Correlation between Crystal and Electronic Structures in Diketopyrrolopyrrole Pigments as Viewed from Exciton Coupling Effects

Diketopyrropyrroles (DPPs) are industrially important red pigments. The color in the solid state changes variously from vivid red to yellowish red, although no noticeable difference is recognized in the molecular spectra in solution. The correlation between the electronic and crystal structures of some DPP pigments has been investigated on the basis of the exciton coupling effects. The polarized reflection spectra on these single crystals revealed that the reflection maximum is hypsochromically shifted as the extent of molecular overlap along the stacking axis is increased. We assumed the present color change as arising from interactions between transition moments and calculated the spectral shifts for the nearest-neighbor molecules. The hydrogen-bond pair ("head-to-tail" arrangement) is found to displace the absorption maximum toward 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.