DynamicsSimulations with Spin-Flip Time-DependentDensity Functional Theory: Photoisomerization and PhotocyclizationMechanisms of cis-Stilbene in ππ* States.

On-the-fly dynamics simulations werecarried out using spin-fliptime dependent density functional theory (SF-TDDFT) to examine thephotoisomerization and photocyclization mechanisms of cis-stilbene following excitation to the ππ* state. A statetracking method was devised to follow the target state among nearlydegenerate electronic states during the dynamics simulations. Thesteepest descent path from the Franck–Condon structure of cis-stilbene in the ππ* state is shown to reachthe S1-minimum of 4,4-dihydrophenanthrene (DHP) via a cis-stilbene-like structure (referred to as (S1)cis-min) on a very flat regionof the S1-potential energy surface. From the dynamics simulations,the branching ratio of the photoisomerization is calculated as trans:DHP = 35:13, in very good agreement with the experimentaldata, trans:DHP = 35:10. The discrepancy betweenthe steepest descent pathway and the significant trans-stilbene presence in the branching ratio observed experimentallyand herein computationally is clarified from an analysis of geometricalfeatures along the reaction pathway, as well as the low barrier of0.1 eV for the pathway from (S1)cis-minto the twisted pyramidal structure on the S1-potential energy surface. It is concluded that ππ*-excited cis-stilbene propagates primarily toward the twisted structuralregion due to dynamic effects, with partial branching to the DHP structuralregion via the flat-surface region around (S1)cis-min. [ABSTRACT FROM AUTHOR]