Long-Range Exciton Diffusion via Singlet Revival Mechanism.

We clarify the mechanism that leads to extended exciton diffusion length in organic materials which exhibit a strong anisotropy of electronic coupling. We analyze the cooperative effects of singlet fission and triplet-triplet annihilation in the exciton diffusion by means of the dynamic Monte Carlo simulations. As a model system, we consider the rubrene crystal which exhibits a long-range exciton diffusion. The deexcitation of the singlet exciton is suppressed by singlet → triplet conversion via singlet fission. Even though the triplet exciton would hardly diffuse along the c-axis in the rubrene crystal (perpendicular to the high mobility plane) because of the small electronic coupling, the regeneration of the singlet exciton via triplet-triplet annihilation enables long-range exciton diffusion along the c-axis. This singlet revival mechanism can extend the overall lifetime and the diffusion length of the exciton, through back-and-forth transitions between an isotropically diffusing singlet exciton and a long-lived triplet exciton.