Vibrationally resolved optical spectra of c i s,t r a n s-1,3,5,7-octatetraene

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Abstract
Vibrationally resolved one photon fluorescence excitation and fluorescence spectra have been measured for cis,trans-1,3,5,7-octatetraene in n-alkane matrices at 4.2 K. Except for absolute intensities and the details of vibronic development, these spectra are similar to those observed for the all-trans isomer. That is, there is a weak S0–S1 transition (0–0 at 28 645 cm−1 in n-hexane) approximately 3490 cm−1 below the strongly allowed S0–S2 transition at 32 134 cm−1. The vibrational development of the S0–S1 absorption and fluorescence is that of a symmetry allowed transition (this transition is symmetry forbidden in the all-trans isomer). Thus, the low intensity of the S0–S1 transition testifies to the importance of multiple excitations in S1. Vibrational modes in S1 are weakly coupled with S2 due to the small energy gap. This coupling is energy dependent. Finally, Franck–Condon factors for the S0–S2 transition have been estimated. These Franck–Condon factors together with the vibrational structure observed for the S0–S1 excitation and fluorescence spectra show that neither S1 nor S2 is significantly distorted with respect to the ground state. Nonetheless, cis–trans photoisomerization competes with fluorescence, even at 4.2 K.