中文摘要：

采用半经典动力学方法模拟了π堆积的胸腺嘧啶体系最低激发态的光物理失活过程.设置激光脉冲仅作用于一个胸腺嘧啶分子T,另一胸腺嘧啶分子T＇保持基态.模拟发现由于T与T＇之间存在π堆积相互作用,导致电荷转移,形成T带负电荷、T＇带正电符的激基复合物.由于相邻分子的空间效应阻碍了激发的T分子到达圆锥相交所必需的强烈扭曲,激基复合物的寿命比单体增长.当分子间距离缩短至0.3 nm后,T分子C_5—C_6键扭曲程度最大,此时发生电荷重组,两个胸腺嘧啶分子均恢复电中性.电荷重组诱导T＇分子发生畸变,并在C_5＇—C_6＇扭曲最大时避免相交,体系衰减至基态,T和T＇分子均恢复平面构型.

英文摘要：

A semiclassical dynamics simulation study was undertaken to determine the photophysical deactivation of the lowest excited state of two stacked thymines.Only one thymine,referred to as T,was excited by a laser pulse and the other molecule,referred to as T,remained in the ground state.The simulation results show that charge transfer between the two thymines because of a rr-stacking interaction leads to the formation of an excimer state,which includes a negative T and a positive T.Additionally,the simulation study indicates that a steric effect of the neighboring bases inhibits the out-of-plane deformation, which is essential in accessing the conical intersection between the lowest electronic-excited state and the ground state.The steric effect eventually leads to a longer electronic-excited state lifetime for the two stacked thymines.The simulation results reveal that when the interbase distance is less than 0.3 nm the molecule T has a remarkable deformation at its C_5 and C_6 sites resulting in charge recombination.The charge recombination ultimately makes the system electronically neutral.On the other hand,the molecule T＇ has a strong twist about its C_5＇—C_6＇ bond in the proximity of the avoided crossing by which the system decays to the ground state.Finally,the two thymine molecules in their ground states recover their planar geometries.