Heavy-atom effect promotes multi-resonance thermally activated delayed fluorescence.

[Display omitted] • Narrowband green emitters are successfully achieved by multi-resonance boron-nitrogen framework. • Heavy-atom effect of sulfur was beneficial to accelerate the reverse intersystem crossing. • Green organic light-emitting diodes with high external quantum efficiency up to 25.5% are achieved. As one type of latest emitters with simultaneous high efficiency and color-purity, the development of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials represents an important advancement for organic light-emitting diodes (OLEDs). We herein present a new strategy to improve the performance of MR-TADF emitters by fusing sulfur element into the B-N based framework, aiming to utilize the non-metal heavy-atom effect in accelerating the reverse intersystem crossing (RISC) process of the emitter. Two compounds, namely 2PTZBN and 2PXZBN, were developed in this work through rigidifying the DABNA-1 skeleton by sulfur or oxygen atoms. The theoretical calculations and photoluminescence studies reveal that the sulfur-incorporated 2PTZBN enable considerable rate constant of RISC (k RISC) up to 2.8 × 105 s−1 in toluene due to larger spin-orbital coupling (SOC) values and smaller singlet–triplet energy splitting (Δ E ST) compared with 2PXZBN. Consequently, organic light-emitting diodes based on 2PTZBN exhibited highly efficient green emission with maximum external quantum efficiency (EQE) of 25.5%. [ABSTRACT FROM AUTHOR]