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

Developing high-resolution organic light-emitting diodes (OLEDs) faces significant challenges, particularly in the blue light region. Multiple resonance thermally activated delayed fluorescence (MR-TADF) has been recognized as a breakthrough in next-generation TADF and high-quality displays due to its narrow bandwidth, exceptional color purity, and high efficiency. Extensive efforts have been devoted to introducing auxiliary groups into the BBCz-SB scaffold to improve OLED efficiency; however, such modifications often lead to undesirable emission shifts from sky-blue to green. Herein, we present a strategic azepine engineering approach that enables deeper blue emission with high efficiency and stability. The azepine-containing compounds tCzAzBN, tCzPAzBN, and tCzPAzBN-d10 are synthesized via mild one-shot borylation in superior yields. Introducing a twisted azepine at the para 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.