Dual-Boron Multi-Resonance Thermally Activated Delayed Fluorescence Emitters Based on Spirofluorene-Rigidified...

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters are a research focus for organic light-emitting diodes (OLEDs) targeting ultra-high-definition displays. Expanding the multi-resonance plane through multi-boron structures enables emission color tuning and spectral narrowing, which rigid indolocarbazole moieties could further reinforce. However, excessive expansion of conjugated aromatic planes leads to severe aggregation-caused quenching (ACQ). Herein, we combined the spirofluorene-based multi-resonance skeleton with indolocarbazoles as the bridging unit to construct a double-boron architecture, aiming to utilize the three-dimensional spatial rigidity of spirofluorene while suppressing intermolecular interactions in aggregated states. Three isomeric emitters were thereby obtained, with emission maxima at 524-565 nm. All emitters exhibit full widths at half-maximum below 20 nm, with the narrowest being 17 nm, and these emitters maintain narrow-band emission even at a high doping concentration of 6 wt%, where film spectral broadening is limited to within 3 nm. OLEDs based on these emitters display narrow bandwidths of 22-26 nm, the highest maximum EQE of 36.2%, and low efficiency roll-off, corresponding to a high EQE of up to 30.2% even under ultra-high luminance of 5 × 10⁴ cd m^-2. This work confirms the feasibility of spirofluorene-modified multi-boron large-plane multi-resonance emitters and provides a reliable molecular design strategy for narrowband OLEDs.