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

Abstract

Accurate prediction of singlet–triplet splittings (ΔE_STs) 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 ΔE_STs 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 ΔE_STs 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 ΔE_STs 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 ΔE_STs of Cz-based TADF emitters.