Benchmarking DFT functionals for excited-state calculations of Donor Acceptor TADF emitters: Insights on the key parameters determining Reverse Inter System Crossing

The importance of intermediate triplet states and the nature of excited states has gained interest in recent years for the thermally activated delayed fluorescence (TADF) mechanism. It is widely accepted that simple conversion between charge transfer (CT) triplet and singlet excited states is too crude, and a more complex route involving higher-lying locally excited triplet excited states has to be invoked to witness the magnitude of the rate of reverse intersystem crossing (RISC) rates. The increased complexity has challenged the reliability of computational methods to accurately predict the relative energy between excited states as well as their nature. Here we compare the results of widely used DFT functionals, CAM-B3LYP, LC-ꞷPBE, LC-ꞷ*PBE, LC-ꞷ*HPBE, B3LYP, PBE0 and M06-2X against a wavefunction-based reference method, Spin-Component Scaling second-order approximate Coupled-Cluster (SCS-CC2) in fourteen known TADF emitters possessing a diversity of chemical structures. Overall, the use of the Tamm-Dancoff Approximation (TDA) together with the CAM-B3LYP, M06-2X and the two ꞷ-tuned range-separated functionals LC-ꞷ*PBE and LC-ꞷ*HPBE demonstrated the best agreement with SCS-CC2 calculations in predicting the absolute energy of the singlet S1, and triplet T1 and T2 excited states and their energy differences. However, consistently across the series and irrespective of the functional or the use of TDA, the nature of T1 and T2 is not as accurately captured as compared to S1. The presented work highlights that despite good agreement of energies, the description of the exact nature of the triplet states should be undertaken with caution.