Water-Triggered Photoinduced Electron Transfer in Acetonitrile-Water Binary Solvent. Solvent Microstructure-Tuned...

The solvent-composition dependence of quenching triplet states of benzophenone (³BP) by anisole in acetonitrile-water (ACN-H₂O) mixtures was investigated by laser flash photolysis over the water mole fraction (x_w) increasing from 0 to 0.92. Single exponential decay of ³BP was observed over the whole composition range. The quenching rate constant consistently increased with the water content but increased far more rapidly with x_w > 0.7. The water-triggered electron-transfer (ET) mechanism was confirmed by a steeply growing quantum yield of the benzophenone ketyl radical anion, escaping back-ET when the partial water volume exceeded the acetonitrile one. The water-content influence on the ³BP quenching rate was described by a kinetic model accounting for the microheterogeneous structure of the ACN-H₂O mixtures and the very different solubility of the reactants in the solvent components. According to the model, the ET mechanism occurs at a rate constant of 1.46 × 10⁹ M^-1 s^-1 and is presumably assisted by the ACN-H₂O hydrogen-bonding interaction.