Methylation‐Engineered MR‐TADF Emitters for BT.2020‐Compliant Deep‐Blue OLEDs with High <i>k</i> _RISC and ACQ Resistance

Abstract Achieving deep‐blue emission with high efficiency and color purity remains a major challenge for next‐generation organic light‐emitting diodes (OLEDs), particularly those targeting the BT.2020 color standard. Herein, a methyl substitution‐induced molecular distortion strategy is proposed to construct deep‐blue multi‐resonance thermally activated delayed fluorescence emitters ( BN‐M2 and BN‐M3 ), that simultaneously enhance spin–orbit coupling and suppress π‐π stacking. Strategic methyl substitution induces significant distortion in the B/N core geometry (dihedral angle > 40°), boosting reverse intersystem crossing rates (up to 2.71 × 10 6 s −1 ) and mitigating aggregation‐caused quenching. The optimized emitters achieve narrowband deep‐blue emission (Commission Internationale de l'Éclairage y coordinate, CIE y = 0.045) and near‐ultraviolet emission (CIE y = 0.035), with a full width at half maximum of 22–24 nm and near‐unity photoluminescence quantum yields (≈100%). Furthermore, OLEDs show record‐high external quantum efficiency with minimal roll‐off: BN‐M3 achieves 34.8% for BT.2020 blue emission, while BN‐M2 reaches 21.4% in the near‐UV spectrum, setting a new benchmark. Notably, the device performance remains stable even at high doping concentrations (up to 15 wt%). This work provides a viable pathway toward realizing BT.2020‐compliant blue OLEDs with both outstanding optoelectronic performance and excellent industrial processability.