Phosphorescent systems based on iridium(III) complexes
Citation for published version (APA):Ulbricht, C. (2009). Phosphorescent systems based on iridium(III) complexes. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR653204
DOI:
10.6100/IR653204
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Phosphorescent systems based
on iridium(III) complexes
systems
based
on
iridium(III)
complexes
Christoph
Ulbricht
Phosphorescent
systems
based
on
iridium(III)
complexes
voor het bijwonen van de openbare verdediging van mijn proefschrif tU
itnodiging
Op dinsdag 17 november 2009 om 16.00 uur . De promotie vindt plaats in het auditorium van de T echnische Universiteit Eindhoven. Aansluitend aan deze plechtigheid is er een receptie waarvoor u van harte bent uitgenodigd.Phosphorescentsystemsbasedon
iridium(III)complexes
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus, prof.dr.ir. C.J. van Duijn, voor een
commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op dinsdag 17 november 2009 om 16.00 uur
door
Christoph Ulbricht geboren te Saalfeld, Duitsland
prof.dr. U.S. Schubert Copromotoren: prof.dr. J.-F. Gohy en
dr. S. Bernhard
This research has been financially supported by the Dutch Polymer Institute (DPI, project #502).
Cover design: Christoph Ulbricht
Printing: Ipskamp Drukkers B.V., Enschede, The Netherlands
Phosphorescent systems based on iridium(III) complexes / by Christoph Ulbricht A catalogue record is available from the Eindhoven University of Technology Library. ISBN nummer: 978-90-386-2050-3
iridium(III)complexes
Kerncommissie: prof.dr. U.S. Schubert (Technische Universiteit Eindhoven) prof.dr. J.-F. Gohy (Technische Universiteit Eindhoven) dr. S. Bernhard (Carnegie Mellon University)
prof.dr. N. Risch (Paderborn University) prof.dr. K. Meerholz (University of Cologne)
prof.dr.ir. R.A.J. Janssen (Technische Universiteit Eindhoven) Overige commissieleden: Dr. A. Winter (Technische Universiteit Eindhoven)
Chapter1
RecentdevelopmentsintheapplicationofphosphorescentIrIIIcomplexes 1
1.1 Introduction 2
1.2 Iridium(III) complex systems – synthesis and properties 3
1.2.1 Neutral iridium(III) complexes 4
1.2.1.1 Bis-cyclometallated iridium(III) complexes 5
1.2.1.2 Tris-cyclometallated iridium(III) complexes 7
1.3 Polymers containing iridium(III) complexes 11
1.3.1 Complex containing polymers – synthesis and properties 11
1.3.2 Dendritic systems – a brief overview 25
1.4 Phosphorescent IrIIIcomplexes in OLED applications 27
1.4.1 General structure of OLEDs 27
1.4.2 Monochromatic OLEDs 30
1.4.3 White OLEDS 32
1.4.3.1 (R-G-B) Strategy 34
1.4.3.2 (B-R-Y-G) Strategy 34
1.4.3.3 (O-B) Strategy 35
1.5 Phosphorescent IrIIIcomplexes: looking beyond OLED applications 36
1.5.1 Luminescent IrIIIcomplexes in LECs
36
1.5.2 Phosphorescent IrIIIcomplexes in oxygen sensing 38
1.5.3 Cationic iridium(III) complexes in bioanalytical applications 39 1.5.4 Iridium(III) complexes for the photocatalytic hydrogen generation 40
1.6 DFT calculations 41
1.7 Conclusions 42
1.8 Aim and scope of the thesis 43
Chapter2
ChargedIrIIIemitterscoordinatingS-shapedterpyridines 59
2.1 Introduction 60
2.2 Synthesis 61
2.2.1 Synthesis of the precursor complexes and the charged emitters 62
2.3.3 Thermal gravimetric analysis 68
2.3.4 Photophysical and electrochemical properties 69
2.4 Conclusions 74
Chapter3
“Clicked”triazoles–newligandsitesforthecoordinationofIrIII 87
3.1 Introduction 88
3.2 Synthesis 89
3.2.1 Synthesis of clicked ligands 89
3.2.2 Synthesis of bis-cyclometallated IrIIIcomplexes 90
3.3 Characterization of the materials 92
3.3.1 Elemental analysis and MALDI-TOF mass spectrometry 92
3.3.2 NMR spectroscopy 94
3.3.3 Photophysical and electrochemical properties 96
3.4 DFT calculations 102
3.5 Conclusions 108
Chapter4
IrIIIphosphorsinpolymericassembliesI–oxetane-functionalizedcomplexes 121
4.1 Introduction 122
4.2 Synthesis of the ancillary ligands and the IrIII phosphors 123
4.3 Analysis 126
4.3.1 NMR spectroscopy 126
4.3.2 IR spectroscopy and MALDI-TOF mass spectrometry 129
4.3.3 Photophysical properties 130
4.3.4 Electrochemical and thermal properties 132
4.4 Crosslinked host-guest system – formation and characterization 133 4.4.1 Copolymerization of the complexes with a crosslinkable host matrix 133
4.4.2 Electroluminescent properties 135
4.5 Device optimization 138
4.6 Conclusions 139
4.7 Addendum 140
4.7.1 Stability of IrIII emitters under acidic conditions 140
Ir phosphorsinpolymericassembliesII–methacrylate-basedsystems 157 5.1 Introduction 158 5.2 Coordinationofpolymer-boundligandsites 159 5.2.1 Synthesisofthepolymerandthereactiveprecursor 159 5.2.2 Preparationofphosphorescentpolymerbycoordination 161 5.2.3 Characterizationofthematerials 161 5.2.3.1NMRspectroscopy 161 5.2.3.2Sizeexclusionchromatography(SEC) 163 5.2.3.3Photophysicalproperties 165 5.2.3 Conclusions 166
5.3 CopolymerizationofanIrIIIcomplexmonomer 167
5.3.1 Synthesisofacomplexandacarbazolemonomer 167 5.3.2 Preparationofphosphorescentpolymersbycopolymerization 169 5.3.3 Characterizationofthematerials 170 5.3.3.1MALDI-TOFmassspectrometryandelementalanalysis 170 5.3.3.2NMRspectroscopy 171 5.3.3.3Sizeexclusionchromatography 173 5.3.3.4Photophysicalproperties 176 5.3.4 Conclusions 179 Summary 187 Samenvatting 190 Curriculumvitae 194 Listofpublications 195 Acknowledgement 197
Recent developments in the application of phosphorescent Ir
IIIcomplexes
Abstract
The recent developments in using iridium(III) complexes as phosphorescent emitters in electroluminescent devices, such as (white) organic 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 summari-zed. 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.
Part of this chapter has been published: C. Ulbricht, B. Beyer, C. Friebe, A. Winter, U. S. Schubert, Adv. Mater. DOI: 10.1002/adma.200803537.
1.1Introduction
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 efficiency of 25%, given by spin statistics, phosphorescent complexes can theoretically achieve 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 high efficient light emitting devices.
In particular, the interest in phosphorescent IrIII complexes is growing rapidly.[5] Very
high luminescence efficiencies and short phosphorescence lifetimes can be realized. However, the most outstanding characteristic of this class of complexes might 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 OLEDs as well as in white organic light emitting device (WOLED) assemblies. But also in other applications they show promising potentials. The research in sensing, biolabeling and photocatalysis also utilizes the attractive features of IrIII complexes.
These developments are supported by the broad diversity of possible structures. The most prominent coordination motifs are cyclometallating ligands, available in a wide range. Tris-cyclometallated homo- and heteroleptic complexes as well as bis-cyclome-tallated ones are the most common phosphorescent IrIII species. A variety of so-called
ancillary ligands gives additional possibilities to define the structure and to tune the properties. Neutral and charged complex species 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 seeking to combine the appealing features of both, phosphor and polymer matrix, within one material. Besides selected examples for the design of new small and dendritic IrIII complexes, a detailed overview of polymer-embedded
phosphors with a focus on the synthetic strategies towards the different polymeric assemblies is given.
1.2Iridium(III)complexsystems–synthesisandproperties
The rich coordination chemistry of IrIII covers a wide range of complexes, including mono-, bis- and tris-cyclometallated species.[8] For the basic synthetic concepts and highlights of earlier examples of IrIII 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 light-emitting electrochemical cells (LECs), the development and optimization of new synthetic approaches towards new types of ligands and cyclometallated IrIII complex systems represent highly active fields of current research. Today 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 PtII species;
as a consequence higher quantum efficiencies can be achieved.[1]
In the following, the recent developments in the preparation of phosphorescent iridium(III) complexes will be highlighted. Tris-cyclometallated complexes or neutral bis-cyclometallated derivatives with ancillary ligands, such as acetylacetonates or picolinates, exhibit high potential for modern OLED applications.[1,16] Charged
bis-cylometallated complexes, mainly with oligopyridines as ancillary ligand, are in particular interesting as emitters in LECs,[1,17] for biolabeling[18-20] or in photocatalytic applications.[9]
A general overview of the most common synthetic strategies towards the various types of phosphorescent IrIII complexes is depicted in Scheme 1.[16] The P-dichloro-bridged
dimer [Ir(C^N)2-P-Cl]2, conveniently prepared from a reaction of the respective ligand
and IrCl3 · x H2O,[21] plays a central role in the coordination chemistry of these
complexes. The chloro-bridge can be split by chelating ligands (and monodendate ligands: e.g. pyridine, DMSO) leading to charged (N^N = 2,2’-bipyridines, 1,10-phe-nanthrolines, etc.; path a) or neutral bis-cyclometallated complexes (L^X = ȕ-diketona-tes, picolinaȕ-diketona-tes, etc.; path b) with preferred trans-N,N configuration of the C^N ligands. The addition of a third cyclometallating ligand results in tris-cyclometallated IrIII
complexes (path c). 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. Another approach to synthesize facial homo- or heteroleptic tris-cyclo-metallated IrIII species utilizes bis-cyclometallated complexes with “labile” ancillary
ligand(s) (e.g. acetoacetonate; path d). In solution, applying thermal or photochemical energy, mer-isomers can be converted into the fac-form (path e).[23,29] The lower
thermodynamic stability of the kinetically favored mer-isomer 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 or IrCl3 · x H2O (path f) is a
common approach to coordinate phenylpyridine (Hppy) and its derivatives.[30]
C N C N Cl Cl Ir N N C C Ir [Ir(C^N)2- -Cl]2 (b) + HL^X (a) + N^N C N C N N N Ir (C^N)2Ir(N^N) C N C N L X Ir (C^N)2Ir(L^X) (c) + N^CH C N N C N C Ir C N N C N C Ir + N^CH (d) Ir(C^N)3 homoleptic Ir(C^N)2(C^N) heteroleptic C N C N O O Ir + N^CH + N^CH mer/ f ac (f) + HC^N Ir(acac)3 or IrCl3· x H2O C N C N N C Ir mer-Ir(C^N)3 (e) h or C N N C N C Ir f ac-Ir(C^N)3 thermodynamical f avored kinetical f avored
Scheme1. Schematic representation of the synthetic strategies utilized for the synthesis of bis-
and tris-cyclometallated IrIII complexes.
1.2.1 Neutral iridium(III) complexes
As already pointed out, neutral iridium(III) complexes are widely used as triplet emitters in OLEDs featuring high external quantum efficiencies. In general, the emission color can be tuned via the combination of cyclometallating and ancillary ligands coordinated to the IrIII core.[1,16] A variety of emissive complexes covering the
whole visible spectra – from blue, over green, yellow and orange, to red – has been introduced. The majority of these phosphorescent IrIII complexes can be assigned to two main categories – bis- and tris-cyclometallated.
1.2.1.1 Bis-cyclometallated iridium(III) complexes
Bis-cyclometallated IrIII 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 analogs. These ancillary ligands provide additional possibilities for the tuning of the electro-optical properties, as well as for the introduction of lateral functionalities.
For the synthesis of neutral bis-cyclometallated acetoacetonate and picolinate com-plexes commonly mixtures of precursor complex, designated ligand, base (e.g. Na2CO3)
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, also much milder reaction conditions can be applied, 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 Na2CO3 at 50 °C for 2 to
6 hours (Scheme 2).[32] [Ir(C^N)2- -Cl]2 C N C N Ir (C^N)2Ir(acac) C N C N Ir (C^N)2Ir(pic) N C N F F F F F N F F Br O O N O O CH3CH2OH, Na2CO3, 50 °C, 4 h O O N HO O CH2Cl2, reflux, 24 h
T. Tsuzuki et al. [32] H. Zhen et al. [36]
C N C N Cl Cl Ir N N C C Ir
Scheme 2. Schematic representation of selected examples for the synthesis of neutral
bis-cyclometallated IrIII complexes.
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 °C in the presence of Na2CO3 the desired functionalized
bis-cyclometallated acetoacetonate complexes were obtained in 74 to 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 hours under heating to reflux. Also, bis-cyclometallated iridium(III) picolinate complexes can be obtained under rather mild conditions. By refluxing precursor and designated ligand for 24 h in dichloromethane Zhen et al. formed the desired picolinate complex (Scheme 2).[36]
While the structure of bis-cyclometallated complexes synthesized via chloro-bridged precursors is usually predetermined, Baranoff et al. observed partial thermal iso-merization of a picolinate complex during vacuum sublimation (Scheme 3).[37] The
isomerization could be reproduced in solution upon thermal treatment, in analogy to the reported mer-fac-isomerization of tris-cyclometallated IrIII complexes. While refluxing in glycerol for 20 hours resulted in 40% of the new isomer, the attempt to induce the isomerization by irradiation (i.e. UV and visible light) was not successful.
Scheme 3. Schematic representation of selected examples of bis-cyclometallated neutral IrIII
complexes (see text for details).
Besides acetoacetone, picolinic acid and the multitude of their derivatives,[38] other structures have recently found application as ancillary ligands. Acetoacetone-resembling acetoacetates with a large variety of residues are accessible from diketene 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-pyridylazolesas ancillary ligands have demonstrated their potential for the tuning of optoelectrical properties
(Scheme 3).[10,39-42] In particular, for the construction of efficient blue-emitting
com-plexes they proved to be promising.
1.2.1.2 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 IrIII
di-mer complex [Ir(ppy)2-P-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
electron-withdrawing or electron-donating substituents;[44-49] extension of the S-conju-gated 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 4). The outstanding role of tris-cyclometallated IrIII complexes
based on phenylpyridine-type derivatives as ligands is underlined by the large number of scientific publications and patents dealing with the synthesis and/or application of the respective complexes, homoleptic as well as heteroleptic ones (June 2009: more than 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 reference is given to recent literature published elsewhere.[1,8-13,16,60]
Scheme4. Schematic representation of selected examples of tris-cyclometallated IrIII complexes
(see text for details).
The two configurational isomers of tris-cyclometallated IrIII 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] Contrary to this finding, an exceptional high quantum efficiency for a
meridonal complex was found by Sun and co-workers for mer-Ir(mppy)3
(mppy = 2-phenyl-4-methyl-pyridine).[61] The synthesis of tris-cyclometallated IrIII
reaction conditions, e.g. refluxing in glycerol or in an excess of ligand.[23,44,62] Utilizing
microwave irradiation, the reaction times could be shortened remarkably.[63,64] However,
further optimization would be desireable to overcome the requirement of large excess of ligand material or the still rather low yields in particular for large systems.
The kinetically favored mer-isomers can be obtained by performing the reactions at lower temperatures (e.g. 120 to 150 °C, inhibiting the formation of the fac-isomers), often supported by admixing of additional base (e.g. triethylamine or Na2CO3). By
utilizing silver salts like 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] However, an excess of silver ions should be avoided in this approach to
prevent the formation of side products significantly lowering the yield of the desired product.[65] A promising approach towards the selective synthesis of mer-isomers was
recently introduced by McGee et al.[66] In this work a reactive P-hydroxy-bridged dimer
complex [(C^N)2Ir-P-OH]2 was used to enable the reaction under mild conditions. The
group of Williams could show that by applying a proper pair of a tridentate bis-cyclo-metallating and a tridendate mono-cyclobis-cyclo-metallating ligand the facile conformation could be predetermined (Scheme 4).[67]
Besides these recent contributions with respect to the synthesis of tris-cyclometallated IrIII complexes in general, also major advances with respect to color tuning have to be
named. In particular, the development of stable and efficient blue emitters still re-presents a major goal. A blue shift of the emission can be realized by widening of the energy band gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Increasing the degree of fluorination on the ligands and simultaneously replacing pyridine by pyrazole, Dedeian et al. could realize green-blue emitting tris-cyclometallated IrIII complexes (Scheme 4).[26] Samuel
and co-workers reported blue emission [Comssion Internationale de l’Eclairage (CIE) x,y-coordinates 0.16 and 0.12] for IrIII complexes bearing three phenyl-[1,2,4]triazoles as cyclometallating ligands (Scheme 4).[68] These systems, however, still suffer from
significant luminescence quenching due to vibrational decay. Furthermore, various effi-cient red-emitting tris-cyclometallated IrIII complexes with dendronized framework are
discussed in a contribution by Zhou et al. (Scheme 4).[50]
1.2.2 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.[21,69] As introduced by Neve and co-workers,[61,70] the commonly used synthetic protocol towards cationic bis-cyclometallated iridium(III)
polypyridyl complexes is based on a bridge-splitting reaction of the respective chloro-bridged iridium dimer complexes under mild conditions. A variety of different 2,2’-bi-pyridine, 1,10-phenanthroline and 2,2’:6’,2’’-terpyridine derivatives has found use as neutral bidendate ligands.[9,20,69,71] The introduction of withdrawing or electron-donating groups on the cyclometallating ligands in combination with lateral (S-con-jugated) 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, –CF3, –C(CH3)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).[9,72,73] Similar to their neutral counterparts the ligand field
stabilization energy (LFSE) in such charged complexes is strongly depending on the position of the substituents with respect to the cyclometallating carbon atom.[32,74,75]
Thompson et al. were able to show that the excited-state properties of bis-cyclo-metallated IrIII complexes can be chemically controlled simply via the nature of the ancillary ligand.[76] The enhancing effect of increased sterical hindrance of the ancillary
N,N-ligand on the PLQYs has been reported by Wu, Wong and co-workers (Scheme 5).[77]
Supported by theoretical investigations, Huang and co-workers recently described their concept of tunable emission via the expansion of the S-conjugated system of the chelating ligand.[78] Related work by Mussini, Roberto, Fantacci and their co-workers deals with the extension of this approach; the photophysical properties of such cationic IrIII complexes could be further fine-tuned via lateral electron-rich or electron-poor
substituents on phenanthroline-type ligands.[79] Functionalized 2,2’:6’,2’’-terpyridine
derivatives and related structures have also been used as bidentate ligand in bis-cyclo-metallated iridium(III) compounds (Scheme 5).[80,81] Highly efficient triplet-triplet intramolecular energy-transfer from a bis-cyclometallated IrIII core to a lateral C
60
-sub-stituent on a functionalized bipyridine ligand was reported by Nierengarten et al.[82]
In continuation of prior work, De Cola and co-workers described new dinuclear iridium(III) complexes obtained by Pd(0)-catalyzed coupling reactions on bromophe-nyl-substituted mononuclear IrIII species (Scheme 5).[83] The same approach was used by Arm and Williams to obtain mixed-metallic assemblies featuring efficient energy transfer from IrIII cores to RuII centers.[84]
Such charged complexes find in particular application in the fields of protein labeling for biomedical analysis, as oxygen sensors, for photocatalytic water-splitting and as active species in LECs.
Scheme5. Schematic representation of selected charged IrIII complexes (see text for details).
To suppress crystallization or aggregation occurring in blended films[85,86] and to
enhance the processability (e.g. via inkjet-printing or spin-coating, by inducing film-forming abilities[87]), cationic IrIII complexes have successfully been introduced into polymeric materials, both non-conjugated[88-90] and conjugated ones.[91-94]
In addition to the complexes bearing three bidentate ligands, analogous structures containing two tridentate ligands[95] (e.g. 2,2’:6’,2’’-terpyridine, 2,6-diaryl-pyridine and
their derivatives) have recently gained attention as potential candidates for applications in areas such as luminescent sensors or materials for directional energy and electron transfer.[96-103] A detailed review on this concept has recently been published by
Williams et al.[104] Iridium(III) mono-terpyridine complexes decorated with
electron-donor or -acceptor groups have furthermore been employed as asymmetric chromo-phores in non-linear optics.[105,106]
Introducing strong ligand-field-stabilizing ligands, such as CN– or CO, the energy gap between HOMO and LUMO can be significantly increased resulting in anionic or cationic iridium(III) complexes with bright blue emission. These recent examples by Fantacci and Nazeeruddin[107,108] as well as Chin[109] and their co-workers have
significantly expanded the toolbox of color-tuning in iridium(III) complexes (Scheme 5).
1.3Polymerscontainingiridium(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 the 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.[110] Host materials can be further on classified
based on their ability to transport holes or electrons. Besides hole- and electron-transporter, a third group – ambipolar charge-transporters – is defined that can readily transport both holes and electrons.[111]
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 and emission), has received increasing attention.[11,112,113] 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.[87,114] 1.3.1 Complex containing polymers – synthesis and properties
There are in principle five general routes to synthesize polymers containing transition metal complexes (Scheme 6): (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”.
(III)(Co)polymerization bycomplexation (II)Complexation at(co)polymers (I)Decorationof(co)polymers (IV)Complexas(co)polymerizationinitiator (Va)(Co)polymerization ofmonomers (Vb)(Co)polycondenzation of“monomers”
Scheme 6. Schematic representation of different general approaches for the synthesis of
(co)polymers that contain transition metal complexes.
(I) In order to “decorate” (co)polymers with complexes, suitable combinations of
functionalities attached to the (co)polymer and the complex, respectively, are utilized.[25,58,59,115,116] Weck and co-workers reacted aldehyde-functionalized heteroleptic
tris-cyclometallated IrIII complexes with amino-group bearing styrene units,
copoly-merized 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” reaction to attach heteroleptic tris-cyclometallated IrIII complexes
equipped with a terminal C-C-triple bond to azide functionalized styrene and N-vinyl-carbazole copolymers (Scheme 7).[59]
Reacting bis-cyclometallated IrIII complexes bearing a vinyl-functionalized 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 endgroup.[25,115,116] Polysiloxanes are generally characterized by a high
gas permeability, and the materials functionalized wih IrIII emitters blended in
Scheme 7. Schematic representation of a selected example for the synthesis of IrIII complex
containing polymeric systems by “decoration” of copolymer (Weck and co-workers).[59]
(II) (Co)polymers bearing suitable ligand sites can be transformed to emitter-equipped
systems by reacting them with proper precursor complexes.[88-91,117-124] One of the
earliest examples of an iridium(III) complex-containing polymer was obtained by Kim and co-workers in an one-pot two steps complexation reaction; after treating Ir(acac)3
with 2-phenylpyridine, the ligand-equipped polymer [poly((2-(4-vinylphenyl)pyri-dine)co-vinylcarbazole)] was added to achieve the formation of polymer-bound tris-cyclometallated IrIII complexes under rather harsh conditions (i.e. 170 °C for 12 h).[117]
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 emission from the complex sites 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[118] as well as Langecker et al.[119] 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-dihexylfluorene-alt-pyridine), with Ir(acac)3 at 250 °C for 12 h, which is
supposed to lead to the formation of mono-cyclometallated IrIII intermediates. In the
second step, 2-phenylpyridine was added, and the mixture was heated again (250 °C for 12 h) to finally obtain the desired complexed copolymer (Scheme 11). Adapting the iridium(III)- and the ligand-feed, the extent of the complexation at the polymer could be varied.[118] Langecker et al. applied a different synthetic procedure.[119] The treatment of
and silver triflate under heating (about 100 °C, 4 days) resulted in the coordination of roughly 50% of the polymer-bound ligand sites (2-phenylpyridine).
Schubert and co-workers reported on a number of charged, polymer-equipped bis-cyclo-metallated IrIII complexes obtained by reacting dimeric chloro-bridged iridium(III) precursor complexes with poly(İ-caprolactone)-functionalized bipyridine[88,89] or
poly-ethylene glycol[90,120] and polystyrene-functionalized terpyridines (here acting as a
bidentate ligand only).[120] Detailed investigations by means of size exclusion
chro-matography (SEC) and matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry proved the formation of well-defined materials. By treating terpyridines tethered to the acrylate moieties of a styrene-block-acrylate-copolymer with iridium(III) mono-terpyridine complexes (at 200 °C for 20 min) Aamer et al. achieved the formation of charged iridium(III) bis-terpyridine complexes at some of the polymer-bound terpyridines.[121] The micellization behavior of the block copolymer in various solvents was investigated.
Deng et al. synthesized random and block co- and terpolymers copolymerizing ligand sites (i.e. styrene-functionalized acetoacetone motifs) with various hole- and electron-transporting monomers (styrene or acrylate derivatives) under nitroxide mediate polymerization (NMP) conditions. Upon complexation with IrIII- or PtII-precursors a broad set of host-guest systems was obtained (Scheme 8).[122] 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%.
Scheme 8. Schematic representation of a selected example for the synthesis of IrIII complex
containing polymeric systems by complexation at a copolymer (Deng et al.).[122]
Cao and co-worker synthesized conjugated fluorene-alt-carbazole polymers bearing charged IrIII 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.[91] By the introduction of charged complex species, implementation of LEC-analogous features, such as improved charge-injection, were attempted.
Multi-layer as well as single-Multi-layer OLEDs were prepared and investigated observing a fast decrease of the maximum external quantum efficiency from 7.3% to 3.4%. Koga et al. described the complexation of methyl methacrylate-styryldiphenylphospine copolymers with chloro-bridged biphenyl cyclooctadiene and C,N-chelated iridium(III) dimer, in different ratios, obtaining the bis-phosphine and the mono-phosphine iridium(III) polymer complexes, respectively.[123,124]
For the “decoration” (I) as well as the complexation (II) at (co)polymers a high conversion at mild conditions is preferable, enabling to work in stoichiometric amounts by avoiding damage and degradation at the polymer. The “clicking” of azides[59] and the coordination to phosphine[123] are two promising examples in this respect. While the
formation of tris-cyclometallated[117-119] and bis-terpyridine iridium(III) complexes[121]
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.[88,89] Steric demands (e.g. in the case of coordination sites in a conjugated backbone,[118,119] polymer blocks[121] or in crosslinked
systems[124]) 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 to vary 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.[91,123]
(III) There are a number of known examples in literature dealing with the
(co)polymerization by complexation, resulting in so-called chain-extended poly-mers.[125] Assemblies based on the “polycomplexation” of bis(terpyridine)s by
RuII,[126,127] NiII,[128] CoII, FeII,[129] and ZnII [130,131,132] as well as poly-PtII-acetylene[133]
and polyferrocene-systems, just to name a few, can be ob-tained this way – but up to now there is no attempt reported to use this method for the formation of IrIII containing polymers. Iridium(III) can form bis-terpyridine complexes similar to RuII and FeII, but
comparably harsh conditions, in particular high temperatures, are usually necessary to obtain the desired complexes in low to moderate yields.
(IV) Another possibility to obtain complex-containing (co)polymers is to use a
re-spective functionalized complex as polymerization initiator. While this approach is described for e.g. RuII complexes,[134] an iridium(III) complex functioning as
polymeri-zation initiator has not yet been reported in literature.
(V) The most widely utilized method to access polymers containing phosphorescent IrIII
complexes is the (co)polymerization,[33,135-147] or (co)condensation,[36,92-94,118,119,148-170] of suitably functionalized complexes. Weck and co-workers used the ring-opening
metathesis polymerization (ROMP) of functionalized exo-norbornene[135,137] or
cyclooctene monomers[136] to synthesize homo- and copolymers equipped with IrIII
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.[135] They demonstrated the copolymerization of a
cyclooctene-functionalized tris-cyclometallated IrIII complex with a
carbazole-func-tionalized cyclooctene.[136] Further on, norbornenes bearing various heteroleptic tris-cyclometallated IrIII complexes were copolymerized with a (bis-carbazole-fluorene)-norbornene derivative.[137,138] Testing several materials in a multilayer device assembly,
a maximum efficiency of 4.9% (at 100 cd·m-2) could be realized.[138]
Ulbricht and Rehmann et al. reported on the synthesis of oxetane-functionalized bis-cyclometallated iridium(III) acetoacetate complexes and the optimization of multi-layer OLEDs employing these emitters, which were covalently incorporated into a crosslinked matrix.[139,140] The hole-transport layers as well as the matrix of the emissive
layer 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/hole-blocking layer on top of the crosslinked HTLs/EML-assembly and by the optimization of the layer thicknesses. Wang et al. as well as Park and co-workers used free radical polymerization initiated by 2,2’-azobis(iso-butyronitrile) (AIBN) to synthesize copolymers possessing iridium(III) complexes.[140,142] In the first case a bis-cyclometallated iridium(III) complex
coordi-nating acrylate as polymerizable ancillary ligand was copolymerized with N-vinyl-carbazole.[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.[142] The expected suppression of phase
segregation in the copolymers compared to the doped analogues was verified by confocal laser scanning microscope (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 high triplet energy. Sato and co-workers investigated iridium(III) complex containing polymeric systems, which were obtained by radical copolymeriza-tion as
well.[33,143-146] They focussed on copolymers from N-vinylcarbazole and
bis-cyclometallated IrIII complexes coordinated to an acetoacetonate or a picolinate ligand,
which were tethered to a styryl- or a vinyl-functionality.[33,143-145] 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 IrIII complexes
possessing an acetoacetonate ligand bearing a styryl-function.[146] The copolymers
functionalized with red, green and blue phosphorescent emitters, respectively, were investigated in respect to their performance in multilayer OLEDs. Recently, Ulbricht et al. described the copolymerization of a bis-cyclometallated IrIII complex coordinating amethacrylate 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 9), respectively.[147] 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[171] as in the case of 3-vinylcarbazole based polymers.[142] 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.
Scheme 9. Schematic representation of a selected example for the synthesis of IrIII complex
containing polymeric systems by copolymerization (Ulbricht et al.).[147]
In order to synthesize conjugated polymers, polycondensation reactions are usually the methods of choice. For the preparation of conjugated polymeric systems containing phosphorescent IrIII complexes, the Suzuki cross-coupling[36,92-94,118,119,148-170] as well as
the Yamamoto coupling[119,148,169,164] found widespread application. In the Suzuki reaction, arylic boronic acids or their esters are cross-coupled with arylic bromines in the presence of a Pd0-catalyst leading to the formation of arylic carbon-carbon bonds
(Scheme 10). Using the Yamamoto method arylic bromines are reacted with equimolar amounts of Ni(cod)2 yielding coupled aromatic systems.
Scheme 10. Schematic representation of a selected example for the synthesis of IrIII complex
containing polymeric systems by copolycondenzation under Suzuki cross-coupling conditions (Cao and co-workers).[151]
Independently from the applied coupling method (i.e. Suzuki or Yamamoto) all reported IrIII 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 (Schemes 10 and 11), 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 IrIII species, respectively. Another possibility is the coordination of a ligand tethered to a
bis-bromo-functionalized aromatic system. Alternatively 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 options to introduce the complex moiety lead to a diversity of conjugated polymeric structures (Schemes 10 and 11). All reported non-conjugated systems can be summarized as polymer-tethered complexes (Schemes 7-9). Analogous conjugated assemblies, where complexes are attached with or without spacer 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 (Scheme 11). This group of materials can be further subclassified depending on the involved ligand(s) (Schemes 10 and 11). The ancillary ligand, e.g. bipyridine or 1,5-phenyl-acetoacetone, in bis-cyclometallated complexes as well as one of the ligands in heteroleptic tris-cyclome-tallated complexes can be incorporated within the backbone. Another possibility is to
apply both cyclometallating ligands of a bis-cyclometallated complex as polycondensa-tion endcappers forming a polymer backbone, which conjugapolycondensa-tion is interrupted by the metal centers of the incorporated complexes. Examples using respective heteroleptic tris-cyclometallated IrIII complexes are not described yet.
Scheme 11. Schematic representation of the general structure and selected examples of IrIII
complex containing polymers – non-conjugated and conjugated.
Non-conjugated and conjugated systems possess distinct features, particularly regarding optical applications. Conjugated polymers are expected to provide better charge trans-port 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 host 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 band gap emitters, i.e. also blue emitters with rather high LUMO
levels.[183] However, the design of conjugated polymers with comparatively high triplet
energy levels is in progress.[110,172]
Besides the complex monomers, mainly two co-building block motifs – 2,7-linked 9,9-difunctionalized-fluorene and/or 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 12).
Scheme 12. Schematic representation of the most widely used co-building blocks in the
synthesis of conjugated polymeric systems hosting phosphorescent IrIII complexes –
2,7-bis(bro-mo/boronic acid ester)-9,9-dialkylfluorene (left) and 3,6-bis(bro2,7-bis(bro-mo/boronic acid ester)-9-alkyl-carbazole (right).
Chen et al. presented some of the earliest examples of iridium(III) complex-containing conjugated polymeric systems. 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(N-carbazolyl-decyl) and 9-hexyl-9-(11,13-di-oxo-tetradecyl) coordinating a bis-cyclometallated iridium(III) complex were the substituents applied.[148] The performance of the obtained copolymers was investigated in solution processed devices (ITO/PEDOT/emissive polymer/Au/Al) exhibiting red emission. Mei et al.,[152] Evans et al.[149] as well as Cao and co-workers[150,151,154] used
the same structural approach (Scheme 10). Utilizing the Suzuki cross-coupling method, conjugated polymers bearing phosphorescent IrIII complexes in the side chain were
obtained. Also here the bis-cyclometallated IrIII complexes were tethered via a ȕ-diketo-nate ligand to the polymeric backbone. Mei et al. cocondensated the same fluorene-complex motif as used by Chen et al.[148] with dialkylfluorene derivatives.[152] As the
relative incorporation rates of the applied co-monomers determine in part their distribu-tion within the forming 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 phosphors within a chain were excluded.[149] 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 spacer-less system considerable 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.[150,151,154] They coordinated
3,6-dibromo-N-(14-trifluoro-11,13-dioxo-tetradecyl)-carbazole as ancillary ligand to form monomer-tethered bis-cyclometallated IrIII complexes. Cocondensations of bis-(boronic acid ester)-dialkylfluorene with dibro-mo-N-alkyl-carbazole and different carbazole-complex monomers resulted in a number of copolymers (Scheme 10).[151] In further investigations, modified assemblies were
prepared exchanging dibromo-N-alkyl-carbazole with 4,7-dibromo-[2,1,3]benzothiadia-zole and dibromo-dialkylfluorene in the coupling mixtures.[151,154] 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 electro-luminescence (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.[150] The other systems consisting of blue (fluorene moieties) and green ([2,1,3]benzothiadiazole moieties) fluorophors, as well as red (tethered IrIII 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.[151,154]
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 IrIII complexes.[153]
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 contributions from all three primary colors and with CIE coordinates close to the equal-energy white point was obtained from a single emissive layer 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
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 accompa-nied by a clear bathochromic shift in absorption and emission. For ancillary ligands as bipyridine and 1,5-bis(phenyl) acetoacetone the effect on the optical properties of the complex can be less pronounced.
Cao and co-workers reported on the incorporation of bis-cyclometallated IrIII complexes
via their ancillary ligands, 1,5-bis(p-bromophenyl)-acetoacetone and 1-(p-bromo-phenyl)-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-alkylcarba-zole, and/or dibromo fluorenone with bis(boronic acid ester)-dialkylfluorene via Suzuki cross-coupling (Scheme 11).[157-161] Besides a number of red-emitting systems,[157,159-161]
white-emitting copolymers were obtained.[158] Instead of benzothiadiazole as in the previous examples of white emitting systems,[153] fluorenone was applied as green-emitting species.
In another variant of this approach, Huang and co-workers synthesized charged bis-cyclometallated IrIII complexes coordinating dibromo-bipyridine or
dibromo-phenan-throline. Applying the Suzuki cross-coupling reaction, bis(boronic acid ester)-dialkyl-fluorene was cocondensated with the complex monomers, dibromo-dialkylester)-dialkyl-fluorene, N-alkylcarbazole, and/or dibromo bis(N-carbazolyl-alkyl) fluorene.[92-94] 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 energy-transfer compared to blended systems. However, luminance and efficiency of devices applying these materials were found to be inferior to assemblies based on neutral IrIII complexes.
Therefore, further optimization is required.
Instead of a symmetrical building block, Kappaun et al. introduced commercially avail-able 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 the Suzuki cross-coupling.[170] Using the pure material in a device only
weak red electroluminescence was obtained. By diluting the material in polyfluorene, white light (CIE: x = 0.30, y = 0.35) was emitted from the device.
Ito et al. presented a first example of heteroleptic tris-cyclometallated IrIII 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 cross-coupling conditions yielded a polymeric material with high complex loading (approximately 50 wt%).[155] A device applying the
red-emissive copolymer as EML showed rather poor performance, which was mainly attributed to concentration quenching. Blending of the copolymer with 4,4’-N,N’-car-bazole-biphenyl (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,[118] Zhang[156] and Langecker et al.[119] made use of the same heteroleptic
tris-cyclometallated complex, fac-bis(2-phenylpyridinato-C2
,N’)[2-(p-bromophenyl)-5-bro-mopyridinato-C2,N']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 IrIII complexes,
Langecker and Schulz et al., respectively, 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 (Scheme 11).[118] Two iridium(III)-containing polymers exhibiting red and greenish-yellow emission, respectively, were obtained. While in PL spectra of solutions only traces of complex emission could be found, energy transfer from the host to the emitter was observed in films. In EL spectra, the host emission was almost completely quenched.
Langecker et al. compared the Suzuki and the Yamamoto coupling reaction with respect to their performance in the synthesis of such complex-containing conjugated systems. The complex monomer was coupled either with just dibromo-dialkylfluorene (Yamamoto) or with additional equimolar amounts of 1,4-bis(boronic acid ester)-2,5-di-alkylbenzene (Suzuki). Due to distinct higher yields and higher degrees of polymeriza-tion, the Yamamoto coupling reaction was found to be the superior method for the synthesis of the desired polymers.[119] In the 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 from host to guest occurs.
In Suzuki cross-coupling reactions, Zhang et al. cocondensated bis(boronic acid ester)-dialkylfluorene with complex monomer and dibromo-di(3-dimethylamino)propyl-fluo-rene. By quarternation of the tethered amino functions with bromoethane, the obtained copolymers were transformed into polyelectrolytes.[156] 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/ca-thode). 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.,[168] Cao and co-workers[36,162,163,165-167] as well as Lee et al.[164] reported on
complex-containing polymers with metal centers interrupting the conjugation. They all used IrIII complexes coordinating two bromo-functionalized cyclometallating ligands and acetoacetonate or an analogue (2,2,6,6-tetramethyl-3,5-heptanedione) as ancillary ligand.
In a first contribution dealing with this polymer class, Sandee et al. applied such bis-cyclometallated IrIII acetoacetonate complexes as endcapper in Suzuki cross-coupling reactions of 2-(boronic acid ester)-7-bromo-dialkylfluorene obtaining acetoacetonate complexes coordinating two cyclometallating macroligands.[168] 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.[36,162,163,165-167] Also
4,7-dibromo-[2,1,3]benzothiadia-zole was used as additional comonomer to tune the optical properties towards white emission (Scheme 11).[167] Polyelectrolytes were obtained by quaternation of fluorene
tethered amino groups[162] analogous to the previously mentioned systems.[156] Besides
acetoacetonate derivatives, picolinate as well as 5-methyl-3-(pyridine-2-yl)-[1,2,4]tria-zolate 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 IrIII complexes with different
kinds of p-dibromo phenyl compounds. Aside from a coupling reaction with a bis-cyclometallated acetoacetonate complex, also condensation reactions with fac-tris(3-bromo-phenyl)-pyridine)iridium(III) were performed resulting in hyperbran-ched structures.[169]
Lee et al. used the Yamamoto coupling reaction to realize further polymer variants. Complex monomer and dibromo-dialkylfluorene were cocondensated, and N-phenyl-4-bromo-1,8-naphthalimide was applied as endcapper.[168] 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 (IrIII complex). The variation of the monomer feed ratios is a general method to optimize the system in respect to high efficiencies, color purity, 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.
1.3.2 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, the dendritic structures can be classified into conjugated[50,51,55,173-179] and
non-conju-gated[31,56,180-182] assemblies (Scheme 13).
Scheme13. Schematic representation of selected dendrimeric IrIII complex centered assemblies:
(1) conjugated system with triaryl motives (Cao and co-workers);[179] (2) non-conjugated system
with peripheral carbazole moities (Jung et al.).[181]
In most cases a heteroleptic or homoleptic facial tris-cyclometallated iridium(III) complex constitutes the core with two[174,175] or, respectively, three[50,51,55,56,173-181] ligand-tethered dendritic arms forming the shell. Li et al. reported on a number of
