Heavy-Atom Effect Promotes Multi-Resonance Thermally Activated Delayed Fluorescence

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 revealed that the sulfur-incorporated 2PTZBN enabled 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 (ΔE_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%.