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

Document Type

Article

Language

eng

Publication Date

2003

Publisher

American Chemical Society

Source Publication

Journal of the American Chemical Society

Source ISSN

0002-7863

Original Item ID

doi:10.1021/ja037867s

Abstract

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.

Comments

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.

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