Conformationally gated switching between superexchange and hopping within oligo-p-phenylene-based molecular wires

We observe well-defined regions of superexchange and thermally activated hopping in the temperature dependence of charge recombination (CR) in a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Ph_n), n = 1-4, and A = perylene-3,4:9,10- bis(dicarboximide) (PDI). A fit to the thermally activated CR rates of the n = 3 and n = 4 compounds yields activation barriers of 1290 and 2030 cm ^-1, respectively, which match closely with theoretically predicted and experimentally observed barriers for the planarization of terphenyl and quaterphenyl. Negative activation of CR in the temperature regions dominated by superexchange charge transport is the result of a fast conformational equilibrium that increasingly depopulates the reactive state for CR as temperature is increased. The temperature dependence of the effective donor-acceptor superexchange coupling, V_DA, measured using magnetic field effects on the efficiency of the charge recombination process, shows that CR occurs out of the conformation with lower V_DA via the energetically favored triplet pathway.