TIME-RESOLVED EPR STUDY ON THE PHOTOCHEMICAL-REACTIONS OF BENZIL

A time-resolved EPR (TREPR) study on the photochemical reactions of benzil in 2-propanol and benzene-triethylamine (TEA) mixed solvents is reported. The emissive signals observed in 2-propanol are assigned to the benzoyl and benzil ketyl radicals, and the absorptive one observed in benzene-TEA is assigned to the benzil anion radical. The reactions are considered to occur from higher excited triplet (T(n)) states through two-photon processes in 2-propanol but from the lowest excited triplet state through a one-photon process in benzene-TEA. TREPR studies are supplemented by optical studies including measurements of transient absorptions, phosphorescence quenching, and fluorescence quenching, whose results support the conclusion drawn from the EPR results. Possible polarization mechanisms to produce emissive signals are considered in detail. It is shown that the direct T1 --> T(n) excitation does not produce the inversion of the polarization regardless of the signs of the zero-field splitting of the T1 and T(n) states. It is then concluded that the emissive signals are due to the reaction from T(n) produced via the S(n) --> T(n) intersystem crossing process. It is also found that the rise of the chemically induced dynamic electron polarization signal of the ketyl radical in 2-propanol consists of fast and slow components. The time profile of the slow rise component strongly depends on the temperature, the concentration of benzil, the excitation laser power, and the oxygen addition. From the analysis of these dependences it is suggested that the free-pair radical-pair mechanism can account for the main features of the slow polarization.