Magnetic Field Effects on Triplet Pair Generated by Singlet Fission in an Organic Crystal: Application of Radical Pair Model to Triplet Pair

Magnetic field effects (MFEs) on triplet pairs generated by singlet fission (SF) in an organic crystal, 1,6-diphenyl-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.