Stepwise Molecular Engineering Toward High‐Performance Deep‐Blue Narrowband OLEDs: Rigidity as the Foundation, Symmetry as the Key

Developing high‐color‐purity deep‐blue emitters that meet the BT.2020 standard remains a critical challenge in organic light‐emitting diodes (OLEDs). Herein, a stepwise molecular engineering strategy for simultaneously accelerating reverse intersystem crossing rate ( k RISC ) and narrowing full‐width at half maxima (FWHM) is proposed for deep‐blue materials. By sequentially enhancing molecular rigidity, strengthening resonant strength, and completing molecular symmetry, a deep‐blue emitter (DBNDICz), constructed on modified diboron multiple‐resonance scaffold, is developed with peak of 452 nm, ultra‐narrow FWHM of 15 nm (0.08 eV) and k RISC of 3.0 × 10 5 s − 1 . The dominant υ 0‐0 transition character of DBNDICz produces a Commission Internationale de I’Éclairage (CIE) coordinates of (0.144, 0.060). Moreover, the expanded conjugated skeleton facilitates horizontal dipole orientation of 93%, leading to a high maximum external quantum efficiency (EQE max ) of 24.2% in bottom‐emitting OLEDs. Impressively, top‐emitting OLED achieves a record‐setting blue index of 514 cd A − 1 CIEy − 1 with CIE (0.146, 0.036) and EQE max of 45.2%, establishing a benchmark for state‐of‐the‐art deep‐blue devices. Additionally, an operational lifetime (LT 50 ) of 154.2 h is achieved in an anthracene‐based host. These results represent one of the best performances reported for deep‐blue OLEDs with CIEy≤ 0.06, providing a robust design paradigm for high‐performance narrowband deep‐blue TADF emitters.