Spirobifluorene fusion in multiple resonance frameworks enables efficient pure‐red OLEDs with narrowband emission and suppressed efficiency roll‐off

Abstract Multiple resonance thermally activated delayed fluorescence (MR‐TADF) materials based on B/N‐fused heteroaromatic frameworks have demonstrated outstanding performance in the blue and green spectral regions; however, achieving efficient and spectrally pure red emission remains challenging, hindering further advances toward high‐performance ultrahigh‐definition OLED displays. Herein, we report a molecular design strategy in which a spirobifluorene fragment is fused into a double‐boron‐embedded polycyclic framework featuring para ‐B–π–B configuration, enabling simultaneous extension of π conjugation and effective suppression of emission broadening. The resulting emitter exhibits pure‐red emission with a narrow bandwidth, which is primarily attributed to the suppression of high‐frequency vibrational modes induced by the rigid spiro‐fused moieties. Time‐resolved photoluminescence studies reveal that incorporation of the spirobifluorene unit enhances the reverse intersystem crossing rate by approximately threefold relative to the parent emitter. Phosphor‐sensitized OLEDs based on this emitter deliver a maximum external quantum efficiency exceeding 40%, with an emission peak at 612 nm and a full width at half maximum of 36 nm. Notably, the devices exhibit exceptionally low efficiency roll‐off, retaining external quantum efficiencies of 35.2% and 31.4% at luminances of 1000 and 10 000 cd m −2 , respectively. These results demonstrate that spiro‐fusion is an effective strategy for balancing spectral purity and triplet exciton utilization in red MR‐TADF emitters.