Magnetic field dependence of singlet exciton fission and fluorescence in crystalline tetracene at 300 K

The magnetic field dependence of singlet exciton fission and fluorescence was studied at 300 K in tetracene crystals in several crystal planes for different field intensities and orientations. The positions of the resonances at high field correspond to those predicted by the zero-field splitting tensor as obtained by EPR, indicating that the outcome of the singlet fission is a pair of free migrating triplet excitons. It is shown that the field dependence and the resonance lineshapes can only be fitted, to within the experimental error, with the kinematic theory of Suna assuming a nearly two-dimensional motion of triplet excitons, restricted to the (a'b) plane. With the assumption of an isotropic triplet exciton diffusion in the (a'b) plane reasonable sets of values can be deduced from the fit for the in-plane hopping rate, for the anisotropy of the diffusion, and for the nearest neighbour triplet-triplet annihilation rate. In many respects, triplet exciton dynamics in tetracene appears to be similar to that in anthracene, as expected from the calculation of the transfer matrix elements and the similarity of the crystal structures. The dependence of the effects on the exciting light intensity are also well accounted for using the usual kinetic equations for the different processes involved during the lifetime of the triplet excitons.