π堆积的腺嘌呤-胸腺嘧啶体系激发态电荷转移和无辐射失活的半经典动力学模拟

A Semiclassical Dynamics Simulation on Charge Transfer and Radiationless Deactivation Excited State of π-Stacked Adenine-Thymine System

采用半经典电子-辐射-离子动力学(SERID)模型模拟了π堆积的腺嘌呤-胸腺嘧啶(A-T)体系激发态的光物理失活过程.设置激光脉冲仅作用于T,模拟发现电子由A转移到T,形成(A+T-)*激基缔合物(exciplex).当分子间距缩短至0.300nm时,由于轨道离域效应产生电荷重组,体系恢复电中性;当A分子的C4-C5扭曲程度最大时,体系通过避免交叉点衰减至基态.Exciplex的失活途径由分子间距离和A分子的变形程序两个因素决定.由于A分子的C4、C5原子位阻较大,难以达到失活所必需的强烈扭曲,因此(A-T)*的寿命比胸腺嘧啶堆积体系(T-T)*显著增长.

A semiclassical electronic radiation ion dynamics （SERID） simulation was used to study the photophysical deactivation of E-stacked adenine and thymine. A laser was only applied to the thymine molecule during the simulations. The results showed that an （A＊T-）＊ type exciplex was formed between excited thymine and unexcited adenine as a consequence of charge transfer. When the intermolecular distance was less than 0.300 nm, the stacked system was recovered to electronic neutrality by charge recombination because of the orbital delocalization effect. When the torsion angle of the C4＇- C5＇ bond of the adenine molecule reached its maximum, the exciplex decayed to its ground state via an avoided crossing. The deactivation channel of the exciplex was found to be dependent on the intermolecular distance and deformation of the adenine molecule. It was difficult for the adenine molecule to undergo strong twist required for deactivation because of the steric hindrance encountered by the C4＇ and C5＇ atoms. Consequently, the lifetime of the A-T exciplex was clearly longer than that of the T-T exciplex.