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

Abstract

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.

Minimum-Energy Conical Intersections by Compressed Multistate Pair-Density Functional Theory

“…The CMS-PDFT method gives the correct topology of closely spaced PESs at and near conical intersections. CMS-PDFT may be compared to extended multistate complete active space second-order perturbation theory (XMS-CASPT2), , which has been demonstrated to be a highly accurate quantum chemical method that can describe important dynamic correlation effects at and near the conical intersections that control quenching and other electronically nonadiabatic processes. − Although CMS-PDFT gives results of comparable quality, including near conical intersections, it is advantageous as it is much less expensive.…”

Excited-State Barrier Controls E → Z Photoisomerization in p-Hydroxycinnamate Biochromophores

“…Although the excited-state dynamics in p -hydroxycinnamates has been studied extensively in solution over the past two decades (see, for example, refs − and references therein), the extent to which solvation perturbs the intrinsic excited-state dynamics remains unclear. Theoretical investigations have suggested that solvation of anionic p -hydroxycinnamates significantly perturbs the S 1 (ππ*) potential energy surfaces and conical intersection seams, − although experimental strategies capable of directly observing photoisomerization in the gas phase are now starting to emerge. − …”