Geometric Accommodation of Atom Repulsions Enables Sub‐10 nm Ultra‐Narrowband Multi‐Resonance Blue Emitters

ABSTRACT Narrowband multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are pivotal for high‐color‐purity organic light‐emitting diodes (OLEDs), yet further bandwidth narrowing typically necessitates extended molecular rigidity that compromises synthetic accessibility and device compatibility. Herein, we report a geometric accommodation strategy to silence vibronic broadening by replacing peripheral six‐membered arenes with compact five‐membered thiophenes. This geometrically adaptive modification alleviates steric H···H repulsion, planarizes the MR skeleton, and selectively suppresses low‐frequency deformation modes. Crucially, the photophysically optimal α ‐borylated topology aligns with the intrinsic regioselectivity of electrophilic borylation, enabling concise synthesis. The resulting α ‐BNTh emitters exhibit intense blue emission with near‐unity photoluminescence quantum yields and ultra‐narrow full widths at half maximum (FWHM) of 8.7–11.7 nm in solution. This molecular‐level spectral sharpening translates effectively to doped films and devices, yielding OLEDs with FWHMs of 8.9–13.1 nm and maximum external quantum efficiencies up to 42.1%. Notably, α ‐BNTh‐CN ‐based devices achieve a sub‐10 nm electroluminescence bandwidth, ranking among the narrowest reported for blue MR‐TADF emitters. This work establishes geometric accommodation via heterocycle engineering as a compact, synthetically viable design principle for ultra‐narrowband MR emitters.