Magnetic Field Effects on Photochemical Reaction in Homogeneous Solution

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We have developed a picosecond laser flash photolysis apparatus with a convenient picosecond pulse laser as an excitation light source and a streak camera to measure transient absorptions. The hydrogen abstraction reaction of triplet 4-methoxy-benzophenone with thiophenol at 265 K was reinvestigated with this apparatus under magnetic fields of 0-1.7 T. The magnetic field effects (MFEs) on this reaction were successfully observed with - 1OO ps time-resolution. The decay rate constant of the generated radical pair was found to increase from 3.42 x 1 09 s^<-1> to 4.15 x 10^9 s^<-1> with increasing magnetic field from 0 T to 1.7 T. The observed magnetic field effects can be explained by the Ag mechanism. Using the simple kinetics model with the Ag mechanism, we tentatively obtained the rate constants of the escape process from the pair (k_<esc>) and the T-S spin conversion process (k_<T-S>) at 0 T and 1.7 T to be 1.97 x 109 s^<-1>, 1.45 x 10^9 s^<-1>, and 2.12 x 10^9 s^<-1>, respectively. From these results, this apparatus is found to be simple and useful for studying MFES on the picosecond spin dynamics.