Mechanistic Photochemistry of Methyl-4-hydroxycinnamate Chromophore and Its One-Water Complexes: Insights from MS-CASPT2 Study

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 120; no. 30; pp. 6014 - 6022
• Main Authors: Xie, Xiao-Ying, Li, Chun-Xiang, Fang, Qiu, Cui, Ganglong
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.08.2016
• ISSN: 1089-5639; 1520-5215; 1520-5215
• DOI: 10.1021/acs.jpca.6b05899

Herein we computationally studied the excited-state properties and decay dynamics of methyl-4-hydroxycinnamate (OMpCA) in the lowest three electronic states, that is, 1 ππ*, 1 nπ*, and S0 using combined MS-CASPT2 and CASSCF electronic structure methods. We found that one-water hydration can significantly stabilize and destabilize the vertical excitation energies of the spectroscopically bright 1 ππ* and dark 1 nπ* excited singlet states, respectively; in contrast, it has a much smaller effect on the 1 ππ* and 1 nπ* adiabatic excitation energies. Mechanistically, we located two 1 ππ* excited-state relaxation channels. One is the internal conversion to the dark 1 nπ* state, and the other is the 1 ππ* photoisomerization that eventually leads the system to a 1 ππ*/S0 conical intersection region, near which the radiationless internal conversion to the S0 state occurs. These two 1 ππ* relaxation pathways play distinct roles in OMpCA and its two one-water complexes (OMpCA-W1 and OMpCA-W2). In OMpCA, the predominant 1 ππ* decay route is the state-switching to the dark 1 nπ* state, while in one-water complexes, the importance of the 1 ππ* photoisomerization is significantly enhanced because the internal conversion to the 1 nπ* state is heavily suppressed due to the one-water hydration.