Novel Photoconductive Polymers

Electrophotography is the only area in which the conductivity of sophisticated organic materials and polymers is exploited in a large scale industrial process today. Photoconductors are characterized by an increase of electrical conductivity upon irradiation. According to this definition photoconductive materials are insulators in the dark and become semiconductors if illuminated. In contrast to electrically conductive compounds photoconductors do not contain free carriers of charge. In photoconductors these carriers are generated by the action of light. 

The discovery of photoconductivity dates back to 1873 when W. Smith found the effect in selenium. Based on this discovery C. F. Carlson developed the principles of the xerographic process already in 1938. 

Photoconductivity in organic polymers was first discovered in 1957 by H. Hoegl, who found that poly(N-vinylcarbazole) (PVK) and charge transfer complexes of PVK with electron acceptors like 2,4,7-trinitrofluorenone act as photoconductors [1], 

Besides the application of photoconductive polymers in photocopiers these materials are also widely used in laser printers in the last years. The third area in which photoconductors are applied is the manufacturing of electrophotographic printing plates. 

The organic 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 weight compounds imbedded in a polymer matrix. These so-called moleculary doped polymers are widely used today. 

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R. M. Podhajny. Novel photoconductive carbzole polymers and photoconductive elements embodying same. US Patent 3 697 264, assigned to R. M. Podhajny, October 10,1972. 

The synthesis of novel materials with high carrier mobilities is one of the major goals in the field of photoconductive polymers. Within the last years different approaches have been pursued to reach this goal. First, the photoconductive properties of conjugated polymers like poly(phenylenevinylene) and poly(methyl phenylsilane) have been investigated [12]. Another approach are liquid crystals which are the topic of the first Chapter. The third way to realize... 

New types of photoconductive polymers containing imide groups have been described [330-332]. They show high photosensitivity and a set of other valuable properties. Takimoto et al. [331] developed novel photoconductive polyimides, whose monomer units are composed of a series of thiophenylene moieties and... 

Both quantum functional materials and molecular devices are considered to be the ultimate functional materials. The former shows a novel property which is specific to the structure and the latter represents the smallest possible functional material. Their properties are closely related to optical and photonic functions. The former shows a quantum size effect, a photoluminescence shift to higher energy which depends on layer thickness, which gives us an idea of a nonlinear optical system. Porphyrin arrays connected with molecular wires show a hole carrier photoconductivity or a photoswitching and a photo-information storage, which suggests an idea of a photoactive neuron model. In this paper, conjugated polymer superlattices and porphyrin arrays connected with molecular wires are described. 

Dissymmetric systems shows exciton splitting of dichroic absorptions and optical activity and photochromic materials having NLO, photoresponsiveness and photorefractivity properties, which occurred due to the presence of azoaromatic and chiral functionalities in the polymers [62]. Finally, novel optically active multifunctional methacrylic copolymers were synthesized, which contained side-chains chiral moieties and linked to a photoconductive carbazolic and to a photochromic azoaromatic chromophores... 

Winiarz JG, Zhang L, Lai M, Friend CS, Prasad PN (1999) Photogeneratitui, eharge transport, and photoconductivity of a novel PVK/CdS-nanoCTystal polymer composite. Chem Phys 245 417-428... 

Wang and Herron [76] have also, demonstrated another new method of doping PVK. By synthesizing small cadmium sulphide clusters (diameter ca 16 A) inside a PVK layer the authors have demonstrated that the resultant composite is capable of generating charges efficiently upon irradiation. In such a composite the semiconductor cluster acts as a sensitizer for the photogeneration of carriers and the polymer serves as carrier transporting medium. The quantum yield 0 and a thermalized separation distance of electron-hole pairs ro were found to be 0.16 and 26 A respectively. The approach can probably be extended to include other semiconductor nanoclusters and to develop a novel class Pf photoconductive materials. 

Ring opening metathesis polymerization of 5- 6-N-carbazolylhexyl)oxy)methyl)-2-norbomene (24a) with various transition metal catalysts has been used for the preparation of the novel carbazolyl-substituted polymer (24b) (Scheme 11.12) [213]. Studies on the photoconductivity of the polymer (19b) including copolymer systems containing tetrahydrofuran moieties are still in progress. 

This book does not intend to cover the whole field of thermotropic liquid crystalline (TLC) materials as it is extremely difficult to cover within a single book. Instead it presents a collection of Chapters written by experts on various exciting topics in the field. Properties of recently developed TLCs (such as banana-type, thiophene-based, and columnar TLCs), phase biaxiaUty, and novel polymeric TLCs are discussed In detail. Solid-state NMR studies to obtain atomistic-level structural and geometrical information of TLCs are presented. Synthesis of liquid crystalUne conjugated polymers, fast switching of nematic materials by an electric field, and photoconducting discotic systems are also presented. 

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