A THEORETICAL-STUDY OF CRYSTALLOCHROMY - QUANTUM INTERFERENCE EFFECTS IN THE SPECTRA OF PERYLENE PIGMENTS

The crystallochromy of perylene pigments has been explored using tight-binding extended Huckel calculations on a one-dimensional infinite stack of perylenes as a function of transverse and longitudinal offset. Although the smallest band gap occurs for perylenes when the molecules are completely eclipsed in the solid state, the general perception in the literature that the degree of ''area overlap'' of successive perylenes in the solid state determines simply the shift of the absorption maximum is not supported by the calculations. The nodal character of the HOMO and LUMO wave functions may be such that the area overlap is large, but the band gap and band broadening is small. This is a quantum interference effect. Calculations on one-dimensional stacks of perylenes of known structure can reproduce the trend both in the wavelength of the absorption maximum and the change in width of the absorption. An analysis of the relationship of band broadening to the photogeneration efficiencies of pigments in photoconductors shows that pigments with substantial band broadening by aggregation are precisely those which benefit most significantly by separating the hole and electron on excitation. Examples of band-broadened pigments with high photogenerating efficiencies are taken from perylenes, squaraines, thiopyrylium, and diketopyrrole photogenerators.