Han Wen-Ge, Lovell Timothy, Liu Tiqing, Noodleman Louis
Department of Molecular Biology, TPC15, Scripps Research Institute, 10550 N. Torrey Pines Rd., La Jolla, CA 92037, USA.
Chemphyschem. 2002 Feb 15;3(2):167-78. doi: 10.1002/1439-7641(20020215)3:2<167::AID-CPHC167>3.0.CO;2-G.
Using spin-unrestricted density functional theory (the VWN Becke-Perdew potential), including broken-symmetry and spin-projection methods, we have obtained the potential-energy curves as a function of the central torsional angle of stilbene in the ground (S0), the first excited triplet (T1), the first excited singlet (S1), and the doubly excited singlet (S2) states. The thermal trans-->cis isomerization of stilbene passes through a diradical broken-symmetry electronic structure around the twisted conformation (90 degrees central torsional angle) in the ground state. Our calculations support the proposed triplet mechanism for sensitized cis [symbol: see text] trans photoisomerization and the nonadiabatic singlet mechanism proposed by Orlandi and Siebrand. On the T1 potential-energy curve, the rotation of the C=C bond for both trans- and cis-stilbene will lead stilbene to the twisted conformation, from which the twisted stilbene will decay to the ground-state surface that is nearly isoenergetic with the T1 surface and has diradical electronic structure in the twisted region. On the S1 potential-energy curve, the energy increases in the direction from trans- to the twisted stilbene, and crosses with the neutral doubly excited S2 potential-energy curve, which has a minimum at the twisted structure and is lower in energy than the zwitterionic doubly excited state. The twisted stilbene around the energy minimum of the neutral doubly excited S2-state will decay onto the ground-state surface from where the rotation of the C=C bond leads the twisted stilbene to either the trans or cis configuration and the isomerization of stilbene is then completed. Similar studies have also been performed on a stilbene derivative with a substituent group, NHCOCH3.
利用自旋非限制密度泛函理论（VWN贝克 - 佩德韦势），包括破缺对称性和自旋投影方法，我们得到了作为芪分子中心扭转角函数的势能曲线，这些势能曲线对应于基态（S0）、第一激发三重态（T1）、第一激发单重态（S1）和双激发单重态（S2）。芪的热反式→顺式异构化在基态下通过扭曲构象（中心扭转角为90度）周围的双自由基破缺对称电子结构进行。我们的计算支持了所提出的敏化顺式[符号：见原文]反式光异构化的三重态机制以及奥兰迪和西布兰德提出的非绝热单重态机制。在T1势能曲线上，反式和顺式芪的C = C键旋转都会使芪转变为扭曲构象，从该构象扭曲芪将衰减到与T1表面几乎等能且在扭曲区域具有双自由基电子结构的基态表面。在S1势能曲线上，能量从反式芪向扭曲芪方向增加，并与中性双激发S2势能曲线相交，该曲线在扭曲结构处有一个最小值且能量低于两性离子双激发态。围绕中性双激发S2态能量最小值的扭曲芪将衰减到基态表面，从该表面C = C键的旋转使扭曲芪转变为反式或顺式构型，芪的异构化随即完成。对具有取代基NHCOCH3的芪衍生物也进行了类似的研究。
