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

Abstract

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.