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-CH ₃CO₂)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-CH₃CO ₂)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 (BNAH) as a sacrificial donor to re-produce ZnTPP from ZnTPP^•+. Therefore, we established a novel system to accumulate [Ru₃(μ₃-O) (μ-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(μ₃- O)(μ-CH₃CO₂)₆L₃]⁰ increased.