Intervalence (Charge-Resonance) Transitions in Organic Mixed-Valence Systems. Through-Space versus Through-Bond Electron Transfer between Bridged Aromatic (Redox) Centers

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American Chemical Society

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Journal of the American Chemical Society

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Intervalence absorption bands appearing in the diagnostic near-IR region are consistently observed in the electronic spectra of mixed-valence systems containing a pair of aromatic redox centers (Ar+/Ar) that are connected by two basically different types of molecular bridges. The through-space pathway for intramolecular electron transfer is dictated by an o-xylylene bridge in the mixed-valence cation radical 3+ with Ar = 2,5-dimethoxy-p-tolyl (T), in which conformational mobility allows the proximal syn disposition of planar T+/T redox centers. Four independent experimental probes indicate the large through-space electronic interaction between such cofacial Ar+/Ar redox centers from the measurements of (a) sizable potential splitting in the cyclic voltammogram, (b) quinonoidal distortion of T+/T centers by X-ray crystallography, (c) “doubling” of the ESR hyperfine splittings, and (d) a pronounced intervalence charge-resonance band. The through (br)-bond pathway for intramolecular electron transfer is enforced in the mixed-valence cation radical 2a+ by the p-phenylene bridge which provides the structurally inflexible and linear connection between Ar+/Ar redox centers. The direct comparison of intramolecular rates of electron transfer (kET) between identical T+/T centers in 3+ and 2a+indicates that through-space and through-bond mechanisms are equally effective, despite widely different separations between their redox centers. The same picture obtains for 3+ and 2a+from theoretical computations of the first-order rate constants for intramolecular electron transfer from Marcus−Hush theory using the electronic coupling elements evaluated from the diagnostic intervalence (charge-transfer) transitions. Such a strong coherence between theory and experiment also applies to the mixed-valence cation radical 7+, in which the aromatic redox S center is sterically encumbered by annulation.


Accepted version. Journal of the American Chemical Society, Vol. 125, No. 51 (2003): 15950-15963. DOI. © 2003 American Chemical Society. Used with permission.

Sergey V. Lindeman was affiliated with the University of Houston at the time of publication.