Photoconductive squaraines

The synthesis of oxo-squaraines and related compounds, including their spectral properties and applications as biomedical probes, photoconducting materials, and photosensitizers are provided in a recent review [56]. 

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. 

The semiconductor selenium is an excellent photoconductor however, its photoresponsiveness decreases sharply at wavelengths over 550 nm, which has motivated research on organic photoconductive materials.1550,1551 Many different organic compounds display photoconductive behaviour, such as phthalocyanines (596 M = metal) or squaraines (597) (Figure 6.33) today more than 90% of xerographic photoreceptors are composed of organic photoconductors. 

Photoconducting and other solid state properties of materials are dependent both upon the intrinsic molecular properties of the material as well as the intermo-lecular interactions that occur in the solid state. The sharp and intense bands of squaraines for example become broad and red-shifted in the solid state [2]. Studies on aggregates can help in developing a molecular level understanding of their solid state properties. In view of this, the aggregation behavior of... 

In an attempt to improve the synthetic yield of high-sensitivity squaraines, e.g., Sq-1 and Sq-8, mixtures of squaraines were synthesized by reacting either squaric acid or dialkyl squarate with a mixture of N,N-dimethylanilines [171-173]. The resulting squaraine mixtures show very similar solution and solid state properties as comparable symmetrical squaraines. More importantly, they also exhibit acceptable xerographic sensitivity. Structural analyses indicated that each of these mixtures consists of two symmetrical squaraines and one unsymmetrical squaraine. The observation of favorable photoconductive properties from these compositions was initially a surprise because it is generally... 

The photoconductivity of USq-5 to -15 have been studied in bilayer xerographic devices [179]. Although fabrication effect, purity and hole-injection efficiency have been shown to be important factors that influence the photoconductivity, USq-13 was identified as the most outstanding squaraine so far. Specifically, USq-13 is shown to have a low dark-decay value (—15 V/s) and high sensitivity in xerographic devices, where Eo values of 3.1 and 1.9 ergs/cm at 600 nm and 790, respectively, are obtained [181]. The sensitivity appears to surpass all squaraines reported in the literature (Table 10.4). In fact, the sensitivity performance of USq-13... 

---