Abstract
To address the spectral broadening and red shift inherent in multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, which are caused by aggregation due to their planar structures, we propose a strategic fusion of spiro groups at specific peripheral positions within the classic narrowband emitter DABNA core. Leveraging positional isomerism, this innovative approach enables the development of two blue TADF emitters: SFX-2BN (O-π-B) and SFX-3BN (O-π-N). The relative positioning of the electron-withdrawing boron (B) and electron-donating nitrogen (N) atoms around the oxygen enables precise emission tuning from sky-blue to pure blue. The rigid spiro structure acts as an intramolecular lock, effectively suppressing detrimental vibrations and π-π stacking, thereby yielding exceptionally narrowband emission (FWHM = 20-22 nm) for both isomers. They exhibit high photoluminescence quantum yields (PLQYs >90%) in thin films. Corresponding OLEDs achieve maximum external quantum efficiencies (EQEs) of 24.8% and 33.4%, respectively. Furthermore, by employing a TADF assistant host, the OLED based on SFX-2BN is improved to 27.5%, while maintaining excellent color purity, with CIE coordinates (0.14, 0.06), which closely approach the BT.2020 blue standard. These results provide key mechanistic insights into the narrowing of emission bands through spiro-group fusion at specific positions within the MR core.