Magnetic Field Effects on Singlet Fission and Fluorescence Decay Dynamics in Amorphous Rubrene

Picosecond time-resolved fluorescence experiments are used to study the dynamics of singlet fission in highly disordered films of rubrene. The fluorescence spectral lineshapes are not temperature-dependent, indicating that intermolecular excitonic effects are absent in these films. The temperature-dependent fluorescence decays in the amorphous films are nonexponential, containing both prompt and delayed components. The kinetics are qualitatively consistent with the presence of singlet fission, but to confirm its presence, we examine the effects of magnetic fields on the fluorescence decay. A quantum-kinetic model is developed to describe how magnetic fields perturb the number of triplet pair product states with singlet character and how this in turn affects the singlet state kinetics. Simulations show that the magnetic field effect is very sensitive to mutual chromophore alignment, and the direction of the effect is consistent with a local ordering for rubrene molecules that participate in fission. From our analysis, the dominant fission rate is 0.5 ns(-1), about 10 times slower than that observed in polycrystalline tetracene films, but we still estimate that similar to 90% of the initially excited singlets undergo fission. Kinetic modeling of our fluorescence decay data and magnetic field dependence reveals that at the low laser intensities used in this experiment geminate triplet pairs do not interact with each other, and that spin-lattice relaxation between triplet sublevels is not complete on the 100 ns time scale. When both exciton fission and fusion are occurring, dynamic measurements in the presence of a magnetic field can elucidate molecular-level details of both processes.