Squaraine pigments

Squaraines are zwitterionic compounds synthesized from squaric adid and N,N-dialkylanilines (Law and Bailey, 1987). 

Loutfy et al. (1983) measured the photoconductivity of a series of squaraines dispersed in a styrene butylmethacrylate copolymer. The generation efficiencies were strongly field dependent. In most materials, a quadratic field dependence was observed. For MSQ, the results were in agreement with Tam. At high fields, the efficiency was approximately 0.70. 

Since the advent of time-of-flight photocurrent techniques in the late 1950s, and photoinduced discharge techniques in the mid-to-late 1960s, there have been many studies of photogeneration in organic materials. While studies lhave been 

In addition to surface-enhanced exciton dissociation and geminate recombination, direct photoexcitation (Northrop and Simpson, 1956 Heilmeier et al., 1963 Harima et al., 1989) and exciton-exciton annihilation (Silver et al., 1963 Jortner et al., 1963 Braun, 1968 Johnston and Lyons, 1968 Foumy et al., 1968 Swenberg, 1969 Braun and Dobbs, 1970 Orlowski and Scher, 1983) arguments have been proposed. In direct photoexcitation, a free electron and free hole are created without the involvement of intermediate states. With the exception of the work of Harima et al. and Orlowski and Scher, there have been few references to direct or exciton-exciton photogeneration processes in the past one and a half decades. 

Pfister and Williams (1973) Schechtman (1976) Borsenberger and Ateya (1978) Stolka and Pai (1978) 

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There are many patents of generation layers utilizing novel squaraine pigments. See, for example, Champ and Vollmer (1987), Kin et al. (1986), Tanaka et al. (1987, 1987a), Law (1993a), and Saeki et al. (1993). 

Environment effects have been described by Law (1987) in studies of a dual-layer photoreceptor with a squaraine-pigment-based generation layer. Seasonal fluctuations in dark decay and charge acceptance were observed and related to humidity. Water adsorption on the pigment particles was suggested as the underlying cause. 

Molecular complexes, such as the complex formed between poly(N-vinylcaibazole) and 2,4,7-trinitro-9-fluorenone, and dye-polymer aggregates were widely used as generation materials in many early applications. Since these materials are not infrared sensitive, there has been increasing emphasis on pigments. The more widely studied are various azo, phthalocyanine, squaraine, and peiylene diimide derivatives. A common feature of all of these materials is that they are polymorphic and exist in many different crystal forms. The properties are thus very sensitive to the conditions used in their preparation. Further, the sensitivity of these materials is strongly field dependent as well as dependent on the transport material. For a review of generation materials, see Law (1993). 

The adsorbed polymer sterically stabilizes particles of 1 in polymer solution, resulting in a stable squaraine-polymer dispersion Analogous steric stabilization of pigment particles via specific... 

Fig. 6. Classes of charge-generating dyes and pigments (a) perylenes, (b) thiapyrylium, (c) phthalocyanines, (d) squaraines, and (e) azo. R and Z are oiganic...

Unsymmetrical azo pigments, 517, 535 Unsymmetrical perylene pigments, 520 Unsymmetrical squaraines, 507 Unsymmetrically substituted phthalocyanines, 696... 

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