An aggregate organic photoconductor. I. Chemical composition, preparation, physical structure, and optical properties.

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Abstract
An aggregate photoconductive film with a broad spectral sensitivity which is useful for electrophotographic applications is described. It consists of an aryl‐substituted thiapyrylium salt (dye) and a dialkylamino‐substituted triphenylmethane (sensitizer‐carrier–transport component, ST) disperesed in poly(4,4′‐isopropylidenediphenyl carbonate). A crystalline complex of the polymer and the dye forms an aggregate as a separate phase. Its formation is accompanied by a red shift in the absorption spectrum and a greatly enhanced photoconductive response which is augmented by the ST component. The aggregate has been simulated by a 2 : 1 complex of dye and 4,4′‐isopropylidenediphenyl‐bis‐phenyl carbonate, a two‐unit model of the polymer. Single crystals of the model complex were grown and its structure was determined from x‐ray‐diffraction data. In the monoclinic (P21/a) crystals, the approximately planar dye cations are arranged 3.44 Å apart in stacks along a and the stacks are arranged in (010) sheets. The carbonate molecules are extended head to tail in chains along a so that substitution of isopropylidene groups for the end hydrogen atoms would result in a polymer. The chains also form (010) sheets and the structure consists of alternating dye and carbonate layers along b. There is no evidence of any bonding interactions between or within the sheets. The model complex crystals are good photoconductors. The optical behavior of the aggregate can be explained as a symmetry‐allowed crystal transition derived from the lowest molecular singlet state of the dye. The photoconductive behavior of aggregate films is described in detail in the accompanying paper, hereafter referred to as Paper II.