Correlation between crystal and electronic structures in diketopyrrolopyrrole pigments as viewed from exciton coupling effects

Diketopyrropyrroles (DPPs) are industrially important red pigments. The col or in the solid state changes variously from vivid red to yellowish red, al though no noticeable difference is recognized in the molecular spectra in s olution. The correlation between the electronic and crystal structures of s ome DPP pigments has been investigated on the basis of the exciton coupling effects. The polarized reflection spectra on these single crystals reveale d that the reflection maximum is hypsochromically shifted as the extent of molecular overlap along the stacking axis is increased. We assumed the pres ent color change as arising from interactions between transition moments an d calculated the spectral shifts for the nearest-neighbor molecules. The hy drogen-bond pair ("head-to-tail" arrangement) is found to displace the abso rption maximum toward longer wavelengths, while the stack pair ("parallel" arrangement) significantly contributes to the hypsochromic shift. The prese nt result is qualitatively in good agreement with the experiment.