Raman intensity and conjugation. III. Conjugation and resonance Raman effect on the Raman intensities of diphenylpolyenes

We have measured the Raman intensities of the phenyl ring vibrations near 1600 cm−1 and the C=C stretching vibrations of the polyene chain of four diphenylpolyenes Ph-(CH=CH)n-Ph with n varying from 1 to 4. All measurements have been carried out in sufficiently dilute CCl4 solutions. The results we obtained can be interpreted quantitatively in terms of a simple relation by which the Raman intensity is given by S=const. fBK. By definition, the fB factor describes quantitatively the resonance Raman effect on the observed intensities whereas the value of the K factor is a measure of the conjugation effect. This relation can be justified empirically for the present investigation. The influence of the conjugation effect on the relative Raman intensities of the phenyl ring vibrations is particularly large. In contrast the conjugation effect on the relative intensities of the C=C stretching vibrations of the polyene chain is extremely small. Both vibrations exhibit clearly a near-resonance type behavior which turns out to be uniform.