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

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• 作者：Xiao-Ying Xie ; Chun-Xiang Li ; Qiu Fang ; Ganglong Cui
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：August 4, 2016
• 年：2016
• 卷：120
• 期：30
• 页码：6014-6022
• 全文大小：571K
• 年卷期：0
• ISSN：1520-5215

文摘

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