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A practical method to calculate time evolutions of magnetic field effects (MFEs) on photochemical reactions involving radical pairs is developed on the basis of the theory of the chemically induced dynamic spin polarization proposed by Pedersen and Freed. In theory, the stochastic Liouville equation (SLE), including the spin Hamiltonian, diffusion motions of the radical pair, chemical reactions, and spin relaxations, is solved by using the Laplace and the inverse Laplace transformation technique. In our practical approach, time evolutions of the MFEs are successfully calculated by applying the Miller-Guy method instead of the final value theorem to the inverse Laplace transformation process. Especially, the SLE calculations are completed in a short time when the radical pair dynamics can be described by the chemical kinetics consisting of diffusions, reactions and spin relaxations. The SLE analysis with a short calculation time enables one to examine the various parameter sets for fitting the experimental date. Our study demonstrates that simultaneous fitting of the time evolution of the MFE and of the magnetic field dependence of the MFE provides valuable information on the diffusion motions of the radical pairs in nano-structured materials such as micelles where the lifetimes of radical pairs are longer than hundreds of nano-seconds and the magnetic field dependence of the spin relaxations play a major role for the generation of the MFE.

Self-quenching of the triplet excited state of thiobenzophenone ((3)TBP*) in molecular solvents, a sodium dodecyl sulfate (SDS) micellar solution, and ionic liquids (ILs) was measured using nanosecond laser flash photolysis. In the molecular solvents and ILs, (3)TBP* single-exponentially decayed, and the decay was accelerated with increasing TBP concentration. Self-quenching rate constants (k(SQ)) obtained in the ILs were smaller than those in the molecular solvents with similar solvent viscosities, indicating that molecular diffusion is inhibited in the ILs. In the SDS micellar solution, the decay of (3)TBP* showed two components. The fast decay component was attributed to self-quenching of (3)TBP* which presents inside micelles containing two or more TBP molecules, while the slow decay component was from deactivation of single (3)TBP*. The decay of (3)TBP* in an IL, N,N,N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)amide (TMPA TFSA), had only one decay component, showing that unlike the micelles, the local structure of TMPA TFSA does not have a long lifetime. The cage lifetime of TMPA TFSA was estimated to be between 17 and 20 ns.

Magnetic field effects (MFEs) on the photoinduced hydrogen abstraction reaction of benzophenone with phenol were investigated in ionic liquids (ILs) with a short alkyl chain (N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide (TMPA TFSA)) and long alkyl chains ((N,N,N-trimethyl-N-octylammonium bis(trifluoromethanesulfonyl)amide (TMOA TFSA) and N-decyl-N,N,N-trimethylammonium bis(trifluoromethanesulfonyl)amide (DTMA TFSA)) by a nanosecond laser flash photolysis technique. In each ionic liquid, escaped radical yield of a benzophenone ketyl radical rapidly increased with increasing magnetic field strength (B) of 0 T < B≤ 0.01 T. At 0.01 T < B≤ 0.4 T, the escaped radical yield almost saturated in TMPA TFSA or gradually increased in TMOA TFSA and DTMA TFSA. At much higher fields of 0.4 T < B≤ 30 T, the yield gradually decreased, resulting in 10-15% decrease at 30 T. The observed MFEs can be explained by the hyperfine coupling and Δg mechanisms together with the relaxation mechanism. On the time profiles of the transient absorption observed for the benzophenone ketyl radical, MFEs were generated in the time range of 0 < t < 0.6 μs. The cage lifetimes of TMOA TFSA and DTMA TFSA were estimated to be at least 120 ns.

Self-trapping and singlet-singlet annihilation of the free excitons in a monomeric (beta) perylene crystal were studied by using femtosecond transient absorption microscopy. The free exciton generated by the photo-excitation of the beta-perylene crystal relaxed to the self-trapped exciton with a rate constant of 7 x 10(10) s(-1). The singlet-singlet annihilation of the free exciton observed under the high excitation density conditions was competed with the self-trapping of the free exciton; we estimated the annihilation rate constant for the free exciton to be 1 x 10(-8) cm(3) s(-1) from the excitation density dependence of the free exciton decay. After self-trapping of the free exciton, no annihilation was observed in the 100 ps time range, suggesting that the diffusion coefficient was reduced drastically by self-trapping. The results show that the major factor limiting the exciton diffusion in the beta-perylene crystal is a relaxation of the free exciton to the self-trapped exciton, and not the lifetime of the exciton. Though the singlet-singlet annihilation rate constants and fluorescence lifetime of the beta-perylene crystal are similar to those of the anthracene crystal, the estimated exciton diffusion length (2 nm) in the beta-perylene crystal is much smaller than that (100 nm) in the anthracene crystal as a result of the exciton self-trapping.