Peripheral mesitylene-modified sky-blue multi-resonance TADF emitter with enhanced horizontal orientation and suppressed aggregation-caused quenching achieving 38.6% external quantum efficiency

Boron-based multiresonance thermally activated delayed fluorescence (MR-TADF) emitters combine intrinsically narrowband emission with near-unity internal quantum efficiency. However, their practical implementation in OLED displays remains limited by aggregation-caused quenching, long delayed fluorescence lifetimes, suboptimal horizontal emissive transition dipole moment ratio (Θ), and poor electrical stability. Among boron-based MR-TADF systems, carbazole-based CzBN has emerged as a representative and widely studied motif due to its facile chemical modifiability, which allows for the development of diverse derivatives aimed at alleviating these limitations. Here, we present MesCzBN, a sky-blue MR-TADF emitter featuring four mesitylene groups appended to the periphery of the CzBN core. The mesitylene “umbrella” sterically shields the MR core and promotes a Θ value of 84% and a high photoluminescence quantum yield of 100% in a carbazole-based host matrix. MesCzBN achieves an external quantum efficiency of 38.6%, a peak electroluminescence wavelength of 494 nm, and a narrow full width at half maximum of 28 nm in a hyperfluorescent device. Peripheral mesitylene modification thus provides a novel sterically wrapping strategy to enhance horizontal molecular orientation and device efficiency in OLEDs, serving as a superior replacement for the benchmark tCzBN with four tert-butyl groups.