The fluorescence and the absorption spectra of 1,8-diphenyl-1,3,5,7-octatetraene. The origin of the transition moments and the interpretation of anomalous intensity distribution

Abstract

The fluorescence and the fluorescence excitation spectra of 1,8-diphenyl-1,3,5,7-octatetraene in octane are observed at 77 K. Special effort has been devoted to obtain the correct absorption spectrum from the fluorescence excitation spectrum. The fluorescence and S1←S0 absorption spectra consist of progressions of totally symmetric C = C and C–C stretching vibrations built on the 0–0 band. These spectral features together with the appearance of the 0–0 band indicate that the molecule is distorted from the C2h point group. No mirror image relationship is observed between the S1←S0 absorption and the fluorescence. Especially, the vibronic band in the absorption involving one quantum of the C = C stretching vibration is observed to be anomalously intense. Such anomalous intensity distribution of the S1←S0 absorption and the lack of the mirror image relationship are analyzed in terms of a mechanism in which both the Herzberg–Teller and Born–Oppenheimer vibronic coupling terms contribute to the transition moment. For the 0–0 and all of the vibronic bands in the absorption and fluorescence, the largest contribution to the transition moment comes from the zeroth order term in the expansion in the nuclear coordinate (i.e., nonvibronic) which is ascribed to the reduction of molecular symmetry. In the fluorescence, the Herzberg–Teller vibronic coupling term contributes destructively with the zeroth order term, whereas the Born-Oppenheimer vibronic coupling term contributes only negligibly. In the absorption, on the other hand, the Born-Oppenheimer vibronic coupling term contributes constructively with the zeroth order term, whereas the Herzberg–Teller vibronic coupling term contributes only a little. The anomalous intensity distribution and the lack of mirror image relationship are mainly ascribed to the difference in sign in the two major terms in the transition moment.