Ultrafast photochemistry of cyclopentadiene: Competing hydrogen migration and electrocyclic ring closure
Ultrafast photochemistry of cyclopentadiene: Competing hydrogen migration and electrocyclic ring closure
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DOI：
10.1016/j.chemphys.2005.05.014
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发表时间：
2005-09-19
期刊：
影响因子：
2.3
通讯作者：
Trushin, SA
中科院分区：
文献类型：
--
作者：
Fuss, W;Schmid, WE;Trushin, SA
通过离解强激光场电离探测，我们发现环戊二烯在240纳米激发后，在37飞秒内从初始的1B(2)态弛豫到暗的2A(1)态；从那里到S₀态呈现双指数衰减（71飞秒和333飞秒）。为了解释这些短时间，我们引入了与光化学周环反应相关的锥形交叉：电环化和氢迁移。从氘同位素效应可以推断出后者反应占主导。从2A(1)态离开后发现的一个反应（19皮秒）归因于最初形成的处于热基态的电环化产物双环[2,1,0]戊烯的热逆反应，重新生成环戊二烯。将该分子与苯和环庚三烯这另外两种分子进行比较，我们指出超快内转换不是由态密度和矩阵元决定的，而是由势能面上的路径决定的；矩阵元方法默认是垂直跃迁。但是可能超快无辐射跃迁总是伴随着类似于化学反应中的原子或基团的大位移，所以在这种情况下光物理是光化学的结果。两者通常有共同的初始路径，一般直到最后一个锥形交叉。（C）2005爱思唯尔有限公司。保留所有权利。
Probing by dissociative intense-laser field ionization, we found that cyclopentadiene relaxes from the initial 1B(2) state, excited at 240 nm, within 37 fs to the dark 2A(1) state; a doubly exponential decay (71 and 333 fs) leads from there to S-0. To explain the short times, we invoke conical intersections which we associate with photochemical pericyclic reactions: electrocyclization and a hydrogen shift. The latter reaction is dominant, as concluded from the deuterium isotope effect. A reaction (19 ps) found after departure from 2A(1) is ascribed to thermal back reaction of the initially formed electrocyclization product bicyclo[2,1,0]pentene in the hot ground state, recovering cyclopentadiene. Comparing this molecule with two others, benzene and cycloheptatriene, we point out that ultrafast internal conversion is not governed by densities of states and matrix elements, but by the pathway on potential surfaces; the matrix-element approach tacitly assumes vertical transitions. But probably ultrafast radiationless transitions are always accompanied by large displacements of atoms or groups similar to that in chemical reactions, so that photophysics is a consequence of photochemistry in such cases. Both have the initial path in common, typically till the last conical intersection. (C) 2005 Elsevier B.V. All rights reserved.