Conformation, distance, and connectivity effects on intramolecular electron transfer between phenylene-bridged aromatic redox centers

Intramolecular electron transfer in the organic mixed-valence cation radical D(ph)(n)D+. [where D = 2,5-dimethoxy-4-methylphenyl and (ph), = poly-p-phenylene] is systematically probed by the structural modification of the molecular conformation, separation distance, and electronic connectivity of the (ph)(n) bridge. Cyclic voltammetry and dynamic ESR line broadening studies afford experimental measures of the energy gap (DeltaE(ox)) and the electron-transfer kinetics (k(ET)) for the D/D+. interaction in a series of methyl-substituted, (poly)phenylene, and bridged-modified ph-X-ph (where X = Cequivalent toC, CH=CH, 0, and CH2CH2 inserts or the (CH3)(2)C tiedown) bridges that comprise the groups A-C donors in Chart 1. Theoretical electron-transfer rates are obtained by the application of the Creutz, Newton, and Sutin (CNS) superexchange model (for the calculation of the electron coupling matrix element H-CNS) to the diagnostic NIR absorptions that wise from the intramolecular bridge-to-redox center (i.e., br --> D+.) charge-transfer transitions. Comparison of the experimental and theoretical electron-transfer rates (k(ET)) indicate that the CNS model is sufficient to provide a mechanistic basis for including conformation, distance and connectivity effects in the design of (poly)phenylene bridges for now organic mixed-valence systems.