Institut für Physikalische und Theoretische Chemie, Johann Wolfgang Goethe-Universität, Max von Laue-Str. 7, 60438 Frankfurt am Main, Germany.
J Phys Chem A. 2009 Oct 29;113(43):11882-7. doi: 10.1021/jp903156j.
Hemithioindigo hemistilbene (HTI) can be selectively photoisomerized from the Z-isomer into the E-isomer and vice versa. Using quantum chemical calculations, we have investigated the mechanism of the photoisomerization in detail. Our calculations demonstrate that two Z- and E-isomers exist in the electronic ground state as well as on the S(1) surface. The S(1) isomers are separated by small energy barriers along the dihedral twisting coordinate, but also a conical intersection with the electronic ground state is present at about 90 degrees twisting angle. Comparison with previously published experimental data reveals that most excited molecules, however, do not isomerize but instead relax to the equilibrium structure of the Z-isomer on the S(1) surface and return back to the ground state via regular fluorescence. Only a small fraction of the excited state population decays via the identified conical intersection and forms ground state E-isomers. This explains the comparably long lifetime of 38 ps of the excited HTI molecules and the observed low quantum yield of photoswitching.
半硫靛红亚甲基二苯乙烯 (HTI) 可以从 Z 型异构体选择性地光异构化为 E 型异构体，反之亦然。使用量子化学计算，我们详细研究了光异构化的机制。我们的计算表明，在电子基态以及 S(1)表面上存在两种 Z 和 E 异构体。S(1)异构体沿着二面角扭曲坐标分离，仅存在小的能量势垒，但也存在与电子基态的锥形交叉，约在 90 度扭曲角度处。与以前发表的实验数据相比，然而，大多数激发分子不会异构化，而是在 S(1)表面上松弛到 Z 型异构体的平衡结构，并通过常规荧光返回基态。只有一小部分激发态种群通过鉴定的锥形交叉衰变，并形成基态 E 异构体。这解释了激发 HTI 分子相对较长的 38 ps 寿命和观察到的光开关低量子产率。
