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) n = 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 (Δ E 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 = C⋮C, CH CH, O, and CH 2 CH 2 inserts or the (CH 3 ) 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 arise 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 new organic mixed-valence systems.