Toward an Accurate Description of Thermally Activated Delayed Fluorescence: Equal Importance of Electronic and Geometric Factors

Obtaining reasonable geometric and electronic structures of excited states is essential for accurately predicting the thermally activated delayed fluorescence (TADF) for the application in organic light-emitting diodes (OLEDs). Both electronic and geometric factors are evaluated using density functionals for reproducing the vertical emission (EVE(S1)) and singlet–triplet splitting energies (ΔEST) of 28 typical TADF molecules. It is found that most TADF molecules (charge-transfer type) can easily twist upon excitation, indicating the importance of constructing a rigid molecular structure for improving the performance of TADF OLEDs. Functionals with insufficient exact exchange will result in the substantial underestimation of the relaxation energy of T1, suggesting that the hybrid functionals such as B3LYP should not be used. Overall, the best approach for calculating EVE(S1) and ΔEST is the descriptor-tuned LC-ωPBELOL functional combining the CAM-B3LYP-optimized excited-state geometries, which shows mean absolute deviations of 0.21 and 0.10 eV, respectively