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 (H(2)Q) by proton-coupled electron transfer (PCET), most likely by concerted electron-proton transfer (EPT). The identity of the transient products ([Ru(bpy)(2)(bpzH(center dot))](2+) and HQ(center dot)) 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(center dot))](2+) functions as a H-atom reductant toward quinone or benzaldehyde with potential implications for net photochemistry and energy conversion.