Peripherally Fused Spiro Structures Enable Narrowband TADF Emitters for Highly Efficient Blue OLEDs

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 design a strategic fusion of spiro groups at specific peripheral positions within the classic narrowband emitter DABNA core. This innovative approach, utilizing positional isomerism, led to 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, resulting in 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, employing a TADF assistant host, the SFX-2BN-based device are enhanced to 27.5%, while maintaining excellent color purity, with CIE coordinates (0.14, 0.06) that closely approach the BT.2020 blue standard. These results reveal key insights into the mechanism of emission band narrowing via spiro-group fusion at specific sites within the MR core.