Comparative study of TDDFT and TDDFT-based STEOM-DLPNO-CCSD calculations for predicting the excited-state properties of MR-TADF.

The time dependent density functional theory (TDDFT) and TDDFT/similarity transformed EOM domain-based local pair natural orbital CCSD (STEOM-DLPNO-CCSD) calculations were explored to estimate their validity in predicting the excited-state properties of multi-resonant thermally activated delayed fluorescence (MR-TADF) materials. Obviously, it was demonstrated that TDDFT calculation is inadequate to provide the quantitative prediction of the lowest singlet excited-state (S), the lowest triplet excited-state (T), and ΔE. On the other hand, TDDFT/STEOM-DNLPNO-CCSD calculation reveals the superior prediction of S, T, and ΔE that are in quantitative agreement with experiments. More importantly, it was found that TD-LC-HPBE/STEOM-DLPNO-CCSD calculation provides the most accurate prediction of S, T, and ΔE. Accordingly, we suggest that TD-LC-HPBE/STEOM-DLPNO-CCSD calculation should be utilized to compute the excited-states properties of MR-TADF materials accurately.