Photosensitized reduction of methyl viologen using eosin-Y in presence of a sacrificial electron donor in water-alcohol mixture

Photoreduction of methyl viologen (MV²⁺) by eosin-Y (EY^2-) in the presence of triethanolamine (TEOA) has been investigated in water-methanol mixture by means of steady-state photolysis and laser-flash photolysis in the visible/near-infrared regions. The complete conversion to the persistent methyl viologen radical cation (MV^-+) was observed in the presence of lower concentrations of EY^2- and excess TEOA. By laser-flash photolysis measurements, electron transfer was confirmed to occur from the triplet state of EY^2- [³(EY^2-)*] to MV²⁺ in the rate constants of ca 2.0 x 10¹⁰ M^-1 s^-1. The rates and efficiencies of production of MV^-+ were found to be dependent on solvent compositions and concentrations of MV²⁺, ionic salt and TEOA. The back electron transfer reaction from MV^-+ to EY^- was retarded in the presence of TEOA, which supports that EY^2- is reproduced by accepting an electron from TEOA. In the presence of excess TEOA, the indirect formation of MV^-+ from EY^-3-, which was produced by accepting an electron from TEOA, was confirmed. The contributions of both the oxidative and reductive routes of ³(EY^2-)* for the MV^-+ formation have been confirmed.