Continuum of Outer- and Inner-Sphere Mechanisms for Organic Electron Transfer. Steric Modulation of the Precursor Complex in Paramagnetic (Ion-Radical) Self-Exchanges

Transient 1:1 precursor complexes for intermolecular self-exchange between various organic
electron donors (D) and their paramagnetic cation radicals (D+•), as well as between different electron
acceptors (A) paired with their anion radicals (A-•), are spectrally (UV−NIR) observed and structurally
(X-ray) identified as the cofacial (π-stacked) associates [D, D+•] and [A-•, A], respectively. Mulliken−Hush
(two-state) analysis of their diagnostic intervalence bands affords the electronic coupling elements (HDA),
which together with the Marcus reorganization energies (λ) from the NIR spectral data are confirmed by
molecular-orbital computations. The HDA values are found to be a sensitive function of the bulky substituents
surrounding the redox centers. As a result, the steric modulation of the donor/acceptor separation (rDA)
leads to distinctive electron-transfer rates between sterically hindered donors/acceptors and their more
open (unsubstituted) parents. The latter is discussed in the context of a continuous series of outer- and
inner-sphere mechanisms for organic electron-transfer processes in a manner originally formulated by Taube
and co-workers for inorganic (coordination) donor/acceptor dyadswith conciliatory attention paid to
traditional organic versus inorganic concepts.

针对各类有机电子给体（D）与其顺磁性阳离子自由基（D+•）之间，以及不同电子受体（A）与其阴离子自由基（A-•）之间发生的分子间自身交换反应，本数据集收录了其瞬态1:1前驱体配合物的相关研究数据。这类配合物通过紫外-近红外（UV−NIR）光谱得以观测，并经X射线衍射（X-ray）结构表征确认为共面（π堆叠）缔合体[D, D+•]与[A-•, A]。
对其特征性价间跃迁带开展穆利肯-哈什（两态）分析，可得到电子耦合元件（HDA）；结合近红外光谱数据提取的马库斯重组能（λ），相关结果均通过分子轨道计算得到验证。
研究发现，电子耦合元件（HDA）数值对氧化还原中心周围的大位阻取代基具有敏感依赖性。由此，通过对给体/受体间距（rDA）的空间位阻调控，位阻受阻的给体/受体与其未取代的母体分子之间展现出显著差异的电子转移速率。
本文结合陶布及其研究团队针对无机（配位）给体/受体二元体系提出的经典理论框架，对有机电子转移过程中连续分布的外层与内层电子转移机制展开讨论，同时兼顾了传统有机化学与无机化学的相关概念差异。