It is a well-known issue that the application of time-dependent density functional theory (TD-DFT) on charge transfer excitations often leads to a severe underestimation if the non-local exchange energy is not properly included. Here we perform a careful first-principles investigation on a specific molecular system that is known to exhibit thermally activated delayed fluorescence (TADF) phenomenon, using three different functions, B3LYP, CAM-B3LYP, and M06-2X. We find that even in functionals with more adequately described or fitted long-range exchange part, the triplet excitation energy levels may still show a large variation, indicating that the performance of a specific functional cannot solely be based on the comparison with the absorption or emission spectra. An inclusion of the solvent effect adds to further complexity. Since the efficiency of TADF is directly related to the energy gaps between low-lying singlet and triplet excited states, the proposed underlying mechanism to illustrate photophysical processes would be intimately associated to the chosen functional and the treatment of the polar medium.
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