Handbook of Polymer Synthesis Second Edition Episode 14 pptx

with polymers because of their homogeneity, good layer-building-properties and term form stability resulting in a long device lifetime.long-The goal of this article is to describe the sc

Polymers for Organic Light Emitting

Devices/Diodes (OLEDs)

O Nuyken, E Bacher, M Rojahn, V Wiederhirn and R Weberskirch

Technische Universita¨t Mu¨nchen, Garching, Germany

an excited singlet state Light emission of the latter is then a result of relaxation processes[4–6] To achieve high electroluminescence efficiencies, the materials have to fulfill severalspecific requirements including low injection barriers at the interface between electrodesand organic material, balanced electron- and hole-density and mobility and high lumines-cence efficiency Furthermore, the recombination zone should be located away from themetal cathode to prevent annihilation of the exited state Since no material known to date

is able to meet all these criteria, modern OLEDs consist — besides the transparentsubstrate (e.g., glass, PET), anode (most commonly indium tin oxide, ITO) and metalcathode (e.g., Mg–Ag-alloy) — of several organic layers for charge injection, transportand/or emission [7,8] (the principal set-up is shown in Scheme 1)

ð1Þ

In such multilayer diodes, each layer can be separately optimized concerninginjection barriers, charge mobility and density and quantum efficiency Much of themotivation for studying organic materials stems from the potential to tailor desirableoptoelectronic properties and process characteristics by manipulation of the primarychemical structure Objecting optimal charge transport, recombination probabilityand light emission and consequently a maximum external efficiency of the device, varioussubstances have been developed, modified and tested in the last few years For holetransport/electron blocking layers, triarylamine- and pyrazoline-structures (see Scheme 2)were found to be most promising [9–11].

ð2Þ

For electron transport/hole blocking purposes, a wide variety of electron-deficientmoieties are well known, e.g., 1,3,4-oxadiazoles [12], 1,2,4-triazoles [13], 1,3-oxazoles,pyridines and quinoxalines [14] (see Scheme 3) Materials with conjugated p-electronsystem (e.g., styrylarylenes, arylenes, stilbenes, oligo- and poly(thiophene)s — seeScheme 3) are widely used as combined charge transport and luminescence layers aswell [12,15]

Basic structures of electron transport=hole blocking materials and oligomericand polymeric materials for charge transport and luminescence

with polymers because of their homogeneity, good layer-building-properties and term form stability resulting in a long device lifetime.

long-The goal of this article is to describe the scope and limitations of synthetic routesthat have been used to produce suitable oligomers and polymers for LED application.The polymers in this article will be discussed on the basis of their backbone structure andthe synthetic strategy of their formation and are divided into completely p-conjugatedpolymers, non-conjugated polymers and polymers with defined segmentation (seeStructure 4)

ð4Þ

Since the discovery of electrically conductive polymers by Heeger, MacDiarmid andShirakawa et al in 1977 [18] — resulting in the Nobel Prize in Chemistry 2000 [19] —p-conjugated systems have a major role in the field of so called ‘plastic electronics’ Keyproperty of these polymers is the conjugated double bond along the polymeric backbone,allowing charge migration after injection via electrodes

A Poly(p-phenylene-vinylene)s (PPV)

The first polymers used for light emitting diodes — discovered by Friend and Holmes et al

in 1990 [20] — and still the most common ones used in recent devices, are completelyp-conjugated poly( p-phenylene-vinylene)s These polymers — which can be used in singlelayer devices as both charge-transport and green emitting materials — will be discussed onthe synthetic strategy of their formation

1 Precursor Routes

Unsubstituted poly(phenylene-vinylene)s (PPVs) are insoluble in any known solvent

To improve solubility and with that processability unsubstituted PPVs were first sized using precursor routes like the so called Wessling- (or sulfonium-) route [21–24].Accordingly, the condensation is performed with solubilized monomers, and a solublepolymeric intermediate is formed The latter is converted to PPV in a final reaction step,that is preferentially carried out in the solid state, allowing the formation of homogeneousPPV films or layers Following this route, a soluble precursor polymer with excellentfilm forming properties is obtained by base induced polyreaction of p-xylylene-a,a0-bisdialkylsulfonium salts After spin coating, the precursor polymer is converted bypolymer analogous heat induced elimination to the corresponding PPVs (Scheme 5)

synthe-ð5Þ

In general, any functionalized poly( p-xylylene) with leaving group in the a-position

to the aromatic moieties can be used as precursor, as long as they fulfill the basicrequirements of OLED-techniques (i.e., solubility, transparency, excellent film formingproperties, good thermal stability after processing, etc.) Commonly used as leavinggroups beside the sulfonium group are halogens [25,26] (so called ‘Gilch-procedure’),hydrohalogenides [27], alkoxides [28] and alkylsulfinyles (known as ‘Vanderzane-procedure’) [29]

To avoid unwanted side reactions and damages of other device-layers during thermalconversion (e.g., by oxidation or reaction with volatile corrosive elimination products),organic-solvent soluble PPV derivatives such as poly(2-methoxy-5-(20-ethylhexyloxy)-p-phenylenvinylene (MEH-PPV) or poly 2,5-dihexyloxy-p-phenylenevinylene (DH-PPV)(Scheme 6) have been developed These materials can be spin-coated from solution afterthe conversion step Another advantage of these PPV-derivatives is the possibility tomodify the electronic properties of the film with different substitution patterns Thereforeall kind of organic substituents have been introduced into the aromatic system to alterthe structure of the aromatic building block, including alkoxy-, alkyl-, cholestanoxy andsilicium containing groups [30–35] (Scheme 6)

ð6Þ

A precursor route not involving heteroatoms in the precursor polymers has alsobeen developed It is based on the oxidation of soluble poly( p-xylylene)s to correspondingPPVs by using stoichiometrical amounts of 2,3-dichloro-5,6-dicyano-1,4-benzochinone(DDQ) (Scheme 7) but is restricted so far to a-phenyl-substituted poly( p-xylylene)s [36]

ð7Þ

Beside spin-coating-based preparation techniques, the so-called deposition-route (CVD) has gained considerable attention as a solvent free preparationprocess Following this route, the starting materials are pyrolized after vaporization,followed by CVD and polymerization of the monomers on the substrate Finally, the

chemical-vapor-halogeno-functionalized poly( p-xylylene) is converted to PPV by polymer-analogousthermoconversion (Scheme 8) [25,37,38].

ð8Þ

2 Polycondensation and C–C-Coupling Routes

Some drawbacks of the precursor routes mentioned above have been overcome by theuse of polycondensation- and C–C-bond-coupling reactions To produce soluble PPV-,poly(thiophene)-, or poly(pyrrol) derivatives for spin coating preparation, various types

of transition metal catalyzed reactions, such as the Heck-, Suzuki-, and reaction, Wittig- and Wittig–Horner-type coupling reactions, or the McMurry- andKnoevenagel-condensation have been utilized

Sonogashira-A typical example of the Pd catalyzed Heck reaction of 1,4-dibromo-2-phenylbenzolwith ethylene to obtain the poly(phenylphenylene vinylene) [39] is depicted in Scheme 9

A common drawback of this reaction-type is the insufficient regioselectivity, resulting in1,1 diarylation of the product (>1%, depending on the substituents) [40]

ð9Þ

In order to avoid this problem, the Suzuki coupling is used as well to obtain varioussubstituted PPVs Therefore an aromatic diboronic acid or ester and dibromoalkylene arereacted in the presence of a Pd catalyst as depicted in Scheme 10 [41]

ð10Þ

synthesized by Knoevenagel condensation of substituted terephthaldehyde with

benzene-1,4-diacetonitriles yielding an alternating copolymer type product (seeScheme 11) [42]

ð11Þ

Schlu¨ter et al described the synthesis of soluble PPV derivatives from substitutedaromatic dialdehydes via McMurry-type polycondensation reaction With this low valenttitanium catalyzed reaction (see Scheme 12), the obtained products are characterized by

a double bond cis/trans ratio of about 0.4 and an average degree of polymerization ofabout 30 [43]

ð12Þ

Phenylic substituents at the vinylene positions — increasing both solubility of thepolymer and stability of the double bond — can be achieved by reductive dehalogenationpolycondensation of 1,4-bis(phenyldichlormethyl)benzene derivatives with chromium(II)-acetate as reducing agent [44] (see Scheme 13)

ð13Þ

A further route leading to unsubstituted PPV was published by Grubbs et al [45],utilizing ring-opening olefin metathesis reaction as shown in Scheme 14 Starting from

bicyclic monomers with bicyclo(2.2.2)octadiene skeleton, the ring-opening metathesispolymerization (ROMP) is performed with Schrock-type molybdenum carbene catalysts.The obtained, well defined, nonconjugated soluble precursors, containing carboxylic esterfunctions, are then thermally converted to the conjugated PPV.

ð14Þ

The Wittig reaction (see Scheme 15) is also a commonly used method for yieldingPPV derivatives from arylene bisphosphonium salts and bisbenzaldehydes Since onlyproducts of moderate molecular weight are obtained, more interest in this reaction isgiven in the field of spacer segregated poly( p-phenylene vinylene)s with defined conjuga-tion length (see III.A) [46] An improvement concerning the degree of polymerization isobtained by the Horner modification of the Wittig procedure (‘Wittig–Horner reaction’).Following this route, the bisphosphonium salt is replaced by bisphosphonates or aromaticbisphosphine oxide monomers [47]

ð15Þ

Due to the side chain induced twist within the main chain the effective conjugationlength is notably effected in soluble PPVs A strategy to overcome this problem and todevelop more rigid conjugated systems has been presented by Davey and co-workers in

1995 who prepared poly(phenylene-ethynylene)-type polymers according to the followingscheme (Scheme 16) [48]

ð16Þ

Oligomers of (o-phenylene-vinylene)s can be obtained using various C–C-couplingand polycondensation methods For higher oligomers and polymers, the Stille-typecoupling of 1,2-diiodobenzene or 1,2-bis(2-iodostyryl)benzene with bis(tri-n-butylstannyl)-ethylene was introduced by Mu¨llen et al [49] (see Scheme 17).

ð18Þ

Heteroaromatic systems, such as the widely used poly(thiophene)s can be obtained bysimple oxidative polymerization of the soluble monomers or oligomers either byelectrochemical means or oxidizing agent such as FeCl3 [50,51] This common route isalso used to synthesize a variety of mono- and dialkyl-, -alkoxy-, and -alkylsulfonic acidsubstituted and therefore soluble poly(thiophene)s [52–57] (Scheme 19) and can also beutilized to obtain poly(pyrrole)s The disadvantage of this polymerization methodshowever is the regiorandom structure of the polymeric product with non-reproducibleproperties

For better defined poly(thiophene) structures a variety of organometallic mediatedsynthesis have been introduced Most widely employed are Grignard-type organo-magnesium compounds in addition to a nickel catalyst Highly regioregular head-to-tail3-alkylpoly(thiophene)s are obtained following the synthetic route of McCullough et al.(see Scheme 20)

ð20Þ

Polymers — prepared via the polymerization of alkylthiophenes — exhibit enhanced conductivity and optical properties when comparedwith regiorandom materials [58,59] Another approach to regioregular alkylpoly(thio-phene)s is the usage of zinc instead of magnesium in nickel- or palladium catalyzedpolymerizations [60,61] Due to the improvements, these synthetic methods are by far themost valuable synthetic routes to these materials In contrast, the regioselective synthesis

2-bromomagnesio-5-bromo-3-of substituted poly(pyrrole)s was not reported to date

Heterocyclic, electron deficient conjugated systems like poly(1,3,4-oxadiazole)s,poly(1,3-oxazole)s and poly(1,2,4-triazole)s are applied in organic light emitting diodes aselectron transport and hole blocking layers The synthetic strategies for their formationare as manifold as the structures themselves, reaching from polymerization of functionalmonomers to polymer analogue formation of the conjugated system (e.g., by ring closuredehydration, dehalogenation, etc.) For further details is referred to the reviews ofSchmidt et al [14] and Feast et al [62]

C Light Emitting Polymers (LEPs) Based on Polyfluorenes

A second important class of p-conjugated polymers are polyfluorenes, which wereobtained the first time by oxidative polymerization of 9-alkyl- and 9,9-dialkylfluoreneswith ferric chloride [63] These polymers showed low molecular weight and some degree ofbranching and non-conjugated linkages through positions other than 2 and 7

A very successful way to improve regiospecificity and to minimize branchingwas the synthesis through transition-metal-catalyzed reactions of monomeric2,7-dihalogenated fluorenes The palladium-catalyzed synthesis of mixed biphenylesfrom phenylboronic acid and aryl bromide discovered by Suzuki et al [64] tolerates a largevariety of functional groups and the presence of water This method can also be used toprepare perfectly alternating copolymers

1 Polyfluorene-Homopolymers

Polyfluorenes with alkyl substituents at C9 are soluble in conventional organic solventssuch as aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran, whichmade them useful to prepare thin films for OLEDs As a consequence many effortshave been undertaken to synthesize a large number of high-molecular-weight, 9-mono-, ordisubstituted very pure fluorene-based polymers

ð21Þ

9,9-Disubstituted 2,7-bis-1,3,2-dioxaborolanylfluorene is allowed to react with avariety of dibromoarenes in the presence of a catalytic amount of (triphenylphosphine)palladium (Scheme 21) The improved process yields high-molecular-weight polymers with

a low polydispersity (<2) in less than 24 h reaction time, whereas the conventional Suzukicoupling process can take up to 72 h and more to deliver polymers of modest molecularweights Optimized LEDs based on these polymers, made by improved Suzuki poly-fluorene chemistry, exhibited light emission exceeding 10,000 cd/m2with a peak efficiency

This alternating copolymer concept has been extended to other conjugated mers as shown in Scheme (22) All synthesized copolymers [66] are of high molecularweight, are highly photoluminescent and their emissive colours can be qualitativelycorrelated to the extent of delocalization in the comonomers For example the thiophenecopolymer emits bluish green light, but the bithiophene copolymer emits yellow light

No other polymer class offers the full range of color with high efficiency, lowoperating voltage and high lifetime when applied in a device The polyfluorene-basedmaterials seem to be very viable for commercial applications A special group within thepolyfluorenes are poly-spiro-derivatives They offer a wide range of accessible colours.Compared to standard polyfluorenes they are morphologically more stable and do notform aggregates as easily (see Scheme 23):

ð23Þ

D Poly(p-phenylene)s

Poly( p-phenylene) (PPP) represents a wide class of interesting conjugated polymers forPLED applications To be exact, the formerly described polyfluorenes also belong to thisclass of polymers Wide bandgaps are typical for PPPs and allow emission of blue light.Since the design of efficient long-lived blue emitters remains a significant challenge to thefield, polymers, such as poly-p-phenylenes, are attractive candidates for consideration

As with the PPVs, most PPPs are characterized by their insolubility and infusibility,properties that were a considerable hindrance towards structural characterization andprocessing Thus research activities were directed to form PPP films via soluble thermallyconverted precursor polymers on the one hand and the development of soluble, substitutedPPPs on the other hand

First attempts to generate poly( p-phenylene) were undertaken by Kovacic et al inthe 1960s [67] He reported the oxidative treatment of benzene with copper(II)-chloride inthe presence of strong Lewis acids (e.g aluminum trichloride) which led to a condensation

of the aromatic rings by forming radical cations as reactive intermediates The benzeneunits are preferentially connected in the 1,4-position, but crosslinking and oxidativecondensation to highly condensed aromates and a maximum degree of condensation ofabout 10 make this reaction interesting only for historical aspects

1 PPPs by Transition-Metal-Catalyzed Condensation Reactions

The availability of newer, more effective methods for aryl–aryl coupling has been animportant driving force for the development of new synthetic strategies for PPPs andother polyarylenes Transition metal catalysis, such as the Pd(0)-catalyzed aryl–arylcoupling developed by Suzuki [63] and nickel(0)-catalyzed or -mediated coupling

according to Yamamoto [68] have been employed most successfully An example for theNi(0)-catalyzed coupling is the coupling of 1,4-dibromo-2-methoxycarbonylbenzene topoly(2-methoxycarbonyl-1,4-phenylene) as a processable PPP precursor [69] The aro-matic polyester PPP precursor is then converted to carboxylated PPP and thermallydecarboxylated to PPP with copper(II)-oxide catalysts (Scheme 24).

n

ð24Þ

A second, very fruitful synthetic principle for structurally homogenous, processablePPP derivatives involves the preparation of soluble PPPs by the introduction of solubi-lizing side groups The pioneering work here was carried out in the late 1980s, whensoluble poly(2,5-dialkyl-1,4-phenylene)s were prepared for the first time [70] The Suzukiaryl–aryl cross-coupling method (Scheme 25), adapted to polymers by Schlu¨ter, Wegner

et al., made it possible to synthesize solubilized PPPs with a dramatically increasedmolecular weight of up to 100 phenylene units

ð25Þ

Soluble PPPs not only contain alkyl substituents, they were also synthesized withalkoxy groups and with ionic side groups like carboxy and sulfonic acid functions, whichare able to form PPP polyelectrolytes [71]

It is also possible to synthesize chiral PPPs as Scherf et al reported [72] They arecomposed of chiral cyclophane subunits, made by a Suzuki-type aryl–aryl cross-couplingreaction of the corresponding diboronic acid and dibromo derivatives The monomerscontaining cyclic –O–C10H20–O– loops were separated into the pure enantiomers and used

to generate the corresponding stereoregular iso- and syndiotactic PPP-derivatives(Scheme 26) The isotactic derivative possesses a chirality of its main chain

ð26Þ

An important aspect concerning the electronic properties of the PPP is the influence

of substituents at the phenylene units In unsubstituted PPP, there is a twist angle of 23

between adjacent phenylene units [73] This seems to be significant, but the p-overlap

is a function of the cosine of the twist angle, so a fair amount of conjugative interactionremains even at 23
If substituents are placed along the PPP backbone (e.g., at the2- and 5-positions), the solubility is enhanced, but the p-overlap is reduced dramatically.The resulting twist angles reach from 60
to 80
depending on the length of the alkylsubstituents [74]

The described facts show the synthetic demands for being able to prepare able, and structurally defined PPPs, in which the p-conjugation remains nearly intact or iseven increased compared to the parent PPP system To realize this principle it is necessary

process-to prepare structures in which the aromatic subunits could be obtained in a planar or onlyslightly twisted conformation in spite of the introduction of substituents One of the firstexamples was the synthesis of polyfluorenes via oxidative coupling of fluorene derivatives

as described above [63] Another possibility to reach this aim is the preparation of

‘stepladder’ PPPs

Monomers like the 2,7-dibromo-4,9-dialkyl-4,5,9,10-tetrahydropyrenes (Scheme 27)represent suitable starting monomers for the realization of such ‘stepladder’ structures.These difunctionalized tetrahydropyrene monomers were first prepared by Mu¨llen et al.[75] and reacted in a Yamamoto-type coupling [76] Reaction of the dibromide with astoichiometric amount of a low-valent nickel(0) complex gave a poly(4,9-dialkyl-4,5,9,10-tetrahydropyrene-2,7-diyl) (PTHP) as a new, completely soluble type of PPP derivative,

in which each pair of neighboring aromatic rings is doubly bridged with ethano linkages.The solubilizing alkyl substituents are attached at such positions on the periphery ofthe molecule that they cannot cause twisting of the main chain The number-averagemolecular weight was Mn¼20,000, corresponding to 46 THP units

n

ð27ÞThe luminescence characteristic of PTHP suggests that it is a potential candidatefor the active component in OLEDs Investigations showed the appearance of a quiteintense blue-green electroluminescence with a quantum yield of up to 0.15% (single layerconstruction ITO/PTHP/Ca)

The ‘stepladder’ concept can be logical continued towards a completely planarladder polymer to minimize the mutual distorsion of adjacent main chain phenylene units.The complete flattening of the conjugated p-system by bridging all the phenylene subunitsshould then lead to maximum conjugative interaction As with the PTHP systems, alkyl

or alkoxy side chain should lead to soluble polymers This idea was realized first in 1991with the first synthesis of a soluble, conjugated ladder polymer [77] The preparation isaccording to a so-called classical route, in which an open-chain, single stranded precursorpolymer was closed to give a double stranded ladder polymer (Scheme 28) In the synthesis

of this LPPP, the precursor polymer is initially prepared by Suzuki aryl–aryl coupling of

an aromatic diboronic acid and an aromatic dibromoketone

ð28Þ

The cyclization to structurally defined, soluble LPPP takes place in a two-stepsequence, consisting of a reduction of the keto group followed by ring closure of thesecondary alcohol groups in a Friedel–Crafts-type alkylation The resulting ladderpolymer has an average molecular weight of 25,000, corresponding to 65 phenylene units.LPPP is characterized by unusual electronic and optical properties as a consequence ofplanarization of the chromophore The absorption maximum undergoes a bathochromicshift to a lmax value of 440–450 nm for the p!p* transition compared to PPP with

lmax¼336 nm [78] The photoluminescence of LPPP in solution is a very intensive blue,but the bulk properties are surprising different: Although efficient LEDs can be assembled,the emission of the solid state film is yellow in the case of photoluminescence andelectroluminescence In comparison to the former descripted PTHPs the quantum yield

is with ca 1% much higher [79]

2 Other Routes to Poly( p-phenylene)s

Recently the most popular synthetic routes to PPPs are the transition-metal-catalyzedcondensation reactions discussed above, but several other syntheses were developed togenerate PPP and its derivatives

About 40 years ago, Marvel et al described [80] the polymerization of5,6-dibromocyclohexa-1,3-diene to poly(5,6-dibromo-1,4-cyclohex-2-ene), followed by athermally induced, solid state elimination of HBr with formation of PPP (Scheme 29).The products, however, indicate some structural defects like incomplete cyclizationand crosslinking

ð29Þ

More than two decades later, Ballard et al developed an improved precursor route,starting from 5,6-diacetoxycyclohexa-1,3-diene (Scheme 30), the so called ICI route[81,82] The soluble precursor polymer is then aromatized thermally to PPP viaelimination of two molecules of acetic acid per structural unit The polymerization ofthe monomer, however, does not proceed as a uniform 1,4-polymerization: beside the