Handbook of Polymer Synthesis Second Edition Episode 13 pdf

Carrier Transport The photogenerated or injected charge carriers move within the polymer under theinfluence of the electric field.. The transport of carriers can now be regarded as a therm

Photoconductive Polymers

P Strohriegl

Universita¨t Bayreuth, Makromolekulare Chemie I, and Bayreuther Institut fu¨r

Makromoleku¨lforschung (BIMF), Bayreuth, Germany

J V Grazulevicius

Kaunas University of Technology, Kaunas, Lithuania

Since 1992 when the first edition of the Handbook of Polymer Synthesis was published

a number of new applications for photoconductive polymers or, to put it correct, chargetransport materials, have appeared The most successful development are organic lightemitting diodes (OLEDs) which right now enter the market as bright displays for cellularphones and car radios Other imortant areas are organic field effect transistors, solar cellsand lasers

For this reason the review has been thoroughly updated mainly in the Sections V.Band V.C which deal with conjugated polymers, a very active research area in which

A Heeger, A McDiarmid and H Shirakawa received the Nobel Prize in 2000 A largenumber of new polymers and up-to-date references have been included

Photoconductivity is defined as an increase of electrical conductivity upon irradiation.According to this definition photoconductive polymers are insulators in the dark andbecome semiconductors if illuminated In contrast to electrically conductive polymersphotoconductors do not have free carriers of charge In photoconductors these carriers,electrons or holes, are generated by the action of light The carriers of electricity can also

be photogenerated extrinsically in an adjacent charge generation layer, and injected intothe polymer which in this case acts as a charge transporting material

Only polymers capable of both producing charge carriers upon exposure to lightand transporting them through the bulk are true photoconductors Polymers that do notabsorb the incident light but accept charges generated in an adjacent material are merelycharge transport materials

The discovery of photoconductivity dates back to 1873 when W Smith found theeffect in selenium Based on this discovery C F Carlson developed the principles of thexerographic process already in 1938 Photoconductivity in polymers was first discovered

in 1957 by H Hoegl [1,2] He found that poly(N-vinylcarbazole) (PVK) sensitized withsuitable electron acceptors showed high enough levels of photoconductivity to be useful

in practical applications like electrophotography As a result of the following activitiesIBM introduced its Copier I series in 1970, in which an organic photoconductor, thecharge transfer complex of PVK with 2,4,7-trinitrofluorenone (TNF), was used for thefirst time [3] The photoconductor was a 13 mm single-layer device It was prepared

by casting a tetrahydrofuran solution containing PVK and TNF onto an aluminumsubstrate [4] Since then numerous photoconductive polymers have been described inliterature and specially in patents The ongoing interest in photoconducting polymers isconnected with an increasing need for low cost, easy to process and easy to form largearea materials

The polymeric photoconductors used in practice are based on two types of systems.The first one are polymers in which the photoconductive moiety is part of the polymer,for example a pendant or in-chain group The second group involves low molecular weightchromophores imbedded in a polymer matrix These so called molecularly doped polymersare widely used today Almost 100% of all xerographic photoreceptors at present aremade of organic photoconductors [5] The main area of application of polymeric photo-conductors is electrophotography [6] Photoconductive polymers are used in photocopiers,laser printers, electrophotographic printing plates, and electrophotographic microfilming.During the last decade, photoconductive or more precisely charge transporting polymershave been widely used in photorefractive composites [7] and in organic light emittingdiodes (OLEDs) [8,9] An upcoming field for the application of charge-transportingpolymers are photovoltaic devices [10,11]

The process of electrophotography is schematically shown in Figure 1 It is acomplex process involving at least five steps [12]

1 Charge In the first step the surface of the photoconductor drum is uniformlycharged by a corona discharge

2 Expose Parts of the photoconductor are discharged by light reflected from animage So the information is transferred into a latent, electrostatic image on thesurface of the photoconductor

3 Develop Electrostatically charged and pigmented polymer particles, the toner,are brought into the vicinity of the oppositely charged latent image transforming

it into a real image

4 Transfer The toner particles are transferred from the surface to a sheet ofpaper by giving the back side of the paper a charge opposite to the tonerparticles

5 Fuse In the last step the image is permanently fixed by melting the tonerparticles to the paper between two heated rolers The photoconductor drum iscleaned from any residual toner and is ready for the next copy

Organic electrophotographic photoreceptors are also widely used in laser printers[13,14] The principal of these printers is almost the same as in a photocopier exceptthe direct generation of the image by a laser instead of the optical system in a copier.Photoreceptors of the laser printers have to absorb in the near infrared range of spectrum.The third area in which photoconductive polymers or polymer composites are applied areelectrophotographic printing plates

The first comprehensive reviews on photoconductive polymers were published byStolka alone [15] and in co-authorship with Pai [16] Chemical aspects of the topic werelater reviewed by several authors [17–19] In the work of Mylnikov photoconductivity

of polymers was reviewed within the framework of semiconductor physics [20], whereasHaarer [21] has concentrated mainly on the transport properties of photoconductivepolymers In their comprehensive book, Borsenberger and Weiss described all aspects ofphotoconductive materials [6]

Photoconductive polymers can be p-type (hole-transporting), n-type transporting), or bipolar (capable of transporting both holes and electrons) Typically,bipolarity can be accomplished by adding electron-transporting molecules such as TNF to

(electron-a donorlike, hole-tr(electron-ansporting polymer such (electron-as PVK Most of pr(electron-actic(electron-al photoconductivepolymers are p-type, however recently much attention is paid to electron-transporting andbipolar polymers [22]

Since the major goal of this chapter is the description of the different classes ofphotoconductive polymers, the underlying physical principles will be only brieflydiscussed For more detailed reviews dealing with photoconductor physics the reader isreferred to the literature [21–24]

The process of photoconduction involves several steps [15]

Figure 1 Principles of the xerographic process (for explanations see text)

A Absorption of Radiation

The first step to a charge carrier generation is the absorption of radiation conductive materials are truly photoconductive only in the range of wavelength ofabsorption Thus PVK is a photoconductor only in the UV range To produce carriers

Photo-by visible light sensitizing dyes or electron acceptors forming coloured charge transfercomplexes must be added

B Generation of Charge Carriers

By the absorption of light the active groups are excited and form closely bound electron–hole pairs The key process that determines the overall photogeneration efficiency is thefollowing field induced separation into free charge carriers This process competes with thegeminate recombination of the electron–hole pair A theoretical description of this process

is provided by Onsager’s [25] theory for the dissociation of ion pairs in weak electrolytes

in the presence of an electric field The model has been successfully applied to amorphousphotoconductors [26] It was found that the photogeneration efficiency, in other wordsquantum yield of the process, is a complicated function of several variables such as electricfield strength, temperature, and separation distance The predicted relationship is in goodagreement with experimental data for doped polymers like N-isopropylcarbazole inpolycarbonate [27], triphenylamine doped polycarbonate [28] and PVK [29,30]

The quantum yields in ‘pure’ photoconductors absorbing in the UV range areusually low and strongly field dependent So at room temperature and an excitationwavelength of 345 nm the quantum yield  for PVK rises from 0.01% at 104V/cm toabout 6% at 106V/cm [28] Substantially higher values for  are obtained in the presence

of complexing additives like dimethyl terephthalate [31,32] The addition of suitableelectron acceptors which form colored charge-transfer complexes is a proven way toincrease the photogeneration efficiency 2,4,7-Trinitrofluorenone (TNF) in combinationwith PVK is so effective that the combination was used in the IBM copier I, the firstcommercial copier with an organic photoconductor

C Injection of Carriers

An injection of carriers only occurs if an extrinsic photogenerator is used together with acharge transporting material Usually dye particles are dispersed in a polymer matrix orevaporated on top of a conductive substrate and then covered with the charge transportingpolymer The carriers are generated in the visible light-absorbing material and injectedinto the polymer

D Carrier Transport

The photogenerated or injected charge carriers move within the polymer under theinfluence of the electric field In this process the photoconductive species, for examplecarbazole groups in PVK, pass electrons to the electrode in the first step and therebybecome cation radicals The transport of carriers can now be regarded as a thermallyactivated hopping process [33–37], in which the hole hops from one localized site toanother in the general direction of the electric field (Figure 2).The moving cation radicalcan accept an electron from the neighboring neutral carbazole group which in turnbecomes a hole, and so on Effectively the hole moves within the material while electrons

only jump among neighboring species Hole transport can therefore be described as aseries of redox reactions among equivalent groups.

During transit, the carriers do not move with uniform velocity but reside most of thetime in localized states (traps) and only occasionally are released from these traps to move

in field direction This trapping process is responsible for the extremely low hole mobilities

in photoconductive polymers For PVK room temperature mobilities from 3  108 to

106cm2/Vs (E ¼ 105V/cm) have been reported [6] Since the transport of holes can bedescribed as a series of electron transfer reactions with a certain activation energy it is notsurprising that the carrier mobility is temperature- and field-dependent

For the characterization of polymeric photoconductors two established methods exist:the Time of Flight (TOF) and the xerographic method Both methods provide informationabout the two fundamental parameters that characterize a photoconductive material:carrier mobility m and quantum yield 

The principle of TOF method is shown inFigure 3.A thin film of photoconductivematerial is sandwiched between a conductive substrate, for example an aluminized

Figure 2 Principles of carrier transport (for explanations see text)

mylar film, and a semitransparent top electrode and connected to a voltage source and

a resistor R Because of the blocking electrodes the source voltage appears across thefilm A thin sheet of charge carriers is generated near the top electrode by a short pulse ofstrongly absorbed light Due to the influence of the applied field the carriers drift acrossthe sample towards the bottom electrode The resulting current is measured in the externalcircuit at the resistor R A typical experimental photocurrent for the polysiloxane 11c(m ¼ 3) with pendant carbazolyl groups is shown in Figure 4 [38]

In the double logarithmic plot of photocurrent versus time the bend at the transittime ttis clearly detectable The effective carrier mobility m is calculated from the transittime according to Equation (1)

Figure 3 Typical time-of-flight (TOF) setup for measuring hole mobilities in polymers

Figure 4 Typical experimental photocurrent of polysiloxane 13 (m ¼ 3) at an electric field of

3  105V/cm (T ¼ 293 K) The arrow marks the transit time

where d denotes the sample thickness and E is the electric field strength With d ¼ 6.7 mm,

E ¼4.6  105V/cm and a transit time tt of 2.8  105ms an effective carrier mobility

of 1  104cm2/Vs is calculated fromFigure 4.Note that for the conjugated trimer (74)with its high mobility the transit time can be seen even in a linear plot of Iphoto vs.time (inset)

The carrier mobility m is temperature- and field-dependent Many theories have beendeveloped to explain the temperature dependence, but no comprehensive model is yetavailable It is still not clear whether the charge carrier mobility follows a simple Arrheniusrelationship (log m ffi 1/T ) as predicted by Gill [33] or if the more complex relationshiplog m ffi 1/T2proposed by Ba¨ssler [39] is valid The relationship between the mobility m andthe electrical field strength E is equally unclear Here Gill’s model predicts a log m ffi E1/2dependence which is consistent with a Pool–Frenkel formalism, whereas Ba¨ssler’scalculations lead to a log m ffi E dependence A detailed description of the differentmodels and results obtained by fitting experimental mobility data to those models isbeyond the scope of this chapter It shall only be pointed out here that the main difficulty

is the limited range of temperature and electric field in which carrier mobilities can bemeasured [38] Additional experiments are necessary to understand the mechanism ofcarrier transport in photoconductive polymers in detail

Several polymer types and classes are known to exhibit photoconductivity Consequently

no preferred method of synthesis exists The known photoconductive polymers areprepared by almost all common methods like free-radical, cationic, anionic, coordina-tion, and ring-opening polymerization, step-growth polymerization, and polymeranalo-gous reactions The only common requirement for all photoconductive materials isthat they have to be of extreme purity It is well known [40–42] that even traces ofimpurities act as traps and have drastic influence on both quantum yield and carriermobility

From the structural point of view the photoconductive polymers described in thischapter can be divided into three groups (Figure 5):

Polymers with pendant or in-chain electronically isolated photoactive groupswith large p-electron systems, for example, aromatic amino groups, like carba-zole or condensed aromatic rings, like anthracene

Polymers with p-conjugated main chain like polyacetylene and phenylenevinylene)

poly(1,4- Polymers with s-conjugated backbone, like organopolysilanes

A Polymers with Pendant or in-Chain Electronically Isolated

Photoactive Groups

An aromatic amino group is a common building block of many known photoconductive

or charge transporting materials Many practical systems used in electrophotographybelong to this category The active groups in these materials are either part of the polymerstructure or low-molecular dopants imbedded in a polymer matrix The later group of

materials of which numerous examples exist especially in the patent literature will not bediscussed here.

Since the discovery of photoconductivity in poly(N-vinylcarbazole) (PVK) [1,2] a variety

of polymers with carbazole groups have been synthesized and their photophysicalproperties have been investigated The main topic of this article is the synthesis ofphotoconductive polymers, so minor attention is given to their photophysical properties.PVK (2b) can be synthesized by free-radical, cationic, or charge-transfer initiatedpolymerization of N-vinylcarbazole (2a) A detailed description of the PVK synthesis isgiven inChapter 2of this handbook

ð2Þ

Poly(N-ethyl-2-vinylcarbazole) (Structure 3a) has been prepared by free-radicalpolymerization, whereas poly(N-ethyl-3-vinylcarbazole) (3b) was synthesized by cationicpolymerization with a boron trifluoride initiator [43] The 2-isomer is reported to exhibit

Figure 5 Different types of photoconductive polymers

higher carrier mobility than PVK, while that of the 3-isomer is lower [44].

ð3Þ

Tazuke and Inoue [45] reported on the synthesis of a polyvinyl derivative having

a pendant dimeric carbazole unit, 1,2-trans-bis(9H-carbazol-9-yl)cyclobutane (DCZB).Poly(trans-1-(3-vinyl-)-carbazolyl)-2-(9-carbazolyl)cyclobutane) (4a) was prepared bycationic polymerization of the corresponding monomer with boron trifluoride Thereaction yielded a polymer of relatively high molecular weight (Mn¼2.5  105, Mw¼5.8  105) Copolymers of the vinyl derivative of DCZB with N-ethyl-3-vinylcarbazolewere also obtained Fluorescence spectroscopy data have indicated that the polymer (4a)does not form excimers The photoconductive properties of polymer (4a) as well as ofits copolymers have been studied by the xerographic technique, both in the presence and

in the absence of the sensitizer TNF [46,47] The photoconductivity of (4a) is increasedcompared to PVK when the charge transfer band of the complex is irradiated Betterphotoconductive properties of (6a) correlate with its photophysical properties Excimerformation is sterically hindered by DCZB groups whereas energy migration occurs effi-ciently in it Charge transfer interaction with TNF is also stronger for (4a) than for PVK.Several polyacrylates and polymethacrylates with pendant carbazole groups havebeen described Poly(2-(N-carbazolyl)ethyl acrylate) (formula 4b) has been prepared byfree radical polymerization of the corresponding monomer [48]

ð4Þ

The polymer exhibits a charge carrier mobility of 7  106cm2/Vs (20
C, 5 

105V/cm) which is higher than in PVK The enhanced carrier mobility in the carbazolecontaining polyacrylate is apparently due to the lack of excimer-forming sites in it Polymer(4b) has also been prepared anionically with ethyl magnesium chloride/benzalaceto-phenone as catalyst [49,50] to yield an almost exclusively isotactic product Due to theinsolubility of the polymer in the toluene/diethyl ether mixture in which the polymerizationwas carried out the molecular weight is low and the product shows a broad molecularweight distribution Nevertheless time of flight measurements show that the carrier mobility

of the isotactic material (1.7  10 cm/Vs, 20 C, 2  10 V/cm) is about six times higherthan the mobility of the atactic polymer The authors concluded that stereoregularstructures enhance the hole drift mobility of pendant-type photoconductive polymers.However, the relatively small increase of the measured mobilities should be interpreted withcaution because it is well known that even traces of impurities may have a drastic influence

on the carrier mobility

A series of polyacrylates and polymethacrylates (5a) in which the carbazolyl groupsare separated from the polymer backbone by alkyl spacers of variable length have beenprepared by different methods as shown in the Scheme 5 [51] The molecular weights ofthe polymers obtained by free-radical polymerization with AIBN in toluene solution arerather low and all polymers exhibit a broad molecular weight distribution The reason isthe low solubility of the polymers in the polymerization solvent toluene In more polarsolvents like tetrahydrofuran the molecular weight is limited by chain transfer reactions.High-molecular weight poly(meth)acrylates (Mw¼100,000–150,000, Mn¼50,000–70,000)were obtained by polymeranalogous reaction of o-hydroxyalkylcarbazoles withpoly(meth)-acryloylchloride IR and1H NMR spectroscopy as well as elemental analysisshow that the reaction yields poly(meth)acrylates with an almost quantitative degree ofsubstitution

ð5Þ

The polyacrylate (6) with a pendant dimeric carbazole unit, carbazol-9-yl)cyclobutane (DCZB), does not show excimer fluorescence and exhibitsimproved hole drift mobility [52] It is obtained by free-radical polymerization of thecorresponding acrylate [53] The molecular weight of the polymer (6) established byvapour pressure osmometry is 46,000 The hole drift mobility of polymer (6) is more thanten times higher than that of PVK or poly(9-ethyl-3-vinylcarbazole).

1,2-trans-bis(9H-ð6Þ

It was established that the elevated hole drift mobility of DCZB polymers is due

to the reduced concentration of trapping sites which are in fact excimer-forming sites.This was confirmed by the temperature and electric field dependencies of the hole mobility.These observations support the idea that charge transport and exciton transport havemany features in common [54]

The cationic polymerization of 2-(N-carbazolyl)ethyl vinyl ether with borontrifluoride etherate or ethylaluminum dichloride as initiator has been described by severalauthors [55–58] (Scheme 7) Low-molar-mass polymers were obtained with both initiators[56] In the case of boron trifluoride etherate the molecular weight (Mn) was 3160, andthe ethyl–aluminum dichloride initiated polymerization yielded poly(2-(N-carbazolyl)ethylvinyl ether) (11b) with Mn¼24,500 At longer reaction times with ethylaluminumdichloride, considerable amounts of insoluble material were formed by cross-linkingreactions The data on the photoconductivity of the polymer (7b) are contradicting

In a steady state measurement Okamoto et al [55] found that the photocurrent in thepolymer (7b) is much lower than that in PVK However xerographic discharge meas-urements carried out by Turner and Pai showed that the samples of the polymer (7b)prepared with boron trifluoride as initiator had carrier mobilities only slightly lower thanthat of PVK [56] The samples of (7b) prepared with ethylaluminum dichloride showed ahigh level of charge trapping that stems from impurities in the polymer film

ð7Þ

Again, it becomes evident that it is almost impossible to compare the results ofphotoconductivity measurements from different authors because of the different methods

of polymer synthesis, purification, and the varying measurement techniques

Gaidelis et al [59] reported that the carrier mobilities of carbazole) (PEPK) (8b) are more than an order of magnitude higher than the valuesreported for PVK This observation later was confirmed by the data of Wada [60] Because

poly-(N-epoxypropyl-of this property PEPK can be used as a charge transporting material in xerocopier drums[61,62] It was also used in electrophotographic microfilming [63] High-molecular-weightPEPK is prepared by substituting halogen atoms of epihalohydrin polymers withcarbazole in organic solvents in the presence of inorganic bases and phenol radical chaininhibitors, like 2,6-di-tert-butyl-p-cresol [64] (Scheme 8)

ð8ÞThe weight average molecular weight (Mw) of PEPK synthesized by such a method

is 440,000

Oligomeric PEPK was produced industrially according to Scheme (9) [65]

ð9ÞApart from hydroxy end groups PEPK (9b) contains also unsaturated end groups [66].Propenylcarbazole groups appear in the oligomer during anionic polymerization of themonomer (9a) as the result of a chain transfer reaction [67] PEPK exhibits the best filmforming properties when its molecular weight (Mw) is in the range from 1000 to 1500 Theglass transition temperature of an oligomer of such molecular weight is 65–75
C.Brominated analogues of PEPK enable to obtain electrophotographic layers ofenhanced electrophotographic photosensitivity [68] The most promising from the point

of view of convenience of synthesis and photoactivity among the brominatedpoly(carbazolyloxiranes) is poly(3,6-dibromo-9-(2,3-epoxypropyl)carbazole) (10a) It issynthesized mainly by cationic ring-opening polymerization of the corresponding oxiranemonomer using Lewis acids [69] or triphenylcarbenium salts [70] as initiators Themolecular weight of the oligomers (10a) usually does not exceed 2000 Because of thepresence of heavy bromine atoms, the glass transition temperature of these oligomers

is higher than that of unbrominated PEPK Their film-forming properties are usuallyinferior to those of PEPK Polymerization via activated monomer mechanism in thepresence of diols allows to prepare bifunctional oligomers of 3,6-dibromo-9-(2,3-epoxypropyl)carbazole having hydroxyl end-groups and a flexible oxyalkylene fragment

in the main chain [71] They show high electrophotographic photosensitivity whensensitized and good film-forming properties [72]

Poly((2-(9-carbazolyl)ethoxymethyl)oxirane) (10b) has been synthesized both bycationic polymerization of the corresponding epoxy monomer with Lewis acids [73],triphenylcarbenium salts [74] and by anionic polymerization initiated with KOH [75] or

by potassium alkalide, potassium hydride, and potassium tert-butoxide [76] Since chaintransfer reactions to (2-(9-carbazolyl)ethoxymethyl)oxirane are not as intense as in thecase of EPK polymerization oligomers (10b) of higher molecular weight can be preparedusing both cationic and anionic initiators Polymerization with potassium hydride yieldspolymers of a degree of polymerization up to 62 Since the carbazole units in (10b)are removed from the main chain compared to PEPK it has a lower glass transitiontemperature and exhibits good film-forming properties in a wide range of molecularweights Xerographic photosensitivity of its layers doped with TNF is lower than that ofthe corresponding layers of PEPK

ð10Þ

A series of polysiloxanes with pendant carbazolyl groups (11c) have been synthesized

by the reaction of poly(hydrogenmethylsiloxane) with various o-alkenylcarbazoles [77]

ð11Þ

Detailed time of flight measurements [78] have shown that the polysiloxane (11c)with the shortest spacer (m ¼ 3) exhibits a carrier mobility which is about one order ofmagnitude higher than that for PVK The data of Goldie et al [79] corroborate thisobservation The activation energy for carrier transport derived from the temperaturedependence of the carrier mobility is 0.6 eV for all the polysiloxanes and for both PVKand N-isopropylcarbazole in a polycarbonate matrix The fluorescence spectra [80] of theextremely pure polysiloxanes prepared starting from synthetic carbazole show that thesepolymers, due to the conformational freedom of the carbazole groups, are free of excimerforming sites.

Thermotropic liquid crystalline side group polymers with carbazolyl groups havebeen reported by Lux et al [81] The idea behind this work was to make a liquid crystallinepolymer with a photoconductive mesogenic unit It should be possible to orient such

a polymer by means of an electric or magnetic field at elevated temperatures where itexhibits a mesophase and to freeze this orientation by cooling down below the glasstransition temperature In the polysiloxanes (12) a carbazole group is incorporated into amesogenic unit The polymers are prepared by a multistep synthesis the last step of which

is the polymer analogous reaction of the mesogenic unit with an alkenyl-terminatedspacer and poly(hydrogenmethylsiloxane) [77] The polymers exhibit broad mesophases,for example polymer (12) with a spacer of three methylene units (m ¼ 3) has a glasstransition at 69
C and a smectic mesophase up to the clearing point at 215
C.Unfortunately, the polysiloxanes show almost no photoconductivity

ð12Þ

The influence of liquid crystalline media on the hole transport of organicphotoconductors has been demonstrated by Ikeda et al [82] They have established thatDCZB dissolved in polymer liquid crystals showed improved hole drift mobility owing

to the orientation of the carrier molecules The same research group [83] has preparedcopolymers of acrylates with side chain mesogens and dimeric carbazoles (13).Incorporation of the DCZB moieties into the copolymers resulted in homogeneousdispersion of carrier groups, but a great extent of destabilization of the liquid crystalline

phase was observed Nevertheless the hole drift mobility was found to be enhanced incopolymer films with more ordered structure of the DCZB moieties, indicating thatorientation of the photoconductive groups is favourable for the charge carrier transport.

ð13Þ

Apart from polymers containing both photoconductive and liquid crystalline sidegroups a lot of attention has been paid to the synthesis of polymers in which bothphotoconductive and nonlinear optical chromophores are present Polymers showing bothsecond-order nonlinear optical and photoconductive properties are photorefractive andhave potential application in data storage and image processing as well as in medicine [84].Carbazolyl-containing photorefractive polymers have been reviewed [85] An example ofsuch functional polymer is given in the Scheme 14 Tamura et al [86,87] have synthesizedpolyacrylates and polymethacrylates having carbazole and tricyanovinylcarbazole sidegroups 5-(N-carbazolyl)pentyl methacrylate and acrylate were polymerized using AIBN

as an initiator The resulting polymer was reacted with tetracyanoethylene to cyanovinylate ca 20% of the carbazole units

tetra-ð14Þ

All polymers discussed above have pendant carbazolyl groups Only few condensates in which the carbazolyl group is part of the main chain have been reported.Tazuke et al [88–90] have synthesized polyurethanes, poly-Schiff bases and polyamidescontaining DCZB moieties in the main chain Polyurethanes containing DCZB moieties

poly-in the mapoly-in chapoly-in (15) were prepared by treatpoly-ing carbazolyl)cyclobutane with the corresponding diisocyanate in the presence of dibutyltindilaurate [88] The molecular weight of the polymer synthesized using hexamethylenediisocyanate as a linking agent was 2700, and that of the polymer prepared with toluylenediisocyanate was 16,000 Polymers (15) exhibit almost exclusively monomer fluorescence

trans-1,2-bis(3-hydroxy-methyl-9-in dilute solution, i.e., they practically have no trans-1,2-bis(3-hydroxy-methyl-9-intramolecular excimer-formtrans-1,2-bis(3-hydroxy-methyl-9-ing sites Theircomplexes with TNF show better photoconductive properties than PVK-TNF

ð15Þ

Polyimines (16) containing DCZB moieties and a spacer of variable number ofmethylene groups have been synthesized by Natansohn et al [91] from trans-1,2-bis(3-formyl-9-carbazolyl)cyclobutane and the corresponding aliphatic diamine Thecharge transfer complexes of the polyamines (16) with tetracyanoethylene and TNFhave been analyzed both in solution and in solid state These polyimines do form chargetransfer complexes with both TNF and tetracyanoethylene, but these complexes have asolution like behavior, i.e., the components are relatively free to move around Chargecarrier transport in the polyimines (16) has been studied by the time-of-flight technique[92] The hole mobility in polyimines (16) is higher than that in PVK

ð16Þ

2 Other Photoconductive Polymers with Non-Conjugated Main Chain

Besides polymers with a carbazole moiety a number of polymers with various pendantaromatic amino groups have been reported Poly(N-vinyldiphenylamine) (17a) and