Coulombic effects on photoinduced electron-transfer reactions between benzophenone derivatives and leuco crystal violet

Abstract

Photo-oxidation of leuco crystal violet (DH) to crystal violet dye (D +) was studied in the benzophenone (BP) derivatives—DH—acetonitrile systems. The BP derivatives with a tetraalkylammonium substituent (R 3N +; I, II and III exhibited much higher photo-oxidizing efficiencies than BP itself. Under deaerated conditions, nearly quantitative photo-oxidation of DH by the absorbed photons was attained by I whereas BP itself showed a poor ability to photo-oxidize DH. Addition of tetraalkylammonium salts (R 4NX), such as cetyltrimethylammonium chloride or tetra- n-butylammonium chloride to the BPDH system also enhanced the quantum yield by a factor of about 10. The enhancement of quantum yield by the use of I, II or III, or by the addition of R 4NX to BP was interpreted as due to stabilization of the BP anion radical or the DH cation radical by ion pairing with the tetraalkylammonium substituent on BP or with the added R 4NX. In the case of I, II and III, the electrostatic repulsion between the DH cation radical and the R 3N + substituent on BP, which enchanced the charge separation efficiency, was the main cause of the high photo-oxidation yields.