This study introduces a B–N covalent bond-involved π-extension strategy to enhance the performance of multi-boron-embedded multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters. By post-functionalizing MR frameworks, the researchers generate high-order B/N-based motifs, which not only increase molecular rigidity to narrow emission linewidth but also promote reverse intersystem crossing (RISC) to mitigate efficiency roll-off. The developed ultra-narrowband sky-blue emitters (with a full-width at half-maximum as small as 8 nm in n-hexane) exhibit significant improvements in photophysical properties compared to their precursors, enabling narrowband OLEDs with external quantum efficiencies (EQE_max) of up to 42.6%, along with reduced efficiency decline at high brightness. This work highlights the effectiveness of the molecular design strategy for advanced MR-TADF emitters and their potential in high-performance optoelectronic devices.This study introduces a B–N covalent bond-involved π-extension strategy to enhance the performance of multi-boron-embedded multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters. By post-functionalizing MR frameworks, the researchers generate high-order B/N-based motifs, which not only increase molecular rigidity to narrow emission linewidth but also promote reverse intersystem crossing (RISC) to mitigate efficiency roll-off. The developed ultra-narrowband sky-blue emitters (with a full-width at half-maximum as small as 8 nm in n-hexane) exhibit significant improvements in photophysical properties compared to their precursors, enabling narrowband OLEDs with external quantum efficiencies (EQE_max) of up to 42.6%, along with reduced efficiency decline at high brightness. This work highlights the effectiveness of the molecular design strategy for advanced MR-TADF emitters and their potential in high-performance optoelectronic devices.