Department of Chemistry and The PULSE Institute, Stanford University, Stanford, California 94305, United States.
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States.
J Phys Chem B. 2020 Jul 2;124(26):5476-5487. doi: 10.1021/acs.jpcb.0c03344. Epub 2020 Jun 17.
The photochemistry of -stilbene proceeds through two pathways: - isomerization and ring closure to 4a,4b-dihydrophenanthrene (DHP). Despite serving for many decades as a model system for photoisomerization, the photodynamics of -stilbene is still not fully understood. We use multiple spawning on a SA-2-CASSCF(2,2) potential energy surface to simulate the nonadiabatic dynamics of isolated -stilbene. We find the cyclization (to DHP and -stilbene) and isomerization (to - and -stilbene) reaction coordinates to be orthogonal; branching between the two pathways is determined on the S excited state within 150 fs of photoexcitation. Trajectory basis functions (TBFs) undergoing cyclization decay rapidly to the ground state in 250 fs, while TBFs moving along the isomerization coordinate remain on the excited state longer, with the majority decaying between 300 and 500 fs. We observe three avoided crossing regions in the dynamics: two along the isomerization coordinate (displaying pyramidalization and migration of an ethylenic hydrogen or phenyl group), and one DHP-like conical intersection along the cyclization coordinate. The isomeric form of the vibrationally hot photoproducts (as determined by measurement 2 ps after photoexcitation) is determined within less than 50 fs of decay to the ground state mediated by passage through a conical intersection. Excess vibrational energy of ground state - and -stilbene is channelled into phenyl torsions (with mostly opposing directionality). Our simulations are validated by direct comparison to experiment for the absorption spectrum, branching ratio of the three photoproducts (44:52:4 -stilbene:-stilbene:DHP), and excited state lifetime (520 ± 40 fs).
异构化和环化生成 4a,4b-二氢菲（DHP）。尽管反式二苯乙烯作为光异构化的模型体系已经使用了几十年，但它的光动力学仍然没有被完全理解。我们使用在 SA-2-CASSCF(2,2)势能面上进行多次分裂来模拟孤立反式二苯乙烯的非绝热动力学。我们发现环化（生成 DHP 和反式二苯乙烯）和异构化（生成顺式和反式二苯乙烯）反应坐标是正交的；在光激发后 150 fs 内，在 S 激发态上决定了两条途径之间的分支。进行环化的轨迹基函数（TBF）在 250 fs 内迅速衰减到基态，而沿着异构化坐标运动的 TBF 在激发态上停留的时间更长，大部分在 300 和 500 fs 之间衰减。我们在动力学中观察到三个避免交叉的区域：两个沿着异构化坐标（显示出三角化和一个烯丙基氢或苯基基团的迁移），以及一个沿着环化坐标的类似 DHP 的锥形交叉。振动热光产物的异构体形式（通过在光激发后 2 ps 进行测量确定）在通过锥形交叉衰减到基态的过程中，在不到 50 fs 的时间内确定。基态顺式和反式二苯乙烯的多余振动能被导向苯环扭转（大部分具有相反的方向性）。我们的模拟通过与实验直接比较来验证，包括吸收光谱、三种光产物（44:52:4 反式二苯乙烯：顺式二苯乙烯：DHP）的分支比以及激发态寿命（520 ± 40 fs）。
