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

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 (R₃N⁺; 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 (R₄NX), 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 R₄NX 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 R₄NX. In the case of I, II and III, the electrostatic repulsion between the DH cation radical and the R₃N⁺ substituent on BP, which enchanced the charge separation efficiency, was the main cause of the high photo-oxidation yields.