Role of the Perfluoro Effect in the Selective Photochemical Isomerization of Hexafluorobenzene
Role of the Perfluoro Effect in the Selective Photochemical Isomerization of Hexafluorobenzene
复制标题
全氟效应在六氟苯选择性光化学异构化中的作用
DOI：
10.1021/jacs.1c01506
复制
发表时间：
2021
影响因子：
15
通讯作者：
Lopez, Steven A.
中科院分区：
文献类型：
--
作者：
Cox, Jordan M.;Bain, Matthew;Kellogg, Michael;Bradforth, Stephen E.;Lopez, Steven A.
六氯苯及其许多衍生物表现出化学选择性光化学异构化，导致高度紧张，杜瓦型双环己烯。虽然与苯六氟化相关的吸收和发射的变化被归因于所谓的“全氟效应”，但六氟苯的电子结构和光化学反应性仍不清楚。现在，我们使用超快时间分辨光谱，多组态计算，和非绝热动力学模拟的组合，开发一个整体的描述的吸收，发射，和光化学动力学的4π-电环六氟苯的闭环和沿反应坐标沿着的电子效应。计算结果表明，吸电子氟取代基通过选择性地稳定σ型激发态，在最低能量的1B 2u（ππ*）和1 E1 g（πσ*）激发态之间产生电子振动耦合.电子振动耦合发生沿着振动模式的e2 usymmetry扭曲的激发态的最小几何形状，导致在实验上广泛的，featureless吸收带，和一个100 nm的斯托克斯位移的荧光形成鲜明对比，苯。最后，电子振动耦合被证明是同时不稳定的反应途径向六氟苯并瓦烯和促进分子振动沿着4π环关闭途径，导致在化学选择性的六氟杜瓦苯。
Hexafluorobenzene and many of its derivatives exhibit a chemoselective photochemical isomerization, resulting in highly strained, Dewar-type bicyclohexenes. While the changes in absorption and emission associated with benzene hexafluorination have been attributed to the so-called “perfluoro effect”, the resulting electronic structure and photochemical reactivity of hexafluorobenzene is still unclear. We now use a combination of ultrafast time-resolved spectroscopy, multiconfigurational computations, and non-adiabatic dynamics simulations to develop a holistic description of the absorption, emission, and photochemical dynamics of the 4π-electrocyclic ring-closing of hexafluorobenzene and the fluorination effect along the reaction coordinate. Our calculations suggest that the electron-withdrawing fluorine substituents induce a vibronic coupling between the lowest-energy1B2u(ππ*) and1E1g(πσ*) excited states by selectively stabilizing the σ-type states. The vibronic coupling occurs along vibrational modes of e2usymmetry which distorts the excited-state minimum geometry resulting in the experimentally broad, featureless absorption bands, and a ∼100 nm Stokes shift in fluorescence—in stark contrast to benzene. Finally, the vibronic coupling is shown to simultaneously destabilize the reaction pathway toward hexafluoro-benzvalene and promote molecular vibrations along the 4π ring-closing pathway, resulting in the chemoselectivity for hexafluoro-Dewar-benzene.