Dependence of Reaction Rates for Bidirectional PCET on the Electron Donor–Electron Acceptor Distance in Phenol–Ru(2,2′-Bipyridine)32+ Dyads

A homologous series of three donor–bridge–acceptor molecules in which a phenolic unit is attached covalently to a Ru(bpy)32+ (bpy =2,2′-bipyridine) complex via rigid rod-like p-xylene spacers was investigated. Photoexcitation at 532 nm in the presence of a large excess of methyl viologen leads to rapid (<10 ns) formation of Ru(bpy)33+. When imidazole base is present in CH3CN solution, intramolecular electron transfer from the phenol to Ru(bpy)33+ occurs, and this is coupled to proton transfer from the phenol to imidazole. All mechanistic possibilities for this proton-coupled electron transfer (PCET) process are considered, and based on a combination of kinetic and thermodynamic data, one arrives at the conclusion that electron and proton release by the phenol occur in concert. By varying the number of p-xylene bridging units, it then becomes possible to investigate the dependence of the reaction rates for concerted proton–electron transfer (CPET) on the phenol–Ru(bpy)33+ distance. A distance decay constant of 0.87 ± 0.09 Å–1 is obtained. This is one of the largest β values reported for electron transfer across oligo-p-phenylene-based molecular bridges, but it is still relatively close to what was determined for “simple” (i. e., not proton-coupled) electron transfer across oligo-p-xylenes. Bidirectional CPET plays a key role in photosystem II. Understanding the distance dependence of such reactions is of interest, for example, in the context of separating protons and electrons across artificial membranes in order to build up charge gradients for light-to-chemical energy conversion.