Abstract
Correlation between electronic and crystal structures of some diketopyrropyrrole (DPP) pigments has been investigated on the basis of the exciton coupling effects. The color changes variously from vivid red to yellowish red on going from p-Cl DPP via DPP to m-CN DPP. Polarized reflection spectra on these single crystals revealed that the reflection maximum is hypsochromically shifted in the above order as the number of atomic contacts between stacking molecules is increased. We assumed the present color change as arising from exciton coupling effects and calculated the spectral shifts for the nearest-neighbor molecules. The hydrogenbond pair (“head-to-tail” arrangement) is found to displace the absorption maximum towards longer wavelengths, while the stack pair (“parallel” arrangement) significantly contributes to the hypsochromic shift. The present result is qualitatively in good agreement with the experiment.
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