Diffusion-Mediated Delayed Fluorescence by Singlet Fission and Geminate Fusion of Correlated Triplets

Abstract

Singlet fission (SF) has been extensively studied through the use of magnetic field effects and analyzed by the Johnson–Merrifield model or its extended models. We extended the Johnson–Merrifield model to study delayed fluorescence originating from diffusion-mediated reversible geminate fusion of triplets; the emission intensity was shown to decay by a A/tβ law, where β = 3/2 in one and three dimensions and β = 1 in two dimensions. Analytical expressions for the amplitude as well as the exponent of the asymptotic power law decay were obtained. We have shown that a wide range of SF materials from polyene-based molecular crystals to amorphous rubrene exhibit A/t3/2 asymptotic decay. The temperature dependence of the asymptotic emission decay supports a mechanism involving delayed fluorescence mediated by diffusion. Moreover, we show that the decay lines at different field strengths will not cross each other when fission occurs effectively. In the opposite limit when fusion occurs effectively the two decay lines will cross each other.