The structural and vibrational properties of streptocyanine dyes, expressed as [(CH3)(2)N(CH)(x)N(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 electron-vibration interaction of the bond-alternation mode, reasonably explains the IR intensity enhancement. In the Raman spectra of SC7 and SC9, a few strong bands appear in solution which are not seen in the polycrystalline state. These Raman bands are considered to arise from the same vibrational modes as the strong IR bands, as in the case of similar Raman bands observed previously for SC5 in solution. The relative Raman intensities of the bands are, however, larger in the spectra of SC7 and SC9. These results provide strong evidence for the validity of the mechanism proposed previously that the electron-vibration interaction in the conjugated chain and the intermolecular interaction with the perchlorate ions (existing at various positions near the conjugated chain in solution) give rise to the strong Raman intensities of these bands. It is also suggested that the appearance of these bands is a good signature of the large vibrational contributions to the polarizability tensors in this type of conjugated molecules. The diagonal and off-diagonal force constants of the CC stretches in the conjugated chains are analyzed. It is shown that the conjugated chains of streptocyanine dyes are more strongly correlated than those of neutral polyene chains.