Ab initio study of cis-trans photoisomerization in stilbene and ethylene

The photochemistry of stilbene is investigated using ab initio quantum chemistry with complete active space self-consistent field (CASSCF) and multireference perturbation theory (CASPT2) methods. We characterize photoisomerization pathways from both the cis and trans isomers, including a minimal energy conical intersection. Similarities to photoisomerization in ethylene are found and emphasized. In contrast to traditional one-dimensional models of stilbene photoisomerization, torsion and pyramidalization are required to reach the minimal energy conical intersection which is expected to dominate in quenching to the ground electronic state. This intersection is characterized as an interaction between charge transfer and covalent states. The present results suggest that the qualitative features of the photoisomerization dynamics elucidated for ethylene can also be expected to apply to stilbene, and call for reconsideration and refinement of the photoisomerization mechanism in stilbene.