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

Abstract

The photophysical, photochemical and electrochemical studies of a mixture of bipyridinyl ruthenium ([Ru^II(bpy)₃]²⁺) and manganese ([Mn^II(L)₃]²⁺ or [Mn₂^III,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 [Mn^II(L)₃]²⁺ are irreversible and lead, by subsequent chemical reactions, to the corresponding di-μ-oxo complex [Mn₂^III,IVO₂(L)₄]³⁺ with a good yield. These latter complexes can be then reversibly oxidized into the stable [Mn₂^IV,IVO₂(L)₄]⁴⁺ species. The luminescence lifetime of the excited state of the photosensitizer [Ru^II(bpy)₃]²⁺* in the presence of variable concentration of manganese complexes has been determined. Using the Stern–Volmer equation, the quenching constant rate k_q have been estimated. It appears that the [Mn^II(L)₃]²⁺ mononuclear complexes quench only very weakly the excited state [Ru^II(bpy)₃]²⁺* since the magnitude of k_q 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 [Mn₂^III,IVO₂(L)₄]³⁺. The k_q 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 [Ru^II(bpy)₃]²⁺* by those binuclear complexes of manganese are presumably energy transfer processes. Finally, the possibility of photocatalytic oxidation of [Mn^II(L)₃]²⁺ and [Mn₂^III,IVO₂(L)₄]³⁺ has been evaluated by continuous irradiation in the presence of the photosensitizer [Ru^II(bpy)₃]²⁺ and an aryl diazonium salt, ArN₂⁺, playing the role of an irreversible electron acceptor. The photooxidation process transforming [Mn^II(L)₃]²⁺ into [Mn₂^III,IVO₂(L)₄]³⁺ by intermolecular electron transfers between photogenerated [Ru^III(bpy)₃]³⁺ and [Mn^II(L)₃]²⁺ occurs for the bpy and dmbpy complexes with a high efficiency. The subsequent photooxidation leading to [Mn₂^IV,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.