Photoreduction of methylviologen using methylacridine orange in the presence of triethanolamine in ethanol-water mixtures

Photoreduction of methylviologen (MV2+) by 10-methylacridine orange (MAO(+)) in the presence of triethanolamine (TEOA) has been investigated in ethanol-water mixed solvents by means of steady-state and laser-flash photolysis in visible/near-IR regions. The complete conversion from MV2+ to MV.+ was observed under the condition of [MV2+]>[MAO(+)] in the presence of an excess TEOA. By laser-flash photolysis measurements, the various rate constants for the electron transfer processes (k(et)) have been determined. The k(et) values for electron transfer from the triplet state of MAO(+) ((3)(MAO(+))*) to MV2+, which is the initial step of oxidative route, are in the order of 10(8) M-1 s(-1). The rate constant of back electron transfer between the ion radicals (MV.+ and (MAO(+))(.+)) was ca. 3 x 10(8) M-1 s(-1), while the back electron-transfer process was suppressed on addition of TEOA. The k(et) values from TEOA to (3)(MAO(+))*, which is the initial step of reductive route, are in the order of 10(5) M-1 s(-1). The k(et) value for electron transfer from (MAO(+))(.-) to MV2+ was estimated to be 4.7 x 10(8) M-1 s(-1). Contributions of the oxidative and reductive routes of (3) (MAO(+))* to the MV.+ formation vary with TEOA concentration. High quantum yields for the MV.+ formation were evaluated. The effects of the ion salt and solvents on the rate constants and quantum yields were found. (C) 2000 Elsevier Science S.A. All rights reserved.