In further calculations, the energy-level arrangements of BTD-FL, BTD-FLBr, and BTD-HeBr are studied (Fig. and Supplementary Fig. ). The excited state levels of the three HLCT emitters exhibit a tiny energy difference (0.02-0.03 eV) between S1 and T2. A small energy difference (0.06-0.11 eV) between S1 and T3 was also found, which implies that the spin-flip routes from higher triplet (T2,3) to singlet (S1) states are feasible. Besides, a low energy level of T1 (1.70 eV) and a large energy gap (0.95-1.00 eV) between T1 and T2 are observed, which can effectively suppress interconversion (IC) transition between them. The energy levels of the molecules can be identified from their prompt and delayed PL spectra at low temperature (Supplementary Fig. ). Moreover, large SOC matrix elements are very important for improving the rate of RISC processes (Fig. and Supplementary Equation ). In the monomer state, the constants of SOC (T2, S1) of three HLCT emitters are small (0.028-0.2 cm -1 ) for all molecules, implying a low possibility for the occurrence of the RISC process (Supplementary Fig. ). However, significantly higher values (0.53-0.74 cm -1 ) are observed for SOC (T3, S1), which triggers an efficient spin-flip occurring. Consequently, the kRISC values are calculated to be 3.61-44.9×10 6 s -1 , which are comparable to those of efficient TADF emitters .