Geometry and Raman spectra of P.R. 255 and its furo-furanone analogue

Fourier transform Raman spectra of two π-isoelectronic compounds 3,6-diphenyl-2,5-dihydro-pyrrolo-[3,4-c]pyrrole-1,4-dione (BPPB, C.I. Pigment Red 255) and 3,6-diphenyl-2,5-dihydro-furo-[3,4-c]furanone (BFFB) with the same 1,4-diphenyl-buta-1,3-diene (DPB) backbone were first time measured in polycrystalline phase. The ground state geometry and vibrational frequencies together with Raman intensities were computed by density functional theory (DFT: B3LYP/6-311G++(d,p)). All intensive observed Raman frequencies were identified as totally symmetric. The difference of carbon-carbon bond lengths of BPPB and BFFB compared to DPB, relating very well with the shifts of CC and C-C stretching modes frequencies, was explained by aromatization of central butadiene unit bounded in diketo-pyrrolo-pyrrole and furo-furanone heterocycles. A strong coupling of modes was observed for BFFB enhancing selectively the intensity of one peak 1593 cm−1 in CC stretching region and one peak 1372 cm−1 in C-C stretching region. CO stretching and N-H bending modes of BPPB are significantly affected by intermolecular hydrogen bonding.