Deep‐Blue Narrowband OLEDs Achieve External Quantum Efficiency Over 40% and Blue Index of 422 by Synergistic π‐Extension and Heavy‐Atom Effect

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters hold great promise for high‐resolution OLEDs, yet achieving both ultranarrow emission and efficient triplet utilization in the deep‐blue region remains challenging. Here, a synergistic molecular design is reported that combines π‐extension and heavy‐atom incorporation to effectively reconcile the trade‐off between color purity and fast reverse intersystem crossing (RISC). In this approach, π‐extension narrows the emission bandwidth and reduces the singlet–triplet energy gap, while the strategic introduction of chalcogen atoms selectively enhances spin–orbit coupling with minimal impact on the emission spectrum. As a result, the new emitter exhibits a peak emission at 453 nm with an exceptionally narrow full width at half maximum (FWHM) of 17 nm and a high RISC rate constant of 3.0 × 10 6 s −1 . When incorporated into a non‐sensitized OLED, the emitter meets the European Broadcast Union (EBU) deep‐blue standard with CIE coordinates as low as (0.140, 0.059), and sustains a brightness exceeding 30,000 cd m −2 . Notably, the device achieves a record‐high external quantum efficiency (EQE max ) of 40.5% with minimal roll‐off—retaining 38.4% and 28.2% at 100 and 1,000 cd m −2 , respectively—and attains a Blue Index (BI) of 422 cd A −1 CIE y −1 . These findings highlight the effectiveness of our tactic in overcoming prior limitations where heavy‐atom doping often compromises color purity, paving the way for next‐generation emitters in advanced display and lighting applications.