A TADF sensitizer moiety decorated MR emitter for enhancing spin-flip and anti-ACQ

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters, mainly constructed from rigid fused polycyclic aromatic hydrocarbon frameworks incorporating boron and nitrogen (B/N), have garnered significant attention attributed to their remarkable optoelectronic properties, such as high efficiency, narrowband emission, and so on. However, the relatively large singlet-triplet energy gap (ΔE _ST), small spin-orbit coupling (SOC) matrix elements and planar rigid framework inherent to MR-TADF materials lead to slow reverse intersystem crossing rates (k _RISC) and aggregation-caused quenching (ACQ), limiting their practical application in organic light-emitting diodes (OLEDs). This study presents an effective molecular design strategy that integrates a bulky thermally activated delayed fluorescence (TADF) moiety (also as a sensitizer) into a multiple-resonance (MR) framework. This hybrid architecture enables the resulting emitter 4TCzBNCN to exhibit bright green emission with a narrow full width at half maximum (FWHM) of approximately 31 nm, alongside a significantly enhanced k _RISC of 4.2 × 10⁶ s^-1 and near-unity photoluminescence quantum yield. Therefore, the solution-processable OLEDs achieve a maximum external quantum efficiency (EQE_max) exceeding 26.8% with an almost unchanged FWHM even at high doping concentration. The results demonstrate that TADF sensitizer decorated MR emitters simultaneously accelerate k _RISC and suppress the ACQ effect, thereby addressing a persistent challenge in conventional MR emitters.