High-performance deep-blue electroluminescence from multi-resonance TADF emitters with a spirofluorene-fused double boron framework

The development of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials in the deep-blue region is highly desirable. A usual approach involves constructing an extended MR-TADF framework; however, it may also intensify aggregate-caused quenching issues and thereby reduce device efficiency. In this study, we develop a molecular design strategy that fuses the MR-TADF skeleton with 9,9 '-spirobifluorene (SF) units to create advanced deep-blue emitters. The SF moiety facilitates high-yield one-shot bora-Friedel-Crafts reaction towards an extended skeleton and mitigates interchromophore interactions as a steric group. Our findings reveal that orbital interactions at the fusion site significantly influence the electronic structure, and optimizing the fusion mode allows for the development of emitters with extended conjugation length while maintaining non-bonding character. The proof-of-concept emitter exhibits narrowband emission in the deep-blue region, a near-unity photoluminescence quantum yield, and a fast kRISC of 2.4 x 105 s-1. These exceptional properties enable the corresponding sensitizer-free OLED to achieve a maximum external quantum efficiency (EQEmax) of 39.0% and Commission Internationale de l'Eclairage (CIE) coordinates of (0.13, 0.09). Furthermore, the hyperfluorescence device realizes an EQEmax of 40.4% with very low efficiency roll-off. Simultaneously extending the pi-skeleton and mitigating interchromophore quenching of MR-TADF emitters resulted in narrowband deep-blue electroluminescence with EQE surpassing 40%.