Molecular Design Opening Multiple Charge Transfer Pathways in Oxygen-Bridged Boron Emitters for High-Efficiency Pure...

In this study, we designed and synthesized two novel oxygen-bridged boron-based thermally activated delayed fluorescence (TADF) emitters, 11-(9H-carbazol-9-yl)-16-phenyl-12-(3-(triphenylsilyl)phenyl)-16H-5,9-dioxa-16-aza-13b-boraindeno[1,2-a]naphtho[1,2,3-fg]anthracene (BOID-Cz-Si) and 9-(12-(3-(triphenylsilyl)phenyl)-5,9,16-trioxa-13b-boraindeno[1,2-a]naphtho[1,2,3-fg]anthracen-11-yl)-9H-carbazole (BOBF-Cz-Si). These compounds produce fluorescent emission through short-range and long-range charge transfer (CT) within the polycyclic aromatic hydrocarbon framework along with additional long-range CT between the donor and acceptor units of the emitter. Indole and benzofuran units were incorporated into the oxygen-bridged boron core as electron-donating moieties to modulate the long-range CT and enhance the multiple resonance (MR)-TADF properties. Furthermore, a bulky tetraphenylsilane group and an auxiliary carbazole unit were incorporated to suppress intermolecular interactions and enhance emission through long-range CT. Both emitters exhibited pure violet emission (peaks near 400 nm) with narrow full width at half-maximum values (approximately 25 nm) in the solution state and high photoluminescence quantum yields (up to 95%). Notably, the BOID-Cz-Si device showed a violet emission peak at 420 nm and a maximum external quantum efficiency of 22.7%, which is one of the highest efficiency values reported in the violet region. These results indicate the promising potential of hybridized oxygen-bridged boron scaffolds, which combine short- and long-range CT within the MR framework with rational donor engineering, as high-efficiency, high-color-purity MR-TADF emitters.