Substituent and Solvent Effects on the Photoisomerization of Cinnamate Derivatives: An XMS-CASPT2 Study

Cinnamate
derivatives show a variety of photo-induced reactions.
Among them is trans–cis photoisomerization, which may involve
the nonradiative decay (NRD) process. The extended multistate complete
active space second-order perturbation (XMS-CASPT2) method was used
in this study as a suitable theory for treating multireference electronic
nature, which was frequently manifested in the photoisomerization
process. The minimum energy paths of the trans–cis photoisomerization
process of cinnamate derivatives were determined, and the activation
energies were estimated using the resulting intrinsic reaction coordinate
(IRC) paths. Natural orbital analysis revealed that the transition
state’s (TS) electronic structure is zwitterionic-like, elucidating
the solvent and substituent effect on the energy barrier of photoisomerization
paths. Furthermore, it was found that the charge on the pyramidalized
carbon atom at the TS structure was strongly correlated with the activation
energy barrier for the cinnamate derivatives. Thus, it seemingly provided
a physical picture of the photoisomerization of cinnamates and was
a good descriptor potentially applicable to molecular design for controlling
the rate constant of the photoisomerization reaction.