Rational Rigid‐Core Design Realizes Efficient and Color‐Pure Sky‐Blue MR‐TADF Emission in Boron‐Nitrogen Functionalized Dibenzo‐Xanthene

ABSTRACT In this work, we report the strategic incorporation of a rigid 14H‐dibenzo[a,j]xanthene ( DBX ) core into the BCz‐BN framework, leading to the development of a novel DBX‐BCz‐BN multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitter. This structural modification preserves the sky‐blue emission color while substantially narrowing the emission profile, reducing the full width at half maximum (FWHM) to 17 nm. In addition, the singlet–triplet energy gap (Δ E _ST ) is lowered from 0.17 eV for BCz‐BN to 0.06 eV, thereby promoting more efficient reverse intersystem crossing (RISC) and improved exciton harvesting. As a result, the corresponding OLED device based on DBX‐BCz‐BN exhibits an external quantum efficiency (EQE) of 22.2%, while maintaining the same emission color. These results demonstrate that rigid‐core engineering via DBX incorporation is an effective strategy for achieving narrowband, efficient, and color‐pure sky‐blue TADF emitters.