Guo Zhenkun, Giokas Paul G, Cheshire Thomas P, Williams Olivia F, Dirkes David J, You Wei, Moran Andrew M
Department of Chemistry, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
J Phys Chem A. 2016 Jul 28;120(29):5773-90. doi: 10.1021/acs.jpca.6b04313. Epub 2016 Jul 13.
The prevalence of ultrafast electron-transfer processes in light-harvesting materials has motivated a deeper understanding of coherent reaction mechanisms. Kinetic models based on the traditional (equilibrium) form of Fermi's Golden Rule are commonly employed to understand photoinduced electron-transfer dynamics. These models fail in two ways when the electron-transfer process is fast compared to solvation dynamics and vibrational dephasing. First, electron-transfer dynamics may be accelerated if the photoexcited wavepacket traverses the point of degeneracy between donor and acceptor states in the solvent coordinate. Second, traditional kinetic models fail to describe electron-transfer transitions that yield products which undergo coherent nuclear motions. We address the second point in this work. Transient absorption spectroscopy and a numerical model are used to investigate coherent back-electron-transfer mechanisms in a transition metal complex composed of titanium and catechol, Ti(cat)3. The transient absorption experiments reveal coherent wavepacket motions initiated by the back-electron-transfer process. Model calculations suggest that the vibrationally coherent product states may originate in either vibrational populations or coherences of the reactant. That is, vibrational coherence may be produced even if the reactant does not undergo coherent nuclear motions. The analysis raises a question of broader significance: can a vibrational population-to-coherence transition (i.e., a nonsecular transition) accelerate electron-transfer reactions even when the rate is slower than vibrational dephasing?
光捕获材料中超快电子转移过程的普遍性激发了人们对相干反应机制的更深入理解。基于传统（平衡）形式的费米黄金规则的动力学模型通常用于理解光诱导电子转移动力学。当电子转移过程比溶剂化动力学和振动退相快时，这些模型在两个方面失效。首先，如果光激发波包在溶剂坐标中穿过供体和受体状态之间的简并点，电子转移动力学可能会加速。其次，传统动力学模型无法描述产生经历相干核运动产物的电子转移跃迁。我们在这项工作中解决第二个问题。利用瞬态吸收光谱和数值模型研究了由钛和儿茶酚组成的过渡金属配合物Ti(cat)3中的相干反向电子转移机制。瞬态吸收实验揭示了由反向电子转移过程引发的相干波包运动。模型计算表明，振动相干产物态可能起源于反应物的振动布居或相干性。也就是说，即使反应物不经历相干核运动，也可能产生振动相干性。该分析提出了一个具有更广泛意义的问题：即使速率慢于振动退相，振动布居到相干性的转变（即非久期转变）能否加速电子转移反应？
