Modeling the Spectroscopy of the Lowest Excited Singlet State of cis,trans-1,3,5,7-Octatetraene: The Role of Symmetry Breaking and Vibronic Interactions

The effects of the introduction of a cis linkage in the simplest polyenic chain containing both a cis and a trans bond on its spectroscopy are studied by using extensive ab initio calculations of the vibronically induced transition moments to the lowest excited singlet state and the first- and second-order vibronic coupling matrix elements between the lowest electronic states. It is shown that the effect of lowering the symmetry from C 2 h to C s is less important than that of vibronic coupling between the lower electronic states. As a result, the S 1 ← S 0 excitation spectrum is found to be dominated by bands arising from Herzberg−Teller borrowing. Support for this conclusion is found in the comparison of the theoretically predicted spectrum with excitation spectra in the solid state and by calculation of the first-order vibronic coupling matrix elements between S 1 and S 2 . The well-known S 0 −S 1 coupling, which leads to the frequency inversion of the completely in-phase C C bond stretching vibration between the two states in all-trans polyenes, is shown to spread over several C C bond stretching vibrations. Finally, it is found that the second-order S 1 −S 2 couplings are not particularly affected in the isomerized molecule, in agreement with previous calculations that showed them in all- trans -octatetraene to be linked to stringlike behavior.