Zhao Li, Zhou Pan-Wang, Zhao Guang-Jiu
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
J Chem Phys. 2016 Jul 28;145(4):044316. doi: 10.1063/1.4959131.
The trans-urocanic acid, a UV chromophore in the epidermis of human skin, was found to exhibit a wavelength dependent isomerization property. The isomerization quantum yield to cis-urocanic is greatest when being excited to the S1 state, whereas exciting the molecule to the S2 state causes almost no isomerization. The comparative photochemical behavior of the trans-urocanic on the S1 and S2 states continues to be the subject of intense research effort. This study is concerned with the unique photo-behavior of this interesting molecule on the S2 state. Combining the on-the-fly surface hopping dynamics simulations and static electronic structure calculations, three decay channels were observed following excitation to the S2 state. An overwhelming majority of the molecules decay to the S1 state through a planar or pucker characterized minimum energy conical intersection (MECI), and then decay to the ground state along a relaxation coordinate driven by a pucker deformation of the ring. A very small fraction of molecules decay to the S1 state by a MECI characterized by a twisting motion around the CC double bond, which continues to drive the molecule to deactivate to the ground state. The latter channel is related with the photoisomerization process, whereas the former one will only generate the original trans-form products. The present work provides a novel S2 state decay mechanism of this molecule, which offers useful information to explain the wavelength dependent isomerization behavior.
反式尿刊酸是人类皮肤表皮中的一种紫外线发色团，被发现具有波长依赖性异构化特性。当被激发到S1态时，异构化为顺式尿刊酸的异构化量子产率最大，而将分子激发到S2态几乎不会引起异构化。反式尿刊酸在S1和S2态上的比较光化学行为仍然是深入研究的主题。本研究关注的是这种有趣分子在S2态上的独特光行为。结合实时表面跳跃动力学模拟和静态电子结构计算，在激发到S2态后观察到三个衰变通道。绝大多数分子通过平面或褶皱特征的最小能量锥形交叉点（MECI）衰变为S1态，然后沿着由环的褶皱变形驱动的弛豫坐标衰变为基态。极小部分分子通过以围绕CC双键的扭转运动为特征的MECI衰变为S1态，这继续驱动分子失活到基态。后一个通道与光异构化过程有关，而前一个通道只会产生原始的反式产物。目前的工作提供了该分子一种新的S2态衰变机制，这为解释波长依赖性异构化行为提供了有用信息。
