Vicinal-Diboron Reinforced [9]Helicene Enables Narrowband Blue Circularly Polarized Emitter With Large Dissymmetry Factor and High Device Efficiency.

Circularly polarized multiple-resonance thermally activated delayed fluorescence (CP-MR-TADF) emitters that simultaneously combine narrowband emission, high efficiency, and pronounced chiroptical activity remain rare because amplification of helicity often compromises the localized electronic structure required for spectral narrowing. Here we report a tetraborated B,N-embedded [9]helicene, B4ONA, constructed through ortho-diboron reinforcement of a multiple-resonance framework. Structural and theoretical analyses reveal that the ortho-diboron configuration strengthens helical rigidity and excited-state confinement, thereby suppressing structural relaxation and vibronic coupling. B4ONA exhibits pure-blue emission at 462 nm with a full width at half-maximum (FWHM) of 20 nm and pronounced chiroptical activity, reaching a high photoluminescence dissymmetry factor of 5.03 × 10-3 in the emissive film. Organic light-emitting diodes (OLEDs) based on B4ONA deliver pure-blue electroluminescence at 464 nm with a FWHM of 21 nm and external quantum efficiencies of up to 34.4%, ranking among the best performances reported to date for blue CP-MR-TADF OLEDs. These findings establish ortho-diboron reinforcement of high-order B,N-embedded helicenes as an effective molecular design strategy for CP-MR-TADF emitters.