Peripheral Modification Strategy of Heavy Atom for High‐Performance Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters

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 (kRISC) of 6.02 × 104 s−1, which is three‐folded higher than BNDBF (2.09 × 104 s−1) without heavy atom. The corresponding green OLED based on BNDBT exhibits an improved external quantum efficiency of 35.5% and lower efficiency roll‐offs at high brightnesses of 100 cd m−2 and 1000 cd m−2, respectively. In addition, the BNDBT‐based OLED maintains high color purity without causing a sharp increase in FWHM as compared with that of BNDBF. This work indicates that introducing the heavy atom at the bottom of the B/N skeleton is an effective strategy to enhance kRISC while maintaining narrow FWHM, thereby achieving high‐performance MR‐TADF emitters.