Rational Rigid-Core Design Realises Efficient and Colour-Pure Sky-Blue MR-TADF Emission in Boron-Nitrogen Functionalised Dibenzo-Xanthene

In this work, we report the strategic incorporation of a 14H-dibenzo[a,j]xanthene (DBX) unit into the BCz-BN framework, leading to the development of a novel DBX-BCz-BN multiresonance (MR) thermally activated delayed fluorescence (TADF) emitter. Interestingly, this structural modification preserves the sky-blue emission colour while significantly narrowing the emission profile by reducing the full width at half maximum (FWHM) to 17 nm. Furthermore, the singlet-triplet energy gap (ΔEST) was notably lowered from 0.17 eV (BCz-BN) to 0.06 eV, which effectively promotes reverse intersystem crossing (RISC) and enhances exciton harvesting. As a result, the corresponding OLED device based on DBX-BCz-BN exhibited an external quantum efficiency (EQE) of 22.2% without any change in the emission colour. These results demonstrate that the introduction of the DBX unit is an effective strategy for achieving a highly stable, narrowband sky-blue TADF emitter.