Theory of Exciton Dynamics in Thermally Activated Delayed Fluorescence

Abstract

AbstractThermally activated delayed fluorescence (TADF) is an upconversion photophysical process based on the population of a bright excited singlet state from an excited triplet via reverse intersystem crossing (rISC), which might be employed to overcome the statistical limitation of electron‐hole recombination in organic light‐emitting diodes. In this work, we describe the intricacies of TADF dynamics by means of a quantum master equation and through perturbative approaches to rate constants of internal conversion and ISC. Simulation of the time evolution from the lowest excited triplet in systems with small energy gaps allows to disentangle the role of different electronic states mediating the population of the singlet manifold and the importance of various rISC mechanisms. Energy gaps between charge transfer (CT) and local excitons (LE), and interstate (hyperfine, spin‐orbit and vibronic) couplings are key factors tuning the feasibility of rISC. Since in molecules LE/CT spin‐orbit coupling is, in general, larger than hyperfine interactions, the rISC rate is largely increased in those situations where either the direct or mediated triplet‐singlet interconversion between states of different character is energetically available. Interestingly, temperature dependence of rISC rate is stronger for the spin‐vibronic coupling mechanism than via hyperfine interaction.