Theoretical investigation of the singlet-triplet splittings for carbazole-based thermally activated delayed fluorescence emitters.

Accurate prediction of singlet–triplet splittings (ΔESTs) is a key issue for the design of thermally activated delayed fluorescence (TADF) emitters. Due to the evident changes between ground- and excited-state electronic structures, the ΔESTs of carbazole (Cz)-based TADF emitters can't be accurately predicted based on the current optimal Hartree–Fock percentage (HF%) (OHF) method. To address this issue, here, we used the adiabatic excitation energy method to accurately predict the ΔESTs of the TADF emitters with different geometries of ground- and excited-states by calculating the minimum potential energy differences between the ground- and excited-states considering the relaxation effect. With the optimized excited states using the B3LYP functional, the theoretically calculated values of ΔESTs based on our method are well consistent with the experimental results for the Cz-based TADF emitters, obviously improved compared with the calculated results based on the OHF method. These results indicate that the adiabatic excitation energy method with the B3LYP functional is a general and accurate way to predict the ΔESTs of Cz-based TADF emitters.