Tunneling Barrier Effects on Photoinduced Charge Transfer through Covalent Rigid Rod-Like Bridges

Four homologous dyads with a phenothiazine donor, rigid variable-length p-xylene bridges, and a ruthenium(II) tris(2,2'-bipyridine) acceptor were synthesized. Photoexcitation of these donor-bridge-acceptor molecules in the presence of excess methylviologen generates a highly oxidizing Ru(III) intermediate, which triggers an intramolecular phenothiazine-to-ruthenium(III) electron transfer that is mediated by the oligo-p-xylene spacers. The rates for this process were determined using transient absorption spectroscopy, and they are found to decrease exponentially with increasing donor-acceptor distance. This decrease occurs with an attenuation factor beta of 0.77 A(-1) and is substantially stronger than for analogous donor-bridge-acceptor molecules where the acceptor is a rhenium(I) tricarbonyl diimine complex (beta = 0.52 A(-1)). This striking finding is interpreted in terms of a larger barrier to hole tunneling in the ruthenium dyads relative to the rhenium systems.