Photoconductive azo pigments

As seen in the structures in Table 10.6, all the photoconductive azo pigments are o-hydroxy substituted. o-Hydroxy substituted azo compounds are known to undergo hydroxy-azo/keto-hydrazone tauto-merism. For instance, 1-phenylazo-2-naphthol (XII) can be a hydroxy-azo or a keto-hydrazone, depending on the solvent, the substituents and the physical state [208,211]... 

Table 10.6. VOL 1 Charge-Transfer Salts, Fullerenes Examples and structures of photoconductive azo pigments ... 

As shown in Scheme 10.4, the design of new azo pigments, by variation of the structures of the aromatic amine and/or the coupler, is almost unlimited. A literature survey indicates that azo pigments from 2-naphthol couplers are consistently found to exhibit moderate to high photoconductivity. Examples of notable aromatic amines and major classes of couplers (VIII, IX, X and XI) are tabulated in Table 10.6... 

A number of reviews have appeared of topical interest. These include photoisomerism of azo dyes, ° photofunctional polysilanes,photochromic pigments, rare earth complexes, pressure sensitive paints, electron-transfer processes,electroactive dendrimers, chiral polyisocyanates, photodefin-able benzocyclobutene, photoconductive polymers,excited states in conjugated polyenes, photosensitive materials and polyazomethanes. ... 

A variety of azo and methine dyes and pigments have been examined in conjunction with PVK and other polymers, but the patent literature indicates that virtually any dye or pigment can sensitize the photoconductivity of PVK into the visible range. When photogeneration proceeds via electronic transitions involving both the polymer and the dye (such as a charge-transfer complex between PVK and the acceptor part of the dye), the process is also considered intrinsic. When these transitions proceed within the dye molecules or pigment particles, the process is termed extrinsic. 

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