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 × 10⁶ 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.