Electron-transfer reactions of the 2-nitrothiophen triplet state studied by laser flash photolysis

A study of the one-electron-transfer reactions of the lowest triplet state of 2-nitrothiophen (3NT) in aqueous solution, at neutral pH, has been undertaken. Absolute rate constants, k2, measured by laser flash photolysis (347.1 nm) for the oxidation of a number of substrates by 3NT, vary from ca. 107 to 1010 dm3 mol–1 s–1: thus[Fe(CN)6]4– is oxidised with a rate constant of k2=(9.45 ± 0.60)× 109 dm3 mol–1 s–1 while that for Tl+ was found to be k2=(9.65 ± 2.29)× 106 dm3 mol–1 s–1. Evidence for complete electron transfer comes, in several cases, from the observation of the oxidized substrate. The correlation of the logarithm of k2 with the free energy change, calculated from the thermodynamic potentials of the reactants, is discussed in terms of current theories of electron transfer. Reaction rate constants for the one-electron reduction of the duroquinone lowest triplet state (3DQ), measured in 30%(v/v) methanol + water solution, are also included in the discussion.