Mechanism of the direct trans→cis photoisomerization of stilbene. Part 1.—Potential energy surfaces of the lowest excited states

Abstract

Potential energy surfaces as a function of two internal coordinates, namely, the rotation angle about the central “double” bond and the twisting angle about the exocyclic “single” bonds, have been obtained for the ground state and the lowest excited states of stilbene by a semiempirical procedure. The reliability of the potential surfaces is discussed, and the barriers to the trans→cis isomerization are shown to be largely unaffected by the choice of the parameters or by the restrictions of the semiempirical approach. In the first excited singlet state, the trans→cis conversion of stilbene appears to be hindered by a potential barrier of 35 kcal mol^–1. These findings favour the “triplet mechanism” for the direct trans→cis photoisomerization of stilbene.