Extending the π‐Skeleton of Multi‐Resonance Emitter toward Pure Green Electroluminescence With High Efficiency and Low Efficiency Roll‐Off

Abstract Organic light‐emitting diodes (OLEDs) based on multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters hold immense potential for ultrahigh‐definition displays due to their narrow emission and high efficiency. However, most OLED devices employing MR‐TADF emitters suffer from severe efficiency roll‐off. Herein, a fused donor‐acceptor molecular design strategy is reported to address this challenge. The designed MR‐TADF emitter, BNNO, integrates a CzBN core with a rigid benzo[5,6][1,4]oxazino[2,3,4‐kl]phenoxazine (NO) donor through a fused π‐conjugated framework. Single‐crystal analysis reveals a unique four‐fold helical conformation stabilized by intramolecular hydrogen bonds, which suppresses high‐frequency vibrational coupling in excited state. Theoretical calculations demonstrate that the introduction of the NO donor can enhance spin‐orbit coupling and provide additional high‐lying reverse intersystem crossing (RISC) channels. BNNO exhibits a narrowband green emission (525 nm, FWHM = 36 nm) with a photoluminescence quantum yield (PLQY) of 97.2% and a minimal singlet‐triplet energy gap (Δ E ST = 0.18 eV). OLEDs employing BNNO achieve state‐of‐the‐art external quantum efficiencies (EQE max ) of 34.9–36.2% at 1–5 wt% doping concentrations. Notably, the device with 1 wt% doping retains an EQE of 30.6% at 1000 cd m −2 , corresponding to an efficiency roll‐off of only 13.1%. This work provides a universal molecular design paradigm for high‐performance MR‐TADF emitters.