AZO PIGMENTS AND THEIR INTERMEDIATES - A STUDY OF THE STRUCTURE - SENSITIVITY RELATIONSHIP OF PHOTOGENERATING BISAZO PIGMENTS IN BILAYER XEROGRAPHIC DEVICES

The structure-sensitivity relationships of several series of bisazo pi gments have been studied in bilayer xerographic devices using a common charge (hole) transporting layer (CTL). For a given bisazo pigment se ries, a correlation is found between the E0.5 value of each pigment an d the Hammett constant sigma(p). The data consistently show that neith er electron-withdrawing substituents nor electron-releasing substituen ts promote high photosensitivity. Factors that influence the sensitivi ty of these pigments in devices, including photogeneration efficiency of the bisazo pigment in the charge generation layer (CGL), charge tra nsfer efficiency from CGL to CTL, and mobility of the holes in CTL, ha ve been considered. Because a common CTL is used and because a differe nt substituent effect would have been observed if interfacial charge t ransfer were a factor, we conclude that the structure-sensitivity rela tionships are simply the result of a substituent effect on the photoge neration efficiency of the pigment. Whereas impurity, particle size, a nd aggregation of the pigment are found to contribute little to the se nsitivity variation, evidence is provided that the variation correlate s to the crystallographic properties. Specifically, the photosensitivi ty is found to depend inversely on the crystallinity of the pigment. I n view of the photogeneration mechanism, we postulate that the structu re-sensitivity relationship originates from the surface area for the p hotogeneration process; the smaller the crystallite size, the larger t he surface area, and consequently the higher the photosensitivity.