Squaraines preparation

Squaraines can be readily synthesized via the reaction of squaric acid (Treibs and Jacob, 1965) or its diester (Law and Bailey, 1986) with aromatic amines. For photoreceptor applications, Law and Bailey (1987) reported that the best results were obtained with squaraines prepared via the diester. This has been ascribed to the formation of a different crystal modification with lower impurity concentrations. Other purification methods have been reported by Lin and... 

A squaraine prepared from N-chlorobenzyl-N-methylaniline and squaric acid has found utility as a generation layer in a dual-layer photoreceptor with an inverted structure for positive charging applications (Yamamoto et al 1986). The photoreceptor showed full-process stability. 

Pham W, Weissleder R, Tung CH (2003) A practical approach for the preparation of monofunctional azulenyl squaraine dye. Tetrahedron Lett 44 3975-3978... 

Griffiths J, Park S (2002) Facile preparative redox chemistry of bis(4-dialkylaminophenyl) squaraine dyes. Tetrahedron Lett 43 7669-7671... 

The effects of the generation-layer fabrication variables on the sensitometry of a dual-layer photoreceptor prepared with bis(4-dimethylaminophenyl) squaraine (X = H in Appendix 2) have been extensively investigated by Law (1987). The charge acceptance, dark discharge, sensitivity, and the residual... 

Unsymmetrical squaraines (Yanus and Limberg, 1986 Kazmaier et al., 1988a) prepared from different aromatic amines have been compared with the corresponding symmetrical compounds (Kazmaier et al., 1988b). In the same... 

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). 

Using a polycondensation approach, Ajayaghoh et al. [63] prepared water-soluble squaraine oligomers by condensation of squaric acid with... 

Some bis-squaraine derivatives 2.211 have been prepared as near-infrared emissive fluorophores, where two squaraine units were linked by oligothiophene spacers (Chart 1.43) [300], These dyes showed absorption bands at 780 and 700 nm with high molar absorptivities. Additionally, by increasing the number of thiophene units, blue shifts in the absorption bands were observed. The noncovalent interactions with human serum albumin (HAS) and BSA have been studied in trizma buffer and showed the formation of 1 1 complexes concomitant with a fluorescence enhancement. 

Many authors agree that products with one or more nitrogen groups as substitutes of the oxygen atoms in the squarate 2 ion should be called rather squaraines. The first of this class of compounds was prepared in 1965 through the reaction of squaric acid 6 and pyrroles, which produced intensely colored condensation products [62]. But the literature also describes the synthesis of a squaraine obtained from resorcinol, which does not possess nitrogen atoms therefore, there is no consensus on the concept of squaraines in the literature [63]. 

Squaraines are usually prepared by condensation of electron-rich aromatic or heteroaromatic compounds such as anilines [45a, 50], phenols [64], and pyrroles [65] with squaric acid 6, as can be seen in Scheme 5.4. 

In xerographic applications, the major technology shortfalls for symmetrical squaraines have been the low synthetic yield, high dark-decay and poor spectral response in the visible region, e.g., < 600 nm. Law and Bailey reported efforts to address these issues via unsymmetrical squaraines, such as USq-5 to -15 in Table 10.5 [178]. The precursors for the synthesis, III-V (structures in Scheme 10.3), were synthesized by a [2 + 2] cycloaddition reaction between p-methoxy-phenylketene, or 3,4-dimethoxyphenylketene or 3,4,5-trimethoxyphenylketene with tetraethoxyethylene, followed by a hydrolysis reaction. Unsymmetrical squaraines can then be prepared by condensing III or IV or V with an aniline derivative. The synthetic and absorption spectral data for the synthesized unsymmetrical squaraines are tabulated in Table 10.5. Studies of... 

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