Dual‐Boron Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters Based on Spirofluorene‐Rigidified Indolocarbazole for Ultra‐Narrowband Luminescence

ABSTRACT 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 × 10 4 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.