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

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.