Peripheral Modification Strategy of Heavy Atom for High-Performance Multi-Resonance Thermally Activated Delayed...

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_RISC) 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 k_RISC while maintaining narrow FWHM, thereby achieving high-performance MR-TADF emitters.