Institute of Physics, University of São Paulo, Rua do Matão 1371. 05508-090, São Paulo, SP, Brazil.
Photonics Institute, Vienna University of Technology, Gußhausstraße 27-29, 1040 Vienna, Austria and Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Straße 17, 1090 Vienna, Austria.
Phys Chem Chem Phys. 2021 Mar 11;23(9):5447-5454. doi: 10.1039/d1cp00041a.
The population and depopulation mechanisms leading to the lowest-lying triplet states of 2-Se-Thymine were studied at the MS-CASPT2/cc-pVDZ level of theory. Several critical points on different potential energy hypersurfaces were optimized, including minima, conical intersections, and singlet-triplet crossings. The accessibility of all relevant regions on the potential energy hypersurfaces was investigated by means of minimum energy paths and linear interpolation in internal coordinates techniques. Our analysis indicates that, after the population of the bright S2 state in the Franck-Condon region, the first photochemical event is a barrierless evolution towards one of its two minima. After that, three viable photophysical deactivation paths can take place. In one of them, the population in the S2 state is transferred to the T2 state via intersystem crossing and subsequently to the T1 state by internal conversion. Alternatively, the S1 state could be accessed by internal conversion through two distinct conical intersections with S2 state followed by singlet-triplet crossing with the T2 state. The absence of a second minimum on the T1 state and a small energy barrier on pathway along the potential energy surface towards the ground state from the lowest triplet state are attributed as potential reasons to explain why the lifetime of the triplet state of 2-Se-Thymine might be reduced in comparison with its thio-analogue.
采用 MS-CASPT2/cc-pVDZ 理论水平研究了导致 2-硒-胸腺嘧啶最低三重态的种群和灭绝机制。优化了不同势能超曲面上的几个关键点，包括最小值、锥形交叉和单重态三重态交叉。通过最小势能路径和内部坐标技术的线性插值研究了势能超曲面上所有相关区域的可达性。我们的分析表明，在 Franck-Condon 区域中 S2 态的种群之后，第一个光化学反应事件是无阻碍地向其两个最小值之一演变。之后，有三种可行的光物理失活途径可以发生。其中一种途径是，S2 态中的种群通过系间穿越转移到 T2 态，然后通过内部转换转移到 T1 态。或者，S1 态可以通过内部转换通过两个不同的与 S2 态的锥形交叉，然后与 T2 态的单重态三重态交叉来获得。T1 态上没有第二个最小值，以及从最低三重态到基态的势能表面上的路径上的小能垒被认为是解释为什么与硫代类似物相比，2-硒-胸腺嘧啶的三重态寿命可能会降低的潜在原因。
