Squaraines absorption characteristics

Among the materials for laser printer utilization, we may consider squarylium dyes and trisazo compounds (Fig. 5). Squaraines, for instance, give rise to photoreceptors with superior electrical characteristics, as they can form different crystalline modifications throughout the introduction of suitable impurities at low concentration [42]. This characteristic is due to the ability of squaraines to form aggregates having very broad absorption spectra (400 to 1000 nm). 

Table 2 Absorption and Emission Characteristics of bis(2,4,6-trihydroxyphenyl)-squaraine (18) in the Presence and Absence of (a) p-cyclodextrin [P-CD] in water and (b) poly(4-vinylpyridine) [P4VP] in methanol...

In a recent study, Ashwell et al. have observed that centrosymmetric squaraine dyes incorporated into LB monolayers showed second harmonic generation efficiencies, which compare favourably with the highest values hitherto reported for LB monolayers of noncentrosymmetric dyes [126]. Based on comparison of the absorption spectra and SHG characteristics of the LB films of squaraine 45 (Structure 15) as well as the effect of deposition pressure on these properties, it has been suggested that formation of noncentrosymmetric aggregates is responsible for these effects. Nonlinear optical studies have also shown that symmetric squaraines have quite large molecular second hyperpolarizabilities [127-131]. 

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