Excited-State Quenching by Proton-Coupled Electron Transfer

The emitting metal-to-ligand charge transfer (MLCT) excited state of [Ru(bpy)2(bpz)]2+ (bpy is 2,2‘-bipyridine; bpz is 2,2‘-bipyrazine) is reductively quenched by hydroquinone (H2Q) by proton-coupled electron transfer (PCET), most likely by concerted electron−proton transfer (EPT). The identity of the transient products ([Ru(bpy)2(bpzH•)]2+ and HQ•) and the kinetics of their formation and disappearance have been established by steady-state emission and time-resolved emission, absorption, and EPR measurements. The protonated, reduced complex [Ru(bpy)2(bpzH•)]2+ functions as a H-atom reductant toward quinone or benzaldehyde with potential implications for net photochemistry and energy conversion.