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 (EY2-) 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 EY2- and excess TEOA. By laser-flash photolysis measurements, electron transfer was confirmed to occur from the triplet state of EY2- [3(EY2-)*] to MV2+ in the rate constants of ca 2.0 x 10(10) 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 EY2- 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(EY2-)* for the MV.+ formation have been confirmed.