Diketopyrrolopyrroles are typical hydrogen-bonded pigments of red color based on the skeleton of 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP). Our previous investigation revealed that the 1:1 mixed crystal (MX-DPP) composed of DPP and its (t-butylphenyl) derivative
(BTB-DPP) gives practically the same electronic spectra as well as X-ray diffraction diagram as those of the hybrid compound of DPP and BTB-DPP (MTB-DPP). In order to elucidate the present coincidence, electronic structure has been investigated on single crystals of MX-DPP and MTB-DPP on the
basis of the structure analysis. The space groups of both crystals are found to be the same (P-1) and the crystallographic parameters are quite similar. In both MX-DPP and MTB-DPP, the same molecular environment (in other words, the “local similarity”) is achieved throughout
the crystal. Furthermore, the polarized reflection spectra measured on single crystals also gave approximately the same results. These experimental facts led us to conclude that the “local similarity” plays the key role in the determination of the crystal and electronic structures
in MX-DPP and MTB-DPP.