Vibrational Spectra and Structures of Long-Chain Streptocyanine Dyes: Effects of Electron−Vibration Interactions and Vibrational Polarizabilities

Abstract

The structural and vibrational properties of streptocyanine dyes, expressed as [(CH3)2N(CH)xN(CH3)2]+ClO4- (called SCx in this paper) with x = 2n + 1 (n = 0−10), are examined by measuring the infrared (IR) and Raman spectra in solution and in the polycrystalline state (for x = 1, 3, 7, 9) and by carrying out density functional calculations at the BHandHLYP/6-31G* level. It is shown that the strong IR bands observed and calculated in the 1800−800 cm-1 region arise from the normal modes containing large contributions from the vibrations along the bond-alternation coordinate of the conjugated chains. As the conjugated chain becomes longer, the strongest IR band shifts toward the lower-wavenumber side, inducing noticeable changes in the spectral pattern, and the total IR intensity increases significantly. The shifts to lower wavenumbers and the changes in the spectral pattern are explained by the decrease in the intrinsic wavenumber of the bond-alternation mode. A two-state model Hamiltonian, which involves e...