电子效应调控丁烯二腈光分子开关的CASSCF和MS-CASPT2理论研究

采用多组态CASSCF方法和MS-CASPT2方法研究了丁烯二腈中性分子及阳离子和阴离子的顺-反异构化机理.结果表明,中性分子和离子态的光顺-反异构化反应经历不同的非绝热跃迁途径:中性丁烯二腈受光激发至S1态后,需克服一个不低于19.7 k J/mol的能垒才有机会到达基态和激发态间的圆锥交叉(S_1/S_0-CI),随后经非辐射跃迁回到基态,S_1/S_0-CI在结构上偏离C=C双键旋转路径,且能量较高,因此会降低旋转速度,阻碍旋转的单向性;丁烯二腈阳离子和阴离子自由基的D_0态和D_1态旋转势能面在90°处相交,优化的D_1/D_0-CI与D_1态中间体的结构和能量均相近,因此从D1态经由D_1/D_0-C_I无辐射跃迁到D_0态的过程无势垒,在此过程中C=C旋转方向性得到最大限度的保持.研究结果证实了电子诱导不仅能降低基态热旋转势垒,而且能够调控光旋转的非绝热跃迁机理.

CASSCF and MS-CASPT2 Studies on an Electron-tunable, 1,2-Dicyanoethylene-based Optical Molecular Switch

CASSCF and MS-CASPT2 mechanistic studies were carried on the cis-trans isomerization processes of 1,2-dicyanoethylene in its neutral, cationic and anionic forms. The results confirmed the importance of e-lectron-induction in reducing the reaction barrier, and more importantly, revealed the different nonadiabatic channels tuned by electron attachment/detachment. The S1→S0 decay of neutral dicyanoethylene need over-come a mild barrier(≥19. 7 kJ/mol) to reach a H-migration-type, namely the S1/S0-CI, which is away from the C C torsional coordinates and may slow down the speed of C C rotation and hurt its directionality;while in cationic and anionic isomerization processes, the D1 and D0 PESs intersect along the rotary path, therefore, the nonadiabatic D1寅D0 decay is barrierless, as result, the directionality of C C rotation is maintained. The study revealed the role of electron induction in tuning the cis-trans photoisomerization, and shed light on the design of light-driven molecular rotary motors.