Magnetic field effects (MFEs) on singlet fission were studied by observing fluorescence from organic crystal of 1,6-diphenyl-1,3,5-hexatriene under magnetic fields of up to 5 T. We found anomalous MFE dips at magnetic fields higher than 2 T, in addition to the known MFEs which saturated around 1 T. The observed results were analyzed by using the stochastic Liouville equation (SLE) in which a distance-dependent exchange interaction (J) in triplet pair, hopping of triplet, and geminate fusion in contacted triplet pair were incorporated. The SLE analysis revealed that the observed dips were caused by a MFE due to the level crossing mechanism and strongly suggested that the contacted triplet pair has a large J, which has been ignored in the previous model of MFEs on the singlet fission. Present results lead to the conclusion that the initial dissociation of the singlet exciton to the contacted triplet pair does not show the MFE and the triplet pair at a separated distance produced by hopping of the triplet plays an important role on the generation of the MFE on the singlet fission.

Magnetic field effects (MFEs) on triplet pairs generated by singlet fission (SF) in an organic crystal, 1,6diphenyl-1,3,5-hexatriene, were studied by steady-state fluorescence measurements under ultrahigh magnetic fields of up to 10 T and by time-resolved fluorescence measurements with subnanosecond time resolution in the presence of magnetic field of 0.5 T. The observed MFEs were analyzed by using the stochastic Liouville equation based on the radical pair model with a modification of the spin Hamiltonian. Excellent agreements between the observed and the simulated MFEs demonstrate that the radical pair model used in the present study can apply to analysis of MFEs on triplet pairs generated by SF in organic materials. Model calculations were performed to clarify how the SF dynamics influences the features of MFE generated in the triplet pairs. The magnitude of the exchange interaction in a correlated triplet pair was precisely determined from the observation of the MFE caused by the level crossing mechanism. We also determined the structure of the correlated triplet pair generated by the SF in 1,6-diphenyl-1,3,5-hexatriene crystal.

The magnetic field effects (MFEs) on photoinduced hydrogen abstraction reactions between benzophenone and thiophenol in an ionic liquid, N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) imide (TMPA TFSI), were studied by a nanosecond laser flash photolysis technique under ultrahigh fields of up to 28 T. Extremely large and anomalous stepwise MFEs were observed for the first time. The escape yield of benzophenone ketyl radical decreased with increasing magnetic field strength (B) at 0 T < B e 2 T. The decrease was almost saturated at 2 T < B e 10 T. At much higher fields (10 T < B e 28 T), the yield decreased again with increasing B, producing a 25% decrease at 28 T.

Magnetic field effects (MFEs) due to the isotropic Δg mechanism were studied for the hydrogen abstraction reactions of 4-methoxy-benzophenone with thiophenol in several solvents at 293 K by a laser flash photolysis technique under ultrahigh magnetic fields of up to 30 T. In 2-propanol, ethanol, and methanol, the yield of the escaped ketyl radical decreased with increasing magnetic field from 0 to 30 T. In 2-methyl-1-propanol, however, the decrease was almost saturated at 20 T and the yield above 20 T reduced to be 2/3 of that at 0 T. Such saturation of the MFEs due to the isotropic Δg mechanism in the reaction through radical pairs in solution was found for the first time.

Experimentally determined quantum yields, decay profiles, and magnetic field effects on fluorescence showed that fluorinated derivatives of diphenylhexatriene were singlet fission materials. The rate constant of singlet fission was estimated as a function of temperature from the initial rate of decay of the fluorescence profiles. The origin of the temperature dependence was discussed in relation to the molecular stacking structure of the crystals.