Control of electron acceptor ability with ligands (L) in photoinduced electron transfer from zinc porphyrin or zinc phthalocyanine to [Ru ₃(μ₃-O)(μ-CH₃COO)₆L ₃]⁺

Photoinduced electron-transfer processes from the excited triplet states of zinc tetraphenylporphyrin (³ZnTPP*) or zinc tetra-tert-butylphthalocyanine (³ZnTBPc*) to oxo-acetato-bridged triruthenium clusters [Ru₃(μ₃-O) (μ-CH₃CO₂)₆-(L)₃]⁺ have been confirmed by nanosecond laser flash photolysis in the visible and near-IR regions. The rise of the transient absorption spectra of the radical cations of ZnTPP and ZnTBPc and the reduced form of the oxo-acetato-bridged triruthenium cluster ([Ru₃(μ₃-O)(μ-CH ₃CO₂)₆(L)₃]⁰) were observed with the concomitant decays of ³ZnTPP* or ³ZnTBPc*. The evaluated rate constants (k_ET) and quantum yields (Φ_ET) for electron-transfer were increased with the order of electron-withdrawing ability of the ligands (L) coordinated to the Ru atoms, 4-cyanopyridine > triphenylphosphine > pyridine > 4-(dimethylamino)pyridine, which is the order of promoting the electron-accepting ability of [Ru₃(μ₃-O)(μ-CH ₃CO₂)₆(L)₃]⁺. The Φ_ET values for ³ZnTPP* were lower than those for ³ZnTBPc*, suggesting the presence of competitive processes such as energy transfer process from ³ZnTPP* to the triplet states of [Ru₃(μ₃-O)(μ-CH₃COO) ₆(L)₃]⁺. For the back electron-transfer process, second-order kinetics indicates that the radical cations of ZnTPP or ZnTBPc and [Ru₃(μ₃-O)(μ-CH₃COO) ₆(L)₃]⁰ return to the original system after solvation in polar solvents at a diffusion controlled limit without side reactions, providing reversible photosensitizing intermolecular electron-transfer systems.