Efficient photoreduction process of [Ru3(mu3-O)(mu-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(mu3-O)(mu-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(mu3-O)(mu-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(mu3-O)(mu-CH3CO2)6L3]+, efficiently generating Ru3(mu3-O)(mu-CH3CO2)6L3]0 with high potential. Although back-electron transfer took place between ZnTPP*+ and Ru3(mu3-O)(mu-CH3CO2)6L3]0 in the diffusion-controlled limit, Ru3(mu3-O)(mu-CH3CO2)6L3]0 accumulates at a steady concentration upon further addition of 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificial donor to re-produce ZnTPP from ZnTPP*+. Therefore, we established a novel system to accumulate Ru3(mu3-O)(mu-CH3CO2)6L3]0 as an electron pool by the excitation of ZnTPP as photosensitizing electron donor in the presence of HV2+ and BNAH as an electron-mediating reagent and sacrificial donor, respectively. With the increase in the electron-withdrawing abilities of the ligands, the final yields of Ru3(mu3-O)(mu-CH3CO2)6L3]0 increased.