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

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 × 10⁶ 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.