Theoretical studies on boron dimesityl-based thermally activated delayed fluorescence organic emitters: excited-state properties and mechanisms

Abstract

Herein, we investigated the luminescence mechanism of thermally activated delayed fluorescence (TADF) of blue-emitting methoxy-carbazole (Cz) substituted and boron dimesityl (BM)-based compounds (CzBM-1, -2, and -3) in cyclohexane using density functional theory (DFT), time-dependent DFT (TD-DFT) and DFT/multireference configuration interaction (DFT/MRCI) methods. On the basis of the optimized minimum-energy structures in the S₀, S₁, and T₁ states, the results show that the lowest energy absorption and emission peaks originate from electronic transitions between the S₀ and S₁ states. These transitions are of charge-transfer characteristics because the HOMO and LUMO are located on the Cz and BM groups, respectively. The well spatial separation of the HOMO and LUMO contributes to a small energy difference (ΔE_ST) between S₁ and T₁, less than 0.13 eV, which promotes the reverse intersystem crossing (rISC) process from T₁ to S₁. At 300 K, the fluorescence emission and ISC rates are comparable (10⁶–10⁷ s⁻¹), indicating that the excitons can deactivate by either emitting fluorescence or hopping to T₁ through ISC. In contrast, the phosphorescence rate from T₁ is negligible compared with the rISC rate (ca. 10¹ s⁻¹vs. 10⁵ s⁻¹), which indicates that the phosphorescence emission channel is blocked, and only the up-conversion TADF emission is recorded. Importantly, k_rISC decreased sharply below 1.0 s⁻¹ at 77 K, which indicates that the rISC process is sensitive to temperature and will be suppressed at low temperatures.