B−N/B−O Contained Heterocycles as Fusion Locker in Multi‐Resonance Frameworks towards Highly‐efficient and Stable Ultra‐Narrowband Emission.

Fusing condensed aromatics into multi‐resonance (MR) frameworks has been an exquisite strategy to modulate the optoelectronic properties, which, however, always sacrifices the small full width at half maxima (FWHM). Herein, we strategically embed B−N/B−O contained heterocycles as fusion locker into classical MR prototypes, which could enlarge the π‐extension and alleviate the steric repulsion for an enhanced planar skeleton to suppress the high‐frequency stretching/ scissoring vibrations for ultra‐narrowband emissions. Sky‐blue emitters with extremely small FWHMs of 17–18 nm are thereafter obtained for the targeted emitters, decreased by (1.4–1.9)‐fold compared with the prototypes. Benefiting from their high photoluminescence quantum yields of >90 % and fast radiative decay rates of >108 s−1, one of those emitters shows a high maximum external quantum efficiency of 31.9 % in sensitized devices, which remains 25.8 % at a practical luminance of 1,000 cd m−2 with a small FWHM of merely 19 nm. Notably a long operation half‐lifetime of 1,278 h is also recorded for the same device, representing one of the longest lifetimes among sky‐blue devices based on MR emitters. [ABSTRACT FROM AUTHOR]