Exciton-Phonon Interaction Model for Singlet Fission in Prototypical Molecular Crystals

In singlet fission (SF), a spin-conserving splitting of one singlet exciton into two triplet excitation states, the transition between localized electronic states can be controlled and modulated by delocalized lattice phonons. In this work, we built an exciton-phonon (ex-ph) interaction model accounting local electronic states coupled with both local molecular vibrations and low frequency intermolecular phonon modes for SF in crystalline tetracene and rubrene. On the basis of the calculated electronic couplings at the equilibrium structure of the molecular dimer, a superexchange path for SF was found for tetracene while couplings between the triplet pair (TT) state and other diabatic states are zero for rubrene due to the high symmetry. Our further ex-ph spectral density analysis and quantum dynamics simulation based on our ex-ph interaction model suggested a thermal-activated mechanism for SF in rubrene crystal via symmetry breaking by nuclear vibration, which is in agreement with recent experiments. It is also shown that thermal fluctuations of electronic couplings in both tetracene and rubrene are mostly in the same order of magnitude at room temperature, and this could be one of the reasons for both tetracene and rubrene to exhibit SF time scales within a close range (hundreds to thousands of femtoseconds) in experiments.