Efficient photoreduction process of [Ru3(μ3-O)(μ-CH3CO2)6L3]+ by electron-mediation via the viologen dication by excitation of a zinc porphyrin

Photoinduced electron-transfer and electron-mediation processes from the excited triplet state of zinc tetraphenylporphyrin (3ZnTPP*) to the hexyl viologen dication (HV2+) in the presence of oxo-acetato-bridged triruthenium clusters, [Ru3(μ3-O)(μ-CH3CO2)6L3]+, have been revealed by the transient absorption spectra in the visible and near-IR regions. By the nanosecond laser-flash photolysis of ZnTPP in the presence of HV2+ and [Ru3(μ3-O)(μ-CH3CO2)6L3]+, the transient absorption bands of the radical cation of ZnTPP (ZnTPP•+) and the reduced viologen (HV•+) were initially observed with the concomitant decay of 3ZnTPP*, after which an extra electron of HV•+ mediates to [Ru3(μ3-O)(μ-CH3CO2)6L3]+, efficiently generating [Ru3(μ3-O)(μ-CH3CO2)6L3]0 with high potential. Although back-electron transfer took place between ZnTPP•+ and [Ru3(μ3-O)(μ-CH3CO2)6L3]0 in the diffusion-controlled limit, [Ru3(μ3-O)(μ-CH3CO2)6L3]0 accumulates at a steady concentration upon further addition of 1-benzyl-1,4-dihydronicotinamide (BN...