Synthesis and characterization of bent-shaped azobenzene ...

Synthesis and characterization of bent-shaped azobenzene monomers: Guest–host
effects in liquid crystals with azo dyes for optical image storage devices
M.R. Lutfor a, *, G. Hegde b , S. Kumar c , C. Tschierske d , V.G. Chigrinov e
a
School of Science and Technology, University Malaysia Sabah, Kota Kinabalu, 88999 Kota Kinabalu, Sabah, Malaysia
b
Liquid Crystal Physics Group, Gothenburg University, Goteborg 4129, Sweden
c
Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore 560 080, India
d
Institute of Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes Str. 2, Halle D-06120, Germany
e
Centre for Display Research, Hong Kong University of Science and Technology, Hong Kong
article info
Article history:
Received 22 May 2009
Received in revised form 2 July 2009
Accepted 3 July 2009
Available online 4 August 2009
Keywords:
Liquid crystals
Bent-shaped monomer
Cis-trans isomerization
Optical materials
Guest–host effects
Optical image storage devices
1. Introduction
abstract
Over the past decade, a large number of bent or banana-shaped
compounds have been synthesized, and their mesophase behavior
was characterized. Most of these compounds contain five aromatic
rings using the resorcinol or its derivatives which has been widely
used as the central unit [1–3]. The B type phases were found in the
bent shaped molecules and smectic and or nematic mesophases
also exhibited in such a compounds [4–8]. In general the mesophases
formed by the banana-shaped compounds are termed as ‘‘Banana’’
(Bn) phases, designated as B1–B8 phases [9], the B3 and B4
phases are crystalline, while the others are mesomorphic [9]. It
must be mentioned here that other nomenclature types like SmCP
(polar tilted smectic) for B 2, Col r (rectangular columnar)/or Col ob
(columnar oblique lattice) for B1 and Smintercal (intercalated smectic)
for B 6 have been used [9]. The most widely studied B 2 phase is
identified as a tilted antiferroelectric polar smectic (SmCPA) phase
with either synclinic (SmCsPA) or anticlinic (SmCaPA) structures
[5].
The banana-shaped molecules of five aromatic rings connected
through azomethine groups showed Smintercal and various B type
phases. It has been seen that the mesophase behavior of azome-
* Corresponding author. Tel.: +6012 8393831; fax: +6088 435324.
E-mail addresses: lutfor73@gmail.com, lutfor@ums.edu.my (M.R. Lutfor).
0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.optmat.2009.07.006
Optical Materials 32 (2009) 176–183
Contents lists available at ScienceDirect
Optical Materials
journal homepage: www.elsevier.com/locate/optmat
Six novel bent-shaped monomers were synthesized such as substituted/or non-substituted 1,3-phenylene
bis-{4-[(4-allyloxy)phenylazo]benzoate} (4a–c) and substituted/or non-substituted 1,3-phenylene
bis-{4-[3-(4-allyloxy-3-fluoro)phenylazo]benzoate} (4d–f) in which azobenzene moiety in the periphery
and substituted/or non-substituted resorcinol as central unit with polymeriable double bonds are linked
at both ends of all the molecules. The mesophase behavior was investigated using polarizing optical
microscopy, DSC and XRD measurements. Four members of the family show an intercalated smectic
(Smintercal) phase and two were crystalline in nature. The trans-form of azo compounds (4a–f) showed
a strong band in the UV region (355–366 nm), which was attributed to the p–p * transition, and a weak
band in the visible region at 455–465 nm due to the n–p * transition. When one of the azo dye (4d) is
mixed with liquid crystal as a guest, showed greater increase in thermal back relaxation time which is
useful for creation of optical image storage devices.
Ó 2009 Elsevier B.V. All rights reserved.
thine liquid crystalline molecules are commonly sensitive if the
substituents are attached at various positions on the aromatic ring
and also if the azomethine linking groups are replaced by different
groups, like ester, thioester or vinylcarboxylate groups [1,6]. There
are numerous study accomplished on the effect of different lateral
substituents on the arms of the bent-core molecules of five-ring
and a lateral fluorine substituent ortho to the terminal n-alkoxy
chain induces interesting electro-optical switching properties to
the mesophases [10–12]. Amaranatha Reddy and Sadashiva [13]
examined the mesomorphic properties and influences of fluorine
substitution at the periphery of the bent-core molecules.
In recent times, number of bent-core molecules containing an
azo (–N@N–) linkage have been reported for the possibility of
photochromism and photoisomerization [14]. Polymerization of
the appropriate bent-core liquid crystals have also been received
significant attention in last several years [15–17]. The crosslinked
liquid crystals polymers derived from acrylates monomers containing
banana liquid crystals have also been reported [18]. Bentcore
liquid crystalline monomers with double bonds at both ends
were used for polymerization to obtained main chain LC polymers
[17–21], among them two materials exhibited a monotropic SmCP
phase [19,21,22] and others form nematic and smectic C phases.
Fodor-Csorba et al. [21] investigated the effect of rigidity and/or
flexibility of the arms of banana shaped monomers with five
aromatic rings linked to the polymerizable terminal chains of the

molecules. There are two series of non-symmetric banana-shaped
compounds combining alkyl and alkenyl terminal groups reported
by Achten et al. [23]. Several oligomeric/polymeric compounds are
reported [24] with vinyl-terminated side chains and a bent core
mesogenic unit of polysiloxane-based liquid crystals [22].
In recent years, a field of research that is growing steadily is that
of photoinduced phenomenon, in which incident light itself brings
about molecular ordering/disordering of the liquid crystalline system
[25]. This particular aspect of photonics, in which light can be
controlled by stimulus, is being proposed as future potential technology
for optical storage devices [26,27]. Molecules containing
azobenzene moiety are well known to show reversible isomerization
transformations upon irradiations with UV and visible light
[28]. Upon absorption of UV light ( 365 nm) the energetically
more stable trans configuration, transforms into a cis configuration.
Photo induced effects reported in the literature [29–31] are
on liquid crystals in which the azobenzene group is either chemically
attached to the molecule of interest or used as a dopant in a
liquid crystal host material. The time taken for the phase transition
to takes place following the isomerization of the photoactive molecules
is not only of significant interest from a basic point of view,
but of concern for application also. In these cases time required for
trans to cis is faster where as the thermal back relaxation time
restoring the nematic phase takes much longer. If, the material
exhibits a nematic–isotropic (N–I) transition and the UV irradiation
is done in the nematic phase, the lowering of the transition
temperature, T NI, could induce an isothermal N–I transition. It is
this photochemically induced transition that is promising for the
optical image-storing systems.
This paper presents synthesis and characterization of six bent
shaped monomers consist of substituted and non-substituted
1,3-phenylene as the core and the rod-shaped azobenzene units
connected to the core via ester linkage with terminal chains having
double bonds at the both wings. In addition, we have studied the
influence of fluorine substitution at the periphery. We also report
here that the addition of bent core azo dye (4d) in liquid crystalline
material increases the thermal back relaxation drastically. A necessary
condition in creating optical storage device is material having
long thermal back relaxation. In the light of this the present article
provides a path for the exploration of systems for obtaining longterm
storage devices.
2. Experimental
2.1. Chemical syntheses
Ethyl 4-(4-hydroxyphenylazo)benzoate (1a) and ethyl 4-
(4-hydroxy-3-fluorophenylazo) benzoate (1b) were synthesized
according to our earlier paper [32]. Ethyl 4-(4-allyloxyphenylazo)benzoate
(2a), ethyl 4-(4-allyloxy-3-fluorophenylazo)benzoate
(2b), 4-(4-allyloxyphenylazo)benzoic acid (3a) and 4-(4-allyloxy-
3-fluorophenylazo)benzoic acid (3b) were synthesized according
to our procedure described elsewhere [33]. 1,3-Dicyclohexylcarbodiimide
(DCC) (Fluka) and 4-(N,N-dimethylamino)pyridine (DMAP)
(Fluka) and silica gel-60 (Merck) were used as received. Dichloromethane
was refluxed over calcium hydride (Fluka) and distilled
out before use.
2.1.1. 1,3-Phenylene bis-[4-(4-allyloxyphenylazo)benzoate] (4a)
Compound 3a (0.580 g, 2.05 mmol) was dissolved in 80 ml of
dry dichloromethane. DMAP (0.025 g, 0.205 mmol) was added
and the mixture was stirred for 30 min. Dry dichloromethane solution
(10 ml) containing resorcinol (0.113 g, 1.02 mmol) was added
to the mixture. DCC (0.453 g, 2.20 mmol) in dry dichloromethane
(10 ml) were added and the mixture was stirred for 24 h. The pre-
M.R. Lutfor et al. / Optical Materials 32 (2009) 176–183 177
cipitate was removed by filtration and the solvent was removed
under reduced pressure. The product was dissolved in dichloromethane
and water. The organic phase was washed with dilute
acetic acid, sodium carbonate solution and water successively
and the solvent was removed under reduced pressure. The compound
was purified by column chromatography over silica gel
using chloroform:methanol (100:1) as eluent. The product was
recrystallized from methanol:chloroform (1:1) to obtained 4a.
Yield: 0.210 g (37%). IR (KBr, mmax, cm 1 ): 3074 (@CH2), 2927
(CH 2), 2862 (CH 2), 1733 (C@O, ester), 1640 (C@C, vinyl), 1594,
1496 (C@C, aromatic), 1248, 1137, 1068 (C–O), 828 (C–H). 1 H
NMR (CDCl 3) d: 8.31 (d, 4H, J = 8.2 Hz, 2 ArH), 7.97 (d, 4H,
J = 7.1 Hz, 2 ArH), 9.94 (d, 4H, J = 7.5 Hz, 2 ArH), 7.50 (t, 1H,
J = 8.2 Hz, ArH), 7.21 (d, 1H, ArH), 7.19 (d, 2H, ArH), 7.05 (d, 4H,
J = 8.9 Hz, 2 ArH), 6.07 (m, 2H, CH@), 5.46 (d, 2H, J = 16.9 Hz,
@CH2), 5.34 (d, 2H, J = 10.3 Hz, @CH2), 4.63 (d, 4H, J = 6.8 Hz,
2 OCH2). 13 C NMR (CDCl3) d: 69.18, 114.59, 115.18, 115.86,
118.28, 119.38, 122.67, 124.42, 125.37, 129.99, 130.29, 131.34,
132.67, 147.17, 151.54, 155.93, 162.01, 164.44. Elemental Analysis
Calc. for C38H30N4O6 (638.668): C, 71.46; H, 4.73; N, 8.77%. Found:
C, 71.36; H, 4.61; N, 8.62%.
2.1.2. 4-Chloro-1,3-phenylene bis-[4-(4-allyloxyphenylazo)benzoate]
(4b)
Compound 4b was prepared by the same method used for
synthesis of 4a. Quantity: compound 3a (0.100 g, 0.354 mmol),
4-chlororesorcinol (0.0255 g, 0.177 mmol), DCC (0.0824 g,
0.40 mmol) and DMAP (0.0048 g, 0.04 mmol). Yield: 0.045 g
(38%). IR (KBr, m max, cm 1 ): 3072 (@CH 2), 2928 (CH 2), 2854 (CH 2),
1740 (C@O, ester), 1642 (C@C, vinyl), 1600, 1500 (C@C, aromatic),
1247, 1138, 1071 (C–O), 836 (C–H). 1 H NMR (CDCl 3) d: 8.34 (d, 4H,
J = 8.2 Hz, 2 ArH), 7.98 (d, 4H, J = 7.6 Hz, 2 ArH), 9.94 (d, 4H,
J = 7.6 Hz, 2 ArH), 7.56 (t, 1H, J = 8.2 Hz, ArH), 7.33 (d, 1H, ArH),
7.20 (d, 1H, ArH), 7.00 (d, 4H, J = 8.6 Hz, 2 ArH), 6.05 (m, 2H,
CH@), 5.46 (d, 2H, J = 16.5 Hz, @CH 2), 5.46 (d, 2H, J = 10.1 Hz,
@CH2), 4.23 (d, 4H, J = 7.7 Hz, 2 OCH2). Elemental Analysis Calc.
for C 38H 29ClN 4O 6 (673.113): C, 67.80; H, 4.33 N, 8.32%. Found: C,
67.63; H, 4.21; N, 8.22%.
2.1.3. 3,5-Bis{[4-(4-allyloxyphenylazo)benzoate]}benzoic acid (4c)
Compound 3a (0.063 g, 0.223 mmol) and 3,5-dihydroxybenzoic
acid (0.0172 g, 0.111 mmol) was dissolved in 40 ml of dry dichloromethane.
DMAP (0.0026 g, 0.022 mmol) and DCC (0.0459 g,
0.223 mmol) were added and the mixture was stirred for 24 h.
The work up and purification procedure was used according to
the method of 4a. Yield: 0.023 g (31%). IR (KBr, mmax, cm 1 ): 3078
(@CH 2), 2929 (CH 2), 2851 (CH 2), 1739 (C@O, ester), 1690 (C@O,
acid), 1641 (C@C, vinyl), 1602, 1501 (C@C, aromatic), 1245, 1138,
1043 (C–O), 832 (C–H). 1 H NMR (CDCl3) d: 8.32 (d, 4H, J = 8.2 Hz,
2 ArH), 7.96 (d, 4H, J = 7.5 Hz, 2 ArH), 9.92 (d, 4H, J = 7.5 Hz,
2 ArH), 7.52 (t, 1H, J = 8.1 Hz, ArH), 7.23 (d, 1H, ArH), 7.21 (d,
2H, ArH), 7.04 (d, 4H, J = 8.6 Hz, 2 ArH), 6.08 (m, 2H, CH@),
5.47 (d, 2H, J = 16.4 Hz, @CH2), 5.35 (d, 2H, J = 10.1 Hz, @CH2),
4.62 (d, 4H, J = 6.9 Hz, 2 OCH 2). Elemental Analysis Calc. for
C39H30N4O6 (682.667): C, 68.61; H, 4.42; N, 8.20%. Found: C,
68.52; H, 4.27; N, 8.02%.
2.1.4. 1,3-Phenylene bis-[4-(4-allyloxy-3-fluorophenylazo)benzoate]
(4d)
Compound 4d was prepared by the same method used for synthesis
of 4a. Quantity: compound 3b (0.104 g, 0.347 mmol), resorcinol
(0.0191 g, 0.173 mmol), DCC (0.0824 g, 0.40 mmol) and
DMAP (0.0048 g, 0.04 mmol). Yield: 0.042 g (36%). IR (KBr, mmax,
cm 1 ): 3075 (@CH 2), 2928 (CH 2), 2854 (CH 2), 1738 (C@O, ester),
1642 (C@C, vinyl), 1609, 1512 (C@C, aromatic), 1275, 1256, 1132,
1011 (C–O), 808 (C–H). 1 H NMR (CDCl 3) d: 8.37 (d, 4H, J = 8.2 Hz,

178 M.R. Lutfor et al. / Optical Materials 32 (2009) 176–183
2 ArH), 7.98 (d, 4H, J = 7.4 Hz, 2 ArH), 7.95 (d, 4H, J = 7.3 Hz,
2 ArH), 7.72 (d, 1H, ArH), 7.41 (d, 1H, ArH), 7.39 (d, 2H, Hz,
ArH), 7.11 (d, 2H, J = 7.8 Hz, 2 Ar-H), 6.06 (m, 2H, CH@), 5.47
(d, 2H, J = 16.8 Hz, @CH 2), 5.33 (d, 2H, J = 9.8 Hz, @CH 2), 4.58 (d,
4H, J = 6.7 Hz, 2 OCH2 ). 13 C NMR (CDCl3) d: 68.80, 70.82,
114.36, 114.95, 119.27, 119.86, 122.72, 124.59, 125.44, 126.18,
130.02, 130.12, 131.24, 131.38, 146.88, 151.66, 155.78, 162.63,
164.74. Elemental Analysis Calc. for C 38H 28F 2N 4O 6 (674.648): C,
67.65; H, 4.17; N, 8.30%. Found: C, 67.51; H, 4.06; N, 8.16%.
2.1.5. 4-Chloro-1,3-phenylene bis-[4-(4-allyloxy-3fluorophenylazo)benzoate]
(4e)
Compound 4e was prepared by same method used for synthesis
of 4a. Quantity: compound 3b (0.098 g, 0.327 mmol), 4-chlororesorcinol
(0.0235 g, 0.163 mmol), DCC (0.0824 g, 0.40 mmol) and
DMAP (0.0048 g, 0.04 mmol). Yield: 0.041 g (35%). IR (KBr, mmax,
cm 1 ): 3070 (@CH2), 2921 (CH2), 2852 (CH2), 1741 (C@O, ester),
1641 (C@C, vinyl), 1607, 1509 (C@C, aromatic), 1241, 1148, 1072
(C–O), 812 (C–H). 1 H NMR (CDCl3) d: 8.35 (d, 4H, J = 8.2 Hz,
2 ArH), 7.98 (d, 4H, J = 7.6 Hz, 2 ArH), 7.87 (d, 4H, J = 7.5 Hz,
2 ArH), 7.55 (t, 1H, J = 8.2 Hz, ArH), 7.33 (d, 1H, ArH), 7.20 (d,
1H, ArH), 7.10 (d, 2H, J = 8.6 Hz, 2 ArH), 6.08 (m, 2H, CH@),
5.46 (d, 2H, J = 16.5 Hz, @CH 2), 5.56 (d, 2H, J = 10.1 Hz, @CH 2),
4.21 (d, 4H, J = 7.7 Hz, 2 OCH2). Elemental Analysis Calc. for
C 38H 27ClF 2N 4O 6 (709.093): C, 64.36; H, 3.81; N, 7.81%. Found: C,
64.25; H, 3.67; N, 7.72%.
2.1.6. 3,5-Bis{[4-(4-allyloxy-3-fluorophenylazo)benzoate]}benzoic
acid (4f)
Compound 4f was prepared by the same method used for synthesis
of 4c. Quantity: compound 3b (0.060 g, 0.200 mmol), 3,5dihydroxybenzoic
acid (0.015 g, 0.100 mmol), DCC (0.041 g,
0.200 mmol) and DMAP (0.0024 g, 0.02 mmol). Yield: 0.024 g
(33%). IR (KBr, mmax, cm 1 ): 3072 (@CH2), 2922 (CH2), 2856 (CH2),
1740 (C@O, ester), 1685 (C@O, acid), 1644 (C@C, vinyl), 1602,
1507 (C@C, aromatic), 1241, 1133, 1065 (C–O), 811 (C–H). 1 H
NMR (CDCl 3) d: 8.32 (d, 4H, J = 8.2 Hz, 2 ArH), 7.97 (d, 4H,
J = 7.5 Hz, 2 ArH), 7.88 (d, 4H, J = 7.5 Hz, 2 ArH), 7.55 (t, 1H,
J = 8.2 Hz, ArH), 7.32 (d, 1H, ArH), 7.21 (d, 1H, J = 8.3 Hz, ArH),
7.10 (d, 2H, J = 8.5 Hz, 2 ArH), 6.07 (m, 2H, CH@), 5.47 (d, 2H,
J = 16.5 Hz, @CH 2), 5.56 (d, 2H, J = 10.2 Hz, @CH 2), 4.20 (d, 4H,
J = 7.6 Hz, 2 OCH2). Elemental Analysis Calc. for C39H28F2N4O6
(718.658): C, 65.18; H, 3.92 N, 7.79%. Found: C, 65.01; H, 3.82; N,
7.66%.
2.2. Characterization
The structures of the intermediates and product were confirmed
by spectroscopic methods: IR spectra were recorded with
1
a Thermo Nicolet Nexus 670 FTIR spectrometer. H NMR
(600 MHz) and 13 C NMR (150 MHz) spectra were recorded with a
Jeol (ECA 600) spectrometer. Compositions of the compounds were
determined by CHN elemental analyzer (Leco and Co). The transition
temperatures and their enthalpies were measured by differential
scanning calorimetry (Perkin DSC 7) with heating and cooling
rates were 10 °C min 1 and melting point of the intermediate compounds
were determined by DSC. Optical textures were obtained
by using Olympus BX50 polarizing optical microscope equipped
with a Linkam THMSE 600 heating stage and a VTO 232 control
unit. X-ray diffraction measurements were carried out using Cu
Ka radiation (k = 1.54 Å) using a 40 kV voltage, 30 mA current from
anode generator (XPERT-PRO) equipped with a graphite monochromator.
X-ray diffraction was carried out in the mesophase obtained
on cooling the isotropic phase. Absorption spectra were
recorded using a Perkin Elmer UV/vis spectrometer (Lambda 25).
2.3. Photoisomerization study
For the preliminary investigation, we have selected one of the
six azo dyes liquid crystals for photoisomerization studies. For
photoisomerization, the liquid crystalline host material which is
called as E7 (room temperature liquid crystal) is physically mixed
with 5% of bent core azo dye (4d) reported here.
Photo absorbance measurements were carried out as a function
of wavelength in the range of 300–700 nm, using a UV–visible
spectrophotometer (Ocean Optics). For these measurements the
samples were sandwiched between two glass plates having uniform
thickness and liquid crystal is filled in isotropic phase. The
UV apparatus used for inducing the photoisomerization is described
below. The UV radiation from an intensity stabilized UV
source with a fiber optic guide was used along with a UV-bandpass
filter (UG 11). The actual power of the radiation passing through
the filter combination, falling on the sample and measured with
a UV power meter kept in the sample position was 1 mW/cm 2 .In
situ measurement has been performed throughout the experiment.
3. Results and discussion
3.1. Synthesis
The intermediates and target compounds 4a–f were prepared
as depicted in Scheme 1. The azobenzene containing rod-like side
arms was prepared from ethyl 4-amino benzoate in which the amino
group is diazotated by sodium nitrite in the presence of three
equivalents of hydrochloric acid and the diazonium salt (A) was
coupled with phenol to yield ethyl 4-(4-hydroxyphenylazo)benzoate
1a. Thus, ethyl 4-(4-hydroxy-3-fluorophenylazo) benzoate 1b
was prepared by same method used for synthesis of 1a where fluorine
was introduced i.e. 2-fluorophenol used instead of phenol.
For introducing the double bonds at the terminals, compound
1a was alkylated with allyl bromide in the presence of potassium
carbonate as base to give ethyl 4-(4-allyloxyphenylazo)benzoate
2a and then compound 2a was base hydrolyzed to yield 4-(4-allyloxyphenylazo)benzoic
acid 3a. Thus, the fluorine containing compounds
2b, 3b were prepared by same method used for synthesis
of 2a and 3a, respectively.
The acid compound 3a was coupled with resorcinol by using
DCC and DMAP to achieved the non-substituted molecule 1,3-phenylene
bis-[4-(4-allyloxyphenylazo)benzoate] 4a. Similarly, compound
3a was coupled with 4-chlororesorcinol to obtained a 4chloro-1,3-phenylene
bis-[4-(4-allyloxyphenylazo)benzoate] 4b.
The compound 4d having fluorine atom at the periphery and compound
4e having chlorine atom at the core and fluorine atom at the
periphery were prepared from acid 3b by same method used for
synthesis of 4a.
Coupling reaction using DCC and DMAP was modified for the
preparation of both compounds 4c and 4f due to core molecules
containing 3,5-dihydroxybenzoic acid, which can be involve in
the coupling reaction. To avoiding unexpected coupling reaction
we have added DMAP to the acid compound 3a/3b for 30 min stirring
to produce active ester and then add 3,5-dihydroxybenzoic
acid to the mixture of 3a/3b to obtain acid substituted compounds
4c and 4f, respectively.
3.2. Mesomorphic properties
DSC study: the differential scanning calorimetry (DSC) was used
to determine the phase transition temperatures (T/°C) and phase
transition enthalpy changes (DH/Jg 1 ). The transition temperatures
form second cooling scans for all compounds have been collected
in Table 1.

Scheme 1. Reagents and conditions: (i) NaNO2, 3 equiv HCl, 2 °C; (ii) NaOH, pH 9, 2 °C; (iii) K2CO3, KI, BrCH2CH@CH2, reflux; (iv) KOH, MeOH; (v) C6H6O2 (resorcinol),
C6H5ClO2 (4-chlororesorcinol), C7H6O4 (3,5-dihydroxybenzoic acid), DCC, DMAP.
Table 1
Phase transition temperature (T/°C) and associated transition enthalpy values (DH/
Jg 1 ) in parentheses given for the second cooling of DSC scans for compounds 4a–f.
Compounds R R 0 Phase transitions
4a H H I 175.2 (5.3) Sm intercal 123.1 (29.5) Cr
4b H Cl I 144.3 (3.7) Smintercal 126.0 [17.4] Cr
4c H COOH I 168 (9.9) Cr
4d F H I 150.5 (6.8) Sm intercal 127.1 (29.7) Cr
4e F Cl I 142.9 (4.4) Sm intercal 126.3 (38.0) Cr
4f F COOH I 165 (8.6) Cr
Abbreviations: Cr = crystal, Sm intercal = smectic phase, I = isotropic phase.
For non-substituted compound 4a, on cooling from isotropic
phase, there are two peaks observed at 175.2 °C (DH = 5.3 Jg 1 )
and 123.1 °C (DH = 29.5 Jg 1 ) which corresponding to I–Smintercal
and Smintercal–Cr transitions (Table 1). For chloro-substituted compound
4b, on cooling two peaks found at 144.3 °C (DH = 3.7 Jg 1 )
and 126.0 °C (DH = 17.4 Jg 1 ) which corresponding to I–Sm intercal
and Smintercal–Cr transitions. Acid substituted compound 4c is
non-mesomorphic, the compound melt at 168 °C and crystallized
at 162 °C.
The corresponding fluoro-substituted compound 4d also displayed
two peaks on cooling at 150.5 °C (DH = 6.8 Jg 1 ) and
127.1 °C (DH = 29.7 Jg 1 ) corresponding to I–Sm intercal and Sm intercal–Cr
transitions (Table 1). For fluoro and chloro-substituted compound
4e show peaks at 142.9 °C (DH = 4.4 Jg 1 ) and 126.3 °C
(DH = 38.0 Jg 1 ) which corresponding to I–Smintercal and Smintercal–Cr
transitions. The fluoro and acid substituted compound 4f is
also non-mesomorphic as like 4c, the compound melt at 165 °C
and crystallization was observed at 160 °C. On heating, single peak
M.R. Lutfor et al. / Optical Materials 32 (2009) 176–183 179
was observed for all compounds. Therefore, four compounds (4a,
4b, 4d and 4e) are monotropic nature and two compounds are
crystalline (4c and 4f).
As can be seen from Table 1, addition of the F atom to the
periphery of the aromatic core (compound 4a vs. 4d) decreases
the mesophase–isotropic transition temperature. Similar effect
was found in our previous reported naphthalene-based materials
and increasing the terminal chain length as expected, decreases
the transition temperature significantly [33]. Compound 4f having
F atom exhibit only crystalline character at lower temperature
than compound 4a. The compound (4b) containing Cl atom to
the aromatic core (compound 4a vs. 4b) decreases the mesophase–isotropic
transition temperature and the compound 4e having
F and Cl atom have lower transitions temperature compared to
all other compounds. Hence, the mesophase stability of our compounds
4a–f is lower than reported compounds [33] and there is
a similar effect of fluorine substitution, leading to lower transition
temperatures for the fluorinated compounds. In contrast to compounds
exhibit nematic phase [33], in this case only monotropic
nature of the intercalated smectic A phase was observed.
3.3. Polarizing optical microscopy (POM) studies
Under the polarizing microscope, upon cooling from the isotropic
phase, a fan-like or focal conic texture as typical for Sm phase
was observed for compounds 4a and 4b. Optical textures observed
for compound 4a at 164 °C and for compound 4b at 130 °C are
shown in Fig. 1. There is no other phase transition on further cooling,
except crystallization. For compound 4b, the clearing temperature
of the Sm phase is much lower than for compound 4a due to

180 M.R. Lutfor et al. / Optical Materials 32 (2009) 176–183
Fig. 1. Optical micrographs of (a) compound 4a at 164 °C, compound 4b at 130 °C (b), compound 4d at 129 °C (c) and compound 4e at 134 °C (d) on cooling from isotropic
phase.
the addition of the Cl-atoms to the central units of the aromatic
core of 4b which decreases the crystal–mesophase transition as
well as the mesophase–isotropic transition temperature. Compound
4c shows crystalline texture on cooling from isotropic
phase.
When cooling from the isotropic phase, a fan-like texture appeared
for compounds 4d and 4e, also as typical for Sm phase.
Optical textures were taken at 128 °C and 134 °C of compound
4d and 4e, respectively (Fig. 1). No other phase transition was observed
on further cooling, except crystallization. For compound 4d
and 4e also show the Sm phase is lower temperature due to the
addition of F atoms to the periphery of the aromatic core of 4d
and F and Cl containing compound of 4e. It was found that the
crystal–mesophase transitions as well as the mesophase–isotropic
transition temperature are very less difference within F and Cl containing
compounds (4b, 4d and 4e) and mesophase was found for
both F or Cl substituted aromatic core in the bent shaped molecules.
Compound 4f also shows crystalline property like compound
4c, therefore, compounds containing acid substituted central core
are non-mesomorphic nature. We found also four compounds
(4a, 4b, 4d and 4e) are monotropic nature under POM and only
one compound 4a is stable when cooling from isotropic phase.
Compound 4a appeared as thermodynamically more stable liquid
crystals phase. Theoretically, the mesophase stability of 4a should
be the highest, because it has no lateral substituent which disturbs
the packing of the aromatic cores. Therefore, compound 4a was
introduced to XRD for detail phase assignment.
3.4. X-ray diffraction studies
The X-ray diffraction studies confirm the phase assignment.
Fig. 2 shows the intensity vs. 2h plot derived from the scanning
of the compound 4a at 165 °C. The diffraction pattern exhibited a
sharp reflection in the small angle region, corresponding to
d = 17.10 Å (165 °C) and a diffuse scattering in the wide-angle region
at d = 4.57 Å. In the wide-angle range we have seen only a diffuse
peak (d = 4.57 Å), which means fluid in a plane structure. The
minimum conformation of compound 4a is a bent-shaped with an
end-to-end distance of 36 Å. X-ray diffraction patterns show that
Fig. 2. Intensity-theta graph derived from the X-ray diffraction pattern of
compound 4a at 165.
the smallest-angle peak corresponds to d = 17.10 Å, which is about
one half of the molecular length in a conformation. Therefore, we
assume that compound 4a exhibited a smectic intercalated phase
which is denoted as Smintercal phase.
Although our synthesized bent-shaped compounds are not suitable
for comparison to other reported compounds due to structural
different, however, we have tried to compare the transition temperatures
and the nature of the mesophases exhibited by resorcinol
based compounds of banana-shaped compounds containing
acrylic monomers without azobenzene groups [21]. Csorba et al.
[21] reported a series of bent-shaped diacrylate monomers, 1,3phenylene
bis[4 0 -(alkenyloxy)biphenylcarboxylate] with different
substituents on the central phenyl ring (H, CH 3,ClorNO 2). No
mesophase was formed either by the unsubstituted or 2-methylsubstituted
derivatives. Each of the chloro-substituted analogues
showed a nematic phase, while 2-nitro-substituent showed a B7
phase at relatively low temperature. Therefore the chemical struc-

ture can affect significantly the mesophase behavior in banana
molecules [21]. The synthesis and mesomorphic properties of a
series of molecules, 1,2-phenylene bis[4-(4-alkyloxyphenylazo)benzoates]
are reported by Prasad [14]. The mesophases
exhibited by these V-shaped azo compounds are identified as
nematic, Sm intercal and crystal E phases. The compounds showed
low transition temperatures and it was found that the CH@N linkage
could be more conducive to mesomorphism compared with
the N@N linkage. Later, another series of bent-shaped liquid crystal
with photo-active azo linkages is reported by Prasad [14] with variable
alkyl groups at both terminals. These compounds show Colr
and SmC AP A mesophases and this SmC AP A showed antiferroelectric
switching characteristics.
However, we have introduced the COO-linkage with the resorcinol
core and azo N@N linkages to the both wings. Hence, the values
of transition temperature of fluorine and chlorine containing
compound 4b, 4d and 4e were less than non-fluorine compound
4a. The observation of banana phases in azo compounds could be
potential from the point of photochromic studies. Hence, the similar
mesophase behavior of the four compounds (4a, 4b, 4d and 4e)
was observed such as monotropic intercalated smectic A phase.
There is a similar transition effect of fluorine and chlorine substitution
(though in a different position), leading to lower transition
temperatures for the fluorinated and chlorinated compounds.
3.5. UV–vis absorption studies
The azobenzene units introduced functional properties into the
bent-core mesogens [33], leading to the possibility of photoisomerization
and photochromic behavior. Solutions with concentrations
of 2.5 10 5 mol/L of 4a–f were prepared in chloroform for UV–
visible absorption studies for preliminary results of the photochemical
properties.
The highest absorptions were observed at 366, 366, 365, 366,
365 and 355 nm of compound4a, 4b, 4c, 4d, 4e and 4f, respectively
and other maximum absorbance about 260 nm and 455–465 nm
for all compounds 4a–f (Fig. 3). The azo containing monomers in
the trans form all show a strong band in the UV-region
( 365 nm) which is attributed to the p–p * transition, and a weak
band in the visible region ( 460 nm) due to the n–p * transition.
The trans form is generally more stable than the cis form, but each
absorbance
1.5
1.2
0.9
0.6
0.3
0
240 300 360 420 480 540
wavelength (nm)
Fig. 3. UV/vis absorption spectra of 4a (red), 4b (blue), 4c (green), 4d (black), 4e
(orange) and 4f (pink line) in chloroform c = 2.5 10 5 mol/L. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)
M.R. Lutfor et al. / Optical Materials 32 (2009) 176–183 181
4b
4e
4f
4a
4d
4c
isomer can be converted into the other by light irradiation of the
appropriate wavelength. Polarized light can induce the reorientation
of azobenzene groups through photochemical trans–cis–trans
isomerization [34,35].
3.6. Photoisomerization studies
Since all the azo dyes shows similar absorption peak, for the
sake of simplicity, one of this azo dye (4d) has been investigated
for guest–host effect in which liquid crystal is the host and bent
core azo dye is the guest, 5% of this azo dye is mixed with 95% of
liquid crystal. Fig. 4 shows the absorbance study as a function of
wavelength at different time intervals with fixed UV intensity.
All these experiments have been carried out at room temperature.
Initially all the molecules are in nematic phase or so called
‘‘trans’’ state. Turning the UV radiation ON, all the molecules transforms
to isotropic phase or so called ‘‘cis’’ state. The liquid crystalline
phase is stabilized by the trans form but is destabilized by the
cis form. Therefore, conversion from trans to cis changes generally
leads to a lowering of the phase transition temperature. So it
changes from order to disorder transition. It has shown in Fig. 4
that after UV radiation is shined on the sample, absorption level
decreases first and then saturates within 80 s of exposure and
achieves a ‘’Photostationary’’ state.
Fig. 5 is obtained from Fig. 4 by taking the peak absorption values
as a different exposure time. One can see the clear changes
from trans state to photo induced cis state. Absorption level decrease
sharply at first and then slowly saturates at around 80 s.
That means, at 80 s almost entire trans isomers is transformed to
cis isomers.
Next shine the sample for 2 min (which is photostationery
state) and then switch off the UV radiation. When the radiation
is switched off (see Fig. 6) and the sample is left in the dark, reverse
isomerization takes place with the sample transforming into the
nematic phase from isotropic phase and consequently absorbance
increases. Absorbance spectrum has been recorded at certain intervals
to check the thermal back relaxation process. After 50,000 s
(around 13 h), system fully transforms to original nematic state.
Fig. 7 is taken from the Fig. 6 which shows the observed phenomena
of Fig. 6 clearly. Initially all cis isomers in isotropic phase
but slowly they start converting back to nematic phase or trans isomers.
It takes very long time (almost 13 h) to convert back to trans
state which is useful for creation of optical image storage devices.
With this idea optical storage device has been constructed. For
the creation of the optical image storage device, it is necessary to
Absorbance (arb.units.)
0.9
0.6
0.3
UV on
365 nm
450 nm
400 500 600
λ (nm)
No UV
1 sec
4 sec
8 sec
16 sec
31 sec
53 sec
72 sec
107 sec
135 sec
Fig. 4. Absorbance study as a function of wavelength for different exposure time.
Intensity used is 1 mW/cm 2 . Within 2 min, it attains photo stationary state.

182 M.R. Lutfor et al. / Optical Materials 32 (2009) 176–183
Absorbance (arb.uni)
0.8
0.7
0.6
0.5
UV ON Dynamics
0 40 80
Time (secs)
120
Fig. 5. The UV ON dynamics of the material we studied here. After 80 s it reached
photo stationery state completely.
Absorbance (arb.units)
1.2
0.9
0.6
0.3
UV off-Thermal Back Relaxation
400 500 600
λ (nm)
Before UV
UV off
50 secs
800 secs
4800 secs
9800 secs
14800 secs
19800 secs
24800 secs
50000 secs
Fig. 6. The thermal back relaxation after shining the system with UV radiation for
2 min and left in the dark. After almost 13 h, the system transform to original
nematic state.
Absorbance (arb.uni)
0.8
0.7
0.6
0.5
UV Off Dynamics
0 200 400 600 800
Time (mins)
Fig. 7. The UV off dynamics (or thermal back relaxation) of the material we studied
here. After 800 min it reaches original nematic state.
have long thermal back relaxation so that stored image lasts longer.
Sample is kept in the nematic phase at room temperature, and
Fig. 8. Optical storage device fabricated using bent core material. Device lasts
longer for almost 12 h. Contrast between the bright and dark state is excellent.
numbered mask is put on top of this. Initially they are in the nematic
phase, UV radiation of wavelength 10 mW/cm 2 coupled with
365 nm UV filter is shined for 10 min for the creation of the display.
Wherever mask is present, it is still in nematic phase whereas
UV exposed area transforms to isotropic phase (see Fig. 8).
This device stayed almost 12 h which is useful for certain applications.
One can get good contrast since transformation is from
nematic to isotropic.
4. Conclusion
The liquid crystals monomers containing azobenzene chromophores
were prepared as target molecules, substituted/or nonsubstituted
1,3-phenylene bis-[4-(4-allyloxyphenylazo)benzoate]
(4a–c) and substituted/or non-substituted 1,3-phenylene bis-[4-
(4-allyloxy-3-fluorophenylazo)benzoate] (4d–f). Four bent-shaped
compounds exhibited Sm intercal mesophases ( 4a, 4b, 4d and 4e).
The compounds can be used for preparation of polymers or silylfunctionalized
bent-core mesogens, whereas the presence of the
azo linkage in these liquid crystals monomer is suitable for photochromism
studies and trans–cis–trans isomerizations cycles under
UV irradiation. Photoisomerization data suggests very long thermal
back relaxation for these kinds of azo dyes and can be possibly
used for optical storage devices. More investigation on these azo
dyes is in progress and will be reported in due course.
Acknowledgments
This research was supported by Fundamental Research Grant
(No. FRGS0006-ST-1/2006), Ministry of Education, Malaysia and
HKUST Grant CERG 612406.
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