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

What Can Be Learned from Magnetic Field Effects on Singlet Fission: Role of Exchange Interaction in Excited Triplet Pairs

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 (<i>J</i>) 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 <i>J</i>, 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 <i>separated</i> distance produced by hopping of the triplet plays an important role on the generation of the MFE on the singlet fission