Institute of Photonics & Quantum Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.
Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
Phys Chem Chem Phys. 2022 Dec 14;24(48):29423-29436. doi: 10.1039/d2cp04789f.
Time-resolved photoelectron imaging and supporting quantum chemistry calculations were used to investigate non-adiabatic excess energy redistribution dynamics operating in the saturated thioethers diethylsulfide, tetrahydrothiophene and thietane. In all cases, 200 nm excitation leads to molecular fragmentation on an ultrafast (<100 fs) timescale, driven by the evolution of Rydberg-to-valence orbital character along the S-C stretching coordinate. The C-S-C bending angle was also found to be a key coordinate driving initial internal conversion through the excited state Rydberg manifold, although only small angular displacements away from the ground state equilibrium geometry are required. Conformational constraints imposed by the cyclic ring structures of tetrahydrothiophene and thietane do not therefore influence dynamical timescales to any significant extent. Through use of a high-intensity 267 nm probe, we were also able to detect the presence of some transient (bi)radical species. These are extremely short lived, but they appear to confirm the presence of two competing excited state fragmentation channels - one proceeding directly from the initially prepared 4p manifold, and one involving non-adiabatic population of the 4s state. This is in addition to a decay pathway leading back to the S electronic ground state, which shows an enhanced propensity in the 5-membered ring system tetrahydrothiophene over the other two species investigated.
采用时间分辨光电子成像和支持的量子化学计算研究了在饱和硫醚二乙硫醚、四氢噻吩和噻吩中运行的非绝热过剩能量再分配动力学。在所有情况下，200nm 的激发在超快(<100fs)时间尺度上导致分子碎片化，这是由 S-C 伸缩坐标上的里德堡到价轨道特征的演化驱动的。还发现 C-S-C 弯曲角也是通过激发态里德堡支通过初始内部转换的关键坐标，尽管只需要相对于基态平衡几何形状小的角度位移。因此，四氢噻吩和噻吩的环状结构施加的构象约束不会以任何显著的方式影响动力学时间尺度。通过使用高强度 267nm 探针，我们还能够检测到一些瞬态(双)自由基物种的存在。这些物种极其短暂，但它们似乎证实了存在两种竞争的激发态碎片化通道-一种直接从最初制备的 4p 支进行，另一种涉及 4s 态的非绝热布居。除了一条导致回到 S 电子基态的衰减途径外，这条途径在五元环系统四氢噻吩中比在研究的其他两种物质中显示出增强的倾向。
