Carbonyl‐Locking Strategy for Multi‐Resonance Emitters Enables Efficient Pure‐Green OLEDs With Ultralow Efficiency Roll‐Off

ABSTRACT Developing multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters that simultaneously achieve bathochromic tuning, narrowband emission, and rapid reverse intersystem crossing (RISC) remains a great challenge for high‐performance organic light‐emitting diodes (OLEDs). Herein, carbonyl units are fused into a multi‐boron v ‐DABNA framework to construct B/N/C═O hybrid MR‐TADF emitters. Within this framework, the electron‐withdrawing carbonyl units stabilize the lowest unoccupied molecular orbital and narrow the bandgap, enabling a bathochromic shift while preserving the alternating frontier‐orbital distribution required for narrowband emission. Meanwhile, the carbonyl‐associated partial n–π* contribution to the excited states enhances spin–orbit coupling and accelerates RISC. The resultant emitters, CO‐ v ‐DABNAMe and CO‐ v ‐DABNAPh, exhibit green emission peaking at 503 and 508 nm with narrow full widths at half‐maximum of only 21 nm, together with high k RISC values exceeding 5.0 × 10 5 s −1 . Binary OLEDs based on CO‐ v ‐DABNAMe and CO‐ v ‐DABNAPh deliver pure‐green electroluminescence with Commission Internationale de l’Éclairage coordinates of (0.18, 0.72) and (0.19, 0.73), accompanied by maximum external quantum efficiencies of 35.3% and 36.2%, respectively. Notably, ultralow efficiency roll‐offs of only 3.4% and 3.9% are achieved at 1000 cd m −2 , representing well‐balanced overall performance among binary pure‐green MR‐TADF OLEDs reported to date.