Photoinduced oxidation of [Mn(L)3]2+ and [Mn2O2(L)4]3+ (L = 2,2′-bipyridine and 4,4′-dimethyl-2,2′-bipyridine) with the [Ru(bpy)3]2+/-aryl diazonium salt system

The photophysical, photochemical and electrochemical studies of a mixture of bipyridinyl ruthenium ([RuII(bpy)3]2+) and manganese ([MnII(L)3]2+ or [Mn2III,IVO2(L)4]3+, L = bpy (2,2′-bipyridine) or dmbpy (4,4′-dimethyl-2,2′-bipyridine)) complexes have been investigated in CH3CN. Electrochemical oxidations of [MnII(L)3]2+ are irreversible and lead, by subsequent chemical reactions, to the corresponding di-μ-oxo complex [Mn2III,IVO2(L)4]3+ with a good yield. These latter complexes can be then reversibly oxidized into the stable [Mn2IV,IVO2(L)4]4+ species. The luminescence lifetime of the excited state of the photosensitizer [RuII(bpy)3]2+* in the presence of variable concentration of manganese complexes has been determined. Using the Stern–Volmer equation, the quenching constant rate kq have been estimated. It appears that the [MnII(L)3]2+ mononuclear complexes quench only very weakly the excited state [RuII(bpy)3]2+* since the magnitude of kq determined for the bpy and dmbpy complexes is about 107 M−1 s−1. In contrast, a strong decrease of the luminescence lifetime is observed by addition of an increasing concentration of [Mn2III,IVO2(L)4]3+. The kq values obtained for the bpy and dmbpy complexes are, respectively, 2.3 × 109 and 2.5 × 109 M−1 s−1. The major quenching pathways of [RuII(bpy)3]2+* by those binuclear complexes of manganese are presumably energy transfer processes. Finally, the possibility of photocatalytic oxidation of [MnII(L)3]2+ and [Mn2III,IVO2(L)4]3+ has been evaluated by continuous irradiation in the presence of the photosensitizer [RuII(bpy)3]2+ and an aryl diazonium salt, ArN2+, playing the role of an irreversible electron acceptor. The photooxidation process transforming [MnII(L)3]2+ into [Mn2III,IVO2(L)4]3+ by intermolecular electron transfers between photogenerated [RuIII(bpy)3]3+ and [MnII(L)3]2+ occurs for the bpy and dmbpy complexes with a high efficiency. The subsequent photooxidation leading to [Mn2IV,IVO2(L)4]4+ is efficient only in the case of the dmbpy complex. The formation of those different species by electrochemical or photochemical ways has been demonstrated and quantified by coupled UV-visible absorption and EPR spectroscopy experiments. The efficiencies of the photoinduced oxidative processes have been correlated to the electrochemical data.