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PROGRESS REPORT 

Recent Developments in the Application of 

Phosphorescent Iridium(III) Complex Systems 

By Christoph Ulbricht, Beatrice Beyer, Christian Friebe, Andreas Winter, and 

Ulrich S. Schubert* 

The recent developments in using iridium(III) complexes as phosphorescent 

emitters in electroluminescent devices, such as (white) organic light-emitting 

diodes and light-emitting electrochemical cells, are discussed. Additionally, 

applications in the emerging fields of molecular sensors, biolabeling, and 

photocatalysis are briefly evaluated. The basic strategies towards charged and 

non-charged iridium(III) complexes are summarized, and a wide range of 

assemblies is discussed. Small-molecule- and polymer-based materials are 

under intense investigation as emissive systems in electroluminescent 

devices, and special emphasis is placed on the latter with respect to synthesis, 

characterization, electro-optical properties, processing technologies, and 

performance. 

1. Introduction 

Phosphorescent transition-metal complexes are attracting significant 

attention with respect to potential applications, in 

particular in organic light emitting devices (OLEDs). [1,2] While 

exciton-based electroluminescence from small fluorophors 

cannot exceed a maximum quantum yield of 25% (according 

to spin statistics), phosphorescent complexes can theoretically 

achieve quantum yields up to 100%; due to heavy atom induced 

spin–orbit coupling, singlet as well as triplet excitons are 

harvested for the emission. [3,4] Combining phosphorescent 

emitters with proper host materials and optimized device set 

ups can result in highly efficient light-emitting devices. 

In particular, the interest in phosphorescent Ir III complexes is 

growing rapidly. [5] Very high luminescence efficiencies and rather 

short phosphorescence lifetimes can be realized. However, the 

most outstanding characteristic of this class of complexes might 

[*] Prof. U. S. Schubert, C. Ulbricht, Dr. A. Winter 

Laboratory of Macromolecular Chemistry and Nanoscience 

Eindhoven University of Technology 

P.O. Box 513, 5600 MB Eindhoven (The Netherlands) 

E-mail: u.s.schubert@tue.nl; www.schuber-group.com 

Prof. U. S. Schubert, C. Ulbricht, B. Beyer, C. Friebe, Dr. A. Winter 

Dutch Polymer Institute (DPI) 

P.O. Box 902, 5600 AX Eindhoven (The Netherlands) 

Prof. U. S. Schubert, B. Beyer, C. Friebe 

Laboratory of Organic and Macromolecular Chemistry 

Friedrich-Schiller-University Jena 

Humboldtstr. 10, 07743 Jena (Germany) 

DOI: 10.1002/adma.200803537 

be the variability of the electro-optical 

properties. Their metal–ligand-based luminescence 

provides the opportunity to tune 

the emission color over the whole visible 

spectrum by varying the attached 

ligands. [6,7] All these criteria make iridium(III) 

complexes highly appealing as 

phosphors in multicolor organic lightemitting 

diodes (OLEDs), but they also 

show promising potential in other applications. 

The research in sensing, biolabeling, 

and photocatalysis also utilizes the attractive 

features of Ir III complexes. 

These developments are supported by 

the broad diversity of possible structures. 

The most prominent coordination motifs 

are cyclometalating ligands, available in a wide range. Triscyclometallated 

homo- and heteroleptic complexes, as well as 

bis-cyclometallated ones, are the most common phosphorescent 

Ir III 

species. A variety of so-called ancillary ligands gives 

additional possibilities to define structure and to tune the 

properties. Neutral and charged complexes are accessible, and the 

ligand design spans from small modifications and functionalizations 

over dendritic layouts to polymeric assemblies. In particular, 

polymers containing iridium(III) complexes are gaining growing 

interest, combining the appealing features of both phosphor and 

polymer matrix within one material. Besides selected examples 

for the design of new small and dendritic Ir III complexes, a 

detailed overview of polymer-embedded phosphors with a focus 

on the synthetic strategies towards the different polymeric 

assemblies is given. 

2. Iridium(III) Complex Systems: Synthesis and 

Properties 

In general, iridium(III) complexes are characterized by the great 

inertness of their saturated coordination sphere requiring harsh 

reaction conditions to substitute the ligands of the commonly 

used starting iridium(III) chloride hydrate. Nonetheless, the rich 

coordination chemistry of Ir III covers a wide range of complexes, 

including mono-, bis-, and tris-cyclometallated species. [8] For 

basic synthetic concepts and highlights of earlier examples of Ir III 

complex systems, several review articles are available. [8–13] With a 

growing interest in phosphorescent iridium(III) complexes as 

emissive species in various applications, e.g., OLEDs or lightemitting 

electrochemical cells (LECs), the development and 

4418 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 4418–4441

www.advmat.de 

optimization of new synthetic approaches towards new types of 

ligands and cyclometallated Ir III complex systems represent 

highly active fields of current research. Today such iridium(III) 

complexes are the most efficient and versatile class of 

phosphorescent emitters produced, [14,15] e.g., they feature shorter 

triplet lifetimes and lesser triplet–triplet annihilation at high 

currents compared to Pt II species; as a consequence higher 

quantum efficiencies can be achieved. [1] Strong spin–orbit 

coupling leads to mixed singlet and triplet metal-to-ligand 

charge-transfer (MLCT) states as well as to mixed ligand-based 

emitting states. The metal–ligand-based emission enables an 

efficient tuning of the emission color by varying the ligands, and 

thus, full-color applications based on phosphorescent Ir III 

complexes can be realized. 

In the following, the recent developments for the preparation 

of iridium(III) complexes utilized in phosphorescent devices 

will be highlighted. Tris-cyclometallated complexes or neutral 

bis-cyclometallated derivatives with ancillary ligands, such as 

acetylacetonates or picolinates, exhibit high potentials for modern 

OLED applications. [1,16] Furthermore, charged bis-cylometallated 

complexes, mainly with oligopyridines as ancillary ligand, are 

particularly interesting as emitters in LECs, [1,17] biolabeling [18–20] 

or photocatalytic applications. [9] 

A general overview of the most common synthetic strategies 

towards the various types of phosphorescent Ir III complexes is 

depicted in Scheme 1. [16] The m-dichloro bridged dimer 

[Ir(C^N) 2 -m-Cl] 2 , conveniently prepared from a reaction of the 

respective ligand and IrCl 3 xH 2 O, [21] plays a central role in the 

coordination chemistry of these complexes. The chloro-bridge 

can be split by chelating ligands leading to neutral (L^X ¼ 

b-diketonates, picolinates, etc.) or charged bis-cyclometallated 

complexes (N^N ¼ 2,2 0 -bipyridines, 1,10-phenanthrolines, etc.) 

with preferred trans-N,N configuration of the C^N ligands (path 

a). The addition of a third cyclometallating ligand results in 

tris-cyclometallated Ir III complexes (path b). With cautious 

control of the reaction conditions, the kinetically preferred 

meridional (mer) or the thermodynamically favored facial (fac) 

isomers are accessible; homoleptic, [22–24] as well as heteroleptic 

ones, [25–28] have been obtained with high selectivity. It has been 

shown that in solution, applying thermal or photochemical 

energy, mer-isomers can be converted into the fac-form (path 

c). [23,29] The lower thermodynamic stability of the kinetically 

favored meridional formation is primarily due to the strongly 

trans-influencing aryl groups opposite to each other (in the 

fac-isomer all three aryl groups are opposite to pyridyl or other 

neutral donor groups). [16] The direct route towards fac-Ir(C^N) 3 

starting from the Ir(acac) 3 precursor, where acac is acetoacetonate 

(path d), or IrCl 3 xH 2 O is a common approach for phenylpyridine 

(Hppy) and its derivatives. [30] 

2.1. Neutral Iridium(III) Complexes 

As already pointed out, neutral iridium(III) complexes are widely 

used as triplet emitters in OLEDs featuring external quantum 

efficiencies up to nearly 20%. In general, the emission color can 

be tuned via the combination of cyclometallating and ancillary 

ligands coordinated to the Ir III core. [1,16] A variety of emissive 

complexes covering the whole visible spectra—from blue, over 

Christoph Ulbricht was born in 

Saalfeld (Germany) and studied 

chemistry at the Friedrich-Schiller- 

University Jena (Germany). He 

graduated in chemistry in 2005. 

Since 2005 he is Ph.D. student in 

the group of Prof. U. S. Schubert 

at the Eindhoven University of 

Technology (The Netherlands), 

where he is working on the design 

of polymeric phosphorescent 

materials. 

Dr. Andreas Winter was born in 

Herne (Germany) and studied 

chemistry at the University of 

Dortmund (Germany), where 

he graduated in organic 

chemistry in 1999. In 2003, he 

received his Ph.D. in chemistry 

(University of Paderborn, 

Germany) for work on applications 

of the Mannich reaction in 

the synthesis of pyridine derivatives 

under supervision of 

Prof. N. Risch, and stayed on as a postdoc. Subsequently, in 

2005 he joined the group of Prof. U. S. Schubert (Eindhoven 

University of Technology, the Netherlands). His research is 

focused on the synthesis of emissive and luminescent 

metallo-supramolecular assemblies. 

Ulrich S. Schubert studied 

chemistry and biochemistry and 

received his Ph.D. for research 

under the supervision of Prof. 

C. D. Eisenbach (Bayreuth, 

Germany) and Prof. G. R. 

Newkome (Florida, USA). After 

a postdoc with Prof. J.-M. Lehn 

(Université Strasbourg, France) 

and a habilitation with Prof. O. 

Nuyken (Technische Universität 

München, Germany), he held a 

temporary position as a professor at the Center for 

NanoScience (TU München) in 1999–2000. From June 2000 

to March 2007 he was Full Professor at the Eindhoven 

University of Technology (Chair for Macromolecular Chemistry 

and Nanoscience), the Netherlands. Since April 2007 he 

is Full Professor at the Friedrich-Schiller-University Jena (Chair 

of Organic and Macromolecular Chemistry), Germany, and 

Part-time Professor in Eindhoven. In addition, he is scientific 

chairman of the cluster HTE of the Dutch Polymer Institute. 


Adv. Mater. 2009, 21, 4418–4441 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4419

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Scheme 1. Schematic representation of the synthetic strategies utilized for the synthesis of cyclometallated iridium(III) complexes and selected examples 

of bis- and tris-cyclometallated neutral iridium(III) complexes (see text for details). Reproduced with permission from [2]. Copyright 2008, Wiley-VCH. 

green, yellow, and orange to red—has been introduced. The 

majority of these phosphorescent Ir III complexes can be assigned 

to two main categories—bis- and tris-cyclometallated. 

2.1.1. Bis-Cyclometallated Iridium(III) Complexes 

Bis-cyclometallated Ir III complexes can be easily obtained from 

the corresponding chloro-bridged dimer complexes. Splitting the 

chloro-bridge and introducing monodendate or bidentate ligands 

provide access to a wide range of neutral and charged complexes. 

Here, the most widely used ligands are acetoacetonate (acac), 

picolinate (pic), bipyridine (bpy), and their structural analogues. 

These ancillary ligands provide additional possibilities for the 

tuning of the electro-optical properties, as well as for the 

introduction of lateral functionalities. 


www.advmat.de 

For the synthesis of neutral bis-cyclometallated acetoacetonate 

and picolinate complexes, the most commonly used conditions are 

often rather harsh. Mixtures of precursor complex, the designated 

ligand, base (e.g., Na 2 CO 3 ), and high boiling alcohols (e.g., 

2-ethoxyethanol) are stirred under reflux for several hours usually 

generating the desired complex in good yields and purity. [6,7,31] 

However, much milder reaction conditions have proved to work out 

as well, reducing the formation of side products, simplifying 

the purification procedure, and providing the opportunity to include 

more sensitive functionalities. Tsuzuki et al. successfully synthesized 

a number of bis-cyclometallated iridium(III) acetoacetonate 

complexes by reacting the respective precursor and acetoacetone in 

a mixture with ethanol and Na 2 CO 3 at 50 8C for 2–6 h. [32] By 

replacing 2-ethoxyethanol with a non-alcoholic solvent in the 

coordination reactions of 2,7-dibromo-fluorene-functionalized 

acetoactone derivatives, Evans et al. were able to avoid undesired 

hydrodebromination side reactions. Performing the reactions in 

acetonitrile at 80 8C in the presence of Na 2 CO 3 , the desired 

functionalized bis-cyclometallated acetoacetonate complexes were 

obtained in 74–81% yield after purification. [33] A synthetic approach 

for iridium(III) bis-cyclometallated acetoacetate complexes introduced 

by DeRosa et al. [34] was adapted for the formation 

of acetoacetonate complexes by Graf et al. [35] Using silver 

trifluoroacetate to split the chloro-bridged dimer, triethylamine as 

base, and acetone as solvent, an acetoacetone derivative, 

11-(2,5-dibromo-4-hexyloxy-phenoxy)-undecane-2,4-dione, was 

coordinated within 3 h of reflux. Also, bis-cyclometallated iridium(III) 

picolinate complexes can be obtained under rather mild 

conditions. By refluxing the precursor and designated ligand for 

24 h in dichloromethane, Zhen et al. formed the desired picolinate 

complex. [36] 

While the structure of bis-cyclometallated complexes synthesized 

via chloro-bridged precursors is usually predetermined, 

Baranoff et al. observed partial thermal isomerization of a 

picolinate complex during vacuum sublimation. [37] The isomerization 

could be reproduced in solution upon thermal treatment, 

in a process analogous to the reported mer–fac isomerization of 

tris-cyclometallated Ir III complexes. While refluxing in glycerol 

for 20 h resulted in 40% of the new isomer, the attempt to induce 

the isomerization by irradiation (i.e., UV and visible light) was not 

successful. 

Besides acetoacetone, picolinic acid, and the multitude of their 

derivatives, other structures have recently found application as 

ancillary ligands (Scheme 1). [38] Acetoacetone-resembling acetoacetates 

with a large variety of residues are accessible from 

diketenes or by transesterification and can be coordinated under 

mild conditions. Complexes with vinyl-, oxetane-, or methacrylate-functionalized 

acetoacetate ligands have found use in the 

formation of more complex systems. The use of various 

2-pyridylazoles as ancillary ligands have demonstrated their 

potential for the tuning of optoelectrical properties 

(Scheme 1). [10,39–42] In particular, they proved to be a promising 

option in the construction of efficient blue-emitting complexes. 

2.1.2. Tris-Cyclometallated Iridium(III) Complexes 

In 1985, Watts and co-workers isolated tris(phenylpyridinato)- 

iridium(III), Ir(ppy) 3 , as an unexpected by-product in the synthesis 

of the chloro-bridged phenylpyridinato Ir III dimer complex, 

[Ir(ppy) 2 -m-Cl] 2 . [43] A general protocol for the synthesis of Ir(ppy) 3 

in high yields, starting from Ir(acac) 3 and Hppy, was described by 

the same group in 1991. [30] Since then, numerous variations of the 

basic Ir(ppy) 3 structure (e.g., introduction of electronwithdrawing 

or electron-donating substituents; [44–49] extension of 

the p-conjugated system; [50–56] replacement of the pyridine ring by 

other N-heteroaromatic rings; [26,57] or lateral functional groups for 

post-complexation modifications [58,59] ) have been reported 

(Scheme 1). The outstanding role of tris-cyclometallated Ir III 

complexes (both homoleptic and heteroleptic) based on phenylpyridine-type 

derivatives as ligands is underlined by the tremendous 

number of scientific publications and patents dealing with the 

synthesis and/or application of the respective complexes (June 

2009: nearly 2000 hits in SciFinder). Therefore, the advances in this 

field cannot be discussed to a full extent here. Only a few selected 

examples will be described in the following, and the reader is 

referred to recent literature published elsewhere. [1,8–13,16,60] 

The two configurational isomers of tris-cyclometallated Ir III 

complexes notably show differences in their photophysical 

properties, i.e., the fac-isomers feature over an order of 

magnitude longer lifetimes and higher quantum efficiencies 

than their meridional counterparts. [23] On the other hand, Sun 

and co-workers described mer-Ir(mppy) 3 (mppy ¼ 2-phenyl-4- 

methyl-pyridine) as an exception to this general concept of only 

low quantum efficiencies for mer-isomers. [61] The synthesis of 

tris-cyclometallated Ir III complexes in the thermodynamically 

favored fac-configuration usually requires harsh reaction conditions, 

e.g., refluxing glycerol or excess of ligand. [23,44,62] Utilizing 

microwave irradiation, the reaction times could be shortened 

remarkably. However, further optimization would be desirable to 

overcome the requirement of large excess of ligand material or 

the still rather low yields, in particular for large systems. [63,64] The 

kinetically favored mer-isomers can be obtained by performing 

the reactions at lower temperatures (e.g., in glycerol at 

120–150 8C) inhibiting the formation of the fac-isomers. By 

utilizing silver salts such as silver triflate to bind the chloride 

ligands of the starting species, much milder conditions can be 

applied to obtain the desired tris-cyclometallated complexes. [26,34] 

A promising approach towards the selective synthesis of 

mer-isomers was recently introduced by McGee and Mann. [65] 

In this work, a reactive m-hydroxy bridged dimer complex 

[(C^N) 2 Ir-m-OH] 2 was used to enable the reaction under mild 

conditions. The group of Williams showed that by substituting 

three bidentate HC^N ligands with a tridentate biscyclometallating 

ligand and a tridendate mono-cyclometallating 

ligand in the coordination sphere of Ir III , the formation of the 

mer-isomers was excluded (Scheme 1). [66] 

Besides these recent contributions with respect to the synthesis 

of tris-cyclometallated Ir III complexes in general, major advances 

with respect to color tuning should be mentioned. In particular, 

the development of stable and efficient blue emitters still 

represents a major goal. A blue shift of the emission can be 

realized by a widening of the HOMO LUMO energy bandgap 

(HOMO ¼ highest occupied molecular orbital; LUMO ¼ lowest 

occupied molecular orbital). By increasing the degree of 

fluorination on the ligands and simultaneously replacing pyridine 

by pyrazole, Dedeian et al. produced green-blue emitting 

tris-cyclometallated Ir III complexes (Scheme 1). [26] Samuel and 

co-workers reported blue emission [Commission Internationale 



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de l 0 Eclairage (CIE) x,y-coordinates 0.16 and 0.12] for Ir III 

complexes bearing three phenyl-[1,2,4]triazoles as cyclometallating 

ligands. [67] These systems, however, still suffer from 

significant luminescence quenching due to vibrational decay. 

Various efficient red-emitting tris-cyclometallated Ir III complexes 

with a dendronized framework are discussed in a contribution by 

Zhou et al. [50] 4-Phenyl-phthalazines were introduced as new 

cyclometallating ligands by Mi et al. [68] and Tong et al. [69] yielding 

red-emitters which show high quantum efficiencies in multilayered 

devices. 

2.2. Charged Iridium(III) Complexes 

Due to their promising photophysical properties, ionic character 

and good solubility in polar organic solvents or even in aqueous 

media, cationic iridium(III) complexes have gained much interest 

in recent years. [21,70] As introduced by Neve and co-workers, [61,71] 

the commonly used synthetic protocol towards cationic biscyclometallated 

iridium(III) polypyridyl complexes is based on a 

bridge-splitting reaction of the appropriate chloro-bridged 

iridium dimer complexes under mild conditions. A variety of 

different 2,2 0 -bipyridine, 1,10-phenanthroline, and 2,2 0 :6 0 ,2 00 - 

terpyridine derivatives has found use as neutral bidendate 

ligands. [9,20,68,72] The introduction of electron-withdrawing or 

electron-donating groups on the cyclometallating ligands in 

combination with lateral (p-conjugated) substituents on the 

polypyridyl ligand enabled the adjustment of the electro-optical 

properties of the complexes. [9] Modifying the cyclometallating 

ligand with different substituents [e.g., F, CF 3 , C(CH 3 ) 3 ] and 

coordinating various chelating ligands to the iridium(III) cores, 

Bernhard and co-workers have prepared a library of luminophores 

featuring high color versatility, a broad range of 

excited-state lifetimes (nanoseconds to several microseconds), 

as well as remarkable photoluminescence quantum yields 

(PLQYs). [9,73,74] Similar to their neutral counterparts, the ligand 

field stabilization energy (LFSE) in such charged complexes is 

strongly dependent on the position of the substituents with 

respect to the cyclometallating carbon atom. [32,75,76] Thompson 

et al. were able to show that the excited-state properties of 

bis-cyclometallated Ir(III) complexes can be chemically controlled 

simply via the nature of the ancillary ligand. [77] The enhancing 

effect of the increased steric hindrance of the ancillary N,N-ligand 

on the PLQYs has been reported by Wu and co-workers 

(Scheme 2). [78] Supported by theoretical investigations, Huang 

and co-workers recently described their concept of tunable 

emission via the expansion of the p-conjugated system of the 

chelating ligand. [79] Related work by Mussini, Roberto, Fantacci 

and their co-workers deals with the extension of this approach; the 

photophysical properties of such cationic Ir III complexes could be 

further fine-tuned via lateral electron-rich or electron-poor 

substituents on phenanthroline-type ligands. [80] Functionalized 

2,2 0 :6 0 ,2 0 -terpyridine derivatives and related structures have also 

been used as the bidentate ligand in bis-cyclometallated 

iridium(III) compounds (Scheme 2). [81,82] Highly efficient 

triplet–triplet intramolecular energy-transfer from a biscyclometallated 

Ir III core to a lateral C 60 -substituent on a 

functionalized bipyridine ligand was reported by Nierengarten 

and co-workers. [83] 

In continuation of prior work, De Cola and co-workers described 

new dinuclear iridium(III) complexes obtained by Pd(0)-catalyzed 

coupling reactions on bromophenyl-substituted mononuclear Ir III 

species (Scheme 2). [84] The same approach was used by Arm and 

Williams to obtain mixed metallic assemblies featuring efficient 

energy transfer from Ir III cores to Ru II centers. [85] 

Such charged complexes find particular application in protein 

labeling for biomedical analysis, as oxygen sensors, and for 

photocatalytic water-splitting. Their potential usage as active 

species in LECs has moved into the focus of current research. 

To suppress crystallization or aggregation occurring in blended 

films [86,87] and to enhance the processability (e.g., via inkjetprinting 

or spin-coating, by inducing film-forming abilities [88] ), 

cationic Ir III complexes have successfully been introduced into 

polymeric materials, both non-conjugated [89–91] and conjugated 

ones. [92–95] 

In addition to the complexes bearing three bidentate ligands, the 

analogous compounds containing two tridentate ligands [96] (e.g., 

2,2 0 :6 0 ,2 00 -terpyridine, 2,6-diaryl-pyridine, and their derivatives) 

have recently gained more attention as potential candidates for 

applications in areas such as luminescent sensors or materials for 

directional energy and electron transfer. [97–104] A detailed review 

about this concept has recently been published by Williams 

et al. [105] Iridium(III) mono-terpyridine complexes decorated with 

electron-donor or -acceptor groups have furthermore been 

employed as asymmetric chromophores in nonlinear optics. [106,107] 

By introducing strong ligand-field-stabilizing ligands, such as 

CN or CO, the energy gap between the HOMO and LUMO can 

be significantly increased resulting in anionic or cationic 

iridium(III) complexes with bright blue emission. Recent 

examples by Fantacci and Nazeeruddin and their 

co-workers [108,109] as well as Chin et al. [110] have nicely expanded 

the toolbox of color-tuning in iridium(III) complexes (Scheme 2). 

3. Polymers Containing Iridium(III) Complexes 

For optical device applications, phosphorescent emitters are 

commonly imbedded within an appropriate matrix. Irrespective 

of the kind of materials used (small molecules and/or 

macromolecules, e.g., polymers), the host should ideally promote 

several tasks: separation of phosphors, charge-injection, charge/ 

energy-transport, and transfer to the phosphorescent species. 

Excessively high concentration or aggregation of the emitters 

often leads to reduced emission efficiency due to concentration 

quenching and triplet–triplet annihilation. For an efficient charge 

injection, the energy barriers to the adjoining layers should not be 

too high. [111] Host materials can be further on classified based on 

their ability to transport holes or electrons. Besides hole- and 

electron-transporters, a third group – ambipolar chargetransporters 

– is defined as host materials that can readily 

transport both holes and electrons. [112] 

The combination of suitable polymeric hosts with small 

emitter molecules, together with additional charge transporting 

molecules within blends, has become a widespread technique in 

the fabrication of polymer light-emitting diodes (PLEDs). 

Blended systems, however, inherently involve the risk of 

undesired phase separation, aggregation, or crystallization, which 

can harm the device performance. Therefore, the design of 

(co)polymers, combining different functions (charge transport 


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Scheme 2. Schematic representation of selected charged iridium(III) complexes recently introduced as promising luminophores in various types of 

application (see text for details). 

and emission), has received increasing attention. [11,113,114] The 

expected benefits are better energy transfer to the emitters (and 

thus higher efficiencies) and higher durability of the device. In 

addition, polymeric materials are of special interest, with respect 

to their flexibility, film-forming properties, and processability 

from solution, e.g., by inkjet printing. [86,115] 

3.1. Complex-Containing Polymers: Synthesis and Properties 

There are in principle five general routes to synthesize polymers 

containing transition metal complexes (Scheme 3): I) ‘‘decoration’’ 

of (co)polymers with complexes, II) complexation at 

(co)polymers, III) (co)polymerization by complexation, IV) 

complex as polymerization initiator, and V) (co)polymerization/ 

condensation of complex ‘‘monomers’’. 

I) In order to ‘‘decorate’’ (co)polymers with complexes, suitable 

combinations of functionalities attached to the (co)polymer and 

the complex, respectively, are utilized. [25,58,59,116,117] Weck and 

co-workers reacted aldehyde-functionalized heteroleptic triscyclometallated 

Ir III complexes with amino-group bearing styrene 

units, copolymerized with N-vinylcarbazole or styrene, forming 

Schiff’s bases, which were further on reduced to chemically more 

inert amines. [58] In another approach, they used a so-called ‘‘click’’ 


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Scheme 3. Schematic representation of different general approaches for the synthesis of 

(co)polymers that contain transition metal complexes. 

reaction to attach heteroleptic tris-cyclometallated Ir III complexes 

equipped with a terminal C C bond to azide-functionalized 

N-vinylcarbazole (Scheme 4) and styrene copolymers. [59] Since 

polyvinylcarbazole (PVK) is a well-known hole transporter and is 

widely used as a polymeric host for phosphors, the 

N-vinylcarbazole copolymers appear highly promising as emissive 

materials in PLEDs. By reacting bis-cyclometallated Ir III 

complexes bearing a vinyl group at the ancillary ligand (i.e., 

pyridine or acetoacetate) and hydride-terminated poly(dimethylsiloxane) 

(PDMS) via hydrosilylation, DeRosa and Köse et al. 

obtained PDMS systems with complexes as endgroups. [25,114,115] 

Polysiloxanes are generally characterized by a high gas permeability, 

and the materials functionalized with Ir III emitters 

blended in polystyrene (PS) exhibited good performance as 

oxygen sensors. 

II) (Co)polymers bearing suitable ligand sites can be 

transformed to emitter-equipped systems by reacting them with 

proper precursor complexes. [87–90,118–125] One of the earliest 

examples of an iridium(III)-complex-containing polymer was 

obtained by Kim and coworkers in a one-pot, two-step 

complexation reaction; after treating Ir(acac) 3 with 

2-phenylpyridine, the ligand-equipped polymer [poly((2-(4- 

vinylphenyl)pyridine)co-vinylcarbazole)] was added to achieve 

the formation of polymer-bound tris-cyclometallated Ir III complexes 

under rather harsh conditions (170 8C for 12 h). [118] 

Photoluminescence (PL) investigations in solutions pointed to an 

intermolecular energy transfer between host and guest rather 

than an intramolecular one. While in dilute solutions only the 

high-energy emission of the polymeric host could be observed, 

the complex emission appeared only at high concentrations. The 

PL of copolymer films showed only a small fraction of host 

emission. Upon electro-excitation, the emission of the host was 

almost completely suppressed. Applying this copolymer as the 

emissive layer (EML) in multilayer devices, a maximum quantum 

efficiency (QE) of 4.4% and a power efficiency of 5.0 lm W 1 

could be achieved. Also, Holdcroft and co-workers [119] as well as 

Langecker and Rehahn [120] performed inter alia 

complexation reactions on conjugated copolymers 

in order to obtain systems equipped with 

tris-cyclometallated fac-iridium(III) complexes. 

Altering the reaction sequence applied by Kim’s 

group, Holdcroft and co-workers treated 

first the polymer, poly(9,9-dihexylfluorenealt-pyridine), 

with Ir(acac) 3 at 250 8C for 12 h, 

which is supposed to lead to the formation of 

mono-cyclometallated Ir III intermediates. In 

the second step, 2-phenylpyridine was added, 

and the mixture was heated again (250 8C for 

12 h) to finally obtain the desired complexed 

copolymer (Scheme 5). By adapting the 

iridium(III)- and the ligand-feed, the extent 

of complexation at the polymer could 

be varied. [119] Langecker and Rehahn [120] 

applied a different synthetic procedure. 

The treatment of poly(2,7-fluorene-co-5,4 0 - 

(2-phenylpyridine)) with a fourfold excess of 

[Ir(ppy) 2 -m-Cl] 2 and silver triflate at about 

100 8C for 4 days resulted in the coordination 

of roughly 50% of the polymer-bound ligand 

sites (2-phenylpyridine). Schubert and co-workers reported a 

number of charged, polymer-equipped bis-cyclometallated Ir III 

complexes obtained by reacting dimeric chloro-bridged iridium(III) 

precursor complexes with poly(e-caprolactone)- 

functionalized bipyridine [87,88] or poly(ethylene glycol) [89,121] 

and PS-functionalized terpyridines (here acting as a bidentate 

ligand only). [121] Detailed investigations by means of size 

exclusion chromatography (SEC) and matrix-assisted laser 

desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry 

showed the formation of well-defined materials. By treating 

terpyridines tethered to the acrylate moieties of a styreneblock-acrylate-copolymer 

with iridium(III) mono-terpyridine 

complexes (at 200 8C for 20 min), Aamer and Tew achieved the 

formation of charged iridium(III) bis-terpyridine complexes at 

some of the polymer-bound terpyridines. [122] The micellization 

behavior of the block copolymer in various solvents was 

investigated. Deng et al. synthesized random and block coand 

terpolymers copolymerizing ligand sites (i.e., styrenefunctionalized 

acetoacetone motifs) with various hole- and 

electron-transporting monomers (styrene or acrylate derivatives) 

under nitroxide mediate polymerization (NMP) conditions. Upon 

complexation with Ir III - or Pt II -precursors, a broad set of 

host–guest systems was obtained (Scheme 4). [121] Testing their 

performance in multilayer device set ups, the best assemblies 

resulted in a white-emitting device with maximum external 

quantum efficiency (EQE) of 4.9% and in a green-emitting device 

with EQE of 10.5%. Cao and co-workers synthesized conjugated 

fluorene-alt-carbazole polymers bearing charged Ir III complexes 

in the side chains by reacting iridium(III) dimer complexes and 

2-(pyridine-2-yl)benzimidazoles, grafted via an alkyl spacer to the 

carbazole units, in different ratios. [92] By the introduction of 

charged complex species, implementation of LEC-analogous 

features, such as improved charge-injection, were attempted. 

Multilayer as well as single-layer OLEDs were prepared and 

investigated. The initial maximum EQE of 7.3% decreased rapidly 

to 3.4%. Koga et al. described the complexation of methyl 


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Scheme 4. Schematic representation of selected examples for the synthesis of polymeric systems containing iridium(III) complexes. 

methacrylate-styryldiphenylphospine copolymers with a chlorobridged 

biphenyl cyclooctadiene and C,N-chelated iridium(III) 

dimer, in different ratios, obtaining the bis-phosphine and 

mono-phosphine iridium(III) polymer complexes, respectively. 

[124,125] 

For the ‘‘decoration’’ (I) and the complexation at (co)polymers 

(II) a high conversion at mild conditions is preferable, enabling 

the use of stoichiometric amounts by avoiding damage and 

degradation at the polymer. The ‘‘clicking’’ of azides [59] and the 

coordination to phosphine [124] are two promising examples in this 


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Scheme 5. Schematic representation of the general structure and selected examples of current polymers, non-conjugated and conjugated, containing 

iridium(III) complexes. 

respect. While the formation of tris-cyclometallated [118–120] and 

bis-terpyridine iridium(III) complexes [122] are usually rather 

demanding with respect to temperature and conversion, other 

types of complexes, e.g., charged bis-cyclometallated complexes 

bearing bipyridine, can often be obtained in high yields at rather 

mild conditions. [89,90] Steric demands (e.g., in the case of 

coordination sites in a conjugated backbone, [119,120] polymer 

blocks, [121] or crosslinked systems [125] ) is an aspect, which might 

need to be considered as well. Complex-free polymers are usually 

easier to investigate by techniques such as SEC, and influences of 

incorporated complexes are easily revealed by comparing final 

and precursor polymers. Both approaches I and II inherently 

involve the possibility of varying the complex content of the final 

polymer by adjusting the feed of the reactive complex or 

precursor, leaving reactive and potential coordination sites at the 

polymer unreacted and uncomplexed, respectively. [92,124] 

III) There is a number of known examples in literature dealing 

with (co)polymerization by complexation, resulting in so-called 

chain-extended polymers. Assemblies based on the ‘‘polycomplexation’’ 

of bis(terpyridine)s by Ru II , [128,129] Ni II , [128] Co II , 

Fe II , [129] and Zn II[130,131] as well as poly-Pt II -acetylene [132] and 

polyferrocene-systems, just to name a few, can be obtained this 

way – but up to now there is no attempt reported to use this 

method for the formation of Ir III containing polymers. 

Iridium(III) can form bis-terpyridine complexes similar to Ru II 

and Fe II , but comparably harsh conditions, in particular high 

temperatures, are usually necessary to obtain the desired 

complexes in low to moderate yields. 

IV) Another possibility for obtaining complex-containing 

polymers is to use a respective functionalized complex as 

(co)polymerization initiator. While this approach has been 

described for Ru II complexes, [133] an iridium(III) complex 


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functioning as polymerization initiator has not yet been reported 

in literature. 

V) The most widely utilized method to access polymers 

containing phosphorescent Ir III complexes is the (co)polymerization, 

[33,134–148] or (co)condensation, [36,93–95,119,120,147–169] of suitably 

functionalized complexes. Weck and co-workers used the 

ring-opening metathesis polymerization (ROMP) of functionalized 

exo-norbornene [134,136] or cyclooctene monomers [135] to 

synthesize homo- and copolymers equipped with Ir III emitters. A 

first proof of the feasibility was gained by polymerizing 

norbornenes bearing tris-cyclometallated (facial and meridional) 

complexes as well as charged bis-cyclometallated iridium(III) 

bipyridine complexes and by performing copolymerizations with 

alkyl-functionalized norbornene. [134] They demonstrated the 

copolymerization of a cyclooctene-functionalized triscyclometallated 

Ir III complex with a carbazole-functionalized 

cyclooctene. [135] Further on, norbornenes bearing various 

heteroleptic tris-cyclometallated Ir III complexes were copolymerized 

with a (bis-carbazole-fluorene)-norbornene derivative. [136,137] 

Testing several materials in a multilayer device assembly, a 

maximum efficiency of 4.9% (at 100 cd m 2 ) could be realized. [137] 

Ulbricht and co-workers [138] and Rehmann et al. [139] reported the 

synthesis of oxetane-functionalized bis-cyclometallated iridium(III) 

acetoacetate complexes and the optimization of multilayer 

OLEDs employing these emitters, which were covalently 

incorporated into a crosslinked matrix. The hole-transport layers 

(HTL) as well as the matrix of the EML consisted of 

oxetane-equipped tetraphenylbenzidine (TPD) derivatives, which 

were crosslinked by photo-induced cationic-ring-opening polymerization 

after deposition from solution. By improving the 

charge balance, the efficiency of the manufactured devices could 

be significantly increased; 18.4 cd A 1 at an operating voltage of 

5 V and a brightness of 100 cd m 2 were achieved. This was 

accomplished by the deposition of a polymeric, electron-transporting, 

and hole-blocking layer (HBL) on top of the 

crosslinked HTLs and EML as well as by the optimization of the 

layer thicknesses. Wang et al. [140] as well as Park and 

co-workers [141] used free radical polymerization initiated by 

2,2 0 -azobis(iso-butyronitrile) (AIBN) to synthesize copolymers 

possessing iridium(III) complexes. In the first case, a biscyclometallated 

iridium(III) complex coordinating acrylate as the 

polymerizable ancillary ligand was copolymerized with 

N-vinylcarbazole. [140] Wang et al. found evidence that this 

copolymer can perform better in devices than blended analogues. 

Park and co-workers copolymerized 3-vinylcarbazoles, bearing 

either a decyl-chain or a bis-cyclometallated iridium(III) 

picolinate complex connected via a dodecyl-spacer in 

9-position. [141] The expected suppression of phase segregation 

in the copolymers compared to the doped analogues was verified 

by confocal laser scanning microscopy (CLSM) studies. The 

performance of the materials in multilayer devices was 

investigated. Using 3-vinylcarbazole-based building blocks 

instead of conventional N-vinylcarbazole led to a reduction of 

carbazole-excimer formation in the polymeric materials upon 

excitation. This provided a higher triplet level for the host material 

compared to PVK and made it more suitable for phosphors with a 

high triplet energy. Sato and co-workers investigated iridium(III) 

complex containing polymeric systems, which were obtained by 

radical copolymerization as well. [33,142–145] They focused on 

copolymers from N-vinylcarbazole and bis-cyclometallated Ir III 

complexes coordinated to an acetoacetonate or a picolinate ligand, 

which were tethered to styryl- or vinyl-functionalities. [33,142–144] 

The authors also reported on a terpolymer emerging from 

the copolymerization of vinyl-functionalized TPD and a 

1,3,4-oxadiazole derivative with bis-cyclometallated Ir III complexes 

possessing an acetoacetonate ligand bearing a styryl-function. [145] 

The copolymers functionalized with red, green, or blue 

phosphorescent emitters were investigated with respect to their 

performance in multilayer OLEDs. Recently, Ulbricht et al. 

described the copolymerization of a bis-cyclometallated Ir III 

complex coordinating a methacrylate-equipped acetoacetate 

ligand with methyl methacrylate by free radical polymerization 

(using AIBN as initiator) and with a methacrylate-carbazole 

derivative applying atom transfer radical polymerization (ATRP) 

conditions (Scheme 4), respectively. [146] Unlike in PVK the 

carbazole-moieties of the ATRP-copolymer were introduced with 

a spacer to the polymeric backbone, which should even more 

suppress the formation of excimers [170] as in the case of 

3-vinylcarbazole based polymers. [141] An almost exclusive emission 

from the complex in highly diluted solutions indicated an 

efficient intramolecular energy transfer from the carbazole units 

to the triplet emitter. 

In order to synthesize conjugated polymers, polycondensation 

is usually the method of choice. For the preparation of conjugated 

polymeric systems containing phosphorescent Ir III complexes, 

the Suzuki cross-coupling [36,93–95,119,120,147–162,164–169] as well as 

the Yamamoto coupling [120,147,163,168] found widespread application. 

In the Suzuki reaction, arylic boronic acids or their esters 

are cross-coupled with arylic bromines in the presence of a 

Pd 0 -catalyst leading to the formation of arylic carbon–carbon 

bonds. Using the Yamamoto method arylic bromines are reacted 

with equimolar amounts of Ni(cod) 2 yielding coupled aromatic 

systems. 

Independently from the applied coupling method (i.e., Suzuki 

or Yamamoto), all reported Ir III complexes used in the formation 

of conjugated polymeric systems are equipped with two arylic 

bromo-functionalities. Depending on the desired structural 

embedding of the complex in the final conjugated polymer 

(Scheme 4 and 5), different possibilities are used to anchor the 

two reactive-sites at the complex. Both functionalities can be 

introduced with the ancillary or a third cyclometallating ligand 

forming the according bis-cyclometallated or heteroleptic 

tris-cyclometallated Ir III species, respectively. Another possibility 

is the coordination of a ligand tethered to a bis-bromofunctionalized 

aromatic system. As an alternative to the 

introduction of both reactive sites by one ligand, chloro-bridged 

iridium(III) precursor complexes bearing one reactive site per 

ligand can be used for the synthesis of bis-cyclometallated and 

heteroleptic tris-cyclometallated iridium(III) complexes possessing 

two bromo-equipped cyclometallating ligands. These 

different possibilities of introducing the complex moiety lead 

to a diversity of conjugated polymeric structures (Scheme 4 

and 5). All reported non-conjugated systems can be summarized 

as polymer-tethered complexes. Analogous conjugated assemblies, 

where complexes are attached with or without spacers to a 

carbazole or a fluorene moiety of a conjugated backbone, were 

described. In other cases, however, one or two of the complex 

ligands are integral parts of the conjugated polymeric backbone 



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(Scheme 5). This group of materials can be further subclassified 

depending on the involved ligand(s) (Scheme 4 and 5). The 

ancillary ligand (e.g., bipyridine or 1,5-bis-phenyl-acetoacetone) 

in bis-cyclometallated complexes as well as one of the ligands in 

heteroleptic tris-cyclometallated complexes can be incorporated 

within the backbone. Another possibility is to apply both 

cyclometallating ligands of a bis-cyclometallated complex as 

polycondensation endcappers, forming a polymer backbone 

where conjugation is interrupted by the metal centers of the 

incorporated complexes. Examples using respective heteroleptic 

tris-cyclometallated Ir III complexes are not described yet. 

Non-conjugated and conjugated systems possess distinct 

features, particularly regarding optical applications. Conjugated 

polymers are expected to provide better charge transport to the 

emitter, but the performance can suffer from the usually rather 

low triplet energy level of the polymeric backbone increasing the 

probability of energy back-transfer from the emitter to the 

polymer. Therefore, most of the current examples dealing with 

conjugated polymeric hosts systems are focused on red triplet 

emitters. In contrast to this, non-conjugated polymers usually 

possess rather high triplet energy levels and can, therefore, be 

seen as more universal host systems, which are also able to deal 

with high bandgap emitters, i.e., also blue emitters with rather 

high LUMO levels. [182] However, the design of conjugated 

polymers with comparatively high triplet energy levels is in 

progress. [111,171] 

Besides complex monomers, mainly two co-building block 

motifs – 2,7-linked 9,9-difunctionalized-fluorene and 3,6-bound 

9-functionalized-carbazole – are used in the formation of these 

conjugated polymeric systems. They are introduced as 

dibromo and/or di(boronic acid ester) derivatives (Scheme 6). 

Chen et al. presented some of the earliest examples of 

iridium(III) complex-containing conjugated polymeric systems. 

By cocondensating fluorene species via Suzuki and Yamamoto 

coupling, a number of 2,7-linked polyfluorenes with varying 

substituents in the 9-position were obtained. Dioctyl, bis(Ncarbazolyl-decyl), 

and 9-hexyl-9-(11,13-dioxo-tetradecyl) coordinating 

a bis-cyclometallated iridium(III) complex were the 

substituents applied. [147] The performance of the obtained 

copolymers was investigated in solution-processed devices 

(ITO/PEDOT/emissive polymer/Au/Al) exhibiting red emission. 

Mei et al., [151] Evans et al. [148] as well as Cao and 

co-workers [149,150,153] used the same structural approach. Utilizing 

the Suzuki cross-coupling method, conjugated polymers 

bearing phosphorescent Ir III complexes in the side chain were 

obtained. Also here the bis-cyclometallated Ir III complexes were 

Scheme 6. Schematic representation of the most widely used co-building 

blocks in the synthesis of conjugated polymeric systems hosting phosphorescent 

iridium(III) complexes – 2,7-bis(bromo/boronic acid ester)-9,9- 

dialkylfluorene (left) and 3,6-bis(bromo/boronic acid ester)-9- 

alkylcarbazole (right). 

tethered via a b-diketonate ligand to the polymeric backbone. Mei 

et al. cocondensated the same fluorene-complex motif used by 

Chen et al. [147] with dialkylfluorene derivatives. [151] As the relative 

incorporation rates of the applied monomers determine in part 

their distribution within the formed copolymer chain, Evans et al. 

applied a system narrowed down to two complementary 

components. Cross-coupling di(boronic ester)-dialkylfluorene 

macromonomers (approximately 15 and 16 fluorene units, 

respectively) and fluorene-complex monomer with each other, 

compositional drifts in the final copolymers, as well as a close 

proximity of complexes within a chain were excluded. [148] The 

respective applied fluorene-complex monomer possessed either 

an octyl-spacer between the fluorene moiety and the complex or a 

spacerless connection. It could be demonstrated that the 

introduction of a non-conjugated spacer between the phosphor 

and the conjugated polymer backbone is beneficial for the 

emission efficiency of the system. The spacerless system 

considerably suffered from triplet energy back-transfer from 

the complex to the polymer. 

Instead of fluorene-complex combinations, Cao and co-workers 

made use of carbazole-bound complexes in their syntheses of 

conjugated polymers with iridium(III) complexes in the side 

chains. [149,150,153] They coordinated 3,6-dibromo-N-(14- 

trifluoro-11, 13-dioxo-tetradecyl)-carbazole as ancillary ligand 

to form monomer-tethered bis-cyclomettalated Ir III complexes. 

Co-condensations of bis-(boronic acid ester)-dialkylfluorene with 

dibromo-N-alkyl-carbazole and different carbazole-complex 

monomers resulted in a number of copolymers 

(Scheme 4). [149] In further investigations, modified assemblies 

were prepared exchanging dibromo-N-alkyl-carbazole with 

4,7-dibromo-2,1,3-benzothiadiazole and dibromo-dialkylfluorene 

in the coupling mixtures. [150,152] Systems consisting of red 

phosphors bound to the fluorene-alt-carbazole backbones 

revealed an efficient energy transfer from backbone to emitter 

upon photoexcitation in films. The electroluminescence (EL) 

spectra exhibited almost no emission originating from the 

polymer matrix even at rather low complex loadings (e.g., 0.5%). 

In an optimized device configuration (multilayer; EML doped 

with electron transporter), a maximum EQE of 4.9% and a 

luminous efficiency of 4.0 cd A 1 with 240 cd m 2 at a bias voltage 

of 7.7 V and peak emission at 610 nm were recorded. [149] The 

other systems consisting of blue (fluorene moieties) and green 

([2,1,3]-benzothiadiazole moieties) fluorophors, as well as red 

(tethered Ir III complexes) phosphors were optimized for the 

emission of white light by adjusting the green and red emitter 

content. White light with CIE coordinates very close to the 

optimum could be obtained. [150,153] 

Wu et al. cross-coupled a mixture of bis(boronic acid 

ester)-dialkylfluorene, three different dibromo-fluorene derivatives, 

and 4,7-dibromo-2,1,3-benzothiadiazole to obtain conjugated 

copolymer systems bearing bis-cyclometallated Ir III complexes. 

[152] The phosphorescent emitter possessed a spacerless 

connection via the five-position of its picolinate ligand to a 

fluorene unit of the polymeric backbone. Similar to Cao’s reports, 

the content of green fluorophor and red phosphor were varied 

within a blue-emitting matrix to obtain white emission. 

To support the charge transport, hole- (triarylamine units) as 

well as electron-transport motifs (oxadiazole derivatives) were 

introduced with fluorene building blocks. White light with 


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contributions from all three primary colors and with the CIE 

coordinates close to the equal-energy white point was obtained 

from a single EML device configuration. A maximum luminance 

efficiency (LE) of 8.2 cd A 1 and a power conversion efficiency 

(PCE) of 7.2 lm W 1 could be realized. 

The monomers differing in their substituents (e.g., complex, 

alkyl, etc.) can be expected to possess similar reactivity, which 

should promote a statistical incorporation within the final 

copolymers (i.e., compositional drifts might be avoided). 

While the non-conjugated anchoring as a side-group can 

usually be expected to have only minor effects on the 

phosphorescent emitter, the incorporation within conjugated 

systems can result in considerable changes in the optical 

behavior. As mentioned previously, extending the conjugation 

on cyclometallating ligands is usually accompanied by a clear 

bathochromic shift in absorption and emission. For ancillary 

ligands such as bipyridine and 1,5-bis(phenyl) acetoacetone, the 

effect on the optical properties of the complex appears to be less 

pronounced. 

Cao and co-workers reported the incorporation of biscyclometallated 

Ir III complexes via their ancillary ligands, 

1,5-bis( p-bromophenyl)-acetoacetone and 1-(p-bromophenyl)- 

acetoacetone, into polymeric structures as monomer and endcapper 

units, respectively. A variety of polymers were obtained by 

cocondensating the complex monomers, the complex endcappers, 

dibromo-dialkylfluorene, dibromo N-alkylcarbazole, 

and/or dibromo fluorenone with bis(boronic acid ester)- 

dialkylfluorene via Suzuki cross-coupling. [156–160] Besides a 

number of red-emitting systems, [156,158–160] white-emitting 

copolymers were obtained. [157] Instead of benzothiadiazole as 

in the previous examples of white-emitting systems, [152] 

fluorenone was applied as green-emitting species (Scheme 5). 

In another variant of this approach, Huang and co-workers 

synthesized charged bis-cyclometallated Ir III complexes containing 

dibromo-bipyridine or dibromo-phenanthroline. Applying 

the Suzuki cross-coupling reaction, bis(boronic acid ester)- 

dialkylfluorene was cocondensated with the complex monomers, 

dibromo-dialkylfluorene, N-alkylcarbazole, and/or dibromo 

bis(N-carbazolyl-alkyl) fluorene. [93–95] Several red-emitting materials 

were obtained and investigated. While blended systems of 

charged complexes within hydrophobic conjugated polymers 

suffered from phase segregation, the imbedding of the complexes 

in the polymer backbone promotes their compatibility. The 

copolymers showed distinctly improved host–guest energytransfer 

compared to blended systems. However, luminance 

and efficiency of devices applying these materials were found to 

be inferior to assemblies based on neutral Ir III complexes. 

Therefore, further device optimization is required. 

Instead of a symmetrical building block, Kappaun et al. 

introduced commercially available 5,7-dibromo-8-hydroxyquinoline 

as ancillary ligand. The obtained complex and dibromo 

dialkylfluorene were reacted with di(boronic acid ester) dialkylfluorene 

under the conditions of Suzuki cross-coupling. [169] 

Using the pure material in a device, only weak red electroluminescence 

was obtained. By diluting the material in 

polyfluorene, white light (CIE: x ¼ 0.30, y ¼ 0.35) was emitted 

from the device. 

Ito et al. presented the first example of heteroleptic 

tris-cyclometallated Ir III complexes implemented via one of their 

ligands into a conjugated polymeric structure. Coupling the 

2,5-bis(2-bromo-dialkylfluorene)-pyridine ligand of the complex 

with bis(boronic acid ester)-dialkylfluorene under Suzuki crosscoupling 

conditions yielded a polymeric material with high 

complex loading (approximately 50 wt%). [154] A device applying 

the red-emissive copolymer as EML showed rather poor 

performance, which was mainly attributed to concentration 

quenching. Blending the copolymer with 4,4 0 -N,N 0 -carbazolebiphenyl 

(CBP) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3, 

4-oxadiazole (PDB) could significantly improve the device output 

from 0.03 to 0.63% maximum EQE. 

Schulz et al., [119] Zhang et al., [155] and Langecker and 

Rehahn [120] made use of the same heteroleptic triscyclometallated 

complex, fac-bis(2-phenylpyridine-C 2 ,N 0 )[2-( p- 

bromophenyl)-5-bromopyridine-C 2 ,N 0 ]iridium(III), to assemble 

their polymer variants, respectively. 

Aside from the previously mentioned complexation reactions 

at polymer-inherent ligand sites towards polymer-imbedded 

heteroleptic tris-cyclometallated Ir III complexes, Langecker and 

Schulz et al., made use of coupling reactions to directly synthesize 

such systems. Schulz et al. cocondensated bis(boronic acid 

ester)-dialkylfluorene with 3,4-dibromothiophene and complex 

monomers. [119] Two iridium(III)-containing polymers exhibiting 

red and greenish-yellow emission, respectively, were obtained. 

While only traces of complex emission could be found in PL 

spectra of solutions, energy transfer from the host to the emitter 

was observed in films. In EL spectra, the host emission was 

almost completely quenched. 

Langecker and Rehahn compared the Suzuki and Yamamoto 

coupling reactions with respect to their performance in the 

synthesis of such complex-containing conjugated systems. The 

complex monomer was coupled either with just dibromodialkylfluorene 

(Yamamoto) or with additional equimolar 

amounts of 1,4-bis(boronic acid ester)-2,5-dialkylbenzene 

(Suzuki). Due to distinctly higher yields and higher degrees of 

polymerization, the Yamamoto coupling reaction was found to be 

the superior method for the direct synthesis of the desired 

polymers. [120] In Suzuki cocondensations, the complex monomers 

appeared to act as endcappers, which was attributed to a 

lower reactivity of the bromine function at the phenyl ring of the 

cyclometallating ligand. Comparable to previous examples, the PL 

spectra from solutions are dominated by host emission. In films, 

energy transfer occurs from host to guest. 

In Suzuki cross-coupling reactions, Zhang et al. cocondensated 

bis(boronic acid ester)-dialkylfluorene with complex monomer and 

dibromo-di(3-dimethylamino)-propyl-fluorene. By quarternation 

of the tethered amino functions with bromoethane, the obtained 

copolymers were transformed into polyelectrolytes. [155] Beside the 

commonly used low work-function cathode material (Ba/Al), high 

work-function Al- and Au-cathodes were applied in the device 

configurations (ITO/PEDOT:PSS/PVK/emissive copolymer/cathode). 

Using a non-quaternized copolymer, similar performances 

were observed – EQEs of 0.54% (Au), 0.69% (Al), and 0.79% (Ba/ 

Al) were obtained. It seems that the amino groups facilitate the 

electron injection from high work-function metals. The quaternized 

materials, however, exhibited much lower device efficiency, 

and no LEC characteristics were found. 

Sandee et al., [167] Cao and co-workers [36,161,162,164–166] as well as 

Lee et al. [163] reported complex-containing polymers with metal 



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centers interrupting the conjugation. They all used Ir III 

complexes containing two bromo-functionalized 

cyclometallating ligands and acetoacetonate or an analogue 

(e.g., 2,2,6,6-tetramethyl-3,5-heptanedione) as an ancillary 

ligand. 

In the first contribution dealing with this polymer class, 

Sandee et al. applied such bis-cyclometallated Ir III acetoacetonate 

complexes as endcappers in Suzuki cross-coupling reactions of 

2-(boronic acid ester)-7-bromo-dialkylfluorene, obtaining acetoacetonate 

complexes coordinating two cyclometallating macroligands. 

[167] Photophysical studies revealed a contribution of the 

connected fluorene moieties to the complex emissive state. 

Moderate efficient green-emitting devices were fabricated. Due to 

the better energy match between the phosphor and the fluorene 

segments, the red-emitting devices showed a significantly better 

efficiency. 

Several reports by Cao and co-workers describe related 

polymeric systems. Besides the complex moieties, mainly 

fluorene and/or carbazole elements were incorporated by Suzuki 

cross-coupling reactions. [36,161,162,164–166] Also, 4,7-dibromo-[2,1, 

3]-benzothiadiazole was used as an additional comonomer 

to tune the optical properties towards white emission 

(Scheme 6). [166] Polyelectrolytes were obtained by quaternation 

of fluorine-tethered amino groups [161] in a manner analogous to 

the previously mentioned systems. [155] Besides the acetoacetonate 

derivatives, picolinate as well as 5-methyl-3-(pyridine-2-yl)- 

1,2,4-triazolate were applied as ancillary ligand. The resulting 

materials showed distinct differences in their device performance. 

[36] In an early description, Cao and co-workers applied 

Yamamoto coupling reactions to cocondensate Ir III complexes 

with different kinds of p-dibromo phenyl compounds. Aside from 

a coupling reaction with a bis-cyclometallated acetoacetonate 

complex, condensation reactions with fac-tris(3-bromo-phenyl)- 

pyridine) iridium(III) were performed resulting in hyperbranched 

structures. [168] 

Lee et al. used the Yamamoto coupling reaction to realize 

further polymer variants. Complex monomer and dibromodialkylfluorene 

were cocondensated, and N-phenyl-4-bromo-1,8- 

naphthalimide was applied as endcapper. [163] Besides the 

introduction of additional functional structures, the use of 

mono-functional species as endcappers enables one to exert an 

influence on the final molar mass and to obtain polymers with 

quenched chain ends. Analogous to similar reports, the emission 

of white light could be tuned by adapting the ratios of the blue 

(fluorene moieties) and green (naphthalimide endcapper) 

flurophores and the red phosphor (Ir III complex). The variation 

of the monomer feed ratios is a general method to optimize the 

system with respect to high efficiencies, color purity (or white 

emission), and/or light output. 

Due to the cocondensation of analogous functionalities (arylic 

bromines), copolymers synthesized by Yamamoto coupling can 

usually be obtained in high molar masses. To realize high molar 

mass copolymers by cross-coupling bis-bromo- with bis(boronic 

acid ester)-arylic compounds in the Suzuki condensation 

reaction, an equimolar ratio of the reactive species is of outmost 

importance. This can be circumvented by the use of monomers 

equipped with both complementary reactive sites. However, the 

cross-coupling of bis(homo-functionalized) comonomer species 

ensures a strictly alternating incorporation within the copolymer 

synthesized excluding undesired homo-coupling of complex 

monomers. 

3.2. Dendritic Systems: A Brief Overview 

As an alternative to the embedding of phosphorescent emitters 

within polymeric systems dendritic assemblies have been 

proposed. Like the polymeric systems, dendritic structures 

can be classified as conjugated [50,51,55,174–180] and nonconjugated 

[31,56,181–183] systems. In most cases, a heteroleptic 

or homoleptic facial tris-cyclometallated iridium(III) complex 

constitutes the core with two [173,174] or three [50,51,55,56,172–180] 

ligand-tethered dendritic arms forming the shell. Lin et al. 

reported on a number of bis(cyclometallated) Ir III complexes 

coordinating dendrimer-functionalized acetoacetonate derivatives 

as ancillary ligand. [181] Further examples for dendritic 

systems with a bis-cyclometallated Ir III complex-core were 

presented by Liang et al. Introducing a dendritic arm with each 

cyclometallating ligand, the final complexes were formed by the 

coordination of 5-methyl-3-(pyridine-2 0 -yl)-1H-1,2,4-triazole as 

ancillary ligand. [178] Usually the core–shell assemblies are 

obtained by the coordination of dendrimer-equipped ligands to 

an iridium(III) metal center. Alternatively, a complex bearing 

reactive sites can be equipped with dendritic arms in 

postcomplexation-functionalization reactions. Using this approach, 

Jung et al. condensated a homoleptic tris-cyclometallated Ir III 

complex, bearing a carboxylic acid group at each ligand, with 

hydroxy-functionalized structures in esterification reactions to 

form three different dendrimer generations with peripheral 

carbazole moieties. [180] Kwong et al. reported on the 

functionalization of bis[(4,6-difluorophenyl)-pyridinato-N,C 20 ]3- 

hydroxypicolinate with different dendritic structures by etherification 

reactions. Three donor–acceptor (phosphor–dendrimer) 

systems connected via the ancillary ligand were obtained. [31] 

Aside from carbazole-derived dendritic structures, [31,177,179–181] 

triphenylamine-based systems [50,178] were applied to function as 

antennas for the transfer of charge and/or energy to the emitting 

center. Based on such tris-cyclometallated Ir III complexes 

(Scheme 1), Zhou et al. fabricated devices exhibiting efficient 

red emission (h ext ¼ 11.6%, h P ¼ 3.7 lm W 1 ) with high color 

purity (CIE x,y-coordinates 0.70, 0.30). [50] 

Samuel and co-workers focused in particular on phenyl-based 

configurations, [55,56,172–177] while You et al. presented a complex 

equipped with tetraphenyl silyl moieties. [51] 

The possibility to construct highly defined and very dense 

assemblies is the most beneficial aspect of dendritic systems. 

Introducing a high density of antenna moieties in close proximity 

to the emitting complex core can also improve the energyharvesting 

and -transfer processes. The bulky dendritic configurations, 

furthermore, support the isolation of the emitting 

centers (i.e., suppression of concentration and triplet–triplet 

quenching). 

However, synthetic effort might be the major drawback of 

the dendritic approach. With the number of generations, the 

synthetic demands can grow significantly; decreasing yields are 

accompanied by an increasing probability of structural defects. In 

contrast to the dendritic approach, compositional variations in 

polymeric systems can be realized with negligible efforts by 


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adapting the feed ratios. Furthermore, a large variety of 

monomers is comparatively easily available. 

4. Phosphorescent Ir(III) Complexes in OLED 

Applications: A Growing Field 

4.1. General Structure of OLEDs 

Due to their very good color reproduction, high color fidelity, low 

power requirement, and very good light efficiency, OLEDs have 

the potential to be superior as batwing radiators in contrast to 

point light sources such as light bulbs and inorganic LEDs. 

Devices can be extremely thin, flat, and transparent and can be 

built up even on flexible substrates. [1,2,182–187] A schematic 

representation of a multilayer OLED and its mode of operation 

are depicted in Figure 1. It comprises an anode, usually indium 

tin oxide (ITO), the HTL, the EML containing a host material and 

the emissive dopant, the HBL, the electron-transporting layer 

(ETL), and the cathode (usually a low work-function metal with an 

electron-injection layer). Examples of charge-transporting materials 

and Ir III emitters applied in OLED configurations are 

summarized in Table 1 and 2. There are also several materials 

available which can be contemporaneously used for different 

layers due to their characteristics resulting from the HOMO and 

LUMO level. The claims for the host materials are well 

summarized by Tsuzuki and Tokito. [202] 

Figure 1 illustrates the idealized working principle of a 

multilayer OLED. The charge carriers, i.e., electrons ( ) and 

holes (þ), are injected under the influence of an external electrical 

field by the cathode and anode into the ETL and HTL, respectively 

(1). The charge transfer is realized via a hopping-mechanism 

between the localized p-electron states. The electrons and holes 

bounce alongside the molecules from one to each other. After 

arriving at the EML, holes and electrons accumulate at the LUMO 

and HOMO of the emissive dopant (3). Thereupon, the 

electron–hole pair formation occurs leading to an exciton 

generation (4). [2,182,185,190] The states of the tightly bound and 

Figure 1. Schematic representation of the general structure of a multilayered 

high-efficiency electrophosphorescent OLED. After the charge 

carrier injection (1), hole and electron transport takes place via a hopping 

mechanism along the HOMO and LUMO levels of the molecules, respectively 

(2). After accumulation of the electrons ( ) and holes (þ) in the active 

layer (3), exciton formation occurs and they collapse under light emission 

(4). Abbreviations: HTL, hole-transporting layer; EML, emissive layer; HBL, 

hole-blocking layer; ETL, electron-transporting layer. 

localized excitons collapse and recombine under emission. [189] In 

a host–guest system, three processes inducing the excitation of 

the phosphor are considered: the long-range Förster-type transfer 

of singlet excitons between matrix and guest, the short-range 

Dexter-type transfer of singlet and triplet excitons generated on 

the host to the dopant, as well as charge trapping and direct 

creation of singlet and triplet excitons on the guest. [190] In 

consequence of the strong spin–orbit coupling derived from the 

heavy metal atoms, both electrogenerated singlet and triplet 

excitons can be utilized for the emission. Therefore, an internal 

efficiency of 100% can be theoretically achieved. [2,10,182,186,191] To 

assure an efficient triplet harvesting on the metal complex, the 

T 1 –S 0 energy of the guest has to be smaller than that of the 

host. [190] Due to the longer lifetime of triplet excitons their 

diffusion lengths are up to two magnitudes larger (>1000 Å) 

compared to singlet excitons (10–100 Å), [185] which can lead to 

exciton leakage. This has to be considered in the construction of 

devices and can be countered by using the right combination of 


Table 1. Selected examples of OLED materials. [a] 

Layer [b] Compounds HOMO [eV] LUMO [eV] 

HTL N,N 0 -Bis(1-naphthyl)-N,N 0 -diphenyl-1,1 0 -biphenyl-4,4 0 -diamine (NPB) [1,10,182,183,192,196,207–210,212,214,216] 5.5 [182,188] 2.5 [182] 

2.4 [216] 

4,4 0 ,4 00 -Tris(N-carbazolyl)-triphenylamine (TCTA) [d] [215] 5.7 [216] 2.3 [216] 

EML [c] 4,4 0 -N,N 0 -Dicarbazolebiphenyl (CBP) [e] [183,184,192,196,207,208,214,218] 5.6 [2] 2.3 [2] 

5.9 [188,208] 2.5 [188,208] 

6.3 [192,217] 3.0 [217] 

p-Bis(triphenylsilyl)-benzene (UGH2) [182,203,216] 7.2 [182,216] 2.8 [216] 

1,3-Bis(9-carbazolyl)-benzene (mCP) [10,182] 5.9 [182,216] 2.4 [182,216] 

HBL Bathocuproine (BCP) [182–184,192,200,208,216,218] 6.5 [182,216] 3.0 [182,216] 

5.9 [188] 2.9 [188] 

ETL 2-2 0 -2 00 -(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) [e] [1,10,200] 6.0 [2] 2.8 [182,216] 

6.3 [182] 3.0 [188] 

Tris(8-hydroxyquinolinato) aluminum (Alq 3 ) [183,184,192,213,214,218] 3.3 [2] 

Bathophenanthroline (Bphen) [e] [209,216] 6.4 [216] 3.0 [216] 

[a] For more information about charge carrier transporting molecular materials and their applications in devices, the reader is referred to the recent review of Shirota and 

Kageyama [112]. [b] ITO has a HOMO level of 4.7 eV, and MgAg for the cathode has a LUMO level of 3.7 eV; the HOMO of LiF/Al is 4.1 eV [180]. [c] Host material for 

SMOLEDs. [d] Also usable as host material [194]. [e] Also hole-blocking ability. 


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Table 2. Selected examples of phosphorescent Ir III dopants for OLED applications. 

Emitter Ir III complex HOMO [eV] LUMO [eV] 

Blue phosphorescent dopant Bis(4 0 ,6 0 -difluoro-phenylpyridinato) iridium(III) picolinate (FIrpic) [196,203,207,218] 

Bis(4 0 ,6 0 -difluoro-phenylpyridinato) iridium(III) tetra(1-pyrazolyl)borate (FIr6) [182,203] 6.1 [182] 3.1 [182] 

Green phosphorescent dopant Bis(2-phenylpyridinato) iridium(III) acetylacetonate [Ir(ppy) 2 (acac)] [207] 5.6 [191] 3.0 [191] 

fac-Tris(2-phenylpyridinato) iridium(III) [Ir(ppy) 3 ] [208–210,215] 5.3 [188] 2.8 [188] 

5.1 [182] 2.6 [182] 

Red phosphorescent dopant Bis(1-(phenyl)isoquinolinato-N,C 2 ) iridium(III) acetylacetonate [Ir(piq) 2 (acac)] [200,209,210,214,215] 5.2 [188] 3.2 [188] 

Bis(2-benzo[b]thiophen-2-yl-pyridinato) iridium(III) acetylacetonate [Ir(btp) 2 (acac)] [196,207,208,213] 

Tris(1-phenylisoquinolato) iridium(III)[Ir(piq) 3 ] [192] 5.1 [192] 3.0 [201] 

5.0 [201] 

charge transport, charge blocking and/or exciton blocking 

layers. [2] In many cases the electron blocking layer (EBL) can 

be dispensed due to the higher mobility of the holes. 

For the fabrication of OLED layers, a diversity of techniques, 

such as vacuum deposition, conformal elastomeric masking, cold 

welding, thermal imaging, organic vapor jet printing, or solution 

processes (e.g., spin-coating and inkjet printing), are used. [184,193] 

Vapor deposition is a very accurate process that allows designing 

complicated structures by applying mask techniques, but it is also 

rather intricate, expensive, and limited to the handling of small 

and thermally stable molecules. In contrast, solution processing 

techniques are rather low-priced, easily applicable to large areas, 

and much less restricted regarding the processed materials. 

While vapor-deposited devices often exhibit superior characteristics, 

solution-processed devices are catching up to show 

comparable performance. [138,192–197] 

Besides the optimization of the employed layers, further 

technical approaches are pursued to improve the device 

outcoupling. [196,197] The application of transparent organic [198] 

or semitransparent metal cathodes [199] leads to so-called 

top-emissive devices with high power and EQE, which enables 

the fabrication of fully transparent displays and lighting 

devices. [197,198] A further possibility to classify OLEDs is by 

the distinguished namely small organic molecules (SMOLEDs), 

polymeric systems (PLEDs), and dendritic structures. [1] 

4.2. Monochromatic OLEDs 

In 2002, Adachi et al. presented a green-emitting device 

containing bis(2-phenylpyridinato-N,C 2 )iridium(III)-acetylacetonate 

[Ir(ppy) 2 (acac)] as dopant featuring a maximum external 

efficiency of 19.0% and a luminous power efficiency of 

h P ¼ (60 5) lm W 1 . [191] A recent example for the great potential 

with respect to device improvement was reported by Zhou et al. 

They prepared OLEDs with iridium(III) acetylacetonates where 

different main group elements were introduced as substituents at 

the cyclometallating ligands (Scheme 1). Depending on the 

nature of the substituent, emission color ranging from bluish 

green ( OPh as substituent) to red ( B(mesityl) 2 as substituent) 

could be realized. Furthermore, when incorporated into devices, 

good power efficiencies (h P ¼ 26.8 to 28.6 lm W 1 ) and external 

quantum efficiencies (h ext ¼ 10.3 to 11.1%) were achieved. [183] Xie 

et al. exploited the positive influence of the exciton-collecting 

effect of a doped HBL. This principle of a double-EML led to 

improvements of the power efficiency up to 25–36%. [200] 

So far, the efficiencies of green triplet emitters are usually 

superior to red [201] and blue ones. [192] The comparably poor 

performance of red emitters can be reasoned by the energy gap 

law. [192] One of the best true red color device emissions could be 

obtained by Ho et al. using neutral acetoacetone complexes with 

9-arylcarbazole motivs in the cyclometallated ligands. These 

devices showed remarkable external quantum efficiencies of 12% 

and power efficiencies of 5.3 lm W 1 . [192] Beside the typical 

compounds (Table 1), the utilization of [Ir(ppy) 2 (acac)] (Table 2) as 

host material was demonstrated, whereas the host emission was 

completely quenched by the dopant [Ir(piq) 3 ] (1 wt%) leading to 

an improvement of the OLED durability due to a lower driving 

voltage. [202] 

The design of stable, efficient ‘‘deep blue’’ light emitters 

remains a challenge. To obtain blue emitters the energy gap 

between the frontier orbitals has to be increased, but this usually 

brings the lowest excited states in proximity to non-emissive 

states, which can become thermally accessible. Therefore, the 

synthesis of a deep blue emitter with high QE is rather 

demanding. Frequently employed iridium(III) complexes for 

blue OLEDs are, for example, the greenish-blue emitting 

bis(4 0 ,6 0 -difluorophenylpyridinato)iridium(III)-picolinate (FIrpic) 

and the sky-blue emitting bis(4 0 ,6 0 -difluorophenylpyridinato) 

iridium(III)-tetra(1-pyrazolyl)-borate (FIr6). Here, for an optimal 

device efficiency, host materials with a wide energy bandgap are 

required (e.g., UGH2, Table 1). [203] Using the complex 

[Ir(dfppy)(fppz) 2 ] (dffpyH: 2-(2,4-difluorophenyl)pyridine, fppzH: 

5-(2-pyridyl)-3-trifluoromethylpyrazole) as dopant enabled the 

construction of a deep blue OLED with coordinates (0.16, 0.18) 

according to CIE. A high EQE of up to 8.5% and a power 

efficiency h P of 8.5 lm W 1 could be obtained. [205] 

4.3. White OLEDs: Growing Interest in the Last Few Years 

The interest in WOLEDs, with focus on applications such as 

display backlights and illumination (Fig. 2), is growing 

significantly. [182,205,206] Merits, such as low driving voltage, high 

efficiency, and freedom in the design (e.g., large surfaces) are very 

appealing. 

As defined by the CIE, the x,y-coordinates of (0.33, 0.33) in the 

chromaticity coordinate system correspond to an ideal white light 

source. Moreover, the color rendering index (CRI) represents a 


www.advmat.de 

doped devices or stacked configurations. [213] Due to the strong 

dependency on the host material and the stability of blue 

phosphorescent emitters, often these are replaced by a blue 

fluorophor. [210,214] The general strategy is to utlize the energy 

from the blue fluorescent emission to excite a green and/or a red 

triplet emitter [196,205,210] or to suppress the triplet–triplet energy 

transfer between the two phosphorescent dopants if this 

fluorescent layer was sandwiched between them. [215] Single 

EML WOLEDs are simple to fabricate, and their CIE 

x,y-coordinates remain stable with changing current densities 

due to the existence of only one recombination zone. The 

drawback of this approach is the comparatively low efficiency. In 

contrast, the multi-EML structured WOLEDs indeed work more 

efficiently, but they show the above mentinoned currentdensity-dependent 

CIE x,y-coordinates because of the recombination-zone 

shift. [213] 


Figure 2. Examples of white OLEDs, which represent an alternative illumination 

source to incandescent light bulbs. (Images reproduced with 

permission of Philips Lighting.). 

quantitative measure describing the color shift which occurs 

when an object is illuminated by the light source and a reference 

source of comparable color temperature. [206] The CRI values can 

range from 0 to 100, whereas 100 indicates no color shift. White 

light sources are referred to daylight. 

Since electrophosphorescent OLEDs generally show very 

high external quantum (h ext ) and power efficiencies (h P ) for 

monochromatic light emission, a combination of them in a single 

device to achieve white light was a natural consequence. Besides 

the right choice of the emitters, the right thickness of each layer is 

essential. [182] Another important factor is the dopant concentration 

in the single layers. [184] Phosphorescent dopant concentrations 

in WOLEDs range between 0.5 and 11 wt%, whereas the 

requirements are in general lower for the green and blue dopants, 

respectively. [183,199,207–210] 

Generally, the layout of the EML ranges between 25 and 

40 nm. [199,202,210,211] A thin thickness is preferred to ensure low 

voltage (

www.advmat.de 


spectrum of the WOLED. After device optimization, good 

performances were obtained: h ext ¼ 13.0%, h P ¼ 12.4 lm W 1 , 

and a CRI-value of 86. Furthermore, a stable white color was 

achieved with a narrow chromaticity range of 0.015 around 

(0.40, 0.40). [196] Divergent from this all-phosphor design, Li et al. 

used an assembly with NPB (Table 1) as the blue-emitting HTL. 

The yellow emission was provided by bis(2-(2-fluorophenyl)-1,3- 

benzothiozolato-N,C 2 )iridium(III) acetylacetonate [(F-BT) 2 Ir(acac)]. 

CIE-coordinates of (0.39, 0.41), power efficiency of 10.3 lm W 1 

and an EQE of 8.4% were achieved. [207] In both aforementioned 

cases, an additional blocking layer between the doped layers was 

necessary to prevent undesired charge or energy transfer. [196,209] 

4.3.3. (O B) Strategy 

Due to the smaller number of dopants, this approach inevitably 

has the inherent advantage of a lower requirement of dye material 

and, as a result, lower costs, fewer masks, and a simpler 

fabrication process. Wong and co-workers recently reported on a 

combination of an orange-emitting iridium(III) phosphor 

and 5-trifluoromethyl-2-[3-(N-phenylcarbazolyl)]-pyridine as blue 

fluorophor for white emission exhibiting high color stability. [205] 

The optimized device configuration yielded a maximum power 

efficiency of 23 lm W 1 and an electroluminescent device 

performance almost invariant from the applied voltage. In the 

range of 6 to 14 V, the CIE x,y-coordinates (0.35, 0.38) varied only 

for (0.01, 0.01). By this, promising devices could be obtained 

that outperform some of the best two-element all-fluorescent and 

all-phosphorescent WOLEDs. Only the comparatively low CRI 

values of 50–60 are still in need of further improvement. [205] In 

similar approaches yellow phosphorescent iridium complexes 

such as bis[2-(4-tert-butylphenyl) benzothiazolato-N,C 2 ]iridium(III) 

[(t-bt) 2 Ir(acac)] and bis(2-(2-fluorophenyl)-1,3-benzothiolato- 

N,C 2 )iridium(III) acetylacetonate [(F-BT) 2 Ir(acac)] and the blue 

fluorescent emitter 4,4 0 -bis (2,2 0 -diphenylvinyl)-1,1 0 -diphenyl 

were applied. [217,218] 

Kwok and co-workers demonstrated recently an all-phosphor 

EML with FIrPic doped in mCP (Table 1) for blue light and an 

orange emitting iridium(III) complex doped in CBP. To achieve 

good device performance, it was necessary to firstly deposit the 

orange-dye-doped layer and afterwards the blue-dye-doped one. 

The thus obtained CIE-coordinates were located at (0.31, 0.41) 

and a power efficiency of h P ¼ 7.6 lm W 1 was determined. [210] 

For comparison to the aforementioned examples, an 

O B-based device constructed by all-fluorescent dopants could 

nearly achieve the white point with CIE-coordinates of (0.33, 

0.34), but with a maximum power efficiency of 3.44 lm W 1 . [188] 

From another fluorescent device possessing a single EML 

with orange-emitting 2,1,3-benzothiadiazole derivatives 

incorporated in polyfluorene as blue emitter, a power efficiency 

of h P ¼ 5.75 lm W 1 and an EQE of h ext ¼ 3.8% with CIE 

coordinates (0.35, 0.34) was obtained, [219] which is already good in 

contrast to other examples. [220] 

However, since the operational lifetime of current blue 

electrophosphorescent emitters in OLEDs is still rather short, 

the color stability of the all-phosphor-doped WOLEDs is still 

limited. Therefore, many examples as already mentioned arose 

using other blue-emitting materials. [186] Thompson and 

co-workers explained that the blue fluorescent emitter is able 

to harness all electrically generated high-energy singlet excitons, 

and the phosphorescent dopants harvest the remained lower-energy 

triplet excitons for the green and red emission. Singlet 

excitons are transferred following a resonant Förster-type process 

onto the lightly doped blue fluorophor as opposed to direct trap 

formation. Placing an undoped host spacer between the 

fluorophor and the phosphors prevents undesired direct energy 

transfer and therefore minimizes the energy loss. This structure 

led to an increase of the power efficiency of 20% by energy 

transfer compared to ideal all-phosphor devices. The improvement 

of the described architecture compared to all-phosphor 

doped WOLEDs is the circumstance that blue emission becomes 

stronger to a smaller extent with increasing driving voltage (less 

efficiency roll-off at high current densities). [186] Chen et al. could 

even show that a color tunability is possible via varying the 

thickness of the undoped host spacer. [218] 

5. Phosphorescent Ir III Complexes: Looking 

Beyond OLED Applications 

5.1. Luminescent Ir III Complexes in LECs 

The so far most prominent application of phosphorescent 

charged Ir III complexes lies in the field of LECs as alternative to 

light-emitting diodes. [17,221–224] In general, LECs are single-layer 

electroluminescent devices consisting of a luminescent material 

in combination with ionic charges clamped between two 

electrodes. Crucial issues, such as efficiency limitation, response 

times, multicolor approaches and device lifetimes, were reviewed 

by Slinker et al. [225] The insensitivity towards the work-function of 

the electrodes is the main characteristic of such devices. As a 

consequence air-stable metals, such as Au or Ag, can be used. By 

applying an external electric field over the device a strong 

interfacial electric field, caused by the displacement of the mobile 

ionic species towards the charged electrodes, is generated. [226] 

The thus separated ions induce doping (i.e., oxidation and 

reduction) of the emissive materials near the electrodes (p-type 

near the anode and n-type near the cathode). These doped regions 

induce ohmic contacts with the electrodes and enhance the 

injection of holes and electrons recombining at the junction 

between p- and n-type regions. Therefore, single-layered LECs 

can be operated at very low voltages with balanced carrier 

injection and high power efficiencies (see also Scheme 2). [227] 

Furthermore, the production process via spin-coating or 

inkjet-printing techniques is comparably simple, since LECs 

feature a wide tolerance to the thickness of the emitting layer. 

Conjugated polymers to which inorganic salts or transition 

metal complexes were added, were conventionally used. [228,229] In 

recent years, the focus has shifted more towards cationic 

phosphorescent transition metal complexes yielding singlecomponent 

LECs. Two advantages over the conventional 

polymer-based LECs are prominent: i) no additional ionconducting 

material is required since the transition metal 

complexes are intrinsically ionic and ii) higher electroluminescence 

efficiencies are reachable due to the phosphorescent nature 

of the complexes. [227] Apart from the intensively studied LECs 

based on [Ru(bpy) 3 ] 2þ and closely related complexes, [1,11,225,230] 


www.advmat.de 

iridium(III)-based complexes have turned out to be highly 

promising candidates for LEC devices due to their high quantum 

efficiencies. [9,77–80,231–234] As a highlight in this respect, the 

green-emitting complex [(ppy-F 2 ) 2 Ir( t Bu 2 -bpy)](PF 6 ) (Scheme 2) 

should be mentioned. It showed in thin solid film [5 wt% in 

poly(methyl methacrylate)matrix] a PLQY close to unity. [235] 

According to Bolink et al., the PL quantum yield in the solid state 

can be considered as a limiting factor for the device efficiency. 

When incorporated into a single-layer LEC, in the present case an 

EQE of almost 15% and a power efficiency of 38 lm W 1 were 

obtained. 

However, the accessible range of emission colors and the 

limited temporal stability (from minutes to a few hours) of ionic 

Ir III complexes compared to their neutral counterparts used in 

OLEDs remain considerable constrictions. In particular, the 

design of efficient blue-emitting charged Ir III complexes with 

respect to the generation of white light by color-mixing is highly 

desirable. Here, the major contributions by the groups of 

Bernhard [73,74,236] and Nazeeruddin [75,237] should be emphasized 

(Scheme 2). Further color-tuning of charged Ir III complexes for 

LECs could be realized via tris-cyclometallated complexes with 

charged groups, e.g., phosphonium salts, in the periphery of the 

cyclometallating ligand (Scheme 2). However, the instability of 

such devices is still a drawback of this approach. [238] As a 

promising step in the development of lighting technology, the 

first example of a white LEC (WLEC) was realized by Wu and 

co-workers [227] by combining a blue-green and a red emitting 

complex in a host–guest system. As depicted in Figure 3, white 

light (x,y-coordinates 0.35 and 0.39, according to CIE) with a good 

EQE of 4% and a power efficiency of 7.8 lm W 1 was obtained. 

Recent studies by Bolink et al. have shown that the second 

crucial issue in LEC technology, i.e., the lifetime of such devices, 

can be significantly improved by introducing bulky shielding 

ligands into the coordination sphere of Ir III (Scheme 2). [239,240] 

LECs in general exhibit a delay between the time when the device 

is turned on and the time when a steady-state light emission is 

reached, the so-called turn-on time. [1] This important parameter 

strongly depends on the mobility of the counterion and can range 

from seconds to hours. [225] Previous attempts to improve the 

device turn-on included the reduction of the EML thickness [241] or 

increasing the applied bias above the turn-on voltage; [242] 

however, in these cases the efficiency was lowered due to exciton 

quenching at the electrodes and the device lifetimes were 

shortened, respectively. Applying a high-voltage pulse and then 

operating the device at lower voltage helped to reduce the turn-on 

Figure 3. White EL emission from a single-layered solid-state LEC based 

on host–guest cationic iridium(III) complexes. Image reproduced with 

permission of [227]. Copyright 2007, The Royal Society of Chemistry. 

time without affecting the lifetime, but this scheme appears not to 

be compatible with all device architectures and applications. [243] 

Changing the nature of the counterion in charged Ir III complexes 

or using ionic liquids as supporting agents could somehow 

reduce the turn-on times, but it could simultaneously increase the 

degradation of the devices. [1,244] Recently, Zysman-Coleman et al. 

introduced Ru II and Ir III complexes with bipyridine ligands 

bearing alkyltriethylammonium side chains to mimic an ionic 

liquid. [245] Thus, the turn-on times of the LECs were remarkably 

lowered by nearly two orders of magnitude compared to 

[(ppy) 2 Ir( t Bu 2 -bpy)](PF 6 ); these turn-on times were reported to 

be the fastest values for devices solely based on heteroleptic Ir III 

complexes. 

5.2. Phosphorescent Ir III Complexes in Oxygen Sensing 

The long-lived triplet excited state of luminescent iridium(III) 

complexes enables the efficient transfer of energy to the triplet 

ground state of molecular oxygen, resulting in luminescence 

quenching and the generation of singlet oxygen. [116] This feature 

of iridium(III) luminophores can be utilized as sensitive oxygen 

probe in, e.g., medicinal, chemical or environmental sensors. 

[34,84,246–248] An experimental array for the measurement 

of oxygen concentration with respect to medicinal applications, 

i.e., for intraocular measurements, was recently introduced by 

Nazeeruddin and co-workers. [249] 

Since the oxygen concentration is in general related to sudden 

changes in the luminescence of the Ir III complex, new materials 

with high quantum yields and long excited-state lifetimes (i.e., up 

to several microseconds) for reliable, sensitive detection are the 

focus of current research. [9] Among others, mixed-ligand 

iridium(III) complexes are highly promising candidates, due to 

their well-tunable excited state properties, high durability, and 

stability towards the generated singlet oxygen. [34] However, for 

successful development of solid-state iridium(III)-containing 

sensors, the compatibility of the luminescent dye with the 

polymer matrix represents a crucial requirement. [115,250,251] 

Polymer-dye incompatibility or high loading can lead to dye 

aggregation and/or phase segregation resulting in self-quenching 

and, therefore, poor sensor performance. [117] Oxygen permeable 

polymers, e.g., PDMS, fluorinated polyacrylates, poly(thionylphosphazene)s, 

and PS, have been widely used as matrix 

materials. [9,117] The most promising approach so far to 

circumvent these limitations is the covalent linkage of the 

iridium(III) dyes to the polymer backbone. DeRosa et al. have 

reported the synthesis of well-defined polymers consisting of 

luminescent Ir III complexes being attached to linear 

PDMS. [25,116,117] While developments in this field are ongoing, 

the processing of these Ir III -PDMS systems with PS resulted in 

permeable blended membranes featuring good material properties, 

i.e., relatively hard and stable films as well as good 

luminescent sensor response to oxygen. 

Beyond the development of novel luminescent probes for 

molecular oxygen, highly sensitive systems based on iridium(III) 

complexes for the sensing of various anions, [252–254] amino 

acids, [255] and alkali [256] as well as heavy transition metal 

cations [257–259] have been introduced in recent years. 



www.advmat.de 


5.3. Cationic Iridium(III) Complexes in Bioanalytical 

Applications 

Besides the previously discussed chemosensory applications, 

their electrochemical and photophysical properties have made 

iridium(III) complexes also highly promising candidates as 

labeling reagents and probes in biological systems. [9,260–262] 

Sensitive, time-resolved detection is enabled due to their intense 

emission and excited-state lifetimes. For applications as labeling 

compounds for luminescence imaging in biomaterials, charged 

iridium(III) complexes can be bound covalently [260–265] or 

non-covalently, e.g., by DNA or protein intercalation, [261,266] to 

biological substrates. Generally, the bound complexes exhibit 

luminescence properties that significantly differ from that of the 

free substrate, due to changes in rigidity and hydrophobicity of 

the surroundings. [267] Therefore, the facile monitoring of 

bioconjunction reactions or the investigation of binding affinities 

can be conducted utilizing such luminescent iridium(III) 

complexes. The progress in this emerging field of bioanalytical 

applications has been topic of a number of recent reviews by Lo 

and co-workers. [18–20] 

The exceeding potential of charged phosphorescent iridium(III) 

complexes in modern biomedical research is highlighted 

by the first example of bioimaging of living cells using such 

complexes. Yu et al. successfully introduced phosphorescent 

complexes into the cytoplasm of cervical cancer cells (HeLa) and 

observed low cytotoxicitiy and reduced photobleaching, compared 

to conventional dyes (Scheme 2, Fig. 4). [268] 

5.4. Iridium(III) Complexes for Photocatalytic Hydrogen 

Generation 

The increasing global energy consumption makes the conversion 

of solar irradiation into convenient and sustainable forms of 

energy one of the main challenges in modern materials research. 

One envisioned goal is to realize artificial photosynthetic systems 

that convert a redox product into its energy and oxidant parent 

materials, thereby storing solar energy for later use. [269,270] 

Among other approaches, [271,272] utilizing solar energy through a 

photocatalytic circle to split water into molecular hydrogen and 

oxygen appears to be the most promising solution. [9] The overall 

process can be considered as two half-reactions, where water is 

oxidized to oxygen and reduced to hydrogen. Mainly the high 

gravimetric energy density and the clean combustion products 

make hydrogen a highly feasible source for energy and therefore, 

much focus is laid on the hydrogen production. In general, such 

photochemical set-ups consist of a transition metal photosensitizer 

combined with an electron relay collecting and storing 

radiant energy and converting protons into molecular hydrogen. 

[273,274] 

Various types of transition metal complexes (e.g., based on Rh I , 

Ru, II or Pt II ) have been employed as photosensitizers, [275–278] but 

in particular heteroleptic iridium(III) complexes have gained 

attraction in this field due to their highly tunable photophysical 

properties. [9] Bernhard and co-workers have introduced Ir III - 

based sensitizers featuring remarkable improvements in the 

production of molecular hydrogen, compared to previously 

investigated Ru II -based systems. [73] A detailed study has mirrored 

Figure 4. Top: Confocal luminescence (a,d) and brightfield images (b,e) of 

living HeLa cells incubated with 20 mM of a green (top) or red iridium(III) 

emitter (bottom) in DMSO/phosphate buffer solution (pH 7, 1:49, v/v) for 

10 min at 25 8C. Overlays of luminescence (l ex ¼ 405 nm) and brightfield 

images are shown in (c) and (f), respectively. Images reproduced with 

permission of [268]. Copyright 2008, The Royal Society of Chemistry. 

Bottom: A 16-well LED photoreactor allowing a parallel array of 16 pressure 

transducers to monitor the kinetics of hydrogen gas evolution. The 

intensity of the light (l ¼ 465 nm) is approximately 130 W cm 1 mimicking 

the solar irradiation. Image reproduced with permission of [9]. Copyright 

2006, Wiley-VCH. 

an enhanced reducing strength, i.e., the ability to donate an 

electron to the electron relay, of the Ir III complex compared to 

related Ru II complexes. A further major achievement by the same 

group was the use of combinatorial synthesis and the 

implementation of the basic measurement set up into a 

high-throughput parallel screening work-flow (Fig. 4) allowing 

simple, fast and reproducible investigation of diverse libraries of 

new iridium(III) photosensitizers (Scheme 2). [269,279,280] 

6. DFT Calculations 

Besides the intense activities in the synthesis and characterization 

of new materials as well as their investigation in various 

applications, the utilization of computational calculations for a 

better understanding of structure–property relationships is 

steadily growing. 


www.advmat.de 

Since the contribution of Hay [281] it became more and more 

common to support experimental work on luminescent iridium(III) 

systems with density functional theory (DFT) calculations. 

[41,75,282–287] Using computational chemistry provides 

considerably more detailed insight into the structure of the 

molecular orbitals involved in essential photophysical processes. 

This enhances the possibilities of selective color tuning. Within 

this field, almost all groups have used Becke’s popular hybrid 

functional B3LYP at a 6–31G*/LANL2DZ or 6–31G**/LANL2DZ 

level. In a recent paper by Nie et al., where calculated geometries 

were compared to experimental data, the authors showed that 

other functionals, mainly those including a high amount of exact 

exchange, can lead to much better results. [288] Hence, if 

computational calculations spread out in iridium(III) chemistry, 

further investigations regarding the quality of the used methods 

in this particular field are required. 

7. Conclusions 

The evolution of phosphorescent iridium(III) complexes offers 

immense opportunities towards their application in the fields of 

OLEDs and related technologies. In this respect, we summarized 

the current developments in the design and preparation of 

various types of small-molecule complexes for light-emitting 

devices. The utilization of advanced mixed-ligand complexes 

brought further achievements with respect to color-tuning, which 

is of utmost importance for potential full-color device applications. 

Contributions from the field of theoretical chemistry, 

precisely predicting the electro-optical properties of the phosphorescent 

emitters, helped to rationalize structure–property 

relationships. In particular, we placed the focus on the covalent 

incorporation of Ir III phosphors into polymers. The combination 

of metal complexes with various types of polymers for PLED 

applications represents a fast growing field of research. Improved 

processability of the materials, homogenity, and enhanced 

flexibility of the device architecture and better intrachain 

energy-transfer are the main benefits from this approach. 

However, the development of conjugated polymers, in particular 

those with high triplet energies, in order to realize blue-light 

emission and raising the external quantum efficiency remain the 

major challenges in this field. Dendritic assemblies with their 

well-defined structures might be alternative emissive systems, 

but the synthetic effort required to produce them is a considerable 

drawback. 

With a growing demand of efficient materials for multicolor 

displays and lighting applications, phosphorescent iridium(III) 

complexes were introduced as promising candidates for highefficiency 

OLED devices. In this respect, we highlighted new 

iridium systems for monochromatic and white light-emitting 

diodes (WOLEDs). The family of red-, green-, and blue-emitting 

complexes for these purposes is continuously growing. 

Besides providing an overview on the wide range of 

applications in OLED technology, we also discuss new application 

prospects. In particular, we discussed the recent improvements of 

light-emitting electrochemical cells based on charged phosphoresencent 

iridium(IIII) complexes. In recent years, the utilization 

of such emitters for photocatalytic water-splitting processes has 

attracted much attention, and today, they appear to be far superior 

than other transition metal complexes in this respect. Bioanlytical 

and sensor applications utilizing the outstanding photophysical 

properties of charged and neutral iridium(III) complexes, 

respectively, are discussed and complete the survey of potential 

applications for phosphorescent iridium(III) complexes. 

Acknowledgements 

Financial support by the Dutch Polymer Institute (DPI), the Nederlandse 

Organisatie voor Wetenschappelijk Onderzoek (NWO, VICI award 

for U.S.S.), and the Fond der Chemischen Industrie is kindly acknowledged. 

Received: November 30, 2008 

Revised: February 10, 2009 

Published online: October 1, 2009 

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