Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
J Phys Chem A. 2013 Sep 5;117(35):8360-7. doi: 10.1021/jp403113u. Epub 2013 Aug 21.
Electron transfer (ET) rate constants from the lowest excited state of the radical anion of benzoquinone, BQ(-•), were measured in THF solution. Rate constants for bimolecular electron transfer reactions typically reach the diffusion-controlled limit when the free-energy change, ΔG°, reaches -0.3 eV. The rate constants for ET from BQ(-•) are one-to-two decades smaller at this energy and do not reach the diffusion-controlled limit until -ΔG° is 1.5-2.0 eV. The rates are so slow probably because a second electron must also undergo a transition to make use of the energy of the excited state. Similarly, ET, from solvated electrons to neutral BQ to form the lowest excited state, is slow, while fast ET is observed at a higher excited state, which can be populated in a transition involving only one electron. A simple picture based on perturbation theory can roughly account for the control of electron transfer by the need for transition of a second electron. The picture also explains how extra driving force (-ΔG°) can restore fast rates of electron transfer.
在四氢呋喃溶液中测量了苯醌自由基阴离子（BQ(-•)*）的最低激发态的电子转移（ET）速率常数。当自由能变化ΔG°达到-0.3 eV 时，双分子电子转移反应的速率常数通常达到扩散控制极限。在这种能量下，BQ(-•)*的 ET 速率常数小一到两个数量级，并且直到-ΔG°达到 1.5-2.0 eV 时才达到扩散控制极限。速率如此之慢可能是因为第二个电子也必须经历跃迁才能利用激发态的能量。同样，溶剂化电子到中性 BQ 的 ET 形成最低激发态的速率较慢，而在更高的激发态下观察到快速 ET，该激发态可以通过仅涉及一个电子的跃迁来填充。基于微扰理论的简单图像可以大致说明需要第二个电子跃迁来控制电子转移。该图像还解释了额外驱动力（-ΔG°）如何恢复快速的电子转移速率。
