Triboron Multiresonant Emitter With Zigzag B/N Alignment Enables Near-Degenerate Singlet-Triplet States for...

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 × 10⁸ 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.