Water-Triggered Photoinduced Electron Transfer in Acetonitrile–Water Binary Solvent. Solvent Microstructure-Tuned Reactivity of Hydrophobic Solutes

[Image: see text] The solvent-composition dependence of quenching triplet states of benzophenone ((3)BP) by anisole in acetonitrile–water (ACN–H(2)O) mixtures was investigated by laser flash photolysis over the water mole fraction (x(w)) increasing from 0 to 0.92. Single exponential decay of (3)BP 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 (3)BP quenching rate was described by a kinetic model accounting for the microheterogeneous structure of the ACN–H(2)O 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 × 10(9) M(–1) s(–1) and is presumably assisted by the ACN–H(2)O hydrogen-bonding interaction.