Efficient Hyperfluorescence OLEDs With Ultrahigh Brightness and Ultralow Efficiency Roll‐Offs Based on New Red Multi‐Resonance TADF Emitters

Abstract Boron‐nitrogen (B‐N) based multi‐resonance (MR) molecules are promising luminescent materials for organic light‐emitting diodes (OLEDs) with high color purity. Recent studies predominantly focus on the strategic modulation of N‐containing fragments to adjust their photophysical properties but often cause undesired spectral broadening while achieving redder emissions. Herein, a simple and effective strategy to design red MR materials is proposed by precisely modulating the electron‐donating strength of O‐containing fragments in B‐N based MR molecular skeleton. The generated molecules show strong red emissions with photoluminescence (PL) peaks at 604–625 nm, narrow full width at half maximum (FWHM) of 30–31 nm (0.099–0.102 eV), and high PL quantum yields ( Φ PL s) of up to 96% in toluene solutions. The hyperfluorescence OLEDs of these MR molecules with phosphorescence sensitizers realize narrow electroluminescence spectra, and provide maximum external quantum efficiency of 32.4%, ultrahigh brightness of 210,500 cd m −2 and ultralow efficiency roll‐offs of only 2.5% and 12.3% at brightness of 1,000 and 10,000 cd m −2 , respectively. This work offers instructive guidance for the design of high‐performance red MR emitters with B‐N molecular skeletons as well as for the fabrication of efficient hyperfluorescence OLEDs with phosphorescence sensitizers.