二芳基乙烯类化合物的光致关环及重排过程的理论研究

二芳基乙烯的光环化反应在分子开关和有机合成领域有重要应用.本文选取Lovo等合成的三个二芳基乙烯类化合物(2-苯基-3-3-2,5-二甲基噻吩]-2-环戊烯酮、2-苯基-3-2-3,5-二甲基噻吩]-2-环戊烯酮及3-苯基-2-3-2,5-二甲基噻吩]-2-环戊烯酮),采用(TD)DFT、CASSCF和CASPT2方法对三个化合物的光环化反应及重排过程进行研究,以探索羰基官能团及杂原子位置的不同对反应机理的影响以及光环化反应对后续重排过程的影响.研究结果表明:二芳基乙烯光环化反应为三个电子态、两类圆锥交叉参与的过程.在三个化合物中,噻吩中S原子位于分子外侧时,光环化产物能量明显高于S位于内侧的情形,关环过程最为困难;当S原子同处内侧时,羰基靠近噻吩一侧较靠近苯环一侧的激发态势能曲线平缓,反应驱动力较小,光环化较为困难.此外,通过重排反应生成的产物以及过渡态和产物能量的分析,发现后续重排过程与光环化步骤发生的难易程度存在一致性,光环化反应越容易发生,重排反应也相应的越容易发生,因此得到的产物产率也越高.

Theoretical study on photo-induced ring closure and rearrangement of diarylethenes

Photocyclozation of diarylethene has wide applications in molecular switches and in organic synthetic chemistry.In this work,we carried out(TD)DFT,CASSCF and CASPT2 calculations to study the reaction mechanism of photo-induced ring closure and rearrangement of three photochromic diarylethene compounds,including 2-phenylcyclopent-3-3-2,5-dimethylthiophen]-2-enone,2-phenylcyclopent-3-2-3,5-dimethylthiophen]-2-enone and 3-phenylcyclopent-2-3-2,5-dimethylthiophen]-2-enone.The effects of different positions of carbonyl group and thiophene-sulfur atom are emphasized.The results show that the cyclozation of diarylethene obeys a three-state,two-conical-intersection mechanism.When the thiophene-S atom is on the outside of the molecule,the ring-closure product is higher in energy than those with an inner S atom,suggesting that the photocyclozation process is more difficult.When S atoms are both on the inner side,the diarylethene with carbonyl group near the thiophene side has less slopped excited-state potential energy surface(PES)than that with a carbonyl group near the benzene ring,thus has lower driving force and is more difficult for cyclozation.In addition,by comparing the barriers of the rate-determining steps in the rearrangement processes,it is found that the tendency of the rearrangement processes is consistent with the photocyclozation reactions,i.e.,the easier photo-induced ring-closure is always accompanied by easier thermal rearrangement,which in together leads to a higher product yield.