title: The excited-state decay mechanism of 2,4-dithiothymine in the gas phase, microsolvated surroundings, and aqueous solution.
abstract: The photophysics of thiothymines has been extensively studied computationally in the past few years due to their significant potential as photosensitizers in photodynamic therapy. However, the corresponding computational studies of the photophysical mechanism of 2,4-dithiothymine are scarce. Herein we have employed the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM methods to systematically explore the excited-state decay mechanism of 2,4-dithiothymine in isolated, microsolvated, and aqueous surroundings. First, we have optimized minima and conical intersections in and between the lowest six excited singlet and triplet states i.e., , , , , and ; then, based on computed excited-state decay paths and spin-orbit couplings, we have proposed several nonadiabatic pathways that efficiently populate the lowest triplet state to explain the experimentally observed ultrahigh triplet-state quantum yield. Moreover, we have found that the excited-state decay mechanism in microsolvated and aqueous environments is more complicated than that in the gas phase. The solute-solvent interaction has significant effects on the excited-state potential energy surfaces of 2,4-dithiothymine and eventually on its excited-state decay mechanism. Finally, the present computational efforts contribute important mechanistic knowledge to the understanding of the photophysics of thiothymine-based photosensitizers.
authors.affiliation: Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China. ganglong.cui@bnu.edu.cn.
authors.forename: Bin-Bin
authors.initials: BB
authors.lastname: Xie
authors.affiliation: Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China. ganglong.cui@bnu.edu.cn.
authors.forename: Qian
authors.initials: Q
authors.lastname: Wang
authors.affiliation: Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China. ganglong.cui@bnu.edu.cn.
authors.forename: Wei-Wei
authors.initials: WW
authors.lastname: Guo
authors.affiliation: Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China. ganglong.cui@bnu.edu.cn.
authors.forename: Ganglong
authors.identifier: http://orcid.org/0000-0002-9752-1659
authors.initials: G
authors.lastname: Cui