Organic light-emitting materials with multi-resonance thermally activated delayed fluorescence (MR-TADF) have shown great potential for realizing highly efficient narrowband organic light-emitting diodes (OLEDs). However, the heavy efficiency roll-off caused by the slow reverse intersystem crossing (RISC) process remains a challenging issue for the further practical application of MR-TADF materials. Here, we develop two TADF emitters, BNDBT and BNDBF, in which the dibenzothiophene and dibenzofuran substituents are attached at the bottom of the B/N frameworks. They all exhibit the similar high photoluminescence quantum yields of 90 % and 87 %. The sulfur-containing material BNDBT exhibits a RISC rate (k