Steric encapsulation of multi-resonance TADF emitters enabling narrowband deep-blue OLEDs with high efficiency and high brightness at elevated doping levels

Deep-blue multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with high efficiency, high color purity, and high brightness are critically important for next-generation OLED displays, yet remain challenging due to severe aggregation and host-guest interactions in the solid state. Herein, we report a core-encapsulating molecular design strategy in which bulky and non-conjugated peripheral groups are introduced to sterically encapsulate a blue-emitting MR core, thereby suppressing intermolecular π-π interactions without perturbing its intrinsic electronic structure. Two new emitters, DNa-BN and QNa-BN, featuring half-encapsulated and fully encapsulated MR-core architectures, respectively, were developed. Owing to its fully encapsulated structure, QNa-BN exhibits pronounced aggregation resistance at high doping concentrations, maintaining photoluminescence quantum yields exceeding 96%, radiative decay rate constants on the order of 108 s−1, and fast reverse intersystem crossing rates (~105 s−1). Consequently, sensitizer-free OLEDs based on QNa-BN deliver narrowband deep-blue emission at 458 nm with a full width at half maximum of 22 nm, CIE coordinates of (0.142, 0.085), a maximum external quantum efficiency (EQEmax) of 34.4%, and a maximum luminance exceeding 20,000 cd m−2. Furthermore, by adopting a hyperfluorescence architecture, the EQEmax is further boosted to 38.7% with significantly suppressed efficiency roll-off. This work demonstrates that steric encapsulation of the MR core provides an effective and general approach to achieving aggregation-resistant, high-efficiency, and high-brightness deep-blue MR-TADF emitters for OLED applications.