The development of organic narrowband emitters faces long-standing challenges in expanding structural diversity, improving synthetic efficiency, elucidating narrowband emission mechanisms, and enhancing overall electroluminescence (EL) performance. Herein, we report a new class of narrowband emitters based on 1,2-BN-heteroarenes, enabled by a systematic design strategy that integrates planar locking, peripheral rotation, and BN-unit extension to tailor vibronic progression in alignment with the principles governing narrowband emission. They are readily synthesized using a borenium species-promoted, amine-directed one-pot borylation in yields over 80%, and exhibit tunable emission colors arising from interplay among locally excited (LE), long-range charge-transfer (CT), and short-range CT states. Representative emitters [B-N]₂ and [B-N]₂-DPA exhibit peak emissions at 460 and 482 nm with ultranarrow full widths at half-maximums (FWHMs) of 16 and 18 nm, respectively, and near-unity photoluminescence (PL) quantum yields. Furthermore, by employing a “hot-exciton layer” design to facilitate exciton dynamics, the corresponding narrowband organic light-emitting diodes (OLEDs) deliver a high maximum external quantum efficiency (EQE) of 29.6%, an exceptionally low efficiency roll-off of 5.7% at 1000 cd m^–2, and superior operational stability compared with the control 1,4-BN-heteroarene. These findings offer new insights into the design of narrowband emitters with diverse structures and high EL performance.