Asymmetric π‐Extended B/N MR‐TADF Emitters Enabling Deep‐Blue OLEDs with Suppressed Efficiency Roll‐Off, Anti‐Quenching and High Horizontal Dipole Orientation

Narrowband deep‐blue multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are crucial for high‐color‐gamut organic light‐emitting diodes (OLEDs), whereas their practical applications are severely restricted by efficiency roll‐off and aggregation‐caused quenching (ACQ). Herein, a twisted‐planar‐twisted (TPT) MR‐TADF emitter named A‐BFCz is synthesized via an asymmetric π‐extension molecular‐design strategy. The asymmetric π‐extended conjugation can retain the locally excited (LE) character of the singlet state and accelerate reverse intersystem crossing (RISC) kinetics via potentially accessible high‐lying RISC pathways, resulting in narrowband deep‐blue emission with a full width at half‐maximum (FWHM) of 17 nm, a fast RISC rate constant approaching 10 6 s −1 , and a near‐unity photoluminescence quantum yield (PLQY). Meanwhile, the dual twisted peripheral substituents on the TPT backbone are beneficial to solid‐state packing behavior by spatially separating adjacent molecules and thus suppressing undesired intermolecular π–π stacking interactions. In addition, the centrally located B–N‐fused rigid scaffold maintains a well‐preserved planar configuration, leading to a horizontal dipole ratio of 99%. Consequently, the non‐sensitized deep‐blue OLEDs based on A‐BFCz achieve a peak external quantum efficiency (EQE) of 42.8%. This work paves the way for the design and optimization of high‐efficiency narrowband deep‐blue MR‐TADF luminescent materials.