<br />
nsðx 0 Þdx 0 ¼ nfðxÞ<br />
Z L<br />
which can be rewritten (analogously to Eq. (31)) in the form<br />
mfðnÞ<br />
Z 1<br />
0<br />
0<br />
nfðx 0 Þ<br />
H dx0 ; ð44Þ<br />
mfðn 0 Þdn 0 ¼ H: ð45Þ<br />
The dependence of dimensionless charge concentration on the slope of the current–voltage characteristics (Eq. (33)) can<br />
be rewritten as<br />
Z 1<br />
HEð1 cÞð2 EcÞ 2 ¼<br />
0<br />
dn 0<br />
mfðn 0 Þ<br />
2<br />
: ð46Þ<br />
From Eqs. (26) and (27) one can get similar equation to Eq. (34)<br />
Z 1<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
dn<br />
mfðnÞ<br />
0<br />
0<br />
mfðn 0 1<br />
¼ ; ð47Þ<br />
Þ 2 Ec<br />
where mfðnÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
p<br />
HEð1 cÞ for n = 1. Comparing the last terms one can get the nonlinear integral equation from which<br />
the dependence mf(n) can be obtained<br />
Z 1<br />
0<br />
dn 0<br />
mfðn 0 Þ ¼<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158 151<br />
H<br />
mfðnÞ Hð2 EÞþm2 f ðnÞ<br />
ð48Þ<br />
½ Š;
152 O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158<br />
where E = 1 for one-dimensional transport of charge carriers (bulk regime). The solution of this equation can be written<br />
in the form [24]<br />
mfðnÞ ¼bn a ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
p<br />
HEð1 cÞn<br />
Ec 1 ; ð49Þ<br />
where coefficient b ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
p<br />
HEð1 cÞ characterizes the relative concentration of space charge in monoenergetic trap at the<br />
extraction contact (n = 1) and coefficient a = Ec 1 characterizes the profile of free charge across the sample. The solution<br />
is valid for Eqs. (46)–(48) and also for monoenergetic trap (E =1,c = 1/2) (45), where mf(n)=(2Hn) 1/2 , see Eq.<br />
(21).<br />
4.2. Exponential distribution of local states<br />
Further we will pay more attention to the insulator with exponential distributions h(E) of local states in energy.<br />
Due to a relatively straightforward derivation, equations describing this distribution have often been employed to fit<br />
experimental data, although no physical justification can be offered for this shape of the DOS function. Thus we<br />
assume<br />
hðEÞ ¼H t exp<br />
E Ec<br />
kT c<br />
; ð50Þ<br />
where Tc is the temperature of the exponential distribution Tc > T.<br />
In real samples one should expect the local states to influence the concentrations of free carriers. Consequently, similar<br />
to the previous case, the set of equations (16) and (17) can be solved if the dependence between the densities of free<br />
and localized carriers is known. If one considers a steady-state situation, these two parameters are coupled via the position<br />
of the quasi-steady-state Fermi level, the density of free carriers being given by Eq. (15), and the density of trapped<br />
carriers of the Fermi–Dirac statistics<br />
where<br />
ntðxÞ ¼<br />
Z EU<br />
EL<br />
hðE; xÞf ½EFðxÞ EŠdE; ð51Þ<br />
1<br />
f ½EFðxÞ EŠ ¼ 1 þ exp EFðxÞ E<br />
ð52Þ<br />
kT<br />
is the Fermi–Dirac function and h(E,x) is the density-of-states (DOS) function, i.e. a function describing the energetic<br />
and spatial distribution of local states between the lower and upper limits (EL and EU, respectively). It is convenient to<br />
introduce a free-to-total ratio parameter H (H 6 1) (39).<br />
Combining Eqs. (51), (52) and (39) one obtains<br />
Z EU<br />
1 ntðxÞ<br />
hðEÞdE<br />
¼ 1 þ ¼ 1 þ<br />
H nfðxÞ EcðxÞ EFðxÞ<br />
EL N b exp 1 þ exp kT EFðxÞ<br />
h i: ð53Þ<br />
E<br />
kT<br />
Let us note that the SCL current–voltage characteristic depends on the position of the quasi-Fermi level near the collecting<br />
electrode EF(L) with respect to energies of distribution of local states [1]. For the sample with local states situated<br />
above the quasi-Fermi level, all traps are shallow, and it follows from Eq. (40) that the parameter H is practically<br />
independent of the position of EF and, consequently, of the voltage applied to the sample (E > Ec)<br />
N b<br />
H ¼ R EU<br />
E Ec<br />
hðEÞ exp EL kT<br />
< 1:<br />
dE<br />
ð54Þ<br />
Equations describing the field intensity and the density of localized carriers at the collecting electrode can be derived<br />
as [33]<br />
F ðLÞ ¼<br />
2l þ 1<br />
l þ 1<br />
U L<br />
;<br />
L<br />
ð55Þ<br />
ntðLÞ ¼<br />
2l þ 1<br />
l þ 1<br />
l<br />
l þ 1<br />
eU L<br />
; 2<br />
eL<br />
ð56Þ<br />
where l = Tc/T > 1 (note that for the exponential distribution c = 1/(l + 1) is independent of voltage). For the exponential<br />
distribution of local states, the current–voltage characteristics for sandwich sample can be written in the form [1]<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong>
Table 3<br />
SCL current–voltage characteristics insulator with shallow traps and traps distributed in energy<br />
Current flow<br />
Shallow traps:<br />
Exponential distribution of traps:<br />
geometry<br />
H = Nc/Ntexp[(Et Ec)/kT]<br />
h(E)=Htexp[(E Ec)/kTc], l = Tc/T >1<br />
D = E/2 D = El/(l +1)<br />
Eð4 EÞ2 U<br />
j ¼ le0erH 8<br />
2<br />
L<br />
L3 h il e0erEl<br />
j ¼ elN c eH tðlþ1Þ<br />
ðlþ1Þ ðlþ1Þ<br />
E U L<br />
2 lþ1 Lð2lþ1Þ Plane-parallel E =1 j ¼ 9<br />
2 U L<br />
8 le0erH<br />
L3 h il e0erl<br />
j ¼ elN c eH tðlþ1Þ<br />
ðlþ1Þ ðlþ1Þ<br />
2lþ1 U L<br />
lþ1 Lð2lþ1Þ Cylindrical E =2<br />
2 U L<br />
j ¼ le0erH<br />
L3 h il e0er2l<br />
j ¼ elN c eH tðlþ1Þ<br />
ðlþ1Þ ðlþ1Þ<br />
2l U L<br />
lþ1 Lð2lþ1Þ Spherical E =3 j ¼ 3<br />
2 U L<br />
8 le0erH<br />
h il e0er3l<br />
j ¼ elN c<br />
2l 1<br />
lþ1<br />
j ¼ leN c<br />
e0er<br />
eH t<br />
l<br />
l<br />
l þ 1<br />
l<br />
2l þ 1<br />
l þ 1<br />
ðlþ1Þ ðlþ1Þ<br />
U L<br />
: ð2lþ1Þ<br />
L<br />
ð57Þ<br />
Utilizing the parameters of fractal structures and c = 1/(l + 1) one can rewrite Eq. (27) in the form:<br />
j ¼ le0erH El<br />
l þ 1<br />
2l þ 2<br />
l þ 1<br />
E<br />
2 2<br />
U L<br />
: 3<br />
L<br />
ð58Þ<br />
Using for the free carrier concentration (Eqs. (15) and (50)), nfL = Nb(nsL/Ht) l and for parameter H ¼ðNb=H l 1Þ<br />
tÞnðl sL<br />
(see (Eq. (54)), where l = Tc/T > 1 we finally get the expression<br />
l<br />
l ðlþ1Þ ðlþ1Þ<br />
e0er El 2l þ 2 E U L<br />
j ¼ leN c<br />
: ð59Þ<br />
eH t l þ 1 l þ 1<br />
ð2lþ1Þ<br />
L<br />
The integral appearing in the denominator of Eq. (53) can be solved only if the DOS function is specified a priori,<br />
thus no general equation exists describing the SCL current–voltage characteristics in this case. One can, however, obtain<br />
approximate solutions for typical distributions applying the zero-temperature approach, i.e., replacing the Fermi–Dirac<br />
function (Eq. (52)) by the Heaviside (step) function<br />
f ðEF EÞ ¼vðEF EÞ: ð60Þ<br />
In other words, one assumes the concentration of carriers in shallow states to be negligibly small compared with<br />
those in deep states. Eqs. (46)–(49) are also valid in this case too, but Eq. (45) must be rewritten in the form<br />
Z 1<br />
mfðnÞ mfðn<br />
0<br />
0 Þdn 0 Hð1 cÞ<br />
¼ ¼ Hl: ð61Þ<br />
c<br />
Eq. (59) represents a general equation of SCL currents for insulator with exponential distribution of local states in energy<br />
independently of electrode geometry. The exact solutions for plane-parallel, cylindrical and spherical electrode<br />
geometries (not simple ones) were derived by Lampert and Mark [1]. Eq. (59) allows to write these solutions in a simple<br />
way, to change only the value of the Euclidean dimension E. The results are given for the insulator with shallow traps<br />
and traps distributed exponentially in energy in Table 3.<br />
5. Surface (gap) electrode configuration<br />
At present there is a strong interest in the construction of thin film field-effect transistors operating in the both<br />
Ohmic and SCL regime. In this case two electrodes (source-drain) with spacing L (channel length) are applied onto thin<br />
film of the thickness a (and vice versa). Because of a specific profile of the electric field in these samples, the expressions<br />
for the SCL currents will differ from those for the bulk sample.<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
5.1. Monoenergetic and exponentially distributed traps<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158 153<br />
L 3<br />
This problem was tackled only for a trap-free case or for the currents controlled by shallow traps. According to Ref.<br />
[36], Eq. (42) should in this case be modified to<br />
I G ¼ 2<br />
2<br />
U L<br />
le0erH W ; ð62Þ<br />
2 p L<br />
eH tðlþ1Þ<br />
ðlþ1Þ U ðlþ1Þ<br />
L<br />
L ð2lþ1Þ
154 O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158<br />
Fig. 5. Sample geometry discussed in this paper: (a) thick and thin sandwich (bulk) structure, (b) thick (u = p/2) and thin<br />
(u = arctan(a/L)) cylindrical segment (arc) structure, (c) gap (film) structure, p/2 > u * > arctan(2a/L).<br />
where W is the channel width (see Fig. 5c) and prefactor (2/p here) depends on the contact shape. The problem was<br />
solved for the thickness limit (a ! 0). Note that in TF FET experiments the active thin film is usually deposited on<br />
the electrode system or thin metal electrodes are deposited on the active semiconductor thin film. Here, the film thickness<br />
a differs from the electrode distance channel length L. On the basis of this analysis Geurst [37] was able to construct<br />
an analytical model of an FET, which goes beyond the usual gradual channel approximation. Later on, Grinberg et al.<br />
[24] solved the SCL currents in thin films for the electrode configuration given in Fig. 5b. They developed the expression<br />
similar to Eq. (62) with factor 0.57 instead of 2/p = 0.63. Note, that in this case the total current is independent of film<br />
thickness a because the decreasing film thickness is compensated by increasing injection.<br />
The results for sandwich bulk (plane-parallel) sample structures were discussed in the last paragraphs for the case of<br />
monoenergetic trap (c = 1/2) or exponential distribution of traps (c =(E D)/E = 1/(l + 1)). The final current terms<br />
were calculated for one-dimensional case (E = 1). The fractal dimensions were ranging in interval D 2h0,1i The ohmic<br />
case was characterized by the fractal dimension D = 1 (paragraph 2), Child’s law and the case of the monoenergetic<br />
shallow trap by D = 1/2 (paragraphs 3 and 4), exponential trap distribution by D = l/(l + 1), and the trap-field-limit<br />
by D ! 1 (paragraph 4). On the basis of the expressions mentioned above one can use for the bulk current<br />
I M<br />
B<br />
¼ Aj ¼ ZWj, where j is the current density (Eq. (42)).<br />
Similarly, for the current of the insulator with traps exponentially distributed in energy one can write<br />
I E<br />
B ¼ jA ¼ le0erHð1 cÞð2 cÞ 2 U 2<br />
L<br />
L 3 ZW ð63Þ<br />
(cf. Eq. (27) for the current density; here E = 1).<br />
The similar situation exists for cylindrical segment sample structures. The final terms for the currents can be<br />
expressed for two-dimensional (cylindrical) case (E =2)as<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
(i) for insulator with traps monoenergetic distributed in energy<br />
I M<br />
2<br />
U L<br />
C ¼ uLWj ¼ le0erH uLW ; ð64Þ<br />
3<br />
L<br />
(ii) for insulator with exponentially traps<br />
I E<br />
C ¼ uLWj ¼ 8le0erHð1 cÞ 3 U 2<br />
L<br />
L 3 uLW ; ð65Þ
where u (cf. Fig. 5b) is the angle at which the current is flowing from surface contact to the sample (for the full cylindrical<br />
case u =2p).<br />
Combining Eqs. (42) and (64) we can get for monoenergetic traps<br />
I M<br />
B<br />
I M ¼<br />
C<br />
9 a<br />
; ð66Þ<br />
8 ðuLÞ<br />
combining Eqs. (63) and (65) we can get for the insulator with traps exponentially distributed in energy<br />
I E<br />
B<br />
I E<br />
ð2 cÞ2<br />
¼<br />
C 8ð1 cÞ 2<br />
a<br />
: ð67Þ<br />
ðuLÞ<br />
For very thin samples, i.e. for small angles, u = arctan(a/L) ! 0, a/(uL) 6 1 and the expression for the current is similar<br />
to that presented in Ref. [24].<br />
The case of the gap sample structure represents the combination of two cylindrical cases (see Fig. 5c). The final term<br />
for current will depend on the type of surface contacts (e.g. edge contact, coplanar strip contact, perpendicular plane<br />
contact, see Ref. [24]). In this case the angle u * P u = arctan[a/(L/2)] and Eqs. (64) and (65) can be rewritten as<br />
I M<br />
G<br />
2<br />
U L 2a<br />
¼ u LWj le0erH arctan 2<br />
L L<br />
I E<br />
G ¼ u LWj<br />
8le0erHð1 cÞ 3 U 2<br />
L<br />
L<br />
2 arctan 2a<br />
L<br />
In these equations the terms<br />
arctanð2a=LÞ ¼f M<br />
G and 8ð1 cÞ 3 arctanð2a=LÞ ¼f E<br />
G<br />
W ; ð68Þ<br />
W : ð69Þ<br />
can be taken as multiplication factor (see [24], cf. 2/p in Eq. (62)).<br />
Using Eq. (69) one can write the terms for the current of the gap sample of the insulator with exponentially distributed<br />
traps in energy (here E =1)<br />
l<br />
ð2lþ1Þ U ðlþ1Þ<br />
I E<br />
G ¼ leN c<br />
e0er<br />
eH t<br />
2l<br />
l þ 1<br />
L<br />
arctan ð2lþ1Þ<br />
L<br />
2a<br />
L<br />
W ; ð71Þ<br />
where c =(E D)/E = 1/(l + 1) and L is the channel length (see Fig. 5c).<br />
5.1.1. Profile of free carrier concentration over the sample thickness<br />
From Eqs. (16), (17) and (39) one obtains (analogously to Eq. (29)) the nonlinear integral equation for the carrier<br />
density<br />
je0er<br />
e 2 l<br />
¼ nfðxÞ<br />
Z L<br />
0<br />
nfðx 0 Þ<br />
uH dx0 ; ð72Þ<br />
which can be rewritten (analogously to Eq. (31)) in the form<br />
mfðnÞ<br />
Z 1<br />
0<br />
mfðn 0 Þdn 0 ¼ uH: ð73Þ<br />
From Eqs. (26) and (27), one can get equivalent equation to Eq. (34)<br />
Z 1<br />
dn<br />
mfðnÞ<br />
0<br />
0<br />
mfðn 0 1<br />
¼<br />
Þ 2 Ec ; where mfðnÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
p<br />
uHEð1 cÞ:<br />
ð74Þ<br />
Comparing the last terms one can get the nonlinear integral equation from the dependence mf(n) can be obtained<br />
Z 1<br />
O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158 155<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
0<br />
dn 0<br />
mfðn 0 Þ ¼<br />
mfðnÞ½ð2<br />
uH<br />
EÞuH þ m2 ;<br />
f ðnÞŠ<br />
ð75Þ<br />
where E = 2 for two-dimensional transport of carrier (cylindrical segment regime, see Fig. 5b). Solution of this equation<br />
can be obtained in the form [24]<br />
mfðnÞ ¼bn a ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
p<br />
uHEð1 cÞn<br />
Ec 1 ; ð76Þ<br />
ð70Þ
156 O. Zmeskal et al. / Chaos, Solitons and Fractals 34 (2007) 143–158<br />
where coefficient b ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
p<br />
uHEð1 cÞ characterizes the relative concentration of space charge in trap at the end of the<br />
sample (n = 1) and coefficient a = Ec 1 characterizes the free charge profile across the sample.<br />
For the gap regime situation, the relative concentration (Eq. (36)) must be rewritten as<br />
mfðnÞ ¼bn a ð1 nÞ b ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
p<br />
uHEð1 cÞ<br />
n<br />
1 n<br />
Ec 1<br />
; ð77Þ<br />
where the first exponent (a) describes the efficiency of the injecting contact and the second exponent (b) the efficiency of<br />
the extracting contact properties. For the special case, when a = b = Ec 1 the profile of free charge relative concentration<br />
across the sample can be obtained. The general coefficient fE of SCLC characteristics can be in this case written<br />
as<br />
fE ¼<br />
bCð2 a bÞ<br />
Cð1 aÞCð1 bÞ<br />
2<br />
; ð78Þ<br />
where C(a,b) is the gamma function.<br />
For b ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />
p<br />
uHEð1 cÞ,<br />
a = Ec<br />
uHE(1 c)(2 Ec)<br />
1 and b = 0 we can get by simple calculation the coefficient fE =<br />
2 (see Eq. (28)).<br />
For the case with extracting contact (see Eq. (77) with the parameters a = Ec<br />
similarly<br />
1 and b =1 Ec), we can get<br />
fE ¼ b sinðpaÞ<br />
pa<br />
2<br />
¼ uHEð1<br />
sin pðEc<br />
cÞ<br />
pðEc<br />
1Þ<br />
1Þ<br />
2<br />
; ð79Þ<br />
where the second term is the error function. For the monoenergetic trap in the gap sample (E =1,c = 1/2 and u 6 p)<br />
the coefficient fE=1 6 2/p. The general equation for SCL current can be then written as<br />
I M<br />
2<br />
2 U L<br />
G 6 le0erH W : 2 p L<br />
Eq. (62) given by Guerst [36] is the limiting case of this equation.<br />
ð80Þ<br />
The dependences of the dimensioneless free carrier concentration on the relative coordinate for all the discussed situations<br />
are shown in Fig. 6. For the bulk regime the dependences for Child’s law, monoenergetic traps and exponential<br />
trap distributions were calculated. There are differences in the concentration of free charge carrier near the cathode. For<br />
the gap electrode configuration the anode was taken as non-Ohmic extracting contact.<br />
6. Conclusion<br />
log [n f (ξ)]<br />
100.0<br />
10.0<br />
1.0<br />
0.1<br />
0.0<br />
0.0 0.2 0.4 0.6 0.8 1.0<br />
Cathode ξ Anode<br />
Fig. 6. Dependences of dimensionless free carrier concentration on the relative coordinate for (a) plane-parallel bulk regimes: Child’s<br />
law ( ), monoenergetic trap (m) and traps exponentially distributed in energy (j), (b) cylindrical bulk regime (traps exponentially<br />
distributed in energy) (d) (full line) and (c) gap thin film regime (traps exponentially distributed in energy (d) (dashed line).<br />
<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong><br />
Using the methodology of E-Infinity the general equations for space-charge-limited currents in trap-free insulators<br />
or wide band-gap semiconductors and in materials with monoenergetic traps and trap distributed exponentially in
energy was derived. The multiplication term of current–voltage characteristic was determined using Fisher’s scaling [23]<br />
and Elnaschie’s E-Infinity Cantorian space time theory as<br />
f E ¼ mð2 gÞ 2 ¼ Eð1 cÞð2 cEÞ 2 ; ð81Þ<br />
where m = D = E(1 c) is the dimension of random walk and g = cE is the anomalous dimension.<br />
This allows us writing one common equation independently of electrode configuration changing only the fractal<br />
dimension D and Euclidean dimension E. By the combination of two cylindrical cases it was possible to derive the<br />
expression for space-charge-limited currents in insulator with monoenergetic traps for the case of samples with surface<br />
(gap) electrode configuration in the form<br />
I M<br />
2<br />
U L 2a<br />
G ¼ u LWj aWj le0erH arctan W ; ð82Þ<br />
2<br />
L L<br />
where l is the charge carrier mobility, e0er is the permittivity of the material, H is the free-to-total ratio parameter, UL is<br />
the applied voltage, L is the electrode distance, the W is width of rectangle electrode and a is the sample thickness. For<br />
trap free case the H =1.<br />
For the insulator with traps exponentially distributed in energy the current is<br />
I E<br />
8le0erð1 cÞ<br />
G ¼ u LWj<br />
3 U 2<br />
L<br />
L 2 arctan 2a<br />
W ; ð83Þ<br />
L<br />
where c is the reciprocal value of the current–voltage characteristics slope in the double logarithmic coordinates,<br />
c = d(lnUL)/d(ln j). This experimental parameter can be expressed as c =(E D)/E, where D is the fractal dimension<br />
of the space charge distribution in the sample and E is the topological (Euclidean) dimension of the contact<br />
configuration.<br />
Acknowledgements<br />
The financial supports by grant FT-TA/036 from the Ministry of Industry and Trade of the Czech Republic and by<br />
the grant A100100622 from the Grant Agency of the Academy of Sciences of the Czech Republic are gratefully<br />
appreciated.<br />
References<br />
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[3] Kao KC, Hwang W. Electrical transport in solids. Pergamon Press; 1981.<br />
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[5] Many A, Goldstein Y, Grover NB. Semiconductors surfaces. Amsterdam: North Holland; 1965.<br />
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[7] Abeles F, editor. Optical properties of solids. Amsterdam: North Holland; 1972.<br />
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[9] Bredsky MH, editor. Amorphous semiconductors. Berlin: Springer; 1979.<br />
[10] Joannopoulos JD, Lecovsky G, editors. The physics of hydrogenated amorphous silicon, vol. I and II. Berlin: Springer; 1979.<br />
[11] Mott NF, Davis EA. Electron processes in non-crystalline materials. Oxford: Clarendon Press; 1979.<br />
[12] Nesˇpu˚rek S, Sworakowski J. Use of space-charge-limited current measurements to determine the properties of energetic<br />
distributions of bulk traps. J Appl Phys 1980;51:2098–102.<br />
[13] Nesˇpu˚rek S, Silinsh EA. Space-charge-limited current theory for molecular crystals with Gaussian distribution of local trapping<br />
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[14] Nesˇpu˚rek S, Smejtek P. Space-charge limited currents in insulators with the Gaussian distribution of traps. Czech J Phys<br />
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[15] Sworakowski J, Nesˇpu˚rek S. Determination of the parameters of traps for current carriers from space-charge-limited currents. J<br />
Electrostat 1979;8:97–101.<br />
[16] Nesˇpu˚rek S, Sworakowski J. A differential method of analysis of steady-state space-charge-limited current–voltage characteristics.<br />
Phys Status Solidi (A) 1977;41:619–27.<br />
[17] Nesˇpu˚rek S, Sworakowski J. A differential method of analysis of steady-state-charge-limited currents: an extension. Phys Status<br />
Solidi (A) 1978;49:K149–52.<br />
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[18] Zmeskal O, Schauer F, Nespurek S. The bulk trap spectros<strong>copy</strong> of solids by temperature modulated space-charge-limited currents<br />
(TMSCLC) in the steady state. J Phys C: Solid State Phys 1985;18:1873–84.<br />
[19] Schauer F, Nespurek S, Zmeskal O. The bulk trap spectros<strong>copy</strong> of solids by temperature modulated space-charge-limited currents<br />
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[20] Schauer F. Space-charge-limited currents for organic solar cells optimisation. Solar Energy Mater Solar Cells 2005;87:235–50.<br />
[21] Nesˇpu˚rek S. Space-charge-limited currents in N,N 0 /diphenyl-p-phenylemediamine and Gaussian distribution of traps. Czech J<br />
Phys 1974;B24:660–70.<br />
[22] Stöckmann F. An exact evaluation of steady-state space-charge limited currents for arbitrary trap distribution. Phys Status Solidi<br />
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[23] El Naschie MS. Fisher’s scaling and dualities at high energy E-Infinity spaces. Chaos, Solitons & Fractals 2001;12:1557–61.<br />
[24] Grinberg AA, Luryi S, Pinto MR, Schryer NL. Space-charge-limited-current in a film. IEEE Trans Electron Dev 1989;36:1162–70.<br />
[25] El Naschie MS. Non-Euclidean spacetime structure and two-slit experiment. Chaos, Solitons & Fractals 2005;26:1–6.<br />
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[27] El Naschie MS. The Feynman path integral and E-infinity from the two-slit Gedanken experiment. Int J Nonlinear Sci Numer<br />
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[28] Mandelbrot BB. Fractal geometry of nature. New York: W.H. Freeman and Co.; 1983.<br />
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<strong>Author's</strong> <strong>personal</strong> <strong>copy</strong>
POLYMERS FOR ADVANCED TECHNOLOGIES<br />
Polym. Adv. Technol. 2006; 17: 673–678<br />
Published online 3 October 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pat.764<br />
Influence of dipolar species on charge transport<br />
in poly[2-methoxy-5-(2 0 -ethylhexyloxy)-p-phenylene<br />
vinylene] y<br />
Petr Toman 1 *, Stanislav Nesˇpu˚ rek 1,2 , Martin Weiter 2 , Martin Vala 2 ,<br />
Juliusz Sworakowski 3 , Wojciech Bartkowiak 3 and Miroslav Mensˇík 1<br />
1 Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky´ Sq. 2, 162 06 Prague 6, Czech Republic<br />
2 Faculty of Chemistry, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic<br />
3 Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland<br />
Received 30 November 2005; Revised 13 April 2006; Accepted 13 April 2006<br />
A theoretical study and experimental evidence for a decrease of the charge carrier mobility in poly[2methoxy-5-(2(-ethylhexyloxy)-p-phenylene<br />
vinylene] (MEH-PPV) under the influence of dipolar<br />
species are presented in this paper. The presence of polar species in the vicinity of an MEH-PPV<br />
chain modifies the on-chain site energies and consequently, due to random mutual positions and<br />
orientations, increases the width of the energy distribution of the chain transport states. The<br />
influence of energetic disorder of these states on the charge carrier mobility was modeled by means<br />
of the Monte Carlo method, based on a numerical solution of the time-dependent Schrödinger<br />
equation in the tight-binding approximation. It was shown that the increasing disorder destroys the<br />
resonance between charge carrier energies on adjacent sites and, therefore, limits the diffusive charge<br />
carrier motion. Copyright # 2006 John Wiley & Sons, Ltd.<br />
KEYWORDS: charge transport; conducting polymers; photochromism; Monte Carlo simulation; quantum chemistry<br />
INTRODUCTION<br />
In the past decades, a great deal of scientific interest has been<br />
devoted to the research into different types of polymer-based<br />
photoswitching systems because of their potential use as<br />
elements of future logic and memory devices. 1–5 Poly(pphenylene<br />
vinylene) (PPV) derivatives possess excellent<br />
semiconducting and luminescent properties accompanied by<br />
relatively easy processing. 6,7 Charge carrier transport in<br />
these materials proceeds predominantly along the conjugated<br />
polymer backbones with the participation of the<br />
interchain hopping. The charge transport along the PPV<br />
chain was theoretically described by Grozema and coworkers<br />
8,9 using the tight-binding approximation model.<br />
They found that the on-chain charge carrier mobility was<br />
governed by the structural (torsional) disorder of the main<br />
chain, giving rise to a broadening of the distribution of<br />
transfer integrals among polymer repeat units. It is known<br />
that the main chain transport in conjugated macromolecules<br />
may be influenced by attaching polar side groups or by<br />
admixing polar additives. 10–12 The introduction of polar<br />
species results in a broadening of the distribution of charge<br />
carrier transport states 13,14 and in creating charge carrier<br />
*Correspondence to: P. Toman, Institute of Macromolecular Chemistry,<br />
Academy of Sciences of the Czech Republic, Máchova St. 7,<br />
120 00 Praha 2, Czech Republic.<br />
E-mail: toman@imc.cas.cz<br />
y 8th International Symposium on Polymers for Advanced Technologies<br />
2005 (PAT 2005), Budapest, 13–16 September, 2005, Part 1.<br />
traps. 15 Both these effects result in a decrease of the charge<br />
carrier mobility.<br />
In this paper, the results of theoretical and experimental<br />
studies on the modulation of charge carrier (hole) transport<br />
in poly[2-methoxy-5-(2 0 -ethylhexyloxy)-p-phenylene vinylene]<br />
(MEH-PPV) doped with the photochromic additive<br />
6-nitro-1 0 ,3 0 ,3 0 -trimethylspiro[2H-1-benzopyran-2,2 0 -indoline]<br />
are presented (Scheme 1). Upon irradiation with light of<br />
an appropriate wavelength, the photochromic additive<br />
undergoes a ring-opening reaction from the closed form of<br />
spiropyran (SP) to the open merocyanine (MR) form. 16,17 The<br />
reversible SP$MR reaction is accompanied by a charge<br />
redistribution resulting in a significant increase of the dipole<br />
moment of the molecule. The interaction of the polar additive<br />
and a hole moving along the polymer chain (charge–dipole<br />
interaction) results in the broadening of the distribution of<br />
local hole energies along the chain; in this way the on-chain<br />
mobility of holes decreases.<br />
EXPERIMENTAL<br />
Thin films of MEH-PPV with SP, typically 150 nm thick, were<br />
prepared by spin coating of chloroform solution onto indium<br />
tin oxide (ITO) electrodes (for electrical measurements) or<br />
onto quartz glass substrates (for optical measurements).<br />
Copyright # 2006 John Wiley & Sons, Ltd.
674 P. Toman et al.<br />
Scheme 1. Photochromic reaction in the SP$MR system.<br />
Vacuum evaporated top Al electrode completed the<br />
sandwich structure for the electrical measurements which<br />
were performed using a Hewlett Packard 4192A impedance<br />
analyzer. The time dependences of conductance and<br />
capacitance were recorded at a constant frequency (1 kHz).<br />
The reaction converting SP into its colored unstable form<br />
(MR) was activated by light (360 20) nm generated by the<br />
mercury discharge lamp HBO-200. The optical decay was<br />
measured in a vacuum in the temperature range 298–350 K.<br />
The maximum of the long wavelength absorption band of<br />
MEH-PPV is situated at 445 nm; an addition of SP increases<br />
the absorption of the sample in the UV region. After the<br />
irradiation of an MEH-PPV sample containing admixed SP<br />
(referred to as MEH-PPV/SP) with UV light, an intense<br />
absorption band appears at 590 nm, associated with the<br />
appearance of the colored MR species. Thermal bleaching in<br />
the dark or during the irradiation of the sample with a He-Ne<br />
laser gradually restores the original spectrum. The kinetics of<br />
coloring and bleaching processes was investigated by<br />
following the temporal evolution of the 590 nm peak. It<br />
was found that the rate of the thermal bleaching process can<br />
be described by the ‘‘stretched exponential’’ function. 18 Time<br />
constants of the decay processes followed the Arrhenius<br />
relation with the activation energy, Ea¼107 kJ/mol, and<br />
frequency factor, 2.9 10 13 sec 1 .<br />
The results of electrical measurements are presented in<br />
Fig. 1. Because MEH-PPV is photoconductive, the conductance<br />
strongly increases under UV illumination due to the<br />
increase of the free charge carrier concentration. However,<br />
the UV light simultaneously triggers the SP> MR reaction,<br />
with high dipole moment of MR species (the dipole moment<br />
changes from 5 D to 12 D, see Ref. 12). The dipolar species<br />
create new charge carrier traps and made the distribution of<br />
transport hopping states broaden. These effects lead to a<br />
decrease of the charge carrier mobility and free charge carrier<br />
Figure 1. Temporal evolution of the conductance and capacitance<br />
of an MEH-PPV sample containing the photochromic<br />
SP. The vertical arrows at times t1 and t2 indicate ‘‘switching<br />
on’’ and ‘‘switching off’’ of the UV illumination, respectively.<br />
concentration. The slow decay of the conductance with time<br />
can be ascribed to the formation of local states due to the SP><br />
MR transformation. After the ‘‘switch off’’ of light (see time t2<br />
in Fig. 1) the conductance strongly decreases due to the<br />
termination of the number of free charge carriers.<br />
The formation of highly polar species during the UV<br />
illumination was simultaneously checked by the capacitance<br />
measurements (see Fig. 1). After the fast increase from 878 to<br />
893 nF at the beginning of the UV illumination, which can be<br />
ascribed to the photodielectric effect due to light generation<br />
of free and trapped charge carriers, a slower increase was<br />
observed. This part of the capacitance kinetics can be<br />
explained by the formation of polar MR species 11 and the<br />
increase of the orientational polarizability. After the ‘‘switch<br />
off’’ of light, a fast decay of the capacitance was observed<br />
followed again by a slow component. The rate of the slow<br />
process was found to approximately follow the rate of the<br />
photochromic reaction detected optically: the Ea value was<br />
found to be 88 kJ/mol, the frequency factor amounting to<br />
1.9 10 11 sec 1 .<br />
MODEL OF CHARGE CARRIER<br />
TRANSPORT<br />
Polar species in the polymer chain vicinity modify the<br />
electrostatic potential due to the charge–dipole interactions.<br />
The change of the electrostatic potential shifts the site<br />
energies of individual polymer repeating units, and consequently<br />
the polymer transport levels are modified. MEH-<br />
PPV is a hole transporting material, hence the energy, en,ofa charge carrier located on an n-th repeat unit (phenylene or<br />
vinylene) is essentially equal to the negative of its first<br />
ionization potential, In, corresponding to the highest<br />
occupied molecular orbital jHOMO>. For the undoped<br />
polymer, the units are treated as separated molecules capped<br />
by hydrogens, then it is possible to calculate their In0 by the<br />
Hartree–Fock (HF) method utilizing the Koopmans’ theorem.<br />
Koopmans’ ionization potential In of a unit interacting<br />
with a polar additive can be expressed as:<br />
In ¼ "n ¼ In0 < HOMOj X<br />
DfijHOMO > (1)<br />
where In0 is the ionization potential of an isolated repeat unit<br />
and Dfi are the changes of the electrostatic potential<br />
describing the charge–dipole interactions of a charge carrier<br />
localized at jHOMO> with all surrounding polar additive<br />
molecules. The change of the shape of jHOMO> induced by<br />
the additive is neglected (frozen orbital approximation).<br />
Since the positions and orientations of the additive with<br />
respect to the polymer chain are essentially random, the<br />
effect results in a broadening of the distribution of transport<br />
states. The most important parameter of this distribution is<br />
its half-width, proportional to the standard deviation s(e) of<br />
the site energies from its average value.<br />
In order to estimate the change of s(e) during the SP$MR<br />
reaction, the equilibrium molecular conformations of SP and<br />
MR were calculated by means of the HF/3-21G (*) method.<br />
The molecules were considered to be isolated. While the<br />
closed form SP assumes only one stable conformation, there<br />
are four almost planar metastable MR forms, differing by<br />
Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17: 673–678<br />
DOI: 10.1002/pat<br />
i
three dihedral angles along the ‘‘bridge’’ connecting the<br />
conjugated rings of the open form, whose values are close to<br />
08 or 1808 (see Ref. 12). The photochromic reaction SP$MR is<br />
accompanied by a redistribution of the atomic charges and<br />
consequently by a change of the dipole moment from 5.5 D<br />
(SP form) to 11–12 D (four possible MR forms). It was found<br />
that there is no significant dependence of the calculated<br />
dipole moments and other electronic properties on the basis<br />
set of atomic orbitals. Since the atomic electronic density<br />
distribution is only slightly influenced by the values of the<br />
dihedral angles, the differences in the electronic properties,<br />
including total energy, of four metastable MR forms are small<br />
and arise mainly from the different positions of the NO 2<br />
group. For this reason, further calculations only for SP and<br />
lowest-energy isomer of MR are considered. While the value<br />
obtained for SP is close to reality, the dipole moment of MR is<br />
probably underestimated since the polar environment<br />
increases the zwitterionic character of MR. Bletz et al. 19<br />
reported the dipole moment of MR measured in a polar<br />
environment to amount to 15–20 D.<br />
The change of the electrostatic potential Df i was calculated<br />
for four different mutual positions of the MEH-PPV tetramer<br />
and an SP/MR additive (above, below, in front of, and<br />
behind the chain, which is assumed to be oriented from left to<br />
right). For each position the distance between the chain and<br />
additive was determined on the basis of the van der Waals<br />
atomic radii. Furthermore, in each position, six main<br />
orientations of the additive molecule were considered. The<br />
orientations were fixed by the additive dipole moment being<br />
Influence of dipolar species on charge transport in MEH-PPV 675<br />
perpendicular, parallel, or skew to the chain. On the whole,<br />
24 calculations were done for each form. The HOMO orbital<br />
of MEH-PPV as well as HOMO orbitals of its oligomers of<br />
any length are localized along the main chain. For this<br />
reason, it is believed that to calculate electrostatic potentials<br />
Df i only ‘‘on-chain’’ is enough, i.e. along a line going through<br />
C–C bonds of vinylenes and crossing phenylenes through<br />
their centers. Figure 2(a) and 2(b) illustrates the on-chain<br />
electrostatic potential changes Dfi for the additive in a<br />
specific position oriented perpendicular and parallel to the<br />
chain, respectively. From all 24 resulting potential curves,<br />
each consisting of 342 calculated points, the standard<br />
deviations s SP(e)¼0.08 eV and s MR(e)¼0.14 eV of the site<br />
energies were calculated. These values provide an estimate<br />
of the broadening of the site energy distribution caused by an<br />
individual randomly placed and oriented additive molecule<br />
(taken as a non-point dipole). The HF calculation gives the<br />
ratio r¼sMR(e)/sSP(e) of the standard deviations of the en<br />
distributions for MR and SP around 1.75 (the estimate for<br />
point dipoles 11.9 and 5.5 D yields the ratio equal to 2.16).<br />
However, it is possible to expect that the real broadening of<br />
the site-energy distribution is for MR even higher than the<br />
calculated one because of its above-mentioned increased<br />
zwitterionic character due to polar environment. Taking into<br />
account both effects one can expect about 2.5-fold increase of<br />
the energetic disorder during the SP> MR reaction.<br />
The charge carrier mobility model of Grozema et al. 8 was<br />
modified in order to describe the charge transport along a<br />
polymer chain under the influence of photochromic polar<br />
Figure 2. The change of the on-chain electrostatic potential of tetramer caused by the one additive<br />
molecule oriented perpendicular (a) and parallel (b) to the chain. Mutual position of the tetramer and<br />
additive molecule is shown in the top part of the figure.<br />
Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17: 673–678<br />
DOI: 10.1002/pat
676 P. Toman et al.<br />
additives. According to the tight-binding approximation, the<br />
polymer chain is modeled by a sequence of N sites<br />
corresponding to the repeat units. The charge carrier motion<br />
on such a chain, in a non-dissipative model, can be described<br />
by the Hamiltonian<br />
H ¼ XN<br />
n¼1<br />
"na þ n an bn;nþ1ða þ nþ1 an þ a þ n anþ1Þ (2)<br />
where an and an þ are annihilation and creation operators of a<br />
charge carrier at an n-th site, en is the energy of a charge<br />
carrier localized at this site, and bn,nþ1 is the transfer integral<br />
between the sites n and nþ1. Both quantities en and bn,nþ1 are<br />
influenced by the random structure of the polymer chain and<br />
its surrounding. It is assumed that the influence of the<br />
additives on the electronic coupling between the polymer<br />
repeat units is small in comparison with the electrostatic<br />
charge–dipole interaction modulating the site energies en.<br />
Hence, for the model in this study the same distribution of<br />
the transfer integrals bn,nþ1 are used as Grozema et al. 8 for the<br />
description of the charge transport on PPV chain. The<br />
influence of substituents on bn,nþ1 distribution was neglected,<br />
but it was considered in the modeling of the en distribution.<br />
In the charge carrier mobility calculations reported in this<br />
paper, the polymer chain consisted of 4000 point centers.<br />
These centers represent alternatively bound phenylenes and<br />
vinylenes, i.e. the model chain consisted of 2000 phenylenevinylene<br />
units. In the case under study, this length is large<br />
enough for the chain to be considered infinite. Randomly<br />
oriented additive molecules, modeled as point dipoles, were<br />
randomly placed in the vicinity of the chain. It was assumed<br />
that no additive molecule was placed at a distance shorter<br />
than 10 A˚ from the chain. This value was estimated from the<br />
chemical structures of MEH-PPV and SP/MR, taking into<br />
account the presence of polymer substituents. The influence<br />
of the additive molecules at a distance more than 50 A˚ from<br />
the polymer chain was neglected. The additive molecules<br />
were placed also beyond the chain ends in order to ensure the<br />
homogeneity of the en distribution along the whole chain.<br />
The concentration of the additive was taken to be<br />
c¼4 10 4 A˚ 3<br />
. For each center representing a repeat unit,<br />
the energetic disorder was calculated as a sum of Coulombic<br />
electrostatic potentials from all additive molecules (cf.<br />
second term on the right side of eqn (1)). With regard to<br />
the value of the minimal distance of an additive molecule<br />
from the chain the size of the polymer repeat units was<br />
neglected. It should be pointed out that the In0 value of a<br />
vinylene unit was calculated to be 1.1 eV higher than that of a<br />
phenylene unit.<br />
The energetic disorder of en was calculated according to<br />
the above-described model for several values of the dipole<br />
moment of the additive m. For a typical value of m¼12 D the<br />
standard deviation of the en distribution is 0.37 eV. Note that<br />
this en distribution arises from the interaction of the hole with<br />
all considered additive molecules. If the mutual interaction<br />
of the additive molecules is neglected, the standard deviation<br />
of the en distribution is proportional to the dipole moment<br />
m of the additive and to the square root of the additive<br />
p ffiffi<br />
concentration, c.<br />
The concentration dependence of the level<br />
of the energetic disorder can be used to adjust the effective<br />
mobility switching. For example, if the additive concentration<br />
is four times lower, then the same degree of energetic<br />
disorder, and consequently the same on-chain mobility,<br />
would require an additive having twice the higher dipole<br />
moment. It should be noted that this model provides a<br />
realistic description of the very strong site to site correlation<br />
of en, which follows from the long-range character of the<br />
Coulombic interaction.<br />
Using the Hamiltonian (eqn (2)) with the molecular<br />
parameters en and bn,nþ1 generated according to the above<br />
described procedure and the hole wave function jC(t)i taken<br />
in the form of a linear combination of the states located at the<br />
individual sites, the time-dependent Schrödinger equation<br />
was numerically integrated. At t¼0, the hole was assumed<br />
localized on a single unit in the middle of the chain. Getting<br />
the wave function in the site representation, it is easy to<br />
calculate the mean-square displacement D 2 (t) of the charge<br />
carrier defined by the relation:<br />
D 2 ðtÞ ¼ cðtÞjn 2 jcðtÞ l 2<br />
(3)<br />
where l¼3.35 A˚ is the inter-unit distance. This quantity is<br />
related to the frequency dependent intramolecular ("onchain")<br />
charge carrier mobility m(v) by the well-known Kubo<br />
formula 20<br />
mðvÞ ¼ ev2<br />
2kBT Re<br />
Z1 D 2 2<br />
3<br />
4 ðtÞ expð ivtÞdt5<br />
(4)<br />
0<br />
where e is the elementary charge, kB is the Boltzmann<br />
constant, T is temperature, and v¼2pf is frequency of the<br />
external field. For the diffusive motion, which is expected in<br />
the long time limit due to the destructive interference even in<br />
the non-dissipative model, D 2 (t)/t. In this case the Kubo<br />
formula can be rewritten to the form of the Einstein relation:<br />
mðv ! 0Þ ¼ e<br />
2kBT<br />
RESULTS AND DISCUSSION<br />
D 2 ðtÞ<br />
t<br />
The dimensionless mean-square displacements D 2 (t)/l 2<br />
calculated for the undoped polymer chain and the chains<br />
doped by additives with dipole moments 6, 12, and 18 D are<br />
shown in Fig. 3. The presented data were obtained by<br />
averaging over 3000 realizations of the transfer integral and<br />
energetic disorder. The time evolution of D 2 (t) shows an<br />
initial rapid increase, which changes after about 10–25 psec to<br />
essentially linear time dependence, corresponding to the<br />
normal diffusive motion. For m¼18 D, D 2 (t) seems to be<br />
constant for t > 20 psec within the accuracy of the model,<br />
which means very small low-frequency mobility for this<br />
value of the additive dipole moment. For this reason, only<br />
dipole moments up to 12 D were considered for further<br />
calculations.<br />
Using the linear fit of D 2 (t) for the time range 25–70 psec,<br />
the mobility values extrapolated to the zero frequency<br />
m(v!0) were calculated according to eqn (5). It follows from<br />
Table 1 that the change of the additive dipole moment from<br />
ca. 6 D to 12 D (corresponding to the calculated change of the<br />
dipole moment during the photochromic reaction SP$MR)<br />
Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17: 673–678<br />
DOI: 10.1002/pat<br />
(5)
Figure 3. The dimensionless mean-square displacement calculated as a function of time for the<br />
undoped polymer chain (a) and the polymer chain doped by additives with different dipole<br />
moments m: (b) 6 D, (c) 12 D, (d) 18 D.<br />
should result in an almost five-fold decrease of the on-chain<br />
mobility. The effect would be even more pronounced if<br />
the experimental value of the MR dipole moment ( 18 D,<br />
see Ref. 19) is considered.<br />
The frequency-dependent hole on-chain mobilities m,<br />
calculated using eqn (4) for different values of the dipole<br />
moments of the additive, are shown in Fig. 4. All curves<br />
exhibit a saturation of m at low frequencies corresponding to<br />
the diffusive charge carrier motion in the long-time limit<br />
(t>25 psec), and a rapid increase for higher frequencies<br />
related to the fast initial hole delocalization (t
678 P. Toman et al.<br />
energies on adjacent sites, and therefore limits the diffusive<br />
charge carrier motion. Thus, the transport properties of a<br />
polymer chain can be reversibly changed by a photochromic<br />
reaction of the additive. The theoretical results were<br />
supported by the measurements of the photochromic change<br />
of conductance and capacitance.<br />
Acknowledgments<br />
This work was supported by the Grant Agency of the Academy<br />
of Sciences of the Czech Republic (Project No. KJB1050301),<br />
by the Academy of Sciences of the Czech Republic (Project<br />
No. T400500402 in the program ‘‘Information Society"), by<br />
the Ministry of Education, Youth, and Sports (Project Nos.<br />
49/2004/CZ and 50/2004/CZ), by the Polish Committee for<br />
Scientific Research (Project Nos. 18/2004/CZ and 19/2004/<br />
CZ), and by the Wroclaw University of Technology.<br />
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11. Sworakowski J, Nesˇpu˚ rek S, Toman P, Wang G, Bartkowiak<br />
W. Reversible mobility switching in molecular materials<br />
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12. Toman P, Bartkowiak W, Nesˇpu˚ rek S, Sworakowski J,<br />
Zales´ny R. Quantum-chemical insight into the design of<br />
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267–278.<br />
13. Valerián H, Brynda E, Nesˇpu˚ rek S, Schnabel W. The effect of<br />
polar additives on charge-carrier transport in polysilylenes.<br />
J. Appl. Phys. 1995; 78: 6071–6078.<br />
14. Nesˇpu˚ rek S, Valerián H, Eckhardt A, Herden V, Schnabel W.<br />
Charge carrier transport in poly(methylphenyl silylene): the<br />
effect of additives. Polym. Adv. Technol. 2001; 12: 306–318.<br />
15. Sworakowski J. Effect of dipolar arrays on the localization of<br />
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16. Dürr H, Bouas-Laurent H. Photochromism. Molecules and<br />
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17. Broo A. Theoretical characterization of a multifunctional<br />
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Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17: 673–678<br />
DOI: 10.1002/pat
Journal of Luminescence 112 (2005) 363–367<br />
Transient photoconductivity and charge generation in thin<br />
films of p-conjugated polymers<br />
Abstract<br />
Martin Weiter a,b, , Heinz Bässler b<br />
a Institute of Physical, Macromolecular and Nuclear Chemistry, Philipps University, Hans-Meerwein-Strasse,<br />
D-35032 Marburg, Germany<br />
b Faculty of Chemistry, Brno University of Technology, Purkynova 118, 61200 Brno, Czech Republic<br />
Available online 23 November 2004<br />
Transient photocurrents in thin films of methyl-substituted poly-phenylene (MeLPPP) and in phenyl-substituted<br />
poly-phenylenevinylene-type copolymer (PhPPV) were observed upon excitation by a laser flash. The questions on the<br />
origin of the optical charge generation and following charge transport in these materials are addressed. It has been<br />
argued that the observed current transient is a convolution of time-dependent charge carrier generation and their<br />
motion. By comparingthe field-dependent photogeneration yield in MeLPPP and PhPPV, transient absorption and<br />
double shot photogeneration, the rate-limiting steps for photoionization in studied polymers were identified.<br />
r 2004 Elsevier B.V. All rights reserved.<br />
PACS: 73.50.Gr<br />
Keywords: Transient photoconductivity; Exciton dissociation; Charge generation<br />
1. Introduction<br />
Photoinduced charge generation and transport<br />
are key elementary processes underlyingthe<br />
function of conjugated polymers. Frequently the<br />
photoionization in conjugated polymers are assumed<br />
to be tractable in terms of Onsager theory<br />
Correspondingauthor. Faculty of Chemistry, Brno University<br />
of Technology, Purkynova 118, 61200 Brno, Czech<br />
Republic. Tel.: +420 541 149 407; fax: +420 541 211 697.<br />
E-mail address: weiter@fch.vutbr.cz (M. Weiter).<br />
ARTICLE IN PRESS<br />
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.jlumin.2004.09.090<br />
www.elsevier.com/locate/jlumin<br />
[1–3]. It implies that intrinsic photogeneration is a<br />
multi-step process, the initial event beingeither<br />
the autoionization of a higher excited optical<br />
Franck–Condon singlet state yielding a coulombically<br />
bound geminate electron–hole pair (GP) or<br />
direct charge transfer [4]. Subsequently, the pair<br />
can recombine geminately or fully dissociate in the<br />
course of temperature and field-assisted diffusive<br />
escape process. Tactially, it has always been<br />
implied that in a bulk system the initial event that<br />
generate GPs is exothermic, i.e. not requiring any<br />
temperature or field assistance. Therefore, the
364<br />
measured field and temperature dependence of the<br />
photogeneration has solely to been attributed to<br />
the diffusive escape of the GP rather than its initial<br />
generation. However, since electric field assisted<br />
dissociation is at least quadratic in field it tends to<br />
vanish at moderate fields. This is at variance with<br />
photoconductivity studies that bear out a roughly<br />
linear field dependence at moderate field. The<br />
straightforward conclusion is that this residual<br />
effect is due to exciton dissociation at impurities<br />
actingas electron scavengers [5]. Based upon<br />
delayed field collection of optically generated<br />
charge carriers in a MeLPPP, it has been argued<br />
that the traditional Onsager approach fails to<br />
describe optical charge carrier generation because<br />
the essential field-assisted step is the primary<br />
dissociation rather than the secondary escape<br />
of the pairs from the coulombic well [6]. Experiments<br />
described in this paper performed with<br />
a broad temperature range will support these<br />
conclusions.<br />
2. Experiments<br />
In our study we focus on the charge carrier<br />
generation in methyl-substituted poly-phenylene<br />
(MeLPPP) and in phenyl-substituted poly-phenylenevinylene-type<br />
copolymer (PhPPV). Transient<br />
photocurrent measurements utilizingtime-of-flight<br />
method were performed on a sandwich cell with a<br />
dielectric layer. It was prepared by spin coatinga<br />
1% (by weight) polymer solution in chloroform<br />
onto an ITO (indium tin oxide) electrode. Some<br />
PhPPV samples were doped with 1% (by weight)<br />
of trinitrofluorene (TNF) as an electron acceptor.<br />
The thickness of the layer was typically 100 nm<br />
(MeLPPP) and 150 nm (PhPPV), respectively. In<br />
order to prevent hole injection, the ITO electrode<br />
was covered by a semitransparent aluminium<br />
layer. As a top electrode a 100 nm thick aluminium<br />
layer was evaporated. Transient photocurrents<br />
were observed upon excitation by a 5 ns flash of<br />
frequency doubled NdYaglaser at photon energy<br />
3.49 eV (355 nm) and/or by a 10 ns flash of dye<br />
laser at photon energy 2.74 eV (453 nm). The<br />
sample was mounted in a cryostat, which allowed<br />
coolingdown to 150 K.<br />
ARTICLE IN PRESS<br />
M. Weiter, H. Bässler / Journal of Luminescence 112 (2005) 363–367<br />
3. Results<br />
The efficiency of the charge generation was<br />
measured as a function of electric field, temperature,<br />
photon dose and polarity of irradiated<br />
electrode. Fig. 1a shows photocurrent transients<br />
in MeLPPP film at an incident photon dose of<br />
40 mJ pulse 1 at different external electric fields.<br />
The rise time of the photocurrent is determined by<br />
the RC time constant of the circuit which is about<br />
30 ns. At times, shorter than 1 ms and at low electric<br />
fields, the current features a plateau in a double<br />
logarithmic representative. At higher fields an<br />
algebraic decay followed by an almost exponential<br />
decay at t41 ms is observed. Fig. 1b shows<br />
complementary data on a PhPPV film at an electric<br />
field of 3.5 10 5 Vcm 1 and different light intensities.<br />
Data representation in double logarithmic<br />
Fig. 1. Transient photocurrents measured at room temperature<br />
(a) on MeLPPP at different electric field at incident photon dose<br />
40 mJ pulse 1 , (b) on PhPPV at different photon dose at electric<br />
field 3.5 10 5 Vcm 1 .
scale reveals a non-exponential decay extending<br />
into the 10 ms region. The shape of the j(t) time<br />
dependence is virtually invariant with light intensity.<br />
The additional measurements also confirm<br />
that the photocurrent is basically independent of<br />
polarity of the diode. The number of collected<br />
charges Q was calculated by integration of the<br />
current. The quantum yield of the photogeneration<br />
was calculated as the ratio of Q and the<br />
number of absorbed photons. Fig. 2 compares the<br />
quantum yield in undoped and doped PhPPV films<br />
and in MeLPPP film at the same incident light<br />
intensity.<br />
To elucidate whether or not there are longerlived<br />
GP we introduce a double-shot technique.<br />
After the initial single-shot excitation (hn1exc ¼<br />
3:49 eV; 35 mJ pulse 1 ), the sample was reexcited<br />
by the second 5 ns pulse (hn2exc ¼ 2:74 eV;<br />
0.7 mJ pulse 1 ) with time delay ranging from<br />
200 ns to 50 ms. The photocurrents measured with<br />
different delay of the second excitation at electric<br />
ARTICLE IN PRESS<br />
M. Weiter, H. Bässler / Journal of Luminescence 112 (2005) 363–367 365<br />
field 7.3 10 5 Vcm 1 are depicted in Fig. 3. In the inset the currents are depicted in double logarithmic<br />
scale. Despite the fact that the photon dose of<br />
the second pulse is 50 times lower than the photon<br />
dose of the first one, the second photocurrent<br />
maximum is much higher than one would expect<br />
from the comparison of the number of absorbed<br />
photons at each pulse. Time relations of the<br />
second photocurrent maximum at different electric<br />
field are shown in Fig. 4, which exhibits a strong<br />
influence of the electric field on the delayed<br />
photogeneration.<br />
Fig. 2. A comparison of the for MeLPPP, PhPPV and PhPPV<br />
doped with 1% TNF at low photon dose (full symbols—ITO<br />
positive bias, open symbols—ITO negative bias).<br />
Fig. 3. Double shots photocurrents with eight different time<br />
delays of the second pulse at electric field 7.3 10 5 Vcm 1 . The<br />
energy of the first pulse at 3.49 eV was 35 mJ pulse 1 , the energy<br />
of the second pulse at 2.74 eV was 0.7 mJ pulse 1 . In the inset the<br />
same situation is depicted in double logarithmic scale.<br />
4. Discussion<br />
A transient current can be either transport or<br />
generation limited. From previous results it is<br />
known that electron transport has never been<br />
observed in these materials in time-of-flight<br />
experiment indicatingthat electrons are trapped<br />
within the time scale of the experiment. In<br />
MeLPPP films hole transport is trap-free yielding<br />
a hole mobility of 2 10 3 cm 2 V 1 s 1 with an<br />
exceptionally weak temperature and field dependence<br />
[7]. InFig. 1a, the expected transit time of<br />
holes is 25 ns, i.e. about three orders of magnitude<br />
shorter than the duration of the photocurrent
366<br />
Fig. 4. Time dependence of the maximum of the second peak of<br />
the photocurrent at different electric field intensities. The<br />
energy of the first pulse at 3.49 eV was 35 mJ pulse 1 , the energy<br />
of the second pulse at 2.74 eV was 0.7 mJ pulse 1 .<br />
pulse. Obviously, the observed transient photocurrent<br />
must be controlled by charge carrier<br />
generation rather than by their transport. This<br />
concurs with the fact that the duration of the<br />
signal is virtually independent at the electric field.<br />
Based upon a value of m ¼ 2 10 6 cm 2 V 1 s 1<br />
for mobility of holes in PhPPV [8], one could<br />
estimate the mean transit time of holes at Fig. 1b<br />
to be 4–40 ms. This could, indeed, be comparable<br />
to the measured duration of the current transient<br />
implyingit to be entirely transport limited.<br />
However, not observingany significant shortening<br />
the current pulse at higher fields cast doubt in this<br />
assignment. In any event, the observed current<br />
transient is a convolution of time-dependent<br />
charge carrier generation, transport and discharge.<br />
Since the measured current transient must<br />
involve dissociation after pulse excitation, the<br />
dissociatingentity has to be a metastable electron–hole<br />
pair produced from a short-lived initial<br />
optical absorption. However, it is open to con-<br />
ARTICLE IN PRESS<br />
M. Weiter, H. Bässler / Journal of Luminescence 112 (2005) 363–367<br />
jecture as to what the rate-determiningstep for<br />
generation of mobile carrier is. It is the primary<br />
field-dependent dissociation of a singlet exciton<br />
into a Coulombically bound GP or its subsequent<br />
escape from its initial potential. In order to<br />
distinguish the above possibilities additional experimental<br />
information is required. In the case of<br />
MeLPPP this was field-modulated picosecond<br />
transient absorption of charge carriers and excitons<br />
simultaneously. By comparingthe field<br />
dependence of the evaluation of the transient<br />
absorption of geminately bound positive polarons<br />
with the microsecond transient photocurrent<br />
signal, we could ascertain that it must be the<br />
field-assisted initial dissociation event which is rate<br />
controlling [9].<br />
In the case of PhPPV the situation is different.<br />
Previous spectroscopic and cw-photoconduction<br />
[10] work on PhPPV without and with controlled<br />
dopingwith TNF showed that a neat PhPPV<br />
contains about 0.04% of dopants, which quench<br />
about 50% of excitations forms on electron–hole<br />
pairs on TNF and PhPPV already. Intentional<br />
dopingby 1% TNF can therefore increase the<br />
photogeneration of charge carriers by about a<br />
factor of 2 only, in agreement with Fig. 2. This<br />
proves that in this case the field assisted step for<br />
photogeneration must be the subsequent escape of<br />
the pair, i.e. an exciton, from its coulombic well<br />
while the initial event is the exciton diffusion<br />
towards the sensitizes followingthe charge transfer<br />
efficiency close to unity. Since this process is<br />
exothermic it does not require an electric field.<br />
Additional support for this argument is that in<br />
PhPPV the photoresponse is larger than in<br />
MeLPPP (Fig. 2) and is associated with a weaker<br />
field dependence although the field quenching of<br />
prompt fluorescence in MeLPPP and PhPPV turns<br />
out to be virtually identical.<br />
Additional probe of longer-lived GP was done<br />
usingdouble-shot technique. The question is<br />
whether or not there are geminate pairs that can<br />
be dissociated optically by a second laser pulse<br />
rather than decay by geminate pair recombination.<br />
After the re-excitation of the sample by the second<br />
laser pulse, the unusually substantial photoresponse<br />
is observed (see Fig. 3). The obvious source<br />
of this enhanced photogeneration must be the
dissociation of the GPs produced by the first pulse.<br />
Since the dissociation of GPs is field enhanced, the<br />
concentration of undissociated GPs has to be<br />
much higher at lower intensities of electric fields.<br />
The slope gradient of the time dependence of the<br />
delayed photocurrent maximum (Fig. 4) at low<br />
field intensities, which corresponds with lifetime of<br />
the bound GPs, proves this notion. This is also in<br />
accordance with the previous delayed fluorescence<br />
experiments on PhPPV that showed that delayed<br />
fluorescence in time scale up to microsecond<br />
originates from the radiative decay of GPs [11].<br />
On the other hand, the concentration of free<br />
carriers increase with increasingfield. Thus, the<br />
influence of the recombination at increasing<br />
intensities of electric field of bimolecular recombination<br />
is enhanced, as it is demonstrated by the<br />
decreasingslope and followingincrease of the<br />
delayed photocurrent maximum in Fig. 4.<br />
5. Conclusion<br />
It is obvious that there are two processes<br />
occurringin the bulk of conjugated polymer that<br />
can give rise to photogeneration within the<br />
spectral range of the S 1 S 0 transition. One of<br />
them is the field-assisted dissociation into geminate<br />
pairs that subsequently can separate fully. This is a<br />
generic process but requires a strong electric field.<br />
The other one is sensitized photogeneration at<br />
either non-intentional or intentional dopants that<br />
can act as electron scavengers. The double-shot<br />
ARTICLE IN PRESS<br />
M. Weiter, H. Bässler / Journal of Luminescence 112 (2005) 363–367 367<br />
experiment proves that after the photoexcitation,<br />
the GP are created and consequently they can<br />
decay in time interval up to 10 ms.<br />
Acknowledgment<br />
We are indebted to V.I. Arkhipov for a<br />
clarifyingdiscussion. We thank U. Scherf for<br />
providingMeLPPP and Covion Organic Semiconductors<br />
for providingPhPPV. This work was<br />
supported by the EU project LAMINATE.<br />
References<br />
[1] L. Onsager, Phys. Rev. 54 (1938) 554.<br />
[2] M. Pope, E. Swenberg, Electronic Processes in Organic<br />
Crystals and Polymers, Oxford University Press, Oxford,<br />
1999.<br />
[3] See K. Mu¨ llen, G. Wegner (Eds.), Electronic Materials, the<br />
Oligomer Approach, Wiley-VCH, Weinheim, 1998.<br />
[4] L. Sebastian, G. Weiser, G. Peter, H. Bässler, Chem. Phys.<br />
95 (1983) 13.<br />
[5] C. Im, E.V. Emelianova, H. Ba¨ ssler, H. Spreitzer, J. Chem.<br />
Phys. 117 (2002) 2961.<br />
[6] D. Hertel, E.V. Soh, H. Bässler, L.J. Rothberg, Chem.<br />
Phys. Lett. 361 (2002) 99.<br />
[7] D. Hertel, H. Ba¨ ssler, U. Scherf, H.H. Horhold, J. Chem.<br />
Phys. 110 (1999) 9214.<br />
[8] J. Veres, C. Juhasz, Phil. Magn. B 75 (1997) 377.<br />
[9] M. Weiter, H. Bässler, V. Gulbinas, U. Scherf, Chem.<br />
Phys. Lett. 379 (2003) 177.<br />
[10] C. Im, E.V. Emelianova, H. Ba¨ ssler, H. Spreitzer, J. Chem.<br />
Phys. 117 (2002) 2961.<br />
[11] A. Gerhard, H. Ba¨ ssler, J. Chem. Phys. 117 (2002) 7350.
Mol. Cryst. Liq. Cryst., Vol. 430, pp. 227–233, 2005<br />
Copyright # Taylor & Francis Inc.<br />
ISSN: 1542-1406 print=1563-5287 online<br />
DOI: 10.1080/15421400590946433<br />
Reversible Formation of Charge Carrier Traps in<br />
Poly(Phenylenevinylene) Derivative due to the<br />
Phototransformation of a Photochromic Additive<br />
Martin Weiter<br />
Martin Vala<br />
Ota Salyk<br />
Oldrˇich Zmeskal<br />
Faculty of Chemistry, Brno University of Technology, Purkynova,<br />
Brno, Czech Republic<br />
Stanislav Nespu˚ rek<br />
Faculty of Chemistry, Brno University of Technology, Purkynova Brno,<br />
Czech Republic and Institute of Macromolecular Chemistry, Academy<br />
of Sciences of the Czech Republic, Prague, Czech Republic<br />
Juliusz Sworakowski<br />
Institute of Physical and Theoretical Chemistry, Wroclaw University<br />
of Technology, Wyb. Wyspianskiego, Wroclaw, Poland<br />
Kinetics of photochromic reaction of spiropyran dissolved in a poly(phenylenevinylene)<br />
derivative MEH-PPV was studied by optical and impedance spectros<strong>copy</strong>.<br />
Spiropyran forms metastable, highly polar merocyanine under illumination with<br />
light of an appropriate wavelength. Due to charge-dipole interactions, charge<br />
carrier traps are formed and affect electrical properties of the polymer matrix,<br />
namely capacitance and photoconductivity.<br />
Keywords: electrical properties; photoconductivity; photochromism; spiropyran<br />
This work was supported by the grant 203=03=133 from the Czech Science<br />
Foundation and by the grant 4 T09A 132 22 from the Polish Committee for Scientific<br />
Research.<br />
Address correspondence to Martin Weiter, Faculty of Chemistry, Brno University of<br />
Technology, Purkynova 118, 612 00, Brno, Czech Republic. E-mail: weiter@fch.vutbr.cz<br />
227
228 M. Weiter et al.<br />
INTRODUCTION<br />
Nowadays multistable molecular systems attract much attention<br />
because of their potential use in optical and electro-optic devices such<br />
as broadband optical modulators, holographic and image-forming<br />
media, rectifiers and switching devices [1–3]. Molecular materials<br />
offer good processibility, low cost and easy modification of mechanical,<br />
optical and electrical properties by minor modification of their chemical<br />
structure. Current research has been mainly focused on systematic<br />
synthesis of novel chromophores in order to prepare doped polymeric<br />
matrices with enhanced electro-optic coefficients. Relatively little<br />
attention has been paid to the examination of electrical properties of<br />
such systems.<br />
Guest molecules will in general have different energy levels from<br />
the host. In particular, if ionization energies and electron affinities<br />
are suitably different, charge carrier traps can be formed. A special<br />
type of traps may occur if guest molecules possess a permanent dipole<br />
moment. The dipole moment contributes to the field acting on surrounding<br />
molecules and influences polarization energy. These dipolar<br />
traps are formed on neighboring host molecules, even though the<br />
impurity itself does not necessarily form a chemical trap. In our earlier<br />
papers [4–6] the concept of an electroactive molecular material was<br />
put forward whose electrical properties would be controlled by optical<br />
switching of photochromic species, either admixed into the polymer<br />
matrix or chemically attached to the polymer chain. The purpose of<br />
the present work is to examine the influence of charge carrier traps<br />
formed by photochromic additive on electrical properties of a pconjugated<br />
polymer matrix.<br />
EXPERIMENT<br />
The studied system consisted of p-conjugated photoconductive poly[2methoxy,<br />
5-(2 0 -ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV)<br />
doped with 5 wt.% of photochromic spiropyran (SP): 6-nitro-1 0 ,3 0 ,3 0 ,trimethylspiro[2H-1-benzopyran-2,2<br />
0 -indoline]. The films, typically<br />
150 nm thick, were prepared by spin coating of chloroform solutions<br />
onto an indium tin oxide (ITO) electrode or quartz glass substrate.<br />
Vacuum evaporated 100 nm thick top Al electrodes completed the<br />
sandwich structures for electrical measurements. Dielectric properties<br />
were studied using a Hewlett Packard 4192 A impedance analyzer.<br />
The time dependences of capacitance and conductance were recorded<br />
at constant frequency (1 kHz). The reaction converting SP into its merocyanine<br />
(M) colored form was activated using a mercury discharge
lamp HBO-200 with band filter (360 20) nm. The measurements<br />
were performed in vacuum in the temperature range 298–350 K.<br />
RESULTS<br />
Reversible Formation of Charge Carrier Traps 229<br />
The photochromic behavior of spiropyran derivatives has been investigated<br />
by many researchers [7–9]. Under irradiation spiropyran exhibits<br />
photochromism as shown in Fig. 1. The compound is stable in<br />
its colorless closed-ring isomeric form, while UV irradiation produces<br />
a metastable open-ring isomer (M) absorbing at 550–600 nm. The<br />
absorption spectra of molecules constituting the studied system are<br />
shown in Fig. 2. The maximum of absorption band of MEH-PPV is<br />
situated at 445 nm, the addition of SP increases the absorption of<br />
the sample in the UV region. After irradiation with UV light the spectra<br />
exhibit a significant absorption band at 590 nm caused by the<br />
colored merocyanine form. The photoinduced color change of SP is<br />
caused by the extension of conjugation in the M isomer [10]. Thermal<br />
fading in the dark or irradiation with a He-Ne laser gradually restores<br />
the original spectrum, the change being fully reversible. The thermal<br />
ring-closure for different temperatures is shown in Fig. 3. The changes<br />
of the reactant concentration were determined following the<br />
absorbance at kmax ¼ 590 nm. The thermal reaction in solutions is a<br />
first-order process; in our experiments the decays were clearly nonexponential,<br />
being better fitted with so-called ‘stretched exponential’<br />
function [11] in which the concentration of active species (nR) decays<br />
with time according to the equation<br />
nRðtÞ ¼nRð0Þ exp ð t=sÞ a ; ð1Þ<br />
where a is a parameter describing the deviation from a purely exponential<br />
decay (a < 1), nR(t) and nR(0) is the concentration of colored<br />
species at time t and s is a time constant presumed to be described<br />
FIGURE 1 Photochromism of the spiropyran molecule.
230 M. Weiter et al.<br />
FIGURE 2 Absorption spectra of the MEH-PPV (dash-dot line), MEH-PPV<br />
mixed with spiropyran before (dashed line) and after UV irradiation (full line).<br />
In the inset the chemical structure of MEH-PPV is depicted (R1 ¼ methyl,<br />
R 2 ¼ 2-ethylhexyl).<br />
by the Arrhenius equation<br />
s A 1 expðEa=RTÞ: ð2Þ<br />
In this equation Ea is the activation energy and A is the pre-exponential<br />
(frequency) factor. Such a behavior can be rationalized assuming that<br />
the process is controlled by a distribution of rate constants. Using the<br />
method previously described [12], we determined the parameters of<br />
the Arrhenius equation. The analysis of the data depicted in Fig. 3 gives<br />
107 kJ=mol for the activation energy of the dominant process and<br />
2.9 10 13 s 1 for the frequency factor.<br />
Dipole moments of stable forms of spiro molecules ranges typically<br />
within (2 5) D depending on substituents attached to their backbones,<br />
whereas the moments of the merocyanine form may exceed<br />
10 D. Thus, during the SP!M photochromic reaction dipolar species<br />
are formed. As was mentioned above, in polymer matrix they can<br />
generate dipolar traps reducing the charge carrier mobility. Consequently,<br />
a decrease of the dark current and=or photocurrent can be<br />
observed [6]. The formation of traps during the irradiation was<br />
checked by impedance measurements. The result is depicted in Fig. 4.
During the illumination of the sample a photocurrent decay was<br />
observed instead of expected current saturation; we attribute the<br />
phenomenon to the formation of the metastable species during the<br />
photochromic reaction. Simultaneously, the capacitance of the system<br />
was found to increase due to the formation of dipolar species. When<br />
the light was switched off, the rapid decrease of the photocurrent<br />
was observed, whereas the capacitance of the system was found to<br />
decrease slowly following the kinetics of the the back ring closure reaction,<br />
M ! SP. The rate constant of the process follows the Arrhenius<br />
equation with Ea ¼ 88 kJ=mol and A ¼ 1.9 10 11 s 1 . The values are<br />
in a qualitatively good agreement with the results obtained from the<br />
optical measurements.<br />
CONCLUSIONS<br />
Reversible Formation of Charge Carrier Traps 231<br />
FIGURE 3 Time evolution of normalized absorbance at 585 nm due to the<br />
dark M!SP reaction. The quotient on the vertical axis is proportional to the<br />
concentration of the reactant. Inset: (concentration time) vs. time dependence.<br />
The position of the maxima correspond to the inverse of the dominant<br />
rate constants [12].<br />
The kinetics of the reversible photochromic reaction merocyanine !<br />
spiropyran, i.e. the slow disappearance of the open-form dipolar species,<br />
was studied using the optical and impedance methods. The rates
232 M. Weiter et al.<br />
FIGURE 4 Time evolution of the conductance and capacitance of the sample<br />
measured by impedance spectros<strong>copy</strong> at room temperature. The vertical<br />
arrows indicate the time when the UV irradiation was switched on and off,<br />
respectively.<br />
and characteristic parameters of the photochromic processes detected<br />
from changes in the capacitance by impedance spectros<strong>copy</strong> qualitatively<br />
follow the parameters of photochromic changes detected optically.<br />
We can conclude that photochromic transformation of<br />
spiropyran produced charge carrier traps affecting the electrical<br />
properties of the polymer matrix, capacitance and photoconductivity<br />
in particular. Thus, an optical signal can be transformed into an electrical<br />
one and a polymeric optron can be, in principle, constructed.<br />
REFERENCES<br />
[1] Dalton, L. R., Steier, W. H., & Robinson, B. H. (1999). J. Mater. Chem., 9, 119.<br />
[2] West, K. S., West, D. P., & Rahn, M. D. (1998). J. Appl. Phys., 84, 5893.<br />
[3] Metzger, R. M. (1999). Acc. Chem. Res., 32, 950.<br />
[4] Nespu˚ rek, S. & Sworakowski, J. (2001). Thin Solid Films, 393, 168.<br />
[5] Nespu˚ rek, S., Toman, P. & Sworakowski, J. (2003). Thin Solid Films, 268, 438–439.<br />
[6] Nespu˚ rek, S., Sworakowski, J., Combellas, C., Wang, G., & Weiter, M. (2004). Appl.<br />
Surf. Sci., 234, 395.<br />
[7] Brown, G. A. (Ed.) (1971). Photochromism, Wiley-Interscience: New York.<br />
[8] Dürr, H. & Bouas-Laurent, H. (Eds.) (1994). Photochromism. Molecules and<br />
Systems, Elsevier: Amsterdam.
Reversible Formation of Charge Carrier Traps 233<br />
[9] Crano, J. C. & Guglielmetti, R. J. (Eds.) (1999). Organic Thermochromic and Photochromic<br />
Compounds, Kluwer: Dordrecht.<br />
[10] Zhi., J. F., Baba, R., Hashimoto, K., & Fujishima, A. (1995). J. Photochem. Photobiol.,<br />
92, 91.<br />
[11] Berlin, Yu., A., Miller, J. R., & Plonka., A. (Eds.) (1996). Special Issue: Rate<br />
Processes with Kinetic Parameters Distributed over Time and Space, Chem.<br />
Phys., 212, 1–246.<br />
[12] Sworakowski, J. & Nespu˚ rek, S. (1998). Chem. Phys. Lett., 298, 21.
Abstract<br />
Plasma polymerisation of methylphenylsilane<br />
O. Salyk a, T, P. Broza a , N. Dokoupil a , R. Herrmann a , I. Kuritka b , J. Prycek a , M. Weiter a<br />
a Faculty of Chemistry, Brno University of Technology, Purkynova 118, Brno, Czech Republic<br />
b Faculty of Technology, Tomas Bata University in Zlin, T. G. Masaryk sq. 275, Zlin, Czech Republic<br />
Available online 12 March 2005<br />
Microwave electron cyclotron resonance plasma afterglow chemical vapour deposition (MW ECR PA CVD) and radio frequency plasma<br />
enhanced CVD (RF PE CVD) of plasma-poly(methylphenylsilane) (p-PMPS) were examined according to plasma composition and resulting<br />
thin film properties. Direct mass spectrometry monitoring and gas chromatography and mass spectros<strong>copy</strong> (GC MS) of cold-trapped plasma<br />
products were used. Deposited samples were measured mainly on luminescence. The low pressure MW ECR plasma creates species for layer<br />
growth by a smaller average number of inelastic collisions per origin of monomer molecule. The exploitation of monomer reaches 23% in<br />
comparison with RF plasma (5–7%) and the deposition rate is larger (1.5 nm s 1 ) in comparison with RF plasma (0.2 nm s 1 ). The inelastic<br />
collision rate is primarily more important for growing layers than the power introduced into plasma. The luminescence spectra of both MW<br />
and RF samples are compared.<br />
D 2005 Elsevier B.V. All rights reserved.<br />
Keywords: Polysilane; PMPS; Plasma deposition; Mass spectros<strong>copy</strong><br />
1. Introduction<br />
Polysilanes are potential materials for both UV and VIS<br />
luminescence application, so they are frequently studied [1].<br />
Their linear j-conjugated chains can be easily broken and<br />
cross-linked and their luminous properties deteriorated by<br />
this way. Short chains of corresponding oligomers exhibit<br />
very similar optical properties [2], as they can be stabilized in<br />
any rigid matrix. Horvath et al. [3] proved the preservation of<br />
luminous centres in an amorphous matrix of surrounding<br />
plasma polymer. Their samples were prepared by radio<br />
frequency plasma enhanced chemical vapour deposition (RF<br />
PE CVD) method. It is known that microwave electron<br />
cyclotron resonance plasma afterglow CVD (MW ECR PA<br />
CVD) method is friendlier to the growing film because of its<br />
less disordering. The difference can be found on luminescence<br />
properties because the IR and UV-VIS absorption<br />
spectros<strong>copy</strong> are not sufficiently sensitive in respect to the<br />
low concentration of luminous centres in examined material.<br />
T Corresponding author.<br />
E-mail address: salyk@fch.vutbr.cz (O. Salyk).<br />
Surface & Coatings Technology 200 (2005) 486–489<br />
0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.surfcoat.2005.02.054<br />
This study is devoted to the comparison of both plasma<br />
depositions resulting in plasma polymer films with<br />
luminous properties.<br />
2. Experimental<br />
www.elsevier.com/locate/surfcoat<br />
Plasma poly(methyl phenyl silane) (p-PMPS) was<br />
deposited in both MW ECR (2.45 GHz) and RF (13.56<br />
MHz) discharge chambers from the methyl-phenylsilane<br />
monomer.<br />
MW apparatus consists of a remote plasma source and the<br />
substrates are placed in the afterglow region. Also monomer<br />
vapour is introduced by a gas bshowerQ in the afterglow<br />
towards the substrate surfaces. Operating pressure is in<br />
principle at the vicinity and below 1 Pa, so the mean free path<br />
of the monomer molecule is comparable to the characteristic<br />
dimension of MW apparatus and monomer molecule can<br />
undergo only few inelastic collisions on its path. The<br />
deposition conditions in the MW ECR PA CVD chamber<br />
were matched on optimum discharge stability and thin film<br />
deposition rate. Hydrogen (30 sccm H2, 1 Pa) in the<br />
mixture with monomer (0.25 Pa) should passivate dangling
Fig. 1. PMPS fragments power dependence of species created in MW<br />
afterglow discharge. Species of 121 mu is molecule depleted by hydrogen<br />
atom, species of 78 mu is benzene and species of 26 mu is acetylene. Dot<br />
lines in no discharge region are only intuitive guides for the eyes.<br />
bonds in the growing layer. The power was varied by<br />
magnetron current set up. The dwelling time of the<br />
reaction mixture is estimated and sustained at 8 s by<br />
setting the valve on the pump inlet. One series of samples<br />
was prepared in a stepwise set up power from 17 to 66 W,<br />
while the substrates were replaced using a carousel holder<br />
without vacuum break. The deposition rate at 17 W was<br />
0.4 nm s 1 , and at other powers remained constant at 1.5<br />
nm s 1 to a finishing thickness of 300 nm.<br />
The RF deposition chamber contains symmetric configuration<br />
with a central hot electrode and two opposite<br />
grounded electrodes with positioned substrates, which are<br />
in direct contact with plasma. The operating pressure/flow<br />
rate was kept in range from 10 to 60 Pa at 30 sccm H2 and<br />
10–40 Pa monomer, so the mean free path of the monomer<br />
molecule can undergo much more inelastic collisions on its<br />
path compared with the previous case. Diffusion is regarded<br />
as prevailing transport mechanism in plasma discharge. The<br />
deposition conditions in the RF PE CVD chamber were not<br />
too far from the volume powder creation regime. The power<br />
was varied by the RF generator set up. One series of<br />
samples was prepared in stepwise set up power from 75 to<br />
140 W.<br />
The fragmentation of monomer was observed by a mass<br />
spectrometer directly connected to the reaction chamber in<br />
both deposition processes. Mass spectra were picked up<br />
during steady conditions at various powers introduced into<br />
the plasma including the state without the discharge. Also,<br />
operation parameters of used apparatuses were controlled in<br />
this manner. The conditions in the discharge were stabilized<br />
after about 10 s after the monomer valve opening. Opening<br />
the shutter started each deposition in order to avoid layer<br />
profile inhomogeneities. In the other case the transient effect<br />
in plasma composition can influence initial growing layer<br />
properties.<br />
A cryogenic trap was used in order to collect plasma<br />
species for further analysis by gas chromatography and<br />
O. Salyk et al. / Surface & Coatings Technology 200 (2005) 486–489 487<br />
mass spectros<strong>copy</strong> (GC MS) method described in [4]. For<br />
this purpose we devised the liquid nitrogen cryogenic trap.<br />
It consists of a cylindrical steel vessel immerged in liquid<br />
nitrogen and attached directly to the discharge chamber by a<br />
valve. To adjust the pumping speed the cryogenic trap was<br />
provided with a small and varying aperture diaphragm.<br />
After sample collection, defined volume of pure argon was<br />
injected into the trap trough the chromatographic septum.<br />
Argon was used as a marker and internal standard for<br />
quantification in GC-MS measurements. For the analysis of<br />
trapped volatile gas species the GC MS chromatograph<br />
TRIO 1000, FISONS INSTRUMENTS with the column<br />
DB-5MSITD ( 30 m, 0.25 mm, 0.25 Am) was used as a<br />
detector of species separated by column served by the<br />
quadruple mass spectrometer. Both gas and liquid phases of<br />
collected sample were analysed.<br />
The films were deposited on Si wafers for IR absorption<br />
and luminescence measurements and quartz glass plates for<br />
UV-VIS spectra picking up.<br />
3. Results and discussion<br />
The mass spectroscopic analysis of reaction gases is<br />
depicted in Figs. 1 and 2 for significant species in both<br />
MW and RF discharges. The most of heavier species<br />
(molecule MPS depleted by H atom—121 mu) are<br />
decomposed stepwise with power and their concentration<br />
decreased, but the highest breakthrough occurred at the<br />
beginning at the transition from non-discharge to the<br />
weakest discharge conditions. In case of MW it can be<br />
explained by the inelastic collision rate, which is at low<br />
pressure early saturated. Also, the deposition rate is early<br />
saturated and remains practically constant in the region<br />
25–70 W. The important splitting products of monomer<br />
molecule were detected benzene (78 mu) and acetylene (26<br />
mu). Benzene is considered as the product of phenyl (77<br />
mu) hydrogenation on the relative long path of species<br />
Fig. 2. PMPS fragments power dependence of species created in RF<br />
discharge.
488<br />
from the reaction chamber to the quadrupole analyzer and<br />
dominates comparing with phenyl. Dissociated hydrogen<br />
for that is available both like the dissociation product of<br />
feeding gas and product of splitting monomer itself. The<br />
power development of benzene and acetylene concentration<br />
can be explained by cascaded decomposition (benzene<br />
and phenyl splits to acetylene) but the power effect is not<br />
so apparent because of statistically low collision rate. In<br />
RF plasma inelastic collision rate due to higher pressure<br />
furthermore continues and acetylene peak is still growing<br />
with power. Toluene (tropylium-mass 91) was observed<br />
both in RF and MW plasmas; i.e. under high and low<br />
pressure condition.<br />
GC MS chromatography enabled analysis of the trapped<br />
plasma reaction products. The phenylsilane is the reminder<br />
of monomer molecule after demethylation. The assumption<br />
of important role of methyl group transfers in polymerisation<br />
processes is supported by the presence of dimethyland<br />
trimethylphenylsilane in the gas phase chromatogram.<br />
Various disilanes and heavier species were detected, which<br />
fact testifies the presence of synthetic reactions in collected<br />
samples. Unfortunately we are not able at present to<br />
interpret GC-MS signals of all detected species unambiguously.<br />
Most of the species detected in samples were trapped<br />
Table 1<br />
Representatives of species detected by GC MS in collected samples both<br />
from RF and MW plasmas<br />
Comments to molecule<br />
identification (highest<br />
m/z + Detected in<br />
of MS spectrum,<br />
significant fragments,<br />
structural units and groups)<br />
RF MW<br />
Benzene + +<br />
Toluene + +<br />
Phenylsilane + +<br />
Methylpenylsilane Monomer (also the only<br />
one relevant peak in<br />
blank samples)<br />
+ +<br />
Dimethylfenylsilane + +<br />
Phenyltrimethylsilane +<br />
Ethyphenyldimethylsilane +<br />
Diphenylsilane + +<br />
Unidentified species 241, Disilane,<br />
dimethyldiphenyl,<br />
+ +<br />
342, Methylphenylsilanyl,<br />
phenylsilane,<br />
methyl, phenyl<br />
+<br />
295, Phenylsilanyl,<br />
biphenyl, alkyl<br />
+<br />
343, Methylphenylsilanyl,<br />
phenyl, alkyl<br />
(4 or more carbons)<br />
+<br />
346, Methylphenylsilanyl,<br />
phenyl, alkyl<br />
(4 or more carbons)<br />
+<br />
389, Phenylsilanyl, phenyl,<br />
alkyl<br />
+<br />
392, Methylphenylsilanyl,<br />
phenyl, alkyl<br />
+<br />
O. Salyk et al. / Surface & Coatings Technology 200 (2005) 486–489<br />
Fig. 3. Excitation and emission spectra of MW ECR CVD films of p-PMPS<br />
for 17 and 66 W of introduced power.<br />
from RF plasma contained Si-phenyl structural unit (see<br />
Table 1).<br />
Let us touch the question of conversion efficiency<br />
measured by GC MS method. In the RF apparatus working<br />
in the optimal regime (from the point of thin film quality)<br />
only small conversions about (5.0F2.0)% were reached,<br />
testifying suppression of the monomer unit destruction by<br />
the high feed gas flow rate. The monomer conversion in<br />
MW ECR PE CVD is approximately three or four times<br />
higher, in this analysis (23.0F5.0)%. The long period of<br />
remaining molecules in the discharge region and its quick<br />
condensation after inelastic collision leads to higher conversion<br />
efficiency and the simpler reaction product composition.<br />
Generally, the observed monomer conversion is<br />
usually above 20% in MW ECR PE CVD in comparison<br />
of common 5–7% in RF PE CVD.<br />
The samples were tested on UV VIS and IR absorption<br />
measurement, but the results were very uniform and cannot<br />
be distinguished from both deposition methods and power<br />
effect. They do not differ from those presented in [3]. The<br />
difference occurred in luminescence measurements. The<br />
Fig. 4. Excitation and emission spectra of RF CVD films of p-PMPS for<br />
140 W introduced power.
comparison of both MW and RF samples is in Figs. 3 and 4.<br />
The excitation spectra of MW samples show excitation peak<br />
at 290 nm corresponding to j–j* transition in dimethylsilane<br />
chain measured at evaporated films [5] and 325 nm of<br />
methylphenylsilane chain at spin coated films (e.g. [6]),<br />
which confirms the presence of such short chains like<br />
luminescent centres. Their measurements did not allow<br />
extension to the larger wavelengths at set emission of 347<br />
nm but existence of 325 nm absorption is apparent.<br />
Emission parts are more complicated, but also contain<br />
corresponding peaks at 350 nm region. Moreover, stronger<br />
luminescence belonging probably to more complicated<br />
luminous centres occurs in the blue region. Emission<br />
intensity was only slightly dependent on mw power until<br />
the power exceeded 50 W. The 66 W case showing the<br />
luminous deterioration is depicted in Fig. 3. All the emission<br />
spectra were measured at the same excitation wavelength<br />
293 nm (of supposed j–j* exciton absorption) and<br />
intensity.<br />
The RF samples exhibit more complicated excitation and<br />
emission spectra in Fig. 4 but the previous mechanism can<br />
also be distinguished. The luminous intensity was usually<br />
comparably weaker.<br />
4. Conclusions<br />
The power dependence of the plasma composition was<br />
monitored by direct mass spectros<strong>copy</strong> and by GC MS<br />
spectros<strong>copy</strong> of cold trapped plasma products. An effect of<br />
inelastic collision rate was concluded. The low pressure<br />
MW ECR plasma products lower number of species for<br />
layer growth than it is in the case of RF plasma; the<br />
exploitation of the monomer and deposition rate is larger in<br />
comparison with RF plasma. The inelastic collision rate is<br />
primarily more important for growing layers than the power<br />
introduced into plasma. In RF plasma, the pressure up to<br />
tens Pa causes many collisions on monomer molecule path<br />
and the number of inelastic ones depends on the power<br />
introduced.<br />
It was observed that the RF PE CVD is a system<br />
producing a large variety of species at used polymerisation<br />
pressure. This fact is reflected in the complicated system of<br />
reactions that lead to disordered bonding for RF plasma<br />
deposited thin film. It is obvious that under such conditions,<br />
only using the regimes of small monomer conversion gives<br />
material with preserved 1D silicon chains [7,8]—see model<br />
in Fig. 5.<br />
O. Salyk et al. / Surface & Coatings Technology 200 (2005) 486–489 489<br />
Fig. 5. Model of short chain in amorphous plasma polysilane matrix.<br />
A sensitive luminescence measurement can detect the<br />
power deterioration. It was demonstrated that the plasmapoly(methylphenylsilane)<br />
p-PMPS exhibits luminescence in<br />
a near UV region similarly as a standard polymer prepared<br />
by chemical synthesis. At the highest power of the<br />
excitation beam the luminescence intensity falls, probably<br />
due to radiation damage of the polymer-like components in<br />
deposited films. UV luminescence together with good<br />
adhesion and degradation resistance made from plasma<br />
polysilanes are feasible and perspective materials for device<br />
construction.<br />
Acknowledgments<br />
The support of the following grant is acknowledged:<br />
Czech Grant agency contract 202/00/518.<br />
I. Kuritkas’ work in Zlin was supported by the project of<br />
the Ministry of Education, Youth and Sports of the Czech<br />
Republic No. MSM 265200015.<br />
References<br />
[1] S. Hayase, Prog. Polym. Sci. 28 (3) (2003) 359.<br />
[2] H. Usui, Thin Solid Films 365 (2000) 22.<br />
[3] P. Horvath, F. Schauer, I. Kuritka, O. Salyk, M. Weiter, N. Dokoupil, S.<br />
Nespurek, V. Fidler, Monatsh. Chem. 132 (1) (2001) 177.<br />
[4] A.M. Wrobel, G. Czeremuszkin, H. Szymanowski, J. Kowalski, Plasma<br />
Chem. Plasma Process. 10 (2) (1990) 277.<br />
[5] N. Tanigaki, H. Kyotani, M. Wada, A. Kaito, Y. Yoshida, Eun-Mi Han,<br />
K. Abe, K. Yase, Thin Solid Films 331 (1-2) (1998) 229.<br />
[6] S. Nespurek, Macromol. Symp. 104 (1996) 285.<br />
[7] F. Schauer, S. Nespurek, P. Horvath, J. Zemek, V. Fidler, Synth. Met.<br />
109 (1–3) (2000) 321.<br />
[8] S. Nespurek, P. Toman, J. Sworakowski, Thin Solid Films 438 (2003)<br />
268.
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JOURNAL OF APPLIED PHYSICS VOLUME 95, NUMBER 2 15 JANUARY 2004<br />
Charge transport in highly efficient iridium cored<br />
electrophosphorescent dendrimers<br />
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Jonathan P. J. Markham and Ifor D. W. Samuel a)<br />
School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews,<br />
Fife, KY16 9SS, United Kingdom<br />
Shih-Chun Lo and Paul L. Burn<br />
The Dyson Perrins Laboratory, Oxford University, South Parks Road, Oxford, OX1 3QY, United Kingdom<br />
Martin Weiter and Heinz Bässler<br />
Institute of Physical, Nuclear and Macromolecular Chemistry and Material Science Center,<br />
Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany<br />
�Received 5 August 2003; accepted 23 October 2003�<br />
Electrophosphorescent dendrimers are promising materials for highly efficient light-emitting diodes.<br />
They consist of a phosphorescent core onto which dendritic groups are attached. Here, we present<br />
an investigation into the optical and electronic properties of highly efficient phosphorescent<br />
dendrimers. The effect of a dendrimer structure on charge transport and optical properties is studied<br />
using temperature-dependent charge-generation-layer time-of-flight measurements and current<br />
voltage (I – V) analysis. A model is used to explain trends seen in the I – V characteristics. We<br />
demonstrate that fine tuning the mobility by chemical structure is possible in these dendrimers and<br />
show that this can lead to highly efficient bilayer dendrimer light-emitting diodes with neat emissive<br />
layers. Power efficiencies of 20 lm/W were measured for a second-generation �G2� Ir(ppy) 3<br />
dendrimer with a 1,3,5-tris�2-N-phenylbenzimidazolyl�benzene electron transport layer. © 2004<br />
American Institute of Physics. �DOI: 10.1063/1.1633336�<br />
I. INTRODUCTION<br />
PROOF COPY 056402JAP<br />
Since early reports of organic electroluminescence �EL�<br />
in small molecules, 1<br />
polymers, 2<br />
and conjugated<br />
dendrimers, 3,4 dramatic improvements in the efficiency and<br />
stability of these materials have been achieved. Now, external<br />
quantum efficiencies �EQEs� of organic light-emitting diodes<br />
�OLEDs� are of the order of 10%–20% 5 and lifetimes<br />
in excess of 10 000 h �Ref. 6� have been reported. In particular,<br />
the development of electrophosphorescent devices 7 has<br />
allowed triplet harvesting and the production of such highly<br />
efficient devices. In this area, iridium-based phosphors have<br />
received much attention due to their highly efficient phosphorescence,<br />
color tunability, and relatively short excited<br />
state lifetime. Both evaporated 8 and solution processed 9 devices<br />
have been reported. Our approach to solution processing<br />
of phosphorescent materials is to use conjugated dendrimers<br />
and we have developed highly efficient single and<br />
bilayer devices using this method. 10,11<br />
Time of flight �TOF� mobility measurements are a powerful<br />
technique for the investigation of charge transport in<br />
organic materials. However, they have not yet been applied<br />
to electrophosphorescent materials. Here, we apply the technique<br />
in the context of electrophosphorescent dendrimers,<br />
which give us scope for the tuning of properties, such as<br />
interchromophore spacing and thus the mobility. An important<br />
feature of this work is the use of the charge-generationlayer<br />
time-of-flight �CGL-TOF� method �explained in Sec.<br />
a� Electronic mail: idws@st-and.ac.uk<br />
II�. For the conventional, bulk-excitation TOF method, a<br />
very thick ��m dimensions� film is usually necessary. For<br />
solution processable materials, this often entails the production<br />
of a drop cast film. Two problems arise in using this<br />
technique. One is that the morphology of the sample may be<br />
very different from that produced by spin coating, and mobility<br />
in organic materials has been shown to be heavily dependent<br />
upon the morphological properties of small molecules<br />
and polymers. The second is that the mobility is<br />
measured upon a sample that is of much greater thickness<br />
than that used for OLEDs. In many cases of dispersive transport,<br />
a thickness dependence of the mobility is observed and<br />
this may call into question the relevance of such measurements<br />
to light-emitting device �LED� design. In the present<br />
work, we overcome both these problems by utilizing a spincoated<br />
layer of �400 nm thickness and a charge-generation<br />
layer of absorptive dye. This enables a well-defined charge<br />
generation point even in a thin sample. It therefore allows the<br />
study of much thinner samples that can be produced by spin<br />
casting with morphological properties of direct relevance to<br />
LED structures. We have recently reported measurements of<br />
nondispersive hole transport in a fluorescent blue-emitting<br />
dendrimer film using the above technique. 12<br />
Conjugated dendrimers consist of a light-emitting core,<br />
dendrons, and surface groups that control the processing<br />
properties. One of the great advantages of this molecular<br />
framework is that each of the components can be tuned independently<br />
to control the color of emission, degree of intermolecular<br />
interaction, and solubility as required for different<br />
applications. Dendrimers can be synthesized in an orderly<br />
0021-8979/2004/95(2)/1/8/$20.00 1<br />
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manner such that the macromolecules are monodisperse and<br />
the structure is precisely known. Thus, there is little issue of<br />
batch to batch reproducibility. It has already been shown that<br />
a dendrimer consisting of a fluorescent core and wide bandgap<br />
dendrons behaves very much like a molecularly doped<br />
polymer albeit with a much more regulated and accurately<br />
controlled interchromophore separation. 13<br />
Charge transport has been studied widely in solutionprocessible<br />
conjugated polymers. It is known for these systems<br />
that different solvents and spinning conditions can produce<br />
very different thin-film morphologies. In addition, even<br />
in relatively ordered materials, such as ladder-type polymers,<br />
sample history has been shown to have a dramatic effect on<br />
the measured charge transport and optical properties. 14 This<br />
may partly explain the wide range of mobilities measured for<br />
some of these materials. 15 The mobility of carriers along<br />
polymer chains can be quite high 16 and, in these cases, the<br />
rate-limiting step is the degree of disorder in the material.<br />
This makes the optimization of the mobility within a neat<br />
organic layer relatively challenging for most spin-coated materials.<br />
However, in contrast, dendrimers allow the precise<br />
control of the core separation. In this article, we demonstrate<br />
that by altering the generation number �or branching� and<br />
attachment point of the dendron to the core we are able to<br />
fine tune the hole mobility and optical properties of these<br />
highly efficient iridium-based dendrimers.<br />
II. EXPERIMENT<br />
FIG. 1. Structures of the three iridium dendrimer systems �from the left-hand side� G1pIr, G1mIr, and G2mIr.<br />
The dendrimers studied here are based on a fac tris�2phenylpyridine�<br />
iridium �Ir(ppy) 3� core, phenylene dendrons<br />
and 2-ethylhexyloxy surface groups. They are:<br />
G1-meta-Ir(ppy) 3 �hence referred to as G1mIr), G1-para<br />
Ir(ppy) 3 (G1pIr), and G2-meta (ppy) 3 (G2mIr). Their<br />
structures are shown in Fig. 1. All are soluble in organic<br />
solvents, such as tetrahydrofuran, toluene, and chloroform.<br />
G1mIr has been previously used to give very high efficiency<br />
solution processed OLEDs. 10,11<br />
CGL-TOF measurements were performed on 300–400<br />
nm thick films spin coated onto cleaned indium-tin oxide<br />
�ITO� substrates �sheet resistance 20 �/�� from 40 mg/ml<br />
chloroform solution. Subsequently, 10 nm of perylene diim-<br />
ide charge-generation layer was evaporated onto the device<br />
covering the whole substrate. The device was completed by<br />
deposition of 100 nm of aluminum to give an active pixel<br />
area of approximately 5 mm 2 . The excitation wavelength was<br />
chosen to pass through the bulk film under study and to be<br />
absorbed by the perylene dye. Charge carriers were generated<br />
within the perylene layer by excitation from a 10 ns<br />
pulse of a frequency-doubled Nd:Yttrium aluminum garnet<br />
laser at a wavelength of 532 nm. The packet of charge carriers<br />
was then swept through the device under an applied<br />
field, and the time for the packet to travel across the device is<br />
known as the transit time (t T). The aluminum electrode was<br />
biased positively and the photocurrent signal detected from<br />
the ITO using the 50 � input of a digital storage oscilloscope.<br />
The applied bias led to the electrons photogenerated<br />
in the perylene diimide layer being removed from the device<br />
at the aluminum electrode and holes being injected into the<br />
dendrimer from the perylene dye and consequently swept<br />
across the device to be annihilated at the ITO electrode.<br />
Thus, the measured photocurrent transients correspond to<br />
hole currents.<br />
Single and bilayer LEDs were fabricated on solution<br />
etched ITO substrates. The ITO was cleaned in ultrasonic<br />
baths of acetone and 2-propanol, before oxygen plasma ashing<br />
under a pressure of 10 �2 mbar at 100 W for 5 min<br />
�Emitech Model K-1050X�. Dendrimer films were spin<br />
coated from 20 mg/ml chloroform solutions to give film<br />
thicknesses of 120 nm for single layer devices and 80 nm for<br />
bilayer devices. Cathode evaporation of 20 nm of calcium<br />
capped with 100 nm of aluminum completed the single layer<br />
devices. Bilayer devices were fabricated by evaporation of<br />
50 nm of 1,3,5-tris�2-n-phenylbenzimidazolyl�benzene<br />
�TPBI� as an electron transporting/hole blocking layer and a<br />
cathode of 1 nm LiF capped with 100 nm aluminum completed<br />
the device. In both cases, the pixel area was approximately<br />
8 mm 2 .<br />
All EL measurements were performed under a vacuum<br />
using a Keithley 2400 source measure unit and calibrated<br />
photodiode. Efficiency calculations were performed by measuring<br />
the light output in the forward direction 17 and bright-
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FIG. 2. �a� G1mIr transient photocurrent at room temperature and field of<br />
1.2�10 6 V/cm. �b� The same transient photocurrent plotted in a double<br />
logarithmic form.<br />
ness measurements were cross checked with a Minolta<br />
LS-100 luminance meter.<br />
III. RESULTS AND DISCUSSION<br />
A. Mobility measurements<br />
The CGL-TOF method was used to measure the mobilities<br />
of G1pIr, G1mIr, and G2mIr iridium cored dendrimers<br />
shown in Fig. 1. A typical transient for a G1mIr film of 330<br />
nm thickness at room temperature and an applied field of<br />
1.2�106 V/cm is shown in Fig. 2�a�. It can be seen that the<br />
transient is weakly dispersive. After the initial photocurrent<br />
peak, there is a long photocurrent tail. The shoulder is close<br />
to the photocurrent peak because the sample is much thinner<br />
than the micron thicknesses usually used in TOF. The mobility<br />
can be calculated from the intersection of the asymptotes<br />
to the two power-law regions of the transients �Fig. 2�b��.<br />
Using Eq. �1�, we calculate a mobility of 4.5<br />
�10�5 cm2 /Vs<br />
�� d2<br />
. �1�<br />
Vttr Here � is the mobility, d the device thickness, V the applied<br />
voltage and t tr the transit time.<br />
The transients shown here are weakly dispersive at room<br />
temperature, however an attempt to analyze them in terms of<br />
the Scher–Montroll 18 theory was not successful. When plotted<br />
on a double logarithmic scale, this theory relates the two<br />
power-law regions of the transient before and after the transit<br />
time by a disorder parameter �, as seen in Eq. �2�<br />
I�t��t ��1���<br />
�t�t tr�<br />
I�t��t ��1���<br />
�t�t tr�. �2�<br />
Thus, if � is constant before and after the transit time then<br />
the slopes of the two regions should add to �2. At room<br />
temperature �298 K� for the iridium dendrimer systems studied<br />
here, this is not the case, as seen in Fig. 2�b�. All transients<br />
for all generations add up to between �1.2 and �1.4.<br />
Deviations from the model have been observed in several<br />
systems, including molecularly doped polymer 19 and conjugated<br />
polymer systems. 14<br />
Monte Carlo simulations 19 have shown that the initial<br />
decay of the dispersive transient is associated with a certain<br />
disorder. The slope of the initial decay for G1mIr �before the<br />
transit time� of �0.08 can be compared with simulated transients<br />
in Ref. 19 and translates into a Gaussian width of the<br />
density of states �DOS�, �ˆ of 4.0. �ˆ is related to the experimentally<br />
accessible � via Eq. �3�<br />
�ˆ � �<br />
. �3�<br />
kT<br />
From �ˆ of 4.0 and T�298 K, we obtain an estimate of �<br />
�103 meV for G1mIr. This is much higher than that measured<br />
for structurally well ordered conjugated polymers,<br />
where a � of 52 meV was reported, but comparable to some<br />
molecularly doped polymer systems �typically in the region<br />
of 80–100 meV� 14 and more disordered conjugated materials,<br />
such as PPV. 20<br />
An important parameter in the understanding of charge<br />
transport is the temperature dependence of the mobility and<br />
its effect on the shape and the scale of the photocurrent transients.<br />
The influence of temperature on the transients is<br />
shown in Fig. 3, measured upon a sample of 430 nm thickness.<br />
It can be seen from Fig. 3�a� that at high temperatures<br />
�343 K�, the transit is nondispersive, with a visible plateau<br />
region characteristic of Gaussian transport, followed by a<br />
broad tail. The transient was measured at a field of 1.2<br />
�105 V/cm. However at low temperatures �203 K� �Fig.<br />
3�b��, the transient is completely dispersive, since carriers do<br />
not attain dynamic equilibrium. The field in this case was<br />
3.25�105 V/cm.<br />
A transition from dispersive to nondispersive transport is<br />
highly significant and can be interpreted in the framework of<br />
earlier simulations based on Monte Carlo methods. 19 These<br />
provide a relation that enables calculation of the temperature<br />
at which such a transition should occur �Eq. �4��<br />
�ˆ 2�� � 2<br />
�44.8�6.7 log d, �4�<br />
kTc� where � is the width of the DOS, Tc is the transition temperature,<br />
and d is the sample thickness in cm. The transition<br />
temperature Tc can be obtained from the change in slope of<br />
3
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FIG. 3. �a� Transient photocurrent for G1mIr measured at 343 K. �b� G1mIr<br />
transient at 203 K.<br />
the graph of the mobility at a given field versus (1000/T) 2 .<br />
Figure 4 shows the temperature dependence of G1mIr at a<br />
field of 1.2�10 5 V/cm. The experimentally measured T c of<br />
293 K is much higher than that reported for a phenyl-amino<br />
substituted PPV where a transition temperature of 153 K was<br />
measured. 14 For a film thickness of 430 nm and a transition<br />
temperature of 293�5 K, this translates to a Gaussian width<br />
of DOS of 99.6�1.7 meV.<br />
In Fig. 5�a�, transients for G1mIr are shown at two different<br />
applied fields, scaled to the transit time. The shapes of<br />
the transients are apparently independent of field, even over<br />
FIG. 4. Mobility vs (1000/T) 2 for G1mIr.<br />
FIG. 5. �a� Room-temperature transient photocurrents of G1pIr at high and<br />
low fields normalized to transit time �b� the same for G1mIr.<br />
fields differing by more than an order of magnitude. In the<br />
case of Gaussian charge carrier transport, the dispersion of a<br />
sheet of carriers migrating through the sample can be calculated<br />
from Eq. �5�, assuming the validity of the Einstein relation<br />
eD��kT<br />
w���kT , �5�<br />
eU<br />
where w is the dispersion, T is the temperature in Kelvin, and<br />
U is the applied voltage in V. Using the values for the data in<br />
Fig. 2, an applied voltage of 40 V �equivalent to a field of<br />
1.2�106 V/cm) and a temperature of 298 K leads to w<br />
�0.045. Experimentally, the dispersion can be obtained from<br />
Eq. �6�<br />
w� t1/2�t tr<br />
, �6�<br />
t1/2 where ttr is the carrier transit time and t1/2 is the time by<br />
when the current has decayed to half of the plateau value,<br />
From Fig. 2, we obtain w�0.399, almost an order of magnitude<br />
greater. If the tail broadening of the transients were<br />
due to thermal diffusion as expected for Gaussian transport, a<br />
change in field by an order of magnitude would result in a<br />
decrease in w by a factor of over 3. This is clearly not seen<br />
here. Earlier simulations have shown that the anomalous<br />
broadening of tails in TOF signals is a characteristic of nondispersive<br />
transport, due to the Gaussian DOS in these<br />
materials. 21 These simulations indicate that the degree of disorder<br />
in the material is closely linked with the dispersion.
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FIG. 6. Transient photocurrents for G1pIr, G1mIr, and G2mIr all scaled by<br />
transit time.<br />
The observation of nondispersive transport, albeit at higher<br />
temperatures, is in marked contrast to highly disordered materials<br />
such as PPV. 22<br />
We have so far examined the transport properties of a<br />
first generation iridium cored phosphorescent dendrimer in<br />
detail. We next consider the effect of changing generation,<br />
which changes the dendrimer core spacing, and investigate<br />
its impact on charge transport. Figure 6 shows traces of<br />
G1mIr and G2mIr normalized to the transit time. G1pIr is<br />
also shown and will be discussed later. Remarkably, the photocurrent<br />
transients are the same for both generations and<br />
overlap when scaled by transit time. This shows that the role<br />
of the dendrons is to simply slow the carrier packet by separating<br />
the core regions, and they have little effect on the tail<br />
broadening of the packet. This is a highly significant result<br />
that illustrates the power of the dendrimer concept to independently<br />
modify material parameters for individual applications.<br />
It can be seen from Fig. 5�b� that transients from<br />
G1pIr at different fields also scale by transit time, however<br />
the tails to these transients are much broader than those seen<br />
for G1mIr. This may indicate a greater level of disorder to<br />
the transport in this material. This can be seen more clearly<br />
when compared to the meta-linked dendrimers in Fig. 6.<br />
The electric-field dependence plotted in the Poole–<br />
Frenkel formalism 23 of the mobility versus E 1/2 is shown in<br />
Fig. 7�a� for all three dendrimer structures. It can be seen that<br />
all three materials give a good fit to this functional form. The<br />
generation number and core configuration allow a fine control<br />
of the mobility. G1pIr displays a zero-field mobility<br />
(� 0) �calculated by extrapolating the fit to the data to zero<br />
field�, of1.9�10 �6 cm 2 /V s. G1mIr has � 0 a factor of 2<br />
lower at 9.7�10 �7 cm 2 /V s. G2mIr is three times lower still<br />
with � 0 of 3.5�10 �7 cm 2 /V s. However, the field dependence<br />
of the three systems is different. Despite having the<br />
highest zero-field mobility, the field dependence of G1pIr is<br />
much lower than G1mIr, resulting in mobility values that<br />
cross at higher fields. G2mIr has a slightly lower field dependence<br />
than G1mIr. Yu et al. 24,25 have used a geometrical<br />
model to explain the origins of field dependence in conjugated<br />
materials. Lower field dependence �correlating with an<br />
increased value of E 0) is borne of a more rigid geometrical<br />
FIG. 7. �a� Poole–Frenkel plot of mobility vs E 1/2 for all three dendrimer<br />
structures. �b� Mobility vs E for all three dendrimer structures. The lines<br />
depict fits to I – V data �discussed in Sec. III C�.<br />
structure, which allows less perturbation to the morphology<br />
under an applied field. The field dependence of the mobility<br />
versus E is plotted in Fig. 7�b�. The line fits are from a<br />
computational model that will be discussed in Sec. III B.<br />
B. Device modeling<br />
As the charge transport measurements using CGL-TOF<br />
are made on films of thickness relevant to LEDs, it is instructive<br />
to compare those results with measurements of current–<br />
voltage (I – V) characteristics made on actual devices. Conjugated<br />
dendrimers provide excellent systems upon which to<br />
study microscopic charge transport properties due to the ability<br />
to fine tune the mobility and the independence of morphology<br />
on preparation conditions. This can be used to test<br />
device models of OLEDs. 26–31 A widely used model has<br />
been developed by Davids et al. 28 This model is different<br />
from the Gaussian disorder model, however, we use it here<br />
because it is simpler and still offers useful insight. A combined<br />
treatment of thermionic emission over a barrier and<br />
tunneling is used to describe the injection of charge into the<br />
organic material, which yields the charge-carrier density at<br />
the electrode. The coupled Poisson, drift, and continuity<br />
equations are then solved through the device using a Poole–<br />
Frenkel field dependent mobility of the form shown in<br />
Eq. �7�<br />
5
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6 J. Appl. Phys., Vol. 95, No. 2, 15 January 2004 Markham et al.<br />
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FIG. 8. I – V curves and model fits for G1pIr, G1mIr, and G2mIr hole only<br />
devices.<br />
��E��� 0e�E/E 0, �7�<br />
where �0 and E0 are the field independent mobility and field<br />
dependence of the mobility, respectively. In our implementation,<br />
the model is simplified by not accounting for the diffusion<br />
of carriers, and we have previously shown good quality<br />
fits using this model on both a conjugated polymer 29 and<br />
fluorescent conjugated dendrimers. 12 Mobility parameters<br />
determined in both of these cases were found to agree with<br />
experimental measurements using TOF at room<br />
temperature. 27 This model has also been used to calculate the<br />
temperature dependence of OLEDs as well as the effect of<br />
oxidation on the mobility values. 29 It has been shown previously<br />
that injection occurs into the core of the dendrimer 12<br />
and thus the barrier to injection is constant for all<br />
generations.<br />
In order to study hole transport in LEDs, we have prepared<br />
hole only devices of neat films of each of the three<br />
dendrimers with ITO and gold electrodes. A positive voltage<br />
was applied to the ITO and the measured current must be due<br />
to holes as the barrier to electron injection is around 2.5 eV<br />
from the gold electrode. Figure 8 shows the I – V characteristics<br />
of 100 nm thick devices. It can be seen that for fields<br />
above 0.8 MV/cm, a given current density requires higher<br />
fields in the sequence G1mIr, G1pIr, G2mIr. The lines indicate<br />
the model fits to the data using the barrier height and<br />
the material parameters �0 and E0 as the fitting parameters.<br />
The fits to the data qualitatively follow the experimental I – V<br />
curves. At lower fields, there are some deviations and this is<br />
believed to be due to the influence of space charge on the<br />
injection, which is not incorporated into the model. 12<br />
The mobility parameters obtained by the model for Eq.<br />
�7� agree very well with the CGL-TOF data. The fits are<br />
shown as the solid lines in Fig. 7�b�, and the mobility measured<br />
by CGL-TOF as point data. This suggests that the<br />
model can provide useful information on charge transport in<br />
dendrimer LEDs. The mobility parameters obtained are<br />
shown in Table I. The barrier height is essentially similar for<br />
all three materials, at �0.47 eV. ITO is reported to have a<br />
work function in the region of 4.8–5.2 eV, 32 depending upon<br />
TABLE I. Fitting parameters for modelling the I–V characteristics of hole<br />
only devices.<br />
Material<br />
Barrier height<br />
�eV� � 0 �cm 2 /V s� E 0 �V/cm�<br />
G1mIr 0.47 9.3�10 �7 8.7�10 4<br />
G1pIr 0.47 2.0�10 �6 1.5�10 5<br />
G2mIr 0.48 3.5�10 �7 1.1�10 5<br />
the exact treatment conditions. The highest occupied molecular<br />
orbital �HOMO� levels for all three dendrimers have been<br />
determined from cyclic voltammetry measurements to be<br />
close to 5.6 eV. 11 Hence, the barrier deduced is consistent<br />
with plasma treated ITO and the measured dendrimer<br />
HOMO.<br />
The EL spectra of the three dendrimers are shown in Fig.<br />
9. Systematic studies of the effect of conjugation on iridium<br />
complexes have also shown that substitution onto the pyridine<br />
ring affects the position of the lowest unoccupied molecular<br />
orbital �LUMO�, while the HOMO level has largely<br />
metal character. 33 Thus, one possible explanation as to the<br />
origin of the 17 nm redshift for G1pIr may be explained by<br />
the lowering of the LUMO level by 0.08 eV, with the HOMO<br />
unaffected. The difference in I – V characteristics is then attributable<br />
to the morphology of the samples and the effects<br />
of the dendron substitution to the core. The values for � 0<br />
obtained using the model are in very good agreement with<br />
those measured by the Poole–Frenkel fits to the CGL-TOF<br />
data. The model also correctly shows the reduced field dependence<br />
of G1pIr.<br />
C. Controlling device performance<br />
by chemical structure<br />
The three materials studied here allow subtle control of<br />
charge transport without major changes to the energy levels<br />
of the system. We achieve this by use of a single molecule<br />
and so avoid the potential risk of phase separation that is<br />
common in blend systems. Single layer devices were fabricated<br />
with 120 nm of neat dendrimer between plasma etched<br />
ITO and calcium/aluminum electrodes. The EQEs are shown<br />
FIG. 9. EL spectra of each of the dendrimers under study.
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J. Appl. Phys., Vol. 95, No. 2, 15 January 2004 Markham et al.<br />
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FIG. 10. EQE vs current density for G1pIr, G1mIr, and G2mIr single layer<br />
devices with Ca/Al cathodes.<br />
in Fig. 10, in both semi-logarithmic �Fig. 10�a�� and double<br />
logarithmic plots �Fig. 10�b��. It can be seen that the quantum<br />
efficiency of G2mIr is the highest, while G1mIr is<br />
greater than G1pIr at low current densities but not at higher<br />
ones. It appears, therefore, that for a single layer device, the<br />
lower the mobility of the carriers, the higher the quantum<br />
efficiency. A single layer device has no confinement mechanism<br />
for charge carriers thus it is logical to reason that the<br />
lower mobility of holes gives them more opportunity to recombine<br />
with electrons before they reach the cathode. This<br />
has been shown by Blom and co-workers 34 on a family of<br />
PPV derivatives, where the increased quantum efficiency of<br />
the lower mobility materials is attributed to a reduction in<br />
quenching from the cathode.<br />
It can also be seen from Fig. 10�b� that for both firstgeneration<br />
dendrimers, the EQE does not saturate with current<br />
density. This continual increase implies that the chargecarrier<br />
balance improves with increasing field. Thus, not only<br />
is the effect of increased generation to slow the mobility of<br />
the majority carriers, but also to aid carrier balance. This has<br />
been observed previously in a family of fluorescent dendrimers,<br />
and the increase in efficiency with generation was attributed<br />
to the improved balance of carrier transport, allowing<br />
relatively mobile holes more opportunity to meet with<br />
trapped electrons. 35<br />
FIG. 11. EQE and power efficiency for dendrimer bilayer devices with<br />
TPBI.<br />
In order to improve the efficiency of OLEDs a heterostructure<br />
of hole and electron transporting materials is often<br />
used. 1 In addition to the greater charge carrier confinement<br />
afforded by such a structure, a build up of charge at the<br />
interface between the two materials leads to a feedback<br />
mechanism allowing easier injection of carriers into the device.<br />
Thus, we fabricated bilayer devices from an 80 nm hole<br />
transporting layer of neat dendrimer and 50 nm of electron<br />
transporting material, TPBI. A LiF/Al cathode completed the<br />
device in this case. The EQEs and power efficiencies of these<br />
devices are shown in Fig. 11. Again, it can be seen that the<br />
dendrimer generation has a large effect on the performance<br />
of these devices. G2mIr shows double the EQE and power<br />
efficiency of the G1 devices. However, in this instance, the<br />
difference between the two first generation dendrimers is<br />
smaller. Excellent power efficiencies of 20 lm/W for neat<br />
G2mIr-based bilayers and 10 lm/W for neat G1mIr-based<br />
devices render these high efficiencies for solution processed<br />
OLEDs with neat emissive layers. This shows that the ability<br />
to fine tune the charge transport by dendrimer generation and<br />
structure is a useful tool for making balanced and efficient<br />
devices.<br />
IV. CONCLUSION<br />
We have reported a detailed study of charge transport in<br />
electrophosphorescent organic semiconductors. The materials<br />
studied here are dendrimers closely related to fac tris�2phenylpyridine�<br />
iridium �Ir(ppy) 3� which is widely used in<br />
OLEDs. We find that we can fine tune the mobility by den-<br />
7
PROOF COPY 056402JAP<br />
8 J. Appl. Phys., Vol. 95, No. 2, 15 January 2004 Markham et al.<br />
drimer generation, and that the transport dynamics can be<br />
related to the structural configuration of the dendrimers. In<br />
addition, we have investigated both the field and the temperature<br />
dependence of these unique materials. The TOF indicates<br />
that energetic disorder can account for the general<br />
transport properties in these materials. A � of 100 meV corresponds<br />
well with that measured for some molecularly<br />
doped polymers and some of the more disordered conjugated<br />
polymers. The transport is weakly dispersive at room temperature,<br />
and a transition to nondispersive transport is seen at<br />
elevated temperatures. A measured transition temperature Tc of 293 K is much higher than that measured for conjugated<br />
polymers. This may be due to specific properties of the metal<br />
PROOF COPY 056402JAP<br />
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complex core or disorder introduced by the morphological<br />
properties of the dendrimer. We show that by increasing the<br />
generation of the dendrimer, we simply lower the mobility<br />
and do not increase the level of disorder in the film, as reflected<br />
in identical levels of tail broadening for the first- and<br />
second-generation material. By increasing the generation, we<br />
are able to improve the balance of charge carriers and are<br />
able to create efficient single layer LEDs. In addition, even<br />
higher efficiencies can be obtained in a bilayer structure,<br />
with the dendrimer architecture having a large impact on the<br />
performance of the device.<br />
ACKNOWLEDGMENTS<br />
The authors thank Dr. J. M. Lupton for supplying the<br />
charge transport model and for helpful advice. They are very<br />
grateful to Professor K. Müllen for providing the perylene<br />
derivative. The authors acknowledge financial support from<br />
EPSRC, SHEFC, CDT Oxford Ltd, the EU project LAMI-<br />
NATE, and the Fond der Chemischen Industrie. One of the<br />
authors �I. D. W. S.� is a Royal Society University Research<br />
Fellow.<br />
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Abstract<br />
A molecular device based on light controlled charge<br />
carrier mobility<br />
Stanislav Nesˇpu˚rek a,b , Juliusz Sworakowski c,* , Catherine Combellas d ,<br />
Geng Wang a , Martin Weiter b<br />
a Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2,<br />
162 06 Prague 6, Czech Republic<br />
b Technical University of Brno, Purkyòova 118, 612 00 Brno, Czech Republic<br />
c Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology,<br />
Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland<br />
d Environnement et Chimie Analytique, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France<br />
Available online 25 June 2004<br />
Electrical properties of binary samples containing photoconducting poly[methyl(phenyl)silylene] with admixed photochromic<br />
spiropyran were investigated. A reversible chemical reaction of the photochromic system, driven by light and producing<br />
highly polar metastable species, manifests itself in a reversible modification of dark currents and photocurrents. The behaviour<br />
of the samples is explained assuming a reversible production and annihilation of dipolar traps associated with the polar<br />
molecules produced in the photochromic process. Use of similar materials as elements of molecular electroactive systems is<br />
postulated.<br />
# 2004 Published by Elsevier B.V.<br />
PACS: 72.20.Jv; 72.80.Le; 73.50.Gr<br />
Keywords: Electrical properties; Photoconductivity; Photochromism; Poly(silylene); Spiropyran<br />
1. Introduction<br />
Applied Surface Science 234 (2004) 395–402<br />
A characteristic feature of molecular materials is<br />
their almost infinite susceptibility to modification of<br />
their physical properties due to minor modifications<br />
of their chemical structures. Among the materials of<br />
interest, multistable molecular systems have recently<br />
*<br />
Corresponding author. Tel.: þ48 71 3203468;<br />
fax: þ48 71 3203364.<br />
E-mail addresses: nespurek@imc.cas.cz (S. Nesˇpu˚rek),<br />
sworakowski@pwr.wroc.pl (J. Sworakowski),<br />
catherine.combellas@espci.fr (C. Combellas).<br />
0169-4332/$ – see front matter # 2004 Published by Elsevier B.V.<br />
doi:10.1016/j.apsusc.2004.05.056<br />
attracted much attention because of emerging prospects<br />
of their application. On the microscopic scale,<br />
their use as elements of logic in molecular-scale<br />
devices was first envisaged by Carter [1].<br />
An example of bistable molecular systems are<br />
photochromic molecules undergoing reversible<br />
changes of the absorption spectra when exposed to<br />
certain types of irradiation and reverting to their<br />
original colours when stored in the dark or irradiated<br />
light of a different wavelength (see, e.g., [2–4]). The<br />
simplest detection of a photochromic reaction is the<br />
optical one as temporal changes in the optical absorption<br />
or reflection can be measured in a straightforward
396 S. Nesˇpu˚rek et al. / Applied Surface Science 234 (2004) 395–402<br />
way. In this way many optical devices can be constructed,<br />
e.g. radiation flux modulators and controllers,<br />
holographic and image-forming media, optical<br />
storages and data display systems. However, in some<br />
devices an electrical response is more desirable; in this<br />
case use of opto-electronic transducers is necessary,<br />
making any device more complex.<br />
An alternative approach is to develop a material<br />
whose electrical properties would be modified in a<br />
controlled way by incident light. Such a material could<br />
be used as a kind of simple and direct ‘opto-electronic<br />
transducer’, on both, micro- and macroscopic scale. In<br />
our earlier papers [5–7], a concept of an electroactive<br />
molecular material has been put forward, whose electrical<br />
properties would be controlled by optical switching<br />
of photochromic species, either admixed into the<br />
polymer matrix or chemically attached to the polymer<br />
chain. The earlier papers [5–7] were focused on properties<br />
of an isolated polymer chain and its potential use as<br />
a molecular switch. In this paper we concentrate on<br />
electrical properties of macroscopic samples built of<br />
such molecules, presenting a direct method to transfer<br />
the photochromic reaction into an electrical response.<br />
2. Theoretical background<br />
2.1. Photochromism<br />
A photochromic process can be schematically<br />
described as a photoreversible reaction which, in its<br />
simplest form, can be written as:<br />
SP @ hn1<br />
hn2;kT M<br />
In general, hn1, the activating radiation (typically from<br />
the UV and/or visible regions), causes a chemically<br />
stable molecule SP (absorbing at a wavelength<br />
l1 ¼ c=n1) to convert to a product M (absorbing at<br />
l2 ¼ c=n2). The product M is thermodynamically less<br />
stable and can revert to the original form SP via a<br />
thermal excitation or upon photoexcitation with l2. In<br />
most systems, the photochromic cycle is usually a<br />
multi-stage process involving several elementary processes<br />
depending on the nature of the system (see, e.g.,<br />
[2–4]).<br />
For the purpose of this paper, photochromic systems<br />
of interest should be selected so as to ensure an<br />
important difference between the dipole moments 1<br />
of the stable and metastable forms or between their<br />
ionisation energies (see the following sections). Spirooxazines<br />
[8,9] and spiropyrans [10,11] (whose dipole<br />
moments are higher in their metastable forms), and<br />
aminoazobenzenes [12] (whose dipole moments are<br />
lower in the metastable forms), can be mentioned in<br />
this context. In the following, we shall focus on<br />
systems containing spiropyran as a photochromic<br />
component. The dipole moments of stable forms of<br />
spiro-molecules amount typically to 2–5 D depending<br />
on substituents attached to their backbones, whereas<br />
the moments of respective metastable forms may<br />
exceed 10 D.<br />
2.2. Transport and trapping of charge carriers<br />
The equation relating the dark current or photocurrent<br />
(j) flowing in a plane-parallel macroscopic<br />
(isotropic) sample can be written as<br />
j ¼ enmFA (1)<br />
where e is the unit charge, n the free charge carrier<br />
concentration, m the effective charge carrier mobility,<br />
F the electric field and A the electroded area of the<br />
sample. Upon changing the concentration of charge<br />
carrier traps one can modify the nm product. Similar<br />
changes can also be seen in the photoconductivity:<br />
illumination of a photoconducting sample results in an<br />
increase of the carrier concentration to n ph , where<br />
nph ¼ gt is the concentration of free carriers generated<br />
by light (g is the number of carriers produced by light<br />
and t the lifetime of charge carriers). The formation of<br />
traps modifies the charge carrier range tmF, and hence<br />
the photocurrent.<br />
Important for a proper design of electroactive molecular<br />
materials is a question concerning the nature of<br />
local centres (traps) capable of localising charge<br />
carriers. Lyons [13,14] demonstrated that in perfect<br />
molecular crystals, the energies of bands for excess<br />
charge carriers, measured with respect to the vacuum<br />
level, are determined by ionisation energies and electron<br />
affinities of constituent molecules, and by the<br />
energy of electrostatic interactions of a carrier<br />
(momentally residing on a given molecule) with elec-<br />
1 Dipole moments will be expressed in Debye units (D)<br />
throughout this paper. 1 D ¼ 3.33 10 30 Cm.
trically neutral surrounding molecules. The latter<br />
parameter is referred to as the polarisation energy<br />
P. Traps for charge carriers occur on sites where,<br />
locally, the molecular ionisation energy or electron<br />
affinity, and/or polarisation energy are different from<br />
the respective values characterising the perfect solid<br />
[15,16].<br />
A particular type of traps may occur if guest molecules<br />
possess a permanent dipole moment. These<br />
molecules may themselves act as chemical traps if<br />
their energy levels fulfil at least one of the relations<br />
mentioned above. Moreover, independently of the<br />
position of the energy levels of the polar dopant, its<br />
dipole moment contributes to the field acting on<br />
surrounding molecules and modifies the local values<br />
of the polarisation energy. Thus the presence of a polar<br />
molecule in a non-polar environment results in producing<br />
local states (dipolar traps) on neighbouring host<br />
molecules, even though the impurity itself does not<br />
necessarily form a chemical trap. Calculations [17,18]<br />
demonstrate, in agreement with results of experiments<br />
performed by several groups [19–24], that parameters<br />
of such traps (their depths, cross-sections, etc.) depend<br />
on the dipole moment of the dopant, as well as on the<br />
concentration and mutual orientation of the dipoles.<br />
The depths of dipolar traps for strongly polar guest<br />
molecules (the dipole moment larger than 10 D) may<br />
exceed 0.5 eV.<br />
The trap-controlled mobilities (m) are reduced with<br />
respect to those measured in perfect solids (m0)bya factor Y expressing the free-to-total charge density<br />
ratio:<br />
m<br />
¼ Y ¼<br />
m0 n<br />
(2)<br />
n þ nt<br />
where nt stands for the density of trapped carriers. In<br />
the simplest case of a nearly perfect solid containing<br />
one set of discrete traps [25] the parameter Y amounts<br />
to<br />
Y ¼ 1 þ M<br />
exp<br />
Nc<br />
Et<br />
(3)<br />
kBT<br />
where Nc is the effective density of transport states, M<br />
the concentration of traps and Et their depth. Note that<br />
the decrease of the mobility is equivalent to a decrease<br />
of the current flowing in the sample.<br />
In disordered solids, apart from the modification of<br />
the trap depths (cf., e.g., [26]), one should also expect<br />
1<br />
S. Nesˇpu˚rek et al. / Applied Surface Science 234 (2004) 395–402 397<br />
a modification of the shapes of the density-of-states<br />
(DOS) functions. The importance of the latter effect<br />
can be estimated basing on the model put forward by<br />
Bässler [27,28]. The model, confirmed by results of<br />
numerous experiments, predicts that the mobilities<br />
should exhibit non-Arrhenius temperature dependences,<br />
their magnitudes decreasing with increasing<br />
widths of the DOS functions. The latter parameter is<br />
related to the concentration and the dipole moments of<br />
the dopant [20–22].<br />
The above discussion demonstrates that the presence<br />
of dipolar species in molecular solids should<br />
result in a reduction of the effective mobilities of<br />
charge carriers due to trap formation and/or broadening<br />
of the distribution of hopping states. The trends<br />
in both cases are similar, though their relative importance<br />
depends on the parameters of the material and<br />
polar additives.<br />
3. Experimental<br />
Poly[methyl(phenyl)silylene] (PMPSi) ((a) in<br />
Scheme 1) was prepared by sodium-mediated Wurtz<br />
coupling polymerisation [29,30] (Mw ¼ 4<br />
10 4 g mol 1 ). Metal-free phthalocyanine (H2Pc) ((b)<br />
in Scheme 1) was obtained from Tokyo Kasei and<br />
purified by sublimation in a temperature gradient.<br />
Photochromic spiropyran, 6-nitro-1 0 ,3 0 ,3 0 -trimethylspiro[2H-1-bezopyran-2,2<br />
0 -indoline] (SP), ((c) in<br />
Scheme 1) was obtained from Aldrich Co. and used<br />
without further purification.<br />
Films of PMPSi with admixed SP (PMPSi-SP) for<br />
electrical measurements were prepared by spin coating<br />
a toluene solution on conductive ITO glasses or<br />
glass substrates. The thicknesses of the films ranged<br />
between 500 nm and 1 mm. The concentration of<br />
admixed SP in the samples used in the experiments<br />
reported in this paper amounted to 5 wt.%. After<br />
deposition, the films were dried at 0.1 Pa and 330 K<br />
for at least 4 h. Top Al electrodes, 40–60 nm thick, and<br />
in some cases H2Pc films, were deposited by vacuum<br />
evaporation.<br />
Current–voltage (j–U) characteristics were measured<br />
in a sandwich configuration using a Keithley<br />
6175A electrometer. For photoconductivity measurements,<br />
the samples were irradiated with a quartz<br />
tungsten halogen lamp equipped with a monochroma-
398 S. Nesˇpu˚rek et al. / Applied Surface Science 234 (2004) 395–402<br />
tor. The reaction converting SP into its coloured form,<br />
merocyanine (M), was activated using a Hg mercury<br />
discharge lamp HBO-200 with water and band (340<br />
20 nm) filters. The measurements were performed at<br />
room temperature.<br />
4. Results<br />
The spectra of molecules constituting the system<br />
studied in this work are shown in Fig. 1. The spectrum<br />
of SP in its stable (non-coloured) form contains four<br />
bands in the UV region peaking at 345, 299, 275 and<br />
240 nm. Illumination of spiropyran with 340 nm light<br />
results in a shift of the 345 nm peak to 375 nm, and in<br />
the appearance of a new band with the maximum at<br />
590 nm. This change is fully reversible: our experiments<br />
showed that the band in the visible region could<br />
be bleached thermally in ca. 80% within ca. 1 h at<br />
room temperature, and completely in ca. 24 h.<br />
In the spectral range covered by our measurements,<br />
the spectrum of neat PMPSi consists of two bands<br />
Scheme 1. Chemical formulae of the materials used in this work.<br />
peaking at 332 and 270 nm. Unfortunately, the positions<br />
of the bands of SP and PMPSi almost coincide,<br />
hence one cannot unequivocally separate effects due<br />
to the photoexcitation of charge carriers in the polymer,<br />
and those associated with the SP ! M photochromic<br />
reaction, as will be shown later.<br />
Fig. 1. Absorption spectra of the materials used in this work. Dashdotted<br />
line: PMPSi in toluene solution; solid line: stable form of SP<br />
(before UV illumination); dashed line: SP after the UV illumination.
Fig. 2. Dark current–voltage characteristics of an ITO/PMPSi-SP/<br />
Al sandwich sample before and after UV illumination. Curve 1—<br />
pristine sample, curve 2—measured immediately after UV<br />
illumination, curves 3 and 4—measured after 3 and 24 h,<br />
respectively, relaxation in vacuum, at room temperature.<br />
Fig. 2 shows a family of dark j–U characteristics<br />
measured on a 600 nm thick sandwich sample ITO/<br />
PMPSi-SP/Al. The characteristics measured on a pristine<br />
sample (curve 1) was ohmic up to 2 V turning to a<br />
superlinear dependence at higher voltages. After the<br />
sample was illuminated for 10 min with a 340 nm<br />
radiation activating the photochromic reaction and<br />
resulting in the formation of a highly polar metastable<br />
form (M) of the photochromic system, the j–U characteristics<br />
(curve 2) exhibited a similar character but<br />
S. Nesˇpu˚rek et al. / Applied Surface Science 234 (2004) 395–402 399<br />
the current was ca. one order of magnitude lower.<br />
When the sample was kept in the dark under vacuum at<br />
room temperature, the current increased (curves 3 and<br />
4), asymptotically reverting to the values measured<br />
before the illumination. It should be noted that this<br />
electrical relaxation qualitatively followed the optical<br />
one.<br />
The results of measurements of the rise and decay of<br />
the photocurrent, carried out on surface samples Al/<br />
PMPSi-SP/Al are shown in Fig. 3a. After the illumination<br />
of the sample with a low-intensity 340 nm<br />
radiation (f ¼ 10 4 Wcm 2 ), a photocurrent was<br />
observed, attaining its steady state value after a few<br />
minutes (curve 1). The photocurrent was found to<br />
increase with the light intensity (curve 2, f ¼ 4<br />
10 4 Wcm 2 ). The photocurrent was also generated<br />
upon illumination of the sample with a more intense<br />
UV light used for the photochromic SP ! M transformation<br />
(curve 3, f ¼ 3 10 2 Wcm 2 ). In this<br />
case, however, the photocurrent kinetics was quite<br />
different: after an abrupt initial rise, the photocurrent<br />
decreased with time, as is shown in the inset to Fig. 3a.<br />
A repeated experiment, performed on the same sample<br />
after formation of the coloured M species, is shown in<br />
the right part of Fig. 3a: the currents 1 0 and 2 0 are ca. 5–<br />
10 times lower than 1 and 2, measured under the same<br />
conditions. A discussion of the results obtained, based<br />
on a simple model, will be given in the following<br />
section of this paper.<br />
Fig. 3. (a) Photocurrent kinetics measured on a surface-type sample Al/PMPSi-SP/Al, in vacuum. Inset: curve 3 re-plotted in double log<br />
coordinates. (b) temporal variation of the parameter Y (free-to-total carrier density ratio) calculated from Eqs. (3) and (6). The curves have<br />
been normalised at t ¼ 0 (i.e., at Y ¼ Y0). The parameter g is proportional to the intensity of absorbed light. Other input parameters are: M0/<br />
N c ¼ 10 5 ; M 0/S ¼ 10 3 ; E t ¼ 0.6 eV; T ¼ 293 K.
400 S. Nesˇpu˚rek et al. / Applied Surface Science 234 (2004) 395–402<br />
Since the optical absorption of SP in the UValmost<br />
coincides with that of PMPSi, the 340 nm light used<br />
in our experiments excites both, PMPSi and SP,<br />
resulting simultaneously in the photocarrier generation<br />
and in the dipolar trap formation. To separate the<br />
effects, and also to test the long-range effect of<br />
charge–dipole interactions, similar photoconductivity<br />
measurements were performed on a surface-type<br />
sample containing a H2Pc film evaporated onto the<br />
PMPSi-SP film (Fig. 4, illumination through the<br />
substrate whose cutoff wavelength was 300 nm, the<br />
geometry is shown in the inset). The samples were<br />
illuminated with two wavelengths: 340 (curve 2) and<br />
625 nm (curves 1 and 3). The latter wavelength is<br />
known to excite the photoconductivity in H2Pc but<br />
not in PMPSi. Moreover, the 625 nm light does not<br />
activate the SP ! M reaction. It is thus obvious that<br />
the photocurrent could be generated only in the H 2Pc<br />
layer. After the irradiation of the PMPSi-SP layer<br />
with UV, the repeated illumination with the 625 nm<br />
light yielded the photocurrent about twice lower that<br />
that measured under identical conditions prior to<br />
exposition of the polymer to UV (curve 3 versus<br />
curve 1 in Fig. 4).<br />
5. Discussion<br />
Fig. 4. Photocurrent kinetics in a two-layer sample, measured in vacuum. Inset: structure of the sample.<br />
The variation of the electrical responses of the<br />
samples under study, shown in Figs. 3 and 4, is<br />
consistent with the assumption of traps being generated<br />
and annihilated upon creation and annihilation of<br />
polar species, and can be semi-quantitatively discussed<br />
on the basis of the model presented below<br />
[31]. We shall consider a system consisting of a<br />
photoconducting polymer in which traps affecting<br />
the mobility of charge carriers are formed by polar<br />
species M, formed upon the photochromic reaction<br />
from weakly polar molecules SP. The respective concentrations<br />
of both forms of the system will be denoted<br />
as M and S. The inspection of the curves 1 and 2 in<br />
Fig. 3a shows that, at low light intensities, the photocurrents<br />
increase with time attaining their equilibrium<br />
values after times of the order of 10 1 to 10 2 s, whereas<br />
the curve 3 decreases after the initial sharp rise, at a<br />
much slower rate (cf. the inset to Fig. 3a). This feature<br />
makes us suppose that the temporal evolution of the<br />
curves is a superposition of a faster kinetics of transport<br />
processes and of a slower kinetics of the chemical<br />
process changing the concentration of traps. Under<br />
this supposition, kinetic equations describing the temporal<br />
evolution of the concentrations of free and<br />
trapped charges can be readily solved assuming that<br />
the chemical reaction producing or annihilating M is<br />
slow with respect to the kinetics of the trapping–<br />
detrapping process (i.e., assuming M ¼ const.). The<br />
solution yields<br />
nðtÞ ¼AIp þðB CIpÞ exp<br />
t<br />
teff<br />
(4)
where Ip is the intensity of light exciting the photoconductivity<br />
in the sample, A, B and C are parameters<br />
independent of the light intensity, and teff a combination<br />
of the trapping, detrapping and recombination<br />
time constants. In the long-time limit, the concentrations<br />
of trapped and free carriers attain their equilibrium<br />
values, reducing the (concentration mobility)<br />
product by the factor Y, defined in Eq. (2). Note that<br />
n(1) should be proportional to the intensity of the<br />
exciting light.<br />
The above equation was derived assuming a constant<br />
concentration of traps. If, as is the case of our<br />
samples, the concentration slowly changes, then the<br />
kinetics of the chemical reaction changing the trap<br />
concentration can be decoupled from the trapping–<br />
detrapping process. It is then sufficient to derive an<br />
expression for the temporal evolution of M and use it<br />
in Eq. (3). The latter expression can be found upon<br />
solving the kinetic equation for the concentrations of<br />
the reacting components of the photochromic system:<br />
dM<br />
dt ¼ kM þ bIchðS0 MÞ (5)<br />
where k is the rate constant of the dark reaction M !<br />
SP, Ich the intensity of the incident light driving the<br />
photochemical reaction SP ! M (note that Ich may,<br />
but does not necessarily have to, be equal to Ip), b<br />
stands for the efficiency of the photochemical reaction,<br />
and the subscript 0 labels the concentrations at<br />
t ¼ 0. The solution of Eq. (5) yields<br />
M ¼ M0 expð kefftÞþ S0g expðkefftÞ 1<br />
(6)<br />
g þ 1 expðkefftÞ<br />
where g ¼ bIch=k and keff ¼ kð1 þ gÞ. It is straightforward<br />
to show that in most cases the solution reduces to<br />
either an exp( kefft)-type dependence or to a<br />
[1 expð kefftÞ]-type one.<br />
Making use of Eqs. (3) and (6), we have calculated<br />
the temporal evolution of the parameter Y for a few<br />
physically plausible cases. The results, shown in<br />
Fig. 3b, reasonably well reproduce the course of the<br />
curves 1–3 shown in Fig. 3a. In the dark and for low<br />
light intensities (i.e., for small g), the dark reaction<br />
prevails. Consequently, the trap concentration<br />
decreases and the factor Y increases. Under a more<br />
intense illumination (i.e., for larger g), strongly polar<br />
metastable species are produced, hence the trap concentration<br />
increases, reducing Y.<br />
S. Nesˇpu˚rek et al. / Applied Surface Science 234 (2004) 395–402 401<br />
The results shown in Figs. 2 and 3 provide an<br />
evidence for the production of traps during illumination<br />
of the polymer samples with the UV light. The<br />
above experiments, however, do not determine<br />
whether these traps are of dipolar nature since it is<br />
equally conceivable that the metastable M molecules<br />
might act as chemical traps. The discrimination can be<br />
sought in differences of the cross sections of the traps<br />
produced: the chemical traps should occur on the M<br />
molecules, their cross sections being of the order of<br />
molecular dimensions [16] whereas the dipolar traps<br />
are created in the vicinity of the dipoles, at distances<br />
determined by long range charge–dipole interactions.<br />
The results shown in Fig. 4 demonstrate that the SP !<br />
M reaction occurring in the polymer layer influences<br />
the transport of charge carriers as far as in the phthalocyanine<br />
film. After the irradiation of the sample with<br />
the 340 nm light, the observed photocurrent is generated<br />
mainly in the PMPSi-SP layer because of a short<br />
penetration depth of light at this wavelength. Simultaneously,<br />
the light creates highly polar metastable M<br />
species, which act as a source of charge carrier traps.<br />
Consequently, the illumination of the sandwich sample<br />
with the UV radiation results in a decrease of the<br />
photocurrent following the kinetics of generation of<br />
traps (Section 2 of the curve). The lifetime of the<br />
dipolar traps is of the order of several hours in the dark<br />
or under illumination with the 625 nm light, hence the<br />
increased concentration of metastable species is also<br />
reflected in a decrease of the photoconductivity of the<br />
pre-irradiated H2Pc film (cf. Sections 1 and 3 in the<br />
figure) upon illumination with the 625 nm light. This<br />
behaviour shows that the traps, influencing not only<br />
the current in the PMPSi-SP layer but also the current<br />
of the neighbour layer, have been created due to longrange<br />
charge–dipole interactions.<br />
6. Final remarks<br />
The results reported in this paper demonstrate a<br />
possibility of a reversible formation and annihilation<br />
of charge carrier traps in molecular materials consisting<br />
of photoconducting polymer containing photochromic<br />
species. Both the dark conductivity and<br />
photoconductivity of PMPSi-SP are reversibly modified<br />
due to the formation and annihilation of dipolar<br />
traps The experiments support the results of the
402 S. Nesˇpu˚rek et al. / Applied Surface Science 234 (2004) 395–402<br />
electrostatic model calculations published elsewhere<br />
[17,18]. Dipolar traps have large cross-sections as<br />
follows from the photoconductivity measured on a<br />
double-layer PMPSi-SP/H 2Pc structure.<br />
It is expected that materials consisting of electroactive<br />
matrices functionalized with suitably chosen<br />
photochromic species (admixed molecules or chemically<br />
attached side groups) may be used as elements of<br />
electro-optical systems or bistable switches. Moreover,<br />
one can envisage a construction of a photo-<br />
FET device in which the gate field is modified by<br />
dipolar fields associated with centres existing in a<br />
photoactive polymer layer.<br />
The stability of the metastable form and the kinetics<br />
of response of the materials should depend on the<br />
nature of the photochromic groups used [2–4] but also<br />
on the polymer matrix, intensity of the exciting light<br />
and other experimental factors.<br />
Finally, it should be noted that the aim of this<br />
contribution was to put forward a general idea allowing<br />
one to design a bistable molecular device capable<br />
of acting on a both, microscopic and macroscopic<br />
scale. A practical realisation of this idea will require a<br />
thorough optimisation of several operational parameters<br />
of the device.<br />
Acknowledgements<br />
The research was supported by the grant OC D14.30<br />
from the Ministry of Education, Youth and Sports of<br />
the Czech Republic, by the grant AV0Z 4050913 from<br />
the Academy of Sciences of the Czech Republic, and<br />
by the grant 4 T09A 132 22 from the Polish Committee<br />
for Scientific Research. The financial support from<br />
European Graduate College ‘‘Advanced Polymer<br />
Materials’’ is gratefully appreciated.<br />
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Abstract<br />
Synthetic Metals 141 (2004) 165–170<br />
Transient photoconductivity in a thin film of a<br />
poly-phenylenevinylene-type conjugated polymer<br />
M. Weiter a,b,∗ , V.I. Arkhipov c,d , H. Bässler a<br />
a Institute of Physical, Macromolecular and Nuclear Chemistry, Philipps University, Hans-Meerwein-Strasse, D-35032 Marburg, Germany<br />
b Faculty of Chemistry, Brno University of Technology, Purkynova 118, 61200 Brno, Czech Republic<br />
c IMEC, Kapeldreef 75, B-3001 Heverlee-Leuven, Belgium<br />
d Institute of Material Science, Darmstadt University of Technology, Petersenstrasse 23, 64281 Darmstadt, Germany<br />
Received 6 August 2003; received in revised form 25 September 2003; accepted 25 September 2003<br />
Dedicated to the memory of Dr. Michael Rice<br />
Transient photocurrents in a 150 nm thick film of a copolymer of a phenyl-substituted poly-phenylenevinylene were observed upon<br />
excitation by a 10 ns laser flash. The signal is due to a superposition of time-dependent generation of mobile holes and their motion. The<br />
essential quantity considered was the number of holes transported in either the neat sample or a film doped with trinitrofluorene as a function<br />
of electric field, temperature, photon dose at different photon energies, and polarity of the irradiated electrode. Based upon complementary<br />
spectroscopic evidence it has been argued that the rate limiting step for charge generation is the dissociation of geminate pairs forming an<br />
electron at an either unintentional or intentional electron acceptor and hole at an adjacent chain. The field and temperature dependence can<br />
be fitted by a recent theory by Arkhipov et al. that differs from the conventional Onsager description.<br />
© 2003 Elsevier B.V. All rights reserved.<br />
Keywords: Conjugated polymer; Photoconductivity; Charge generation<br />
1. Introduction<br />
In the early days of research into conjugated polymers,<br />
it was believed that the commencement of photoconductivity<br />
right at the onset of optical absorption is a signature of<br />
those materials behaving like an inorganic semiconductor in<br />
which the optical and electrical gap are identical [1]. Meanwhile,<br />
there is abundant evidence against this notion. Most<br />
experimentalists [2,3] and theoreticians [4–6] concur that the<br />
binding energy of a singlet exciton, i.e. the energy needed to<br />
split an exciton to a Coulumbically unbound charges, is no<br />
less than 0.5 eV. The ubiquitous increase of the photoconductive<br />
yield at about 1 eV above the absorption threshold<br />
[2,7] is analogous to the behavior of conventional molecular<br />
crystals such an anthracene [8]. It indicates that the excess<br />
electronic energy can be used to overcome the coulombic<br />
binding energy of a Frenkel-type neutral exciton via either<br />
autoionization or dissociation while the excited chain segment<br />
is still vibrationally “hot” [7,9].<br />
∗ Corresponding author. Tel.: +420-541-149-407;<br />
fax: +420-541-211-697.<br />
E-mail address: weiter@fch.vutbr.cz (M. Weiter).<br />
0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.synthmet.2003.09.018<br />
This article focuses on photoconductivity within the spectral<br />
range of the S1 ← S0 transition, i.e. below the energy<br />
at which instantaneous dissociation of a singlet (S1) exciton<br />
prior to vibrational cooling occurs [10]. There is clear<br />
evidence, though, that also vibrationally relaxed excitons<br />
can dissociate, provided that the required excess energy is<br />
supplied by either a strong electric field or a sensitizer. The<br />
realization of the former case has been proven by (i) field<br />
quenching of the fluorescence [11], (ii) transient absorption<br />
under an electric field that can monitor the correlated decrease<br />
of the population of S1 excitons and the growth of<br />
the polaron signal [12] and (iii) transient photoconductivity<br />
in a ladder-type poly(para-phenylene) (MeLPPP) that is<br />
characterized by an exceptionally low degree of disorder<br />
and high degree of chemical purity [13]. However, since<br />
electric field assisted dissociation is at least quadratic in<br />
field it tends to vanish at moderate fields. This is at variance<br />
with photoconductivity studies that bear out a roughly<br />
linear field dependence at moderate field. The straightforward<br />
conclusion is that this residual effect is due to exciton<br />
dissociation at impurities acting as electron scavengers<br />
[14]. At times, it is difficult to distinguish both phenomena.<br />
By comparing results of transient photoconductivity in
166 M. Weiter et al. / Synthetic Metals 141 (2004) 165–170<br />
a poly-phenylenevinylene-based copolymer (PhPPV) and<br />
previous results on MeLPPP, we delineate characteristic<br />
features of both processes paying special attention to the<br />
properties of PhPPV that has been used before when studying<br />
sensitized photogeneration with the spectral range of<br />
the S1 ← S0 transition [14].<br />
2. Experiment<br />
PhPPV was synthesized via GILCH polymerization by<br />
COVION Organic Semiconductor GmbH [15]. The structure<br />
formula of the studied PhPPV derivative is shown in Fig. 1<br />
together with the optical density of a PhPPV film. Transient<br />
photocurrent measurements were performed on a sandwich<br />
cell with a polymer layer of a thickness of ca 150 nm. It<br />
was prepared by spin coating of a 1% polymer solution in<br />
chloroform onto an indium tin oxide (ITO) electrode. In order<br />
to prevent exciton-induced hole injection the ITO was<br />
covered by a semitransparent aluminum layer with an optical<br />
density of 0.46. A 100 nm thick Al-layer served as a<br />
top electrode. The sample capacitance was typically 2.2 nF.<br />
Some samples were doped with 1% and 10 wt.% of trinitrofluorene<br />
(TNF) as an electron acceptor. The experiments<br />
were done with either a frequency doubled Nd:YAG laser<br />
at photon energy 3.49 eV (355 nm) or a dye laser at photon<br />
energy 2.74 eV (453 nm). Since the calculated quantum efficiencies<br />
for photogeneration did not differ by more than factor<br />
2, which is the experimental accuracy set by calculating<br />
the reflection losses at various sample interfaces, most of the<br />
data were taken upon excitation at hνexc = 3.49 eV where<br />
hot exciton dissociation was still a small effect. The sample<br />
was mounted in a cryostat which allowed cooling down to<br />
150 K. In order to minimize space charge effect that may<br />
occur upon periodic excitation all experiments were done in<br />
the single shot regime.<br />
Optical density<br />
1.2<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
Wavelength (nm)<br />
600 550 500 450 400 350<br />
0.0<br />
2.0 2.5 3.0 3.5<br />
Energy (eV)<br />
Fig. 1. Absorption spectrum of the PhPPV film. In the inset, the structure<br />
formula of PhPPV is shown.<br />
3. Results<br />
Fig. 2 shows photocurrent transients in a 150 nm PhPPV<br />
film at 295 K at electric fields of 3.5×10 5 and 1.3×10 6 V/cm<br />
and different photon doses while Fig. 3 shows equivalent<br />
data on a PhPPV film doped with 1% of TNF at both polarities<br />
of the excited ITO:Al electrode. The rise time was<br />
determined by the RC time constant of the circuit that was<br />
about 30 ns. Data representation in double logarithmic scale<br />
reveals a non-exponential decay extending into the 10 �s<br />
regime. The shape of the j(t) time dependence is virtually<br />
invariant with light intensity. Regarding the pulse shape the<br />
Fig. 2. Photocurrent transients measured in a 150 nm thick PhPPV film<br />
at 295 K at different photon doses (hνexc = 3.5 eV) and an electric field<br />
of (a) 3.5 × 10 5 V/cm, (b) 1.3 × 10 6 V/cm.<br />
photocurrent (mA)<br />
10 0<br />
10 -1<br />
10 -2<br />
10 -8<br />
10 -3<br />
F=10 6 Vcm -1<br />
F=10 5 Vcm -1<br />
10 -7<br />
10 -6<br />
time (s)<br />
Fig. 3. Photocurrents measured in PhPPV film doped with 1% trinitrofluorene<br />
at a photon dose 55 �J at different electric fields and polarity. Solid<br />
(dashed) lines refer to positive (negative) bias to ITO.<br />
10 -5<br />
10 -4
Fig. 4. (a) The number of charges collected as a function of electric<br />
field at different photon doses. (b) The same data but normalized to the<br />
capacitor charge.<br />
only field dependence noted is that at higher fields the long<br />
time tail of the photocurrent is somewhat diminished. Fig. 3<br />
confirms that the current is basically independent of polarity<br />
of the diode.<br />
The number of charges Q, collected by the electric field<br />
was calculated by integrating the current up to a time,<br />
Fig. 5. Number of charges collected per pulse at different electric field<br />
as a function of laser intensity.<br />
M. Weiter et al. / Synthetic Metals 141 (2004) 165–170 167<br />
Yield<br />
10 -1<br />
10 -2<br />
10 -3<br />
10 -4<br />
MeLPPP<br />
PhPPV<br />
PhPPV + 1 % TNF<br />
10 5<br />
Field (Vcm -1 )<br />
Fig. 6. A comparison of the for MeLPPP, PhPPV and PhPPV doped<br />
with 1% TNF at low photon dose (full symbols: ITO positive bias; open<br />
symbols: ITO negative bias).<br />
typically 20 �s, when the current has decayed to the background<br />
level. This appears to be a reasonable compromise<br />
in order to avoid divergence of the integral over an algebraic<br />
decay function. In Fig. 4a, Q is plotted as a function<br />
of electric field at different photon doses. It is instructive to<br />
normalize Q to the calculated capacitor charge (Fig. 4b). It<br />
is obvious that at a high photon dose the collected charge<br />
saturates at the capacitor charge. This is also borne out<br />
by Fig. 5, which shows Q as a function of the incident<br />
intensity for different electric field. At low intensities Q is<br />
strictly linear with light intensity. Fig. 6 compares the field<br />
dependence of Q measured in undoped PhPPV, PhPPV<br />
doped with 1% TNF film and in MeLPPP film at the same<br />
incident light intensity. It shows that doping increases photoconduction<br />
by a factor of 2–3. This is in agreement with<br />
previous results [14]. The temperature dependence of Q at<br />
selected values of electric field is presented in Fig. 7. At<br />
Fig. 7. Temperature dependence of the quantum yield in a PhPPV film<br />
at different electric field as a function of temperature. Calculated curve<br />
was taken from [24] (see text).<br />
10 6
168 M. Weiter et al. / Synthetic Metals 141 (2004) 165–170<br />
E = 2.2 × 10 5 V/cm, Q is weakly temperature dependent<br />
features activation energy of about 0.07 eV for T>200 K.<br />
At lower temperatures, Q tends to saturate.<br />
4. Discussion<br />
A transient photocurrent can either be transport or generation<br />
limited. The former case applies if the generation time of<br />
is much shorter than the transit time experimental signatures<br />
being the plateau current of the time of flight signal is proportional<br />
to the product of n(F)µ(F)F while the transit time<br />
is given by d/µ(F). Here, n(F) and µ(F) are the field dependent<br />
number of generated charge carriers and their mobility,<br />
respectively. In the case of relevant experiments on MeLPPP<br />
the distinction was straight forwarded because the hole mobility<br />
could be measured and estimated carrier transit times<br />
were several orders of magnitude shorter than the response<br />
time of the transient current. Based upon a value of µ = 2×<br />
10 −6 cm 2 /(V s) for holes in PhPPV measured on a �m-thick<br />
sample at a moderate electric field [16] one could estimate<br />
the mean transit time of holes generated homogenously thorough<br />
the bulk of a 150 nm thick film at electric fields ranging<br />
from 10 5 to 10 6 V/cm to be 4–40 �s. This could, indeed, be<br />
comparable to the measured duration of the current transient<br />
implying it to be entirely transport limited. However, not<br />
observing any significant shortening of the current pulse at<br />
higher fields cast doubt on this assignment. It is known that<br />
in energetically random dielectric such as a film of PhPPV<br />
the motion of charges is associated with dispersion, notably<br />
at early stage of transport when carriers have not reached<br />
quasi-equilibrium [17]. It may well be that in a sample as thin<br />
as 150 nm the effective hole mobility is higher. In any event,<br />
the observed current transient is a convolution of time dependent<br />
charge carrier generation, transport and discharge.<br />
In the absence of additional experimental information, only<br />
the time-integrated photocurrent will be analyzed. As long<br />
as no bi-molecular recombination of charge carriers occurs<br />
and the range of carriers is solely given by generation and<br />
discharge at the electrodes, this quantity is a direct measure<br />
of the number of charge carriers generated. The first condition<br />
is fulfilled at low light intensities (see below). That<br />
the second condition is also met is proven by the fact that<br />
in time of flight experiments on micrometer thick samples<br />
[16] a carrier arrival signal is observed. However, this argument<br />
applies to holes only because the electron mobility<br />
is trap-limited and, concomitantly, is much lower than the<br />
hole mobility. This implies that on the time interval 0–10 �s<br />
only holes contribute to the time integrated current pulse.<br />
Since the measured current transient must involve dissociation<br />
after pulse excitation the dissociating entity has to<br />
be a metastable electron–hole pair produced from a short<br />
lived initial optical excitation. However, it is open to conjecture<br />
what the rate-determining step for generation of mobile<br />
carrier is. Is it the primary field dependent dissociation<br />
of a singlet exciton into a Coulumbically bound geminate<br />
electron–hole pair (GP) or is it its subsequent escape from<br />
its initial potential? Solely, based upon the field dependence<br />
of the generation yield no unambiguous assignment can be<br />
made. The reason is that any experimental superlinear field<br />
dependence of Q could be fitted by the conventional Onsager<br />
model either in its 1D [18] or 3D [19] version by<br />
choosing an empirical value for the so-called thermalization<br />
distance of the GP regardless whether or not the conceptual<br />
basis of the data analysis is valid. In order to distinguish the<br />
above possibilities one requires additional experimental information.<br />
In the case of MeLPPP this was field-modulated<br />
picosecond transient absorption of charge carriers and excitons<br />
simultaneously. By comparing the field dependence<br />
of the evolution of the transient absorption of geminately<br />
bound positive polarons with the transient photocurent signal<br />
we could ascertain that it must be the field-assisted initial<br />
dissociation event which is rate controlling [13]. In this<br />
case, dissociation create more loosely bound GP that can<br />
subsequently be ionized quite easily without requiring too<br />
much of assistance by either electric field or temperature.<br />
In the case of PhPPV, the situation is different. Previous<br />
spectroscopic [20] and cw-photoconduction [14] work<br />
on PhPPV without and with controlled doping with trinitrofluorene<br />
showed that neat PhPPV contains about 0.04%<br />
of dopants. They quench already about 50% of excitations<br />
and form electron hole pairs on TNF and PhPPV. Intentional<br />
doping by 1% TNF can, therefore, increase the photogeneration<br />
of charge carriers by about a factor of 2 only. This is<br />
in agreement with Fig. 6. It proves that in this case the field<br />
assisted and rate-limiting step for photogeneration must be<br />
the subsequent escape of the pair from its Coulombic well<br />
rather than exciton diffusion towards to the sensitizer that<br />
captures an electron. The latter process is exothermic and it<br />
does not require a promoting electric field. Additional support<br />
for this argument is that in PhPPV the photoresponse<br />
is larger than in MeLPPP (Fig. 6), and in addition, is associated<br />
with a weaker field dependence although the field<br />
quenching of prompt fluorescence in MeLPPP and PhPPV<br />
turns out to be virtually identical [21]. Therefore, the exciton<br />
binding energy in both systems had also to be the same<br />
implying that the amount of intrinsic field−assisted exciton<br />
breaking has to be the same, too, at variance with photogeneration<br />
at moderate fields as documented by Fig. 4.<br />
At first glance we should expect that the field dependence<br />
of Q is tractable in terms of Brown’s [22] adaptation of<br />
Onsager’s theory of the dissociation/recombination for geminate<br />
pair with finite lifetime. However, in this theory both<br />
the field and temperature dependences of the dissociation<br />
yield are related via the value of the initial intra-pair separation.<br />
Since the measured activation energy of the yield at<br />
the lower electric field is as low as 0.07 eV that value should<br />
be 6 nm. This is incompatible with the fact that there is no<br />
indication that the photocurrent will saturate, not even at a<br />
field as large as 2.5 × 10 6 V/cm. At the same time, the extrapolated<br />
values of the yield at either infinite temperature
and field are different whereas conventional Onsager theory<br />
predicts limT →∞ϕ = limF→∞ϕ = ϕ0, where ϕ0 is the initial<br />
yield for GP formation for an optical excitation. This<br />
problem has been recognized for some time already [23] and<br />
has been taken as evidence that classic Onsager-type theories<br />
are inappropriate for conjugated polymers. Recently,<br />
Arkhipov et al. [24] presented a theory that is able to explain<br />
this phenomenon. It rests upon the notion that in a<br />
GP, consisting of a localized electron at a dopant with high<br />
electron affinity and a hole on the polymer, the latter must<br />
not be treated as a localized point charge but rather than<br />
a charge that can execute an oscillatory motion in the potential<br />
well inside the segment of the polymer determined<br />
by the Coulombic attraction. Therefore, the hole is not at<br />
rest but is associated with a kinetic energy, that depends on<br />
the effective mass of the hole. The situation is analogous<br />
to the motion a diatomic molecule inside the bottom of a<br />
Morse potential. Dissociation can occur whenever the hole<br />
can cross the potential barrier. The predicted temperature<br />
dependence at a filed of 3×10 5 V/cm of the process is much<br />
weaker than in a system in which the charges are fully localized<br />
because the energy needed for dissociation is diminished<br />
by the kinetic energy and agreement with experiment is<br />
gratifying.<br />
In Fig. 8, the Q(F) data for the photogeneration yield<br />
in the PhPPV film doped with 1% TNF (see Fig. 6) are<br />
compared with the prediction of theory under the premise<br />
that all primary excitations will migrate to a dopant. The<br />
parameter is the relative effective mass of the hole on a<br />
polymer chain. It turns out that a perfect fit is obtained for<br />
meff/m0 ≈ 2. Such a value of meff/m0 for a �-conjugated<br />
polymer is at variance with theory [6] and electro-reflection<br />
Fig. 8. Comparison of the carrier photogeneration yield at different electric<br />
fields and parametric in the on-chain effective mass, calculated by<br />
Arkhipov et al. (Fig. 3 in [24]) and experimental data for PhPPV doped<br />
with 1% TNF at low photon dose of 4.1 �J.<br />
M. Weiter et al. / Synthetic Metals 141 (2004) 165–170 169<br />
experiments on single crystalline poly-diacetylenes featuring<br />
a Franz-Keldysh effect [25]. On the other hand, there is<br />
experimental evidence that even the on-chain charge carrier<br />
mobility, probed via microwave techniques [26] is of the<br />
order of 0.1 cm 2 /(V s), i.e. comparable with that of molecular<br />
crystals at 295 K. This implies that meff/m0 must be of<br />
the order of unity. We conjecture that the low value of the<br />
effective mass applies only upon instantaneous generation<br />
of a charge, i.e. at times shorter than the electron-phonon<br />
time.<br />
We shall briefly comment on the observation that the<br />
collected charge saturates at a level given by the capacitor<br />
charge. The reason is that the generated electrons are<br />
localized at acceptor sites and established a reservoir of recombination<br />
centers for holes. As a consequence, the hole<br />
range becomes less than the film thickness and there is redistribution<br />
of the internal electric field. This phenomenon<br />
has already been addressed in [13].<br />
5. Conclusion<br />
It is obvious that there are two processes occurring in<br />
the bulk of conjugated polymer that can give rise to photogeneration<br />
within the spectral range of the S1 ← S0<br />
transition. One of them is the field assisted dissociation<br />
into geminate pairs that subsequently can separate fully.<br />
This is a generic process in conjugated polymers but requires<br />
a strong electric field. The other one is sensitized<br />
photogeneartion at either non-intentional or intentional<br />
dopants that can act as electron scavengers. Its magnitude<br />
depends on the degree of doping although a concentration<br />
of 0.1% of electron acceptors may be enough to overcompensate<br />
the intrinsic dissociation at least at moderate<br />
electric fields. Unfortunately, it is notoriously difficult<br />
to distinguish both processes unless additional spectroscopic<br />
information is available. The field dependence of<br />
the yield alone is insufficient for deciding upon the prevalence<br />
of either process because data fitting in terms of an<br />
Onsager-like process is often ambiguous even if values<br />
for the initial separation of a hypothetic GP may be realistic.<br />
In summary, we emphasize that in neither case the<br />
Onsager description is applicable because in the intrinsic<br />
case the field assistance in the primary event of exciton<br />
breaking is disregarded in the Onsager formalism, while in<br />
the other case GP dissociation is aided by the finite energy<br />
of the charge carrier—usually the hole—that resides on<br />
a segment of the conjugated polymer next to the counter<br />
charge.<br />
Acknowledgements<br />
We thank Covion Organic Semiconductors for providing<br />
PhPPV. This work was supported by EU-project LAM-<br />
INATE and the Fond der Chemishen Industrie.
170 M. Weiter et al. / Synthetic Metals 141 (2004) 165–170<br />
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Abstract<br />
Transient photoconductivity in a film of ladder-type<br />
poly-phenylene: failure of the Onsager approach<br />
M. Weiter a,*,1 ,H.B€assler a , V. Gulbinas b , U. Scherf c<br />
a Institute of Physical, Macromolecular and Nuclear Chemistry, Philipps University, Hans-Meerwein-Strasse,<br />
D-35032, Marburg, Germany<br />
b Institute of Physics, A. Gostauto 12, LT-2600 Vilnius, Lithuania<br />
c Bergishe Universit€at Wuppertal, Chemistry Department, Gauss Strase 20, D-42119, Wuppertal, Germany<br />
Received 20 May 2003; in final form 14 August 2003<br />
Published online: 6 September 2003<br />
By comparing the field dependent evolution of the transient absorption of geminate pairs generated of singlet excitons<br />
in a film of ladder-type poly-phenylene and the field dependent transient photocurrent it is argued that the rate<br />
limiting step for photoionization is the initial field assisted dissociation into geminately bounded electron–hole pairs<br />
rather than its ultimate escape from the coulombic well.<br />
Ó 2003 Elsevier B.V. All rights reserved.<br />
1. Introduction<br />
Chemical Physics Letters 379 (2003) 177–182<br />
Meanwhile it is widely accepted that, in general,<br />
the opto-electronic properties of p-conjugated<br />
polymers resembles those of molecular crystals [1].<br />
This does not mean that all relevant features are<br />
bare replicas of those of molecular crystals.<br />
Among these is the notion that photoionization is<br />
tractable in terms of Onsager theory [2,3]. It implies<br />
that intrinsic photogeneration is a multi-step<br />
process, the initial event being either the autoionization<br />
of a higher excited optical Franck–Con-<br />
* Corresponding author. Fax: +420-541-211-697.<br />
E-mail address: weiter@fch.vutbr.cz (M. Weiter).<br />
1 Permanent address: Faculty of Chemistry, Brno University<br />
of Technology, Purkynova 118, 612 00, Brno, Czech republic.<br />
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.cplett.2003.08.043<br />
www.elsevier.com/locate/cplett<br />
dom singlet state yielding a coulombically bound<br />
geminate electron–hole pair (GP) or direct charge<br />
transfer [4]. Subsequently the pair can recombine<br />
geminately or fully dissociate in the course of<br />
temperature and field assisted diffusive escape<br />
process. The spectroscopically accessible energy<br />
difference between the lowest charge transport<br />
state and the singlet excited of the parent molecule<br />
plays a key role in that process. In conventional<br />
molecular crystals it is as large as about 0.5 eV [4].<br />
This precludes thermal dissociation of the vibrationally<br />
relaxed singlet excitation unless that excitation<br />
is generated near an electrode which can act<br />
as a acceptor of one of the charge comprising the<br />
initial exciton. Tacitly, it has always been implied<br />
that in a bulk system the initial event that generate<br />
GPs is exothermic, i.e., not requiring any
178 M. Weiter et al. / Chemical Physics Letters 379 (2003) 177–182<br />
temperature or field assistance. Therefore the<br />
measured field and temperature dependence of the<br />
photogeneration has solely to been attributed to<br />
the diffusive escape of the geminate electron–hole<br />
pair rather than its initial generation. Binary<br />
charge transfer (CT) systems behave similarly except<br />
that excited state of the system is the charge<br />
transfer state with a finite lifetime [5].<br />
Based upon delayed field collection of optically<br />
generated charge carriers in a ladder-type polyphenylene<br />
(MeLPPP) it has been argued that the<br />
traditional Onsager approach fails to describe<br />
optical charge carrier generation because the essential<br />
field-assisted step is the primary dissociation<br />
rather than the secondary escape of the pairs<br />
from the coulombic well [6]. Experiments described<br />
in this Letter support this conclusion.<br />
However, the current work avoids the experimentally<br />
demanding delayed field collection technique.<br />
Instead we combined transient photoconductivity<br />
with transient spectros<strong>copy</strong> in order to disentangle<br />
primary and secondary dissociation, the additional<br />
advantage being that there is no minimal time<br />
delay between generation of photocarriers and<br />
their probing. Experiments performed with a<br />
broad temperature range will support the conclusion<br />
that the conventional Onsager formalism fails<br />
to account for photogeneration in p-conjugated<br />
polymers although the previous notion of sequential<br />
dissociation of a primary optical excitation<br />
remains valid.<br />
2. Experiments<br />
Transient photocurrent measurements were<br />
performed on typically 100 nm thick layer of the<br />
MeLPPP prepared by spin coating of a 1% polymer<br />
solution in chloroform onto a indium tin oxide<br />
(ITO) electrode. In order to prevent injection<br />
the ITO was covered by a semitransparent aluminium<br />
layer with an optical density 0.46. One<br />
hundred nanometer thick top layer of aluminium<br />
completed the sandwich structure with a capacitance<br />
1.5 nF. Most of the experiments were done<br />
upon exciting the sample by a frequency-doubled<br />
Nd/Yag laser (hmexc ¼ 3:5 eV, i.e., kexc ¼ 355 nm).<br />
Although the hmexc exceeds the energy of the singlet<br />
exciton (ES1 ¼ 2:7 eV) is it still below the theoretical<br />
energy at which the efficiency of cw photoconduction<br />
at low light intensities features a step<br />
increase due to hot exciton dissociation [7].<br />
The sample was mounted in a cryostat which allowed<br />
cooling down to 150 K. All experiments<br />
were done in the single shot regime making sure<br />
that any remaining trapped space charge has been<br />
erased.<br />
3. Results<br />
Fig. 1 shows a photocurrent transients in a 100<br />
nm thick MeLPPP film at a 295 K at a incident<br />
photon dose of 40 lJ/pulse, equivalent to an<br />
absorbed photon dose of 6 10 12 photons, and at<br />
different external electric fields. The rise time is<br />
determined by the RC time constant of the circuit<br />
that was about 30 ns. At times shorter than 1 ls<br />
and at low electric fields the current features a<br />
plateau in a double logarithmic representative. At<br />
higher fields an algebraic decay followed by an<br />
almost exponential decay at t > 1 ls is observed.<br />
At t > 10 ls a weak algebraic tail is noticeable.<br />
The number of collected charges Q was calculated<br />
by integration of the current. Plots of Q as a<br />
Fig. 1. Transient photocurrents measured at different external<br />
electric field and at room temperature. The incident photon<br />
dose was 40 lJ/pulse equivalent to an absorbed photon dose of<br />
6 10 12 photons per sample area.
Number of generated charges<br />
10 11<br />
10 10<br />
10 9<br />
F[Vcm -1 ], Q=C·V/e<br />
2·10 6 , 1.9·10 11<br />
1·10 6 , 1.0·10 11<br />
5·10 5 , 4.7·10 10<br />
2·10 5 , 1.9·10 10<br />
0.1 1 10 100 1000<br />
Fig. 2. Dependence of the number of generated charges on<br />
photon dose parametric in the electric field at negative bias to<br />
the ITO electrode. The solid lines indicate the calculated<br />
number of capacitance charges ðQ ¼ CV Þ.<br />
Number of generated charges<br />
10 11<br />
10 10<br />
10 9<br />
10 8<br />
10 4<br />
420 µJ/pulse<br />
44 µJ/pulse<br />
4,1 µJ/pulse<br />
10 5<br />
Q=C·V/e<br />
10 6<br />
Fig. 3. Number of generated charges as a function of the<br />
applied electric field parametric in the excitation intensity.<br />
The straight line indicates the dependence of the corresponding<br />
capacitance charge on the electric field.<br />
function of the photon dose and electric field are<br />
presented in Figs. 2 and 3. Experiments within<br />
the temperature range from 310 to 150 K turn<br />
out that Q is temperature insensitive. This concurs<br />
with previous experiment on cw photoconductivity<br />
[7].<br />
M. Weiter et al. / Chemical Physics Letters 379 (2003) 177–182 179<br />
4. Discussion<br />
A transient current can be either transport or<br />
generation limited. From previous results is known<br />
that in freshly prepared MeLPPP films hole<br />
transport is trap-free, time of flight signals yielding<br />
a hole mobility of 2 10 3 cm2 V 1 s 1 with an<br />
exceptionally weak temperature and field dependence<br />
[8]. This is a consequence of the low degree<br />
of structural disorder. On the other hand electron<br />
transport has never been observed in a time of<br />
flight experiment indicating that electrons are<br />
trapped. It turns out that at the lowest electric field<br />
employed for the recording the signals in Fig. 1,<br />
the expected transit time of holes is 25 ns, i.e.,<br />
comparable to the RC-time of the circuit, i.e.,<br />
about three orders of magnitude shorter than the<br />
duration of the photocurrent pulse. Obviously, the<br />
observed transient photocurrent must be controlled<br />
by charge carrier generation rather than by<br />
their transport. This concurs with the fact that the<br />
duration of the signal is virtually independent at<br />
the electric field, at variance with the notion of it<br />
being determined by transport. It also indicates<br />
that this signal decay cannot be limited by electron<br />
transport. Detrapping of the negative space charge<br />
must occur on the longer time scale.<br />
The obvious source of the transient photocurrent<br />
must be the final dissociation of metastable<br />
geminately bound electron–hole pairs (GPs) whose<br />
existence has been well established by (i) delayed<br />
field collection experiment [6], (ii) delayed fluorescence<br />
[9], (iii) thermally stimulated luminescence<br />
[10] and (iv) two color photoconduction [11].<br />
At a given time t the photocurrent is determined<br />
by the product of the number of GPs which survived<br />
until time t and the rate constant of their<br />
subsequent escape kesc from the coulombic potential<br />
iðtÞ ¼eNGPðtÞkesc: ð1Þ<br />
Q ¼<br />
The total number of collected charge is<br />
Z T<br />
0<br />
i dt ¼ g dissg escNph; ð2Þ<br />
where g diss is the initial yield of dissociation of<br />
singlet excitations, Nph produced by the laser and<br />
g esc is the fraction of GP, i.e., g diss Nph is the
180 M. Weiter et al. / Chemical Physics Letters 379 (2003) 177–182<br />
number of GP which escape geminate recombination.<br />
In principle, both g diss and g esc can be<br />
functions of the applied electric field and temperature.<br />
In the delayed field experiment this product<br />
has been disentangled by applying a collecting field<br />
pulse after a delay time of 100 ns after the generation<br />
pulse. In the present experiment generated<br />
charge carriers are collected and their number will<br />
be compared to the number of GP generated as<br />
monitored by the transient absorption within time<br />
domain of 200 fs to 500 ps which is too short<br />
for their subsequent escape from the coulombic<br />
potential [12]. This procedure is superior to that<br />
employed in [6] where the difference between the<br />
generation and collection field vanished at high<br />
electric field. Moreover, the currents experiments<br />
were extended towards lower photon dose thus<br />
avoiding any possible effect which bimolecular<br />
annihilation of singlet excitons during the laser<br />
pulse might have.<br />
Fig. 2 indicates that the number of charge collected,<br />
Q, increase linearly until it saturates at high<br />
intensities at a values close to the value of the<br />
capacitance charge CV . The linear field dependence<br />
of Q at high pump intensities bears out this<br />
more clearly (top curve in Fig. 3). This behavior<br />
can easily be explained in terms of GP dissociation<br />
yielding quickly moving holes and trapped electrons.<br />
The latter create a negative space charge<br />
that acts as recombination centers. Invoking<br />
Langevin-type recombination it is easy to show<br />
that the probability of a hole to recombine with an<br />
amount of quasi-stationary negative charge of CV ,<br />
is close unity [13]. While at low laser intensity and<br />
homogenous excitation within the film one dissociating<br />
GP will contribute 0.5 elementary charge –<br />
this is because the range of a hole is half of the<br />
sample thickness – the according range must decrease<br />
as the number of GPs, having dissociated<br />
already, approaches the number of capacitor<br />
charges. A quantitative explanation is presented in<br />
Appendix A.<br />
The message derived for the intensity dependence<br />
of Q is that meaningful information on<br />
monomolecular dissociation of singlet excitons<br />
requires experiments with laser intensities sufficiently<br />
low that bimolecular reaction of neither<br />
singlet excitons nor charge carrier can occur.<br />
Based upon earlier studies in the prompt fluorescence<br />
at variable light intensities this limit is 1 lJ/<br />
pulse at photon energy of 2.73 eV [14,15]. Data<br />
measured at 4.1 lJ/pulse at hmexc ¼ 3:5 eV meet this<br />
criterion. In Fig. 4 field dependence of the yield of<br />
holes collected is compared with the yield of generated<br />
GPs by field-assisted dissociation of relaxed<br />
singlet excitons as monitored by transient pump–<br />
probe experiments [12]. Note that in figure the<br />
yield rather the number of collected charges is<br />
shown. The proportionality of both dependences is<br />
striking. It confirms that (i) the rate limiting field<br />
assisted step is the dissociation of relaxed singlet<br />
excitons into GPs rather than their subsequent<br />
escape from coulombic well, (ii) the former process<br />
proceeds on a 200 fs to 0.5 ns time scale while<br />
the dwell time of GPs extends to the microsecond<br />
regime, and (iii) approximately 10% of initially<br />
Yield<br />
10 0<br />
10 -1<br />
10 -2<br />
10 -3<br />
10 4<br />
10 -4<br />
10 5<br />
10 6<br />
10 1<br />
10 0<br />
10 -1<br />
10 -2<br />
10 7<br />
10 -3<br />
Peak photocurrent [mA]<br />
Fig. 4. Charge generation efficiency (full symbols, left coordinate)<br />
and the photocurrent peak (open symbols, right coordinate)<br />
versus applied electric field, parametric in the excitation<br />
energy (squares: 4.1 lJ/pulse, circles: 44 lJ/pulse). The stars<br />
indicate the field dependent evolution of the transient absorption<br />
of geminate pairs generated by the dissociation of singlet<br />
excitons at photon dose of 14 lJ/cm 2 [12].
generated GP are liable to complete dissociation at<br />
a field of 3 MV/cm.<br />
The field dependence of the yield of fully dissociated<br />
GPs is weaker at pump intensity beyond<br />
the threshold at which bimolecular annihilation<br />
becomes the dominant channel for the decay of<br />
singlet excitons. In this case the yield has to be<br />
estimated by normalizing the Q to the number of<br />
singlet pairs generated. The plot is included in<br />
Fig. 4. At low electric field the yield g exceeds the<br />
value measured at low intensity while at high fields<br />
both dependences intersect. The latter observation<br />
is due to the onset of bimolecular recombination<br />
of mobile holes and trapped electrons and shortens<br />
the hole range (see above). The former effect is<br />
generic and confirms the notion that if the initially<br />
dissociating state is a highly excited singlet state<br />
generated by singlet–singlet annihilation is more<br />
efficient [16] and requires less stimulation by an<br />
electric field.<br />
Recording the time resolved photocurrent allows<br />
extracting additional information on the escape<br />
process of the initially generated GPs. Fig. 4<br />
includes data on the photocurrent maximum at a<br />
time compared to RC-time constant of the circuit.<br />
It turns out that the field dependence is very similar<br />
to the field dependence of the number of<br />
charges collected. Based upon Eq. (1) this implies<br />
that within the limit of experimental uncertainty,<br />
the field dependence of the escape rate of GP from<br />
initial coulombic well must be vanishingly small.<br />
Combined with the experimental results that the<br />
amount of charge collected is independent of<br />
temperature, it indicates that it is that fraction of<br />
GPs which have been generated close to the maximum<br />
of the superimposed coulombic and external<br />
potential which contributes most to the photocurrent,<br />
be keeping in mind that conjugated<br />
polymers are disordered hopping systems. The<br />
above conclusion can be cast into a schematic diagram<br />
for photoionization (Fig. 5). Depending<br />
upon the applied electric field an excitons can<br />
transfer one its charge to polymer segments within<br />
the coulombic well. At zero electric field only very<br />
few excitons can form a tightly bound GP unless<br />
the initial dissociation event involves bimolecular<br />
collision of two singlet excitons. Under the latter<br />
circumstance delayed fluorescence, arising from<br />
M. Weiter et al. / Chemical Physics Letters 379 (2003) 177–182 181<br />
Fig. 5. Schematic diagram of the photoionization in organic<br />
hoping system with a exciton binding energy of 0.7 eV and a<br />
Gaussian width of the density of states distribution of 0.05 eV<br />
in a extended electric field of 2 MV/cm and assuming a dielectric<br />
constant of 3.5.<br />
geminate pair recombination, can be observed.<br />
Upon increasing the electric field some excitons<br />
can dissociate into metastable GPs. Most of them<br />
recombine nonradiatively because the external<br />
potential overcompensate the energy difference<br />
between the GP and singlet exciton but can be<br />
revitalized and emit delayed fluorescence when the<br />
electric field is turned off [9]. GPs that managed to<br />
find an acceptor state close to the transport energy<br />
of the disordered semiconductor can diffuse and<br />
have the option of being collected by either the<br />
applied electric field or the retarding coulombic<br />
field. According to Eq. (1) the transient photocurrent<br />
at time t is the product of the number of<br />
GPs having survived up to time t and their escape<br />
rate kescðtÞ. At moderate times, i.e., 30<br />
ns 6 t 6 0:5 ls, when most of the GP are still alive,<br />
iðtÞ is still controlled by the time dependence of kesc<br />
whereas at longer times it is the exhaustion of the<br />
reservoir, the ultimate dissociation rate constant<br />
being of the order of 10 6 s 1 . It is worth noting<br />
that the time dependence of kesc is very different<br />
from the rate for ultimate GP collapse, monitored,<br />
for instance, by the decay of the delayed fluorescence,<br />
because the latter refers to metastable GPs<br />
rather than to those GP that are liable to escape
182 M. Weiter et al. / Chemical Physics Letters 379 (2003) 177–182<br />
only. When replotting the data of Fig. 1 on a<br />
semilogarithmic scale it becomes obvious that the<br />
photocurrent pulse is almost exponential indicating<br />
that there is little dispersion in the escape rate<br />
because MeLPPP is a fairly ordered material. This<br />
explains why delayed fluorescence features a<br />
power decay whereas the time dependence of kescðtÞ<br />
can be rather weak and of an external electric field.<br />
5. Conclusions<br />
In summary the experiments prove that the field<br />
dependence of the intrinsic photogeneration in the<br />
p-conjugated polymer MeLPPP is controlled by<br />
the initial step of the dissociation of a relaxed<br />
singlet exciton into a geminate pair rather than its<br />
escape from the coulombic potential. Therefore it<br />
is illegitimate to analyze the field (and temperature)<br />
dependence of the yield in terms of OnsagerÕs<br />
theory although the functional form of g can accidentally<br />
be similar. This conclusion is not specific<br />
for a conjugated polymer but is generic for organic<br />
solids in which the energy gap between the geminate<br />
electron–hole pair state and the parent singlet<br />
state is sufficiently close so that it can be compensated<br />
by a large electric field.<br />
Acknowledgements<br />
Experimental assistance by Dr. Chan Im is<br />
greatly appreciated. This work was supported by<br />
the EU-project LAMINATE, the Volkwagen–<br />
Stiftung and the Fond der Chemischen Industrie.<br />
Appendix A<br />
If a hole is injected from the anode into a dielectric<br />
containing a negative space charge N (per<br />
unit area) it can either recombine or get discharged<br />
at the cathode. The probability for recombination<br />
is<br />
Prec ¼ krec=ðkrec þ s 1<br />
tr Þ; ðA:1Þ<br />
where str is the transit time of the hole and krec is<br />
the recombination rate, i.e., krec ¼ cN =d, where c<br />
is the bimolecular reaction constant and d the<br />
sample thickness. Since str ¼ d 2 =lV and making<br />
use of the Langevin ratio c=l ¼ e=ee0<br />
Prec ¼ 1 þ e0eF<br />
eN<br />
1<br />
: ðA:2Þ<br />
If N is determined by the capacitor charge, i.e.,<br />
N ¼ ee0F =e, Prec ¼ 1=2. The calculation could<br />
easily be extended to include spatial dispersion of<br />
the origin of the hole generation and explains why<br />
the number of collected holes must saturate at the<br />
close to capacitor charge.<br />
References<br />
[1] see K. M€ullen, G. Wegner, Electronic Materials, the<br />
Oligomer Approach, Wiley-VCH, Weinheim, 1998.<br />
[2] L. Onsager, Phys. Rev 54 (1938) 554.<br />
[3] M. Pope, C.E. Svenberg, Electronic Processes in Organic<br />
Crystals and Polymers, second ed., Oxford University<br />
Press, Oxford, 1999.<br />
[4] L. Sebastian, G. Weiser, G. Peter, H. B€assler, Chem. Phys.<br />
95 (1983) 13.<br />
[5] C.L. Braun, J. Chem. Phys. 80 (1984) 4157.<br />
[6] D. Hertel, E.V. Soh, H. B€assler, L.J. Rothberg, Chem.<br />
Phys. Lett. 361 (2002) 99.<br />
[7] S. Barth, H. B€assler, U. Scherf, K. M€ullen, Chem. Phys.<br />
Lett. 288 (1998) 147.<br />
[8] D. Hertel, H. B€assler, U. Scherf, H.H. Horhold, J. Chem.<br />
Phys. 110 (1999) 9214.<br />
[9] B. Schweitzer, V.I. Arkhipov, U. Scherf, H. B€assler, Chem.<br />
Phys. Lett. 313 (1999) 57.<br />
[10] A. Kadashchuk, Yu. Skryskevski, A. Valkin, N. Ostapenko,<br />
E.V. Emelianova, V.A. Arkhipov, H. B€assler,<br />
Phys. Rev. B 63 (2001) 115205.<br />
[11] J.G. M€uller, U. Scherf, U. Lemmer, Synth. Met. 119 (2001)<br />
395.<br />
[12] V. Gulbinas, Yu. Zaushitsyn, V. Sundstr€om, D. Hertel, H.<br />
B€assler, A. Yartsev, Phys. Rev. Lett. 89 (2002) 107401.<br />
[13] H. Vestweber, H. B€assler, J. Gr€uner, R.H. Friend, Chem.<br />
Phys. Lett. 256 (1996) 37.<br />
[14] D. Hertel, Yu.V. Romanovski, B. Schweitzer, U. Scherf,<br />
H. B€assler, Synth. Met. 116 (2001) 1391.<br />
[15] A. Haugeneder, M. Neges, C. Kallinger, W. Spirkl, U.<br />
Lemmer, J. Feldmann, M.C. Amann, U. Scherf, J. Appl.<br />
Phys. 85 (1999) 1124.<br />
[16] C. Silva, A.S. Dhoot, D.M. Russell, M.A. Stevens, A.C.<br />
Arias, J.D. MacKenzie, N.C. Greenham, R.H. Friend,<br />
S. Setayesh, K. Mullen, Phys. Rev. B 64 (2001) 125211.
Monatshefte fuÈr Chemie 132, 177±183 (2001)<br />
Luminescence in Organic Silicons Prepared<br />
from Organic Precursors in Plasma Discharges<br />
Pavel HorvaÂth 1 , FrantisÏek Schauer 1; , Ivo KurÏitka 1 , Ota Salyk 1 , Martin<br />
Weiter 1 , Norbert Dokoupil 1 , Stanislav NesÏpuÊrek 1;2 , and Vlastimil Fidler 3<br />
1 Faculty of Chemistry, Brno University of Technology, CZ-61200 Brno, Czech Republic<br />
2 Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, CZ-16206<br />
Prague, Czech Republic<br />
3 Faculty of Nuclear Sciences, Czech Technical University, CZ-11519 Prague, Czech Republic<br />
Summary. The photoluminescence of plasma-prepared polysilanes during the change from linear<br />
1D Si chains to an amorphous 3D Si network was studied. The excitonic absorption band with a<br />
maximum at 353 nm in 1D Si experiences a blue shift and broadening upon introduction of<br />
branching and networking defects. With the gradual transition from 1D to 3D structure, an extensive<br />
redistribution of oscillator intensity along the absorption edge, accompanied by a decrease of the<br />
resolution of the - band, was observed. In the short wavelength region of the excitation spectra<br />
there is an enormous increase of excitonic emission at 328 nm. This effect is tentatively attributed to<br />
the excitation of the phenyl group or to the phenyl-silicon bond as con®rmed by effusion spectra of<br />
the phenyl species.<br />
Keywords. Silicon polymers; Plasma polymers; Luminescence; Effusion.<br />
Introduction<br />
Recently, many research groups have concentrated on the preparation of polysilanes<br />
by unconventional means, trying to produce materials with high ordering and/or<br />
increased dimensionality. Many attempts have been directed to the preparation of<br />
oriented ®lms of poly-(dimethylsilanediyl) by vacuum sublimation. Suzuki et al. [1]<br />
used UV laser ablation to deposit poly-((methyl)-phenylsilanediyl) (PMPSi).<br />
Watanabe et al. [2] employed laser chemical vapour deposition (CVD) of a<br />
monomeric chlorosilane for depositing of PMPSi with a laser tuned for attacking the<br />
Si±Cl bond preferentially (266 nm). Finally, Nagai et al. [3] used radio-frequency<br />
(RF) CVD for depositing silicon polymers of different dimensionality. In this work,<br />
emphasis is laid on the in¯uence of the dimensionality changes from 1D to 3D on the<br />
photoluminescence of radio frequency plasma prepared polysilylanes.<br />
Corresponding author
178 P. HorvaÂth et al.<br />
Results and Discussion<br />
The purpose of this study was the investigation of the effects of structural transition<br />
from 1D to higher dimensions; therefore, solution-prepared PMPSi was studied<br />
®rst as the reference. In Fig. 1, absorbance measurements taken from Ref. [4] and<br />
the corresponding photo luminescence (PL) emission spectrum are shown. The<br />
absorption spectrum of a solid ®lm of PMPSi as determined from re¯ection<br />
measurements [4] consists of three main regions with peaks at 332, 270, and<br />
Fig. 1. Absorption and photoluminescence (PL) emission spectra of a solid ®lm of poly-((methyl)phenylsilanediyl)<br />
(PMPSi) prepared from solution (PL spectrum excited at ˆ 280 nm at room<br />
temperature)<br />
Fig. 2. Absorption and non-corrected excitation PL spectra of a solid ®lm of poly-((methyl)phenylsilanediyl)<br />
(PMPSi) prepared from solution (PL excitation spectra taken for photo<br />
luminescence at ˆ 360 nm at room temperature)
Luminescence in Organic Silicons 179<br />
194 nm. It has been shown [5] that the ®rst long-wavelength electronic transition<br />
originates mainly from delocalized - transitions. The energy of this transition<br />
depends strongly on the length of the molecule and its conformation. The peak at<br />
shorter wavelength ( ˆ 270 nm) is associated with - transitions in the benzene<br />
ring. The origin of the shortest wavelength peak (around ˆ 200 nm) is not yet<br />
clear; ®rst calculations point at excitations of -electrons of benzene rings. There is<br />
also a very weak tail up to 450 nm similar to that observed in Ref. [6]. This could<br />
arise from the Si-branching structure as follows from model studies. The PL<br />
emission spectrum (excitation wavelength 280 nm) in Fig. 1 shows two bands: a<br />
sharp one with a maximum at 353 nm which is of excitonic nature (related to -<br />
transitions) and a broad one with its maximum situated at about 500 nm. In Fig. 2,<br />
the corresponding PL excitation spectra for photo luminescence at ˆ 360 nm are<br />
shown. The optical transitions - ( 345 nm) and - ( 265 nm) are<br />
visible, as well as the shortest wavelength peak (around ˆ 200 nm) [7]. Besides, a<br />
very sharp peak occurred at ˆ 223 nm in the PL excitation spectrum; it will be<br />
dealt with in some detail later.<br />
The PL of the plasmatically prepared PMPSi depends drastically on the<br />
preparation conditions. In Fig. 3, typical excitation (a) and emission (b) PL spectra<br />
taken from Ref. [8] for different ratios of partial pressures of hydrogen and<br />
dichloromethylphenylsilane monomer are given. Both types of spectra exhibit<br />
optical transitions typical for both 1D and 3D structures. The excitonic PL is<br />
observed for samples prepared under partial pressures of monomer above 20 Pa. In<br />
the emission spectrum of these samples, the exciton band is located at ˆ 360±<br />
390 nm (Si - ). The optimized sample, prepared under partial pressures of 60 Pa<br />
for H2 and 20 Pa for the monomeric silane, exhibits excitonic bands at 380 (Si<br />
- ), 327, and 285 nm (phenyl - ), and also a tail in the long wavelength region;<br />
these features are typical for disordered materials. In the emission spectrum, an<br />
exciton band with a maximum at 390 nm (Si - ) and an imperfection centred at<br />
470 nm can be observed. Excitation spectra were detected for photoluminescence at<br />
ˆ 480 nm, emission spectra were measured with an excitation wavelength of<br />
280 nm. The occurrence of the excitonic band supports the assumption of a low ratio<br />
of Si atoms bound to three further Si moieties in the plasmatically prepared material<br />
as e.g. found in chemically prepared branched poly-(hexyl-(methyl)-silylanediyl)<br />
structures [9]. The arti®cially introduced branching points at Si centres lead to a<br />
total suppression of excitonic luminescence for concentrations above 1±2% due to<br />
reduction of coherence length along the Si±Si chain and excessive exciton trapping<br />
and decay at the branching points.<br />
It is well known [10] that hydrogen surface reactions exert basically twofold<br />
in¯uence on plasmatically grown materials: ®rst, the hydrogen mediates etching<br />
during the deposition process, improving the ®nal properties of the ®lms by<br />
removing the energetically weak bonds and thus leading to more compact and voidfree<br />
material; second, the increased concentration of hydrogen in the ®lms<br />
deteriorates the microstructure and forms hydrogen related defects. Thus, usually an<br />
optimum of microphysical properties at a speci®c hydrogen concentration is<br />
observed.<br />
Absorbance and emission PL spectra of a plasmatically prepared sample are<br />
shown in Fig. 4. A comparison with the corresponding measurements on PMPSi
180 P. HorvaÂth et al.<br />
Fig. 3. Excitation (a) and emission (b) photoluminescence (PL) spectra of plasmatically prepared<br />
PMPSi at different partial pressures of hydrogen and of the dichloromethylphenylsilane monomer<br />
(PH2 ‡ Pmono). Excitation spectra were detected for photoluminescence at 480 nm. The excitation<br />
spectrum of the sample prepared at PH2 ˆ 60 Pa and Pmono ˆ 20 Pa exhibits bands at 380 nm (Si -<br />
), 327 nm, and 285 nm (phenyl - ); a tail in the long wavelength region is typical for disordered<br />
materials. In the emission spectrum, an exciton band with a maximum at 390 nm (Si - ) and an<br />
imperfection PL centred at 470 nm can be observed. Excitation spectra were detected at 480 nm,<br />
emission spectra were measured for excitation at 280 nm<br />
prepared from solution (Fig. 2) reveals the presence of all typical optical transitions:<br />
the excitonic transitions (Si - ) are well visible both in absorption ( ˆ 310 nm)<br />
and PL emission ( ˆ 325 nm) spectra. In the excitation spectrum, the -<br />
transition ( ˆ 258 nm) and the short-wavelength peak at ˆ 223 nm are present.<br />
We also studied in some detail the PL properties of plasmatically prepared<br />
PMPSi excited in the short wavelength region ( ˆ 210±230 nm). In Fig. 5, the
Luminescence in Organic Silicons 181<br />
Fig. 4. Absorption and emission PL spectra of a solid plasma-prepared ®lm of poly-((methyl)phenylsilanediyl)<br />
(PMPSi) (excitation at 280 nm at room temperature)<br />
Fig. 5. PL emission spectra for plasmatically prepared PMPSi for excitation wavelengths of 280 and<br />
210 nm at room temperature<br />
effect of different excitation wavelengths is given, resulting in two PL emission<br />
bands. The PL emission spectrum with an excitation wavelength of 280 nm contains<br />
both the Si main chain - exciton at 325 nm and a broad defect luminescence<br />
centred at 440 nm as expected from previous experiments. The PL emission<br />
spectrum obtained with an excitation wavelength of 210 nm differs considerably ±<br />
the Si main chain - excitonic emission at ˆ 328 nm increases enormously in<br />
intensity, and the defect luminescence is nearly missing. Figure 6 shows the<br />
excitation PL spectra recorded at ˆ 338 nm (a) and emission spectra obtained by<br />
excitation at ˆ 210 nm (b) for plasmatically prepared PMPSi of different ®nal
182 P. HorvaÂth et al.<br />
Fig. 6. PL spectra of plasmatically prepared PMPSi ®lms; (a) excitation spectra recorded at<br />
ˆ 340 nm, (b) emission spectra excited with ˆ 210 nm; the curves refer to different ®nal<br />
temperatures of the fractional heating in thermal desorption spectros<strong>copy</strong> as described in Ref. [8]<br />
temperatures of fractional heating in the thermal desorption spectros<strong>copy</strong> as<br />
described in Ref. [8], where the effused species are detected by mass spectros<strong>copy</strong><br />
under controlled heating. The PL spectra of the virgin samples, measured as<br />
prepared, are depicted with the parameter 25 C; the subsequent maximum temperatures<br />
are indicated. It is worth mentioning that the virgin sample exhibits the<br />
most pronounced luminescence which decreases after annealing (the luminescence<br />
measurements were performed at room temperature). The effect is strictly
Luminescence in Organic Silicons 183<br />
correlated with the effusion of the benzene rings in the range of temperatures of<br />
350±400 C [8]. Thus, we tentatively attribute the localization of the PL site excited<br />
at 210±230 nm to the phenyl group or the phenyl-silicon bond. Similar effects have<br />
been described in connection with spectral narrowing of stimulated mirrorless<br />
emission in conjugated polymers [12, 13], but effects of super¯uorescence [14] have<br />
also to be considered.<br />
Experimental<br />
The reference PMPSi was prepared by Wurtz coupling polymerization as described by Zhang and<br />
West [13]. The low-molecular weight fractions were extracted with boiling diethyl ether. The residual<br />
polymer, obtained in ca. 17% yield, possessed an unimodal but broad molecular mass distribution of<br />
Mw ˆ 4 104 . Thin ®lms were prepared from toluene solution by casting on borosilicate glass.<br />
The CVD reactor was an 13.56 MHz capacitively coupled radio-frequency discharge unit with a<br />
maximum power of 1.5 W cm 2 . For the deposition, a mixture of hydrogen (0±60 Pa) and<br />
dichloromethylphenylsilane (5±30 Pa) was used. The substrate temperature was kept at 80 C. The<br />
substrates for IR absorption were silicon polished wafers; for UV absorption and luminescence<br />
measurements, quartz and borosilicate glass were used. UV/Vis absorption spectra were recorded with<br />
a Hitachi U300 instrument; for IR spectros<strong>copy</strong>, a Nicolet Impact 400 FTIR assembly was used;<br />
luminescence both in steady-state and transient regimes was measured with an Edinburgh Science FF/<br />
FL900 spectrometer, for swelling experiments, a home built apparatus was used; desorption<br />
measurements were carried out on a mass spectrometer Leybold 200 pumped by a Balzers<br />
turbomolecular pump; scanning photoelectron spectros<strong>copy</strong> was performed on XPS and UPS VG-<br />
Scienti®c ADES 400 instruments.<br />
Acknowledgements<br />
This paper presents results achieved in connection with the COST 518 project. Support from the<br />
grant A1050901 of the grant agency of the Academy of Sciences of the Czech Republic is<br />
acknowledged.<br />
References<br />
[1] Suzuki M, Nakata Y, Nagai H, Goto K, Nishimura O, Okutani T (1998) Mater Sci Eng A146:36<br />
[2] Watanabe A, Kawato T, Matsuda M, Fujitsuka M, Ito O (1998) Thin Sol Films 312: 123<br />
[3] Nagai H, Nakata Y, Suzuki M, Okutani T (1998) J Mater Sci 33: 1897<br />
[4] NavraÂtil K, SÏik J, HumlicÏek J, NesÏpuÊrek S (1999) Opt Mater 12: 105<br />
[5] Harrah LA, Zeigler JM (1987) Macromolecules 20: 610<br />
[6] Ito O, Terazima M, Azumi A, Matsumoto N, Takeda K, Fujino M (1989) Macromolecules 22:<br />
1718<br />
[7] NesÏpuÊrek S, Schauer F, Kadashchuk A, this issue<br />
[8] Schauer F, NesÏpuÊrek S, HorvaÂth P, Zemek J, Fidler V (2000) Synth Metals 109: 321<br />
[9] Wilson WL, Weideman TW (1991) J Phys Chem 95: 4568<br />
[10] Cf. e.g. Davis EA: Hydrogen in Silicon (1996) J Non-Crystall Solids 198±200: 1<br />
[11] Zhang X-H, West R (1984) J Polym Chem Ed 22: 159<br />
[12] Lemmer U (1998) Polym Adv Technol 9: 476<br />
[13] Frolov SV, Gellermann W, Ozaki M, Yoshino K, Vardeny ZV (1997) Phys Rev Lett 78: 729<br />
[14] Siegman AE (1986) Lasers. University Science Books, Mill Valley, CA<br />
Received July 10, 2000. Accepted (revised) September 8, 2000
Ž .<br />
Journal of Non-Crystalline Solids 227–230 1998 669–672<br />
ž /<br />
Metastable states in poly methylphenylsilylene induced by UV<br />
radiation and electron beam<br />
R. Handlir a,) , F. Schauer a , S. Nespurek a,c , I. Kuritka a , M. Weiter a , P. Schauer b<br />
a<br />
Faculty of Chemistry, Technical UniÕersity of Brno, Veslarska 230, 637 00 Brno, Czech Republic<br />
b<br />
Institute of Scientific Instruments of the Academy of Sciences of the Czech Republic, Brno, Czech Republic, KraloÕopolska 112, 616 00<br />
Brno, Czech Republic<br />
c<br />
Institute of Macromolecular Chemistry of the Academy of Sciences of the Czech Republic, HeyroÕskeho 2, 162 06 Prague 6, Czech<br />
Republic<br />
Abstract<br />
We have examined and measured the density of states Ž DOS. in a prototypical polyŽ silylene. -polyŽ methylphenylsilylene.<br />
Ž PMPSi. using the method of post-transit hole emission signals from traps using the time of flight Ž TOF. photoconductivity<br />
method. The main goal of our measurements was to correlate the metastable states produced both by UV radiation and<br />
electron beam and to determine their basic parameters as to their energies and susceptibility for annealing. In the course of<br />
measurements we discovered in accordance with our previous observations the fully reversible states around and deeper than<br />
0.55 eV. q 1998 Elsevier Science B.V. All rights reserved.<br />
Ž .<br />
Keywords: Poly methylphenylsilylene ; UV radiation; Electron beam<br />
1. Introduction<br />
Amorphous silicon and polyŽ silylenes. are members<br />
of the larger class of silicon backbone solids.<br />
The latter mentioned backbone polymers are of interest<br />
because of their electrical, photo-electronic, and<br />
non-linear optical properties. Also, their optical and<br />
electrical properties differ from structurally analogous<br />
carbon-based p-conjugated systems such as<br />
polyŽ ethylene. or polyŽ styrene . , resembling rather<br />
fully p-conjugated systems such as polyŽ acetylene . .<br />
The quantum generation efficiency and the charge<br />
carrier drift mobility of the order of 10 y4 cm 2 V y1<br />
)<br />
Corresponding author. Fax: q42 5 4321 1101; e-mail:<br />
handlir@fch.vutbr.cz.<br />
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.<br />
Ž .<br />
PII: S0022-3093 98 00339-1<br />
y1 s w1–3 x,<br />
large for organic polymeric photo conductors,<br />
stimulated interest in its photo stability and<br />
electronic structure. In this paper, we report the<br />
measurements of the metastable states produced both<br />
by UV radiation and electron beam, and the determination<br />
of the basic parameters as to their energies<br />
and susceptibility for annealing in a prototypical<br />
polyŽ silylene. -polyŽ methylphenylsilylene. Ž PMPSi.<br />
using the method of post-transit hole emission signals<br />
from traps using time of flight Ž TOF. photoconductivity.<br />
2. Material properties and experiment<br />
Ž .<br />
PMPSi Scheme 1 was prepared by Wurtz coupling<br />
polymerization, as described by Zhang and
670<br />
( )<br />
R. Handlir et al.rJournal of Non-Crystalline Solids 227–230 1998 669–672<br />
Scheme 1.<br />
West wx 4 . The low-molecular weight fractions were<br />
extracted with boiling diethyl ether. The residual<br />
polymer, obtained in ca. 17% yield, possessed an<br />
unimodal but broad molecular mass distribution, Mw s4.10 4 .<br />
Thin films for photoconductivity measurements<br />
Ž thickness ranges from 2 to 4 mm. were prepared<br />
from a toluene solution by casting on conducting<br />
indium tin oxide or Au covered borosilicate glasses.<br />
The top semitransparent Al electrodes were prepared<br />
by vacuum evaporation. Before deposition the polymers<br />
were purified three times by precipitation in<br />
methanol and toluene solution and centrifuged Ž1200<br />
rpm, 145 min . . After deposition the films were dried<br />
at 10y3 Pa at 330 K for at least 4 h.<br />
Infrared absorption spectra are given in Fig. 1.<br />
Tentative assignments for the vibrational bonds are<br />
based on the earlier assignments by Zhang and West<br />
wx 4 and completed by authors.<br />
Fig. 2 gives data on ultra-violet visible Ž UV-VIS.<br />
absorption and luminescence spectra in solution and<br />
Fig. 1. Infrared spectra of PMPSi.<br />
Fig. 2. Absorption spectra of PMPSi in tetrahydrofurane solution<br />
Ž curve 1: – P – . and in the solid state Ž curve 2: . .<br />
Curves 3 Ž – PP – . and 4 Ž — — . represent luminescence spectra in<br />
tetrahydrofurane solution and in the solid state, respectively.<br />
in the solid state. The exciton peak at 3.7 eV is a<br />
consequence of the Ž s–s ) . Žbonding<br />
and antibonding<br />
molecular orbitals. absorption of all Ž Si. nseg- ments in the amorphous and polycrystalline sample,<br />
the fluorescence maximum at 3.5 eV provides a<br />
better estimate for one-photon absorption of the<br />
longest segments. The ‘size’ of the excitation is<br />
;25 bonds from the radiative power measurements<br />
wx 5 . The second exciton band with peak at about 4.4<br />
eV occurs in the Ž p–p ) . transitions in the phenyl<br />
side groups.<br />
In the transient photoconductivity Nd–YAG laser<br />
with a dye laser and a doubler were used to produce<br />
a single shot Ž ls330 nm . . Transient photocurrent<br />
was detected with low noise converter and digitizing<br />
oscilloscope. The vacuum temperature controlled<br />
cryostat was used both for the measurements and<br />
light-soaking experiments. A pulse modulated electron<br />
microscope Ž Us10 kV and Is1 nA. provided<br />
with the spectral electroluminescence apparatus was<br />
used for the electron induced metastability. For controlled<br />
light-soaking, both the water filtered Xe lamp<br />
Ž 75 W. and Hg lamp Ž 200 W. were used.<br />
3. Results<br />
The samples were irradiated by UV light around<br />
l s350 nm Ž using a bandpass filtered Xe lamp.<br />
inc<br />
and molecular weight of the polymer was measured
( )<br />
R. Handlir et al.rJournal of Non-Crystalline Solids 227–230 1998 669–672 671<br />
Fig. 3. Reciprocal value of the molecular weight of PMPSI vs.<br />
irradiation time Ž ls350 nm.Ž B . . fs represents the quantum<br />
efficiency of polymer main chain scission. The lines are drawn as<br />
guides for the eyes.<br />
by means of gel permeation chromatography. It is<br />
evident from Fig. 3 that the average molecular weight<br />
decreases with irradiation time. At the same time the<br />
shift of the position of the maxima of the absorption<br />
was observed Žfrom<br />
336 nm to 320 nm after 20 min<br />
irradiation . . This shift supports the results concerning<br />
the Si–Si bond scission after UV irradiation.<br />
From the flash photolysis measurements wx 6 , it<br />
follows that the induced absorption consists of maximum<br />
at 360 nm and two broader smaller maxima at<br />
about 423 and 460 nm. The absorbance in the main<br />
maximum was determined as 0.009 cmy1 . The 360nm<br />
absorption is assigned to the radical-cation Žposi-<br />
tive polaron. on the basis of the spectral data in the<br />
g-irradiated rigid matrix at 77 K wx 7 . The smaller<br />
Fig. 4. The time evolution of the degradation both for the UV ŽXe<br />
filtered, 75 W. radiation Ž I. and electron beam Ž 10 kV, 1 nA.<br />
Ž n . . The lines are drawn as guides for the eyes.<br />
absorption at 460 nm is attributed to both silyl<br />
radical;SiR and silylene biradical wx<br />
2<br />
8 .<br />
The time evolution of the degradation both for the<br />
UV Ž Xe. radiation and electron beam are in Fig. 4. It<br />
is necessary to point out that both plots differ in the<br />
output quantity—in the UV degradation experiments<br />
it was the amplitude of the TOF photocurrent signal<br />
and in the electron beam experiment it was an<br />
integral electroluminescence signal. The recovery of<br />
both signals after an anneal was, in both cases, at<br />
nearly original magnitudes.<br />
4. Discussion<br />
The main goal of the experiments was to determine<br />
the properties of the metastable states. For this<br />
purpose we used the post-transit spectros<strong>copy</strong> de-<br />
scribed in Ref. wx 9 . The light soaking time evolution<br />
of the total collected charge, Q , the dispersion<br />
coll<br />
coefficient, a, and the value of the photocurrent at<br />
the transit time, I , are depicted in Fig. 5. Also,<br />
max<br />
the respective quantities after the anneal at T s370<br />
a<br />
K for t s20 min are given. It is obvious that all the<br />
a<br />
quantities decrease with the light soaking, but the<br />
dependencies for Q and a have a similar depencoll<br />
dence compared to that of the photocurrent at the<br />
transit time I . Besides, both exhibit different anmax<br />
nealing behavior as complete recovery of the photocurrent<br />
at the transit time I is achieved, whereas,<br />
max<br />
Fig. 5. The irradiation induced metastable states: time evolution<br />
the total collected charge Q Ž I . coll , the dispersion coefficient a<br />
Ž `. and the value of the photocurrent at the transit time I Ž n . max ,<br />
also given are the values after anneal at 370 K for 10 min<br />
Ž Bv' . . The lines are drawn as guides for the eye.
672<br />
( )<br />
R. Handlir et al.rJournal of Non-Crystalline Solids 227–230 1998 669–672<br />
Fig. 6. The irradiation induced metastable states: time evolution of<br />
the metastable DOS function in the time interval tgŽ 0,10. min,<br />
also given is the DOS after anneal at 370 K for 20 min Ž – PP –. .<br />
Q and a after anneal do not reach the original<br />
coll<br />
magnitudes.<br />
The irradiation induced metastable states expressed<br />
by the density of states Ž DOS. function are<br />
in Fig. 6. The evaluation and results are in accor-<br />
dance with our previous paper w10 x.<br />
The progressive<br />
increases in DOS at the energy, 0.55 eV, with indication<br />
of some still deeper radiation generated states.<br />
The remarkable point is that those states after anneal<br />
at 370 K for 20 min do not anneal completely Žsee<br />
a<br />
curve in Fig. 6 . . The light-induced metastable states<br />
have been previously observed in transient photocur-<br />
rent hole experiments by Naito et al. w11,12 x.<br />
They<br />
interpret the metastable changes by creation of con-<br />
formational changes in Si backbone. In Ref. w13 x,<br />
the<br />
authors calculated and observed photocreated<br />
metastable states in organopolysilane solids using<br />
light induced ESR. They found two types of light-induced<br />
centers, one for lower photo excitation Žca.<br />
3.5<br />
eV. due to the Si skeleton stretching forces and the<br />
other higher photo excitation Ž over 4.8 eV. creates<br />
weak bonds in several places of the Si skeleton.<br />
5. Conclusions<br />
Ž.<br />
The main conclusions are: 1 the observed phenomena<br />
are interpreted as the metastability in polysi-<br />
lylanes connected with the change of the conformation<br />
length that was correlated with the blue shift of<br />
the maximum energy of the absorption correspond-<br />
) Ž.<br />
ing to s–s Si transitions; 2 the light soaking<br />
time evolution of the total collected charge, Q , the<br />
coll<br />
dispersion coefficient, a, and the value of the pho-<br />
tocurrent at the transit time, I , causes decrease of<br />
max<br />
all the quantities mentioned; Ž. 3 the temperature<br />
anneal T s370 K for t s20 min recovers the<br />
a a<br />
quantity I , whereas only partial recovery of the<br />
max<br />
quantities Q and a is achieved.<br />
coll<br />
Acknowledgements<br />
The support of the Czech Grant Agency contract<br />
102r97r0105 and Grant Agency of the Academy of<br />
Sciences contract 4050603r1997 is acknowledged.<br />
References<br />
wx 1 A. Eckhardt, W. Herden, S. Nespurek, W. Schnabel, Philos.<br />
Mag. B 71 Ž 1995. 239.<br />
wx 2 S. Nespurek, Macromol. Symp. 104 Ž 1996. 285.<br />
wx 3 R.G. Kepler, J.M. Zeigler, L.A. Harrah, S.R. Kurtz, Phys.<br />
Rev. B 35 Ž 1982. 2819.<br />
wx 4 X.-H. Zhang, R. West, J. Polym. Sci., Polym. Chem. Ed. 22<br />
Ž 1984. 159.<br />
wx 5 L.A. Harrah, J.M. Zeigler, in: C.E. Hoyle, J.M. Torkelson<br />
Ž Eds. . , Photophysics of Polymer, American Chemical Soc.,<br />
Washington, DC, 1987.<br />
wx 6 C.P. Penado, T. Wollny, W. Schnabel, Radiat. Phys. Chem.<br />
40 Ž 1992. 101.<br />
wx 7 S. Irie, H.H. Horri, M. Irie, Macromolecules 13 Ž 1980. 1355.<br />
wx 8 Y. Ohsaho, J.R.G. Thorpe, C.M. Phillips, J.M. Zeigler, R.M.<br />
Hochstrasser, J. Phys. Chem. 13 Ž 1989. 4408.<br />
wx 9 G.F. Seynhaeve, R.P. Barclay, G.J. Adreaenssens, J.M. Marshall,<br />
Phys. Rev. B 39 Ž 1989. 10196.<br />
w10x F. Schauer, R. Handlir, S. Nespurek, Adv. Mater. Opt.<br />
Electron. 7 Ž 1997. 61.<br />
w11x H. Naito, S. Zhang, M. Okuda, T. Dohmaru, J. Appl. Phys.<br />
76 Ž 1994. 3612.<br />
w12x H. Naito, S. Kodama, Q.Z. Kang, M. Okuda, J. Non-Cryst.<br />
Solids 198–200 Ž 1996. 653.<br />
w13x K. Takeda, K. Shiraishi, M. Fujiki, M. Kondo, K. Morigaki,<br />
Phys. Rev. B 50 Ž 1994. 5171.
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43