Lin Jiaxing, Balamurugan D, Zhang Peng, Skourtis Spiros S, Beratan David N
‡Department of Physics, University of Cyprus, Nicosia 1678, Cyprus.
∥Freiburg Institute for Advanced Studies (FRIAS), University of Freiburg, Freiburg 79104, Germany.
J Phys Chem B. 2015 Jun 18;119(24):7589-97. doi: 10.1021/jp511429z. Epub 2015 Feb 4.
The frontiers of electron-transfer chemistry demand that we develop theoretical frameworks to describe the delivery of multiple electrons, atoms, and ions in molecular systems. When electrons move over long distances through high barriers, where the probability for thermal population of oxidized or reduced bridge-localized states is very small, the electrons will tunnel from the donor (D) to acceptor (A), facilitated by bridge-mediated superexchange interactions. If the stable donor and acceptor redox states on D and A differ by two electrons, it is possible that the electrons will propagate coherently from D to A. While structure-function relations for single-electron superexchange in molecules are well established, strategies to manipulate the coherent flow of multiple electrons are largely unknown. In contrast to one-electron superexchange, two-electron superexchange involves both one- and two-electron virtual intermediate states, the number of virtual intermediates increases very rapidly with system size, and multiple classes of pathways interfere with one another. In the study described here, we developed simple superexchange models for two-electron transfer. We explored how the bridge structure and energetics influence multielectron superexchange, and we compared two-electron superexchange interactions to single-electron superexchange. Multielectron superexchange introduces interference between singly and doubly oxidized (or reduced) bridge virtual states, so that even simple linear donor-bridge-acceptor systems have pathway topologies that resemble those seen for one-electron superexchange through bridges with multiple parallel pathways. The simple model systems studied here exhibit a richness that is amenable to experimental exploration by manipulating the multiple pathways, pathway crosstalk, and changes in the number of donor and acceptor species. The features that emerge from these studies may assist in developing new strategies to deliver multiple electrons in condensed-phase redox systems, including multiple-electron redox species, multimetallic/multielectron redox catalysts, and multiexciton excited states.
电子转移化学的前沿领域要求我们开发理论框架，以描述分子系统中多个电子、原子和离子的传递。当电子通过高势垒长距离移动时，氧化或还原的桥局域态的热占据概率非常小，电子将在桥介导的超交换相互作用的促进下从供体（D）隧穿到受体（A）。如果D和A上稳定的供体和受体氧化还原态相差两个电子，则电子有可能从D相干地传播到A。虽然分子中单电子超交换的结构-功能关系已经确立，但操纵多个电子相干流动的策略在很大程度上还不为人知。与单电子超交换不同，双电子超交换涉及单电子和双电子虚拟中间态，虚拟中间态的数量随着系统规模的增加而迅速增加，并且多类路径相互干扰。在本文所述的研究中，我们开发了用于双电子转移的简单超交换模型。我们探讨了桥结构和能量学如何影响多电子超交换，并将双电子超交换相互作用与单电子超交换进行了比较。多电子超交换在单氧化和双氧化（或还原）的桥虚拟态之间引入了干扰，因此即使是简单的线性供体-桥-受体系统也具有类似于通过具有多个平行路径的桥进行单电子超交换的路径拓扑结构。本文研究的简单模型系统展现出丰富性，通过操纵多个路径、路径串扰以及供体和受体物种数量的变化，适合进行实验探索。这些研究中出现的特征可能有助于开发在凝聚相氧化还原系统中传递多个电子的新策略，包括多电子氧化还原物种、多金属/多电子氧化还原催化剂和多激子激发态。
