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 ([Ru-II(bpy)(3)](2+)) and manganese ([Mn-II(L)(3)](2+) or [Mn-2(III,IV) O-2(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-mu-oxo complex [Mn-2(III,IV) O-2(L)(4)](3+) with a good yield. These latter complexes can be then reversibly oxidized into the stable [Mn-2(IV,IV) O-2(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 10(7) M-1 s(-1). In contrast, a strong decrease of the luminescence lifetime is observed by addition of an increasing concentration of [Mn-2(III, IV) O-2(L)(4)](3+). The k(q) values obtained for the bpy and dmbpy complexes are, respectively, 2.3 x 10(9) and 2.5 x 10(9) 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 [Mn-2(III,IV) O-2(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 [Mn-2(III,IV) O-2(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 [Mn-2(IV,IV) O-2(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.