Engineering Nitrogen/Carbonyl MR-TADF Emitters: Spiro-Lock and Tert-Butyl Synergy in Narrowband Blue Emission

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with rigid planar structures are promising for their exceptional color purity and outstanding device efficiency. However, as an important MR unit, rigidly interlocked nitrogen/carbonyl-based blue materials often face challenges like spectral broadening, red-shifting, and reduced efficiency compared to nitrogen/boron system. Herein, a peripheral modification strategy incorporating tert-butyl groups via a spiro-lock framework is used to synthesize four molecules: QAO-TF, TQAO-TF, TQAO-F, and TSOQ. The spiro-lock structure solidifies the molecular framework, narrows the emission bandwidth, and elevates the photoluminescence quantum yield to over 96%. Meanwhile, the peripheral tert-butyl groups introduce steric hindrance, isolating the luminescent core and suppressing intermolecular interactions in the solid state, thereby improving device efficiency while maintaining narrowband emission. Notably, TQAO-F shows an electroluminescence peak at 476 nm with a 25 nm full width at half maximum (FWHM) and an external quantum efficiency (EQE) of 31.7%. TSOQ, with its oxygen-induced charge effect, achieves narrowband pure blue emission with an FWHM of 20 nm, surpassing 30% EQE without sensitizers. This overall performance suggests its potential to rival the classic nitrogen/boron system.