Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of the Ministry of Education (MOE), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.
J Chem Phys. 2013 Dec 28;139(24):244304. doi: 10.1063/1.4853715.
Simulation of the excited state dynamics of all-trans-1,3,5,7-octatetraene has been performed to investigate the ultrafast radiationless S2 → S1 internal conversion process. Multireference configuration interaction with single excitation method has been employed to optimize the equilibrium structure of the excited states, as well as the S2/S1 conical intersection, and to investigate the non-adiabatic molecular dynamics of the S2/S1 state transition. At the conical intersection, the molecule is found to be distorted from the original planar trans structure to a nearly perpendicular conformation around C3-C4 bond, with the torsion angle being about 107°. Such structural change can result in mutual approaching of states S2 and S1 in energy, and drastically increase the nonadiabatic coupling between the two states by destroying the inter-state symmetry prohibition in the electronic wavefunctions. Surface-hopping molecular dynamics simulations are performed to describe the non-adiabatic process. Upon the Franck-Condon excitation to the S2 state, the molecule quickly twists its C3-C4 bond and approaches the conical intersection region, where it can undergo efficient internal conversion to S1. The decay time constant (τ) of S2 state is estimated to be around 251 fs by fitting the occupation number of average fraction of trajectories using an exponential damping function. This value is reasonably consistent with previous experimental measurements of around 300-400 fs.
已对全反式-1,3,5,7-辛四烯的激发态动力学进行了模拟，以研究超快无辐射 S2 → S1 内转换过程。采用多参考组态相互作用与单激发方法对激发态的平衡结构以及 S2/S1 双锥交叉点进行了优化，并对 S2/S1 态跃迁的非绝热分子动力学进行了研究。在双锥交叉点处，分子从原始的平面反式结构扭曲到 C3-C4 键附近几乎垂直的构象，扭转角约为 107°。这种结构变化可以导致 S2 和 S1 态在能量上的相互接近，并通过破坏电子波函数中的态间对称禁阻，极大地增加两个态之间的非绝热耦合。采用表面跳跃分子动力学模拟来描述非绝热过程。当 Franck-Condon 激发到 S2 态时，分子迅速扭曲其 C3-C4 键并接近双锥交叉区域，在那里它可以有效地进行 S1 的内转换。通过使用指数衰减函数拟合平均轨迹分数的占据数来估算 S2 态的衰减时间常数（τ）约为 251 fs。这个值与之前约 300-400 fs 的实验测量值相当一致。
