Vibrational analysis of a strongly correlated system, pentamethine streptocyanine dye, based on observed infrared and Raman spectra and density functional calculations

Vibrational analysis is carried out for the organic (cationic) part of a pentamethine streptocyanine dye, [(CH₃)₂N(CH)₅N(CH₃) ₂]⁺ClO₄^- (alias SC5), by measuring its infrared and Raman spectra in solution and in the polycrystalline state and by calculating the vibrational force field and the IR and Raman intensities by the ab initio molecular orbital and density functional methods. It is found that a reasonable set of structural parameters and vibrational force field can be obtained for the SC5 organic part at the BHandHLYP/6-31G* level. The observed features of the IR and Raman spectra, including relative intensities, are well reproduced by the calculations at this theoretical level. Two strong IR bands observed in the 1600-1200-cm^-1 region arise from the delocalized b₁ modes along the bond-alternation coordinate of the conjugated chain. The strong IR intensities are explained by large charge fluxes induced by these modes due to the strong electron-vibration interaction. These modes also appear in the Raman spectrum in solution because of the interaction with the perchlorate ion existing at an asymmetric position near the conjugated chain. A delocalized a₁ mode of the conjugated chain gives rise to a strong Raman band. Examination of the IR and Raman intensities and the vibrational force constants clearly shows that the conjugated chain of the SC5 organic part is a strongly correlated system. A detailed analysis of the origin of the IR and Raman intensities shows that the potential energy distribution is not necessarily a good indicator of the origin of intensities.