Triphenylene‐Involved π‐Extension Combining With Phenyl‐Blocking Enhances the Stability of MR‐TADF Emitter: Top‐Emitting OLED Realizes EQE Approaching 60% With BT.2020 Green Gamut and Long Lifetime

ABSTRACT The realization of ultrahigh definition displays necessitates pure‐green organic light‐emitting diodes (OLEDs) with simultaneously exceptional efficiency, high color purity, and long‐term operational stability. Herein, we develop a series of pure‐green multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters based on the boron/nitrogen‐embedded polycyclic aromatic hydrocarbon (BN‐PAH), designed through a rational moderate π‐extension and peripheral phenyl blocking strategy. The molecular designs afford narrowband pure‐green emission with a full‐width at half‐maximum (FWHM) of nearly 20 nm and high photoluminescence quantum yields ( Φ PL s, up to 95%). By systematically blocking the redox‐active positions with phenyl groups, the emitters exhibit significantly enhanced electrochemical and photochemical stability. In bottom‐emitting OLEDs, the optimized emitter BN‐Tpl‐Ph achieves a maximum external quantum efficiency (EQE max ) of 33.8% and long operational lifetime (LT80 = 4012 h at 1000 cd m −2 ). Notably, in a top‐emitting OLED configuration, BN‐Tpl‐Ph delivers a pure‐green emission with a Commission Internationale de l'Eclairage (CIE) y‐coordinate of 0.78, a high EQE max up to 59.2%, and an LT80 of 409 h at 5000 cd m −2 . This work reveals the effectiveness of molecular design strategies that combine moderate π‐extension with peripheral phenyl blocking for developing high‐performance pure‐green MR‐TADF emitters.