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

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 kRISC values exceeding 5.0 × 105 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.