Asturiol David, Lasorne Benjamin, Robb Michael A, Blancafort Lluís
Institut de Química Computacional, Parc Científic i Tecnològic de la Universitat de Girona, Edifici Jaume Casademont, Pic de Peguera 15 (la Creueta), 17003 Girona, Spain.
J Phys Chem A. 2009 Sep 24;113(38):10211-8. doi: 10.1021/jp905303g.
The photodynamics along the main decay paths of thymine after excitation to the lowest pi,pi* state have been studied with MS-CASPT2 calculations and semiclassical CASSCF dynamics calculations including a surface hopping algorithm. The static calculations show that there are two decay paths from the Franck-Condon structure that lead to a conical intersection with the ground state. The first path goes directly to the intersection, while the second one is indirect and involves a minimum of the pi,pi* state, a small barrier, and a crossing between the pi,pi* and n,pi* states. From the static calculations, both paths have similar slopes. The dynamics calculations along the indirect path show that, after the barrier, part of the trajectories are funneled to the intersection with the ground state, where they are efficiently quenched to the ground state. The remaining trajectories populate the n,pi* state. They are also quenched to the ground state in less than 1 ps, but the static calculations show that the decay rate of the n,pi* state is largely overestimated at the CASSCF level used for the dynamics. Overall, these results suggest that both direct and indirect paths contribute to the subpicosecond decay components found experimentally. The indirect path also provides a way for fast population of the n,pi* state, which will account for the experimental picosecond decay component.
通过MS - CASPT2计算和包括表面跳跃算法的半经典CASSCF动力学计算，研究了胸腺嘧啶激发到最低π,π态后沿主要衰变路径的光动力学。静态计算表明，从弗兰克 - 康登结构出发有两条衰变路径，它们会导致与基态的锥形交叉。第一条路径直接通向交叉点，而第二条路径是间接的，涉及π,π态的一个极小值、一个小势垒以及π,π态和n,π态之间的一次交叉。从静态计算来看，两条路径具有相似的斜率。沿间接路径的动力学计算表明，越过势垒后，部分轨迹会汇聚到与基态的交叉点，在那里它们被有效地猝灭到基态。其余轨迹则填充到n,π态。它们也会在不到1皮秒的时间内被猝灭到基态，但静态计算表明，在用于动力学的CASSCF水平下，n,π态的衰变率被大大高估了。总体而言，这些结果表明直接和间接路径都对实验中发现的亚皮秒衰变成分有贡献。间接路径还为n,π*态的快速填充提供了一种方式，这将解释实验中的皮秒衰变成分。
