The Importance of High-Frequency Modes in the Prediction of RISC Rates for TADF Molecules

The reverse intersystem crossing (RISC) rate determines the efficiency of dyes displaying thermally activated delayed fluorescence (TADF). Such a rate can be predicted at the full quantum level by considering all the vibrational normal modes or adopting an approximated methodology which relies on single classical modes. We evaluated the importance of considering all of the vibrational degrees of freedom in computations for targeting the design of novel emissive materials from first principles. We computed the RISC rate for 17 molecules of interest for TADF by comparing a full quantum mechanics treatment based on Fermi's golden rule with Marcus-based semiclassical approaches. The results are quantitatively and sometimes qualitatively different in the two approaches, especially when the reorganization energy is small, a common occurrence for molecules exhibiting TADF. The importance of high-frequency modes varies across the set of molecules considered, suggesting that their evaluation should become part of the molecular design process.