Exploring the Structure of an Exchange-Coupled Triplet Pair Generated by Singlet Fission in Crystalline Diphenylhexatriene: Anisotropic Magnetic Field Effects on Fluorescence in High Fields

Magnetic field effects (MFEs) on triplet pairs generated by singlet fission (SF) in a single crystal of 1,6-diphenyl-1,3,5-hexatriene were studied by steady-state fluorescence measurements under high fields up to 8 T. In the MFE curves, which are fluorescence intensity plotted against the magnetic field, we found three dips at the magnetic fields of 2.2, 3.0, and 4.4 T. These dips are explained with the level crossings of spin states in an exchange-coupled triplet pair generated by SF in the crystal. We also found that dip depths varied with the direction of the magnetic field. The structure of the exchange-coupled triplet pair was determined from the analysis on the observed anisotropies in dip depths. We also discuss a dip shape in coherent SF systems. In coherent SF systems, the electronic interaction between a singlet exciton and an exchange-coupled triplet pair induces dip splitting as well as SF itself.