Distance Dependence of Bidirectional Concerted Proton–Electron Transfer in Phenol-Ru(2,2′-bipyridine)32+ Dyads

Proton-coupled electron transfer (PCET) was investigated in three covalent donor–bridge–acceptor molecules with different bridge lengths. Upon photoexcitation of their Ru(bpy)32+ (bpy=2,2′-bipyridine) photosensitizer in acetonitrile, intramolecular long-range electron transfer from a phenolic unit to Ru(bpy)32+ occurs in concert with release of the phenolic proton to pyrrolidine base. The kinetics of this bidirectional concerted proton–electron transfer (CPET) reaction were studied as a function of phenol–Ru(bpy)32+ distance by increasing the number of bridging p-xylene units. A distance decay constant (β) of 0.67±0.23 Å−1 was determined. The distance dependence of the rates for CPET is thus not significantly steeper than that for ordinary (i.e., not proton coupled) electron transfer across the same bridges, despite the concerted motion of oppositely charged particles into different directions. Long-range bidirectional CPET is an important reaction in many proteins and plays a key role in photosynthesis; our results are relevant in the context of photoinduced separation of protons and electrons as a means of light-to-chemical energy conversion. This is the first determination of β for a bidirectional CPET reaction.