Triboron Multiresonant Emitter With Zigzag B/N Alignment Enables Near‐Degenerate Singlet–Triplet States for High‐Performance Non‐Sensitized OLEDs With Mild Roll‐Off

ABSTRACT Boron/nitrogen‐embedded polycyclic frameworks are attractive emitters in organic light‐emitting diodes (OLEDs) because they combine narrowband emission with thermally activated delayed fluorescence. Linear extension of multiresonant frameworks with zigzag boron/nitrogen alignment is predicted to enable bathochromic emission, reduced singlet–triplet energy gaps, and enhanced oscillator strengths, yet experimental access to higher‐order structures remains challenging. Here we report a site‐programmable, stepwise borylation strategy for constructing a triboron DABNA‐extended framework with zigzag boron/nitrogen alignment, L‐DABNA‐TriB. Our combined theoretical and experimental studies reveal how progressive linear extension from mono‐ to tri‐boron frameworks modulates the electronic structure, leading to bandgap narrowing, strengthened radiative transition, and near‐degenerate singlet and triplet excited states. Importantly, L‐DABNA‐TriB in toluene exhibits yellow–orange emission at 559 nm with a narrow full width at half‐maximum of 33 nm/0.13 eV, a singlet–triplet energy gap of ca. 4 meV, and a radiative decay rate of 1.5 × 10 8 s −1 . A non‐sensitized OLED based on L ‐DABNA‐TriB achieves a maximum external quantum efficiency of 38.0% and retains 35.2% at 1000 cd m −2 . This work establishes zigzag boron/nitrogen‐aligned linear extension as an effective molecular design strategy for narrowband emitters with efficient exciton harvesting at long wavelengths.