Simulating thermally activated delayed fluorescence exciton dynamics from first principles†

Abstract

Delayed fluorescence pathways are a proven method to achieve significant efficiency gains in a myriad of technologies such as light-emitting diodes, multi-resonance effects leading to superirradiance or hyperafterglow/hyperfluorescence, and molecular logic. Scalability and the lack of low-cost materials hinder the search for optimised materials due to both time and financial constraints. A theoretical toolkit which could predict the properties of unknown materials could overcome this limitation. In this proof-of-concept work, we highlight a robust methodology which can predict the properties of an albeit unknown material with a high degree of efficacy with respect to experimental measurements. We first model the photophysical exciton dynamics of bay-site oxygen-fused quinolino[3,2,1-de]acridine-5,9-dione (OQAO) in the monomer-phase using density functional theory as a case study; an existing pathway of thermally activated delayed fluorescence (TADF) remains highly inefficient; an exciton has a 0.18% probability of undergoing a cycle of TADF. A reevaluation using a simplified dimer, where OQAO is paired with a resonant-emitter perylene, highlights that charge-transfer and multi-exciton phenomena are nearly non-existent. Paired homodimers were found to increase the efficiency by more than 70-fold. The kinetics for both monomer and dimer systems were then exported to an in-house Monte Carlo sampling codebase; while the monomer displayed minimal delayed fluorescence, the dimer was vital in capturing it. Evidence also suggested that exciton hopping plays an important role in the TADF process. This first-of-its-kind comprehensive study serves as a stepping stone highlighting that robust modelling of TADF systems is achievable.