Hydrogen-bond strengthening upon photoinduced electron transfer in ruthenium-anthraquinone dyads interacting with hexafluoroisopropanol or water.

Quinones play a key role as primary electron acceptors in natural photosynthesis, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, the influence of hydrogen-bond donating solvents on the thermodynamics and kinetics of intramolecular electron transfer between Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and 9,10-anthraquinone redox partners linked together via one up to three p-xylene units was investigated. Addition of relatively small amounts of hexafluoroisopropanol to dichloromethane solutions of these rigid rodlike donor-bridge-acceptor molecules is found to accelerate intramolecular Ru(bpy)(3)(2+)-to-anthraquinone electron transfer substantially because anthraquinone reduction occurs more easily in the presence of the strong hydrogen-bond donor.

Accelerated hole transfer across a molecular double barrier.

We report on a dyad in which photoinduced hole transfer through a non-uniform molecular double barrier is more than one order of magnitude more rapid than hole transfer across a comparable uniform (rectangular) tunneling barrier.

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.

Electron tunneling through oligo-p-xylene bridges.

A series of rigid rodlike molecules having a phenothiazine donor, oligo- p-xylene bridges, and a rhenium(I) tricarbonyl phenanthroline acceptor were synthesized and studied in the context of long-range electron transfer. By optical absorption spectroscopy, the p-xylene bridges are found to have essentially length-independent HOMO-LUMO energy gaps, which is in clear contrast to oligo- p-phenylene spacers. Nanosecond time-resolved luminescence spectroscopy reveals an exponential decrease of electron transfer rates with increasing donor-acceptor distance; the attenuation factor beta is 0.

A fast-moving [2]rotaxane whose stoppers are remote from the copper complex core.

[reaction: see text] A new copper-complexed rotaxane is described. It consists of a two-coordination site ring threaded by a sterically non-hindering 2,2'-bipyridine derivative. An electrochemical signal (oxidation or reduction of the copper center, Cu(I) or Cu(II)) induces rearrangement of the system.