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+center dot), as well as between different electron acceptors (A) paired with their anion radicals (A(-center dot)), are spectrally (UV-NIR) observed and structurally (X-ray) identified as the cofacial (pi-stacked) associates [D, D+center dot] and [A(-center dot,) A], respectively. Mulliken-Hush (two-state) analysis of their diagnostic intervalence bands affords the electronic coupling elements (H-DA), which together with the Marcus reorganization energies (lambda) from the NIR spectral data are confirmed by molecular-orbital computations. The H-DA 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 (r(DA)) 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 dyadswith conciliatory attention paid to traditional organic versus inorganic concepts.