Strategic azepine engineering realizes highly efficient and stable blue narrowband light-emitting diodes

position of the MR core induces a blue-shifted emission and a narrow singlet-triplet energy gap. Notably, tCzPAzBN with phenyl-substituted azepine exhibits a higher horizontal ratio, boosting the OLED efficiency to an external quantum efficiency (EQE) of 35.2%. Furthermore, deuterium substitution in tCzPAzBN-d10 markedly enhances the operational lifetime of the OLED. Incorporating a newly developed TADF assistant dopant enables the corresponding hyperfluorescence device to achieve an EQE exceeding 40%, thereby setting a new benchmark among single-boron-based blue OLEDs. This azepine-centered molecular design provides a robust platform for developing MR-TADF materials that meet the demands of next-generation OLEDs in terms of color definition, high efficiency, and long-term operational stability.