Narrowband multi-resonance pure-red emitters <i>via</i> enhanced molecular orbital delocalization for high-performance organic light-emitting diodes

Multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials with pure-red gamut are in demand for high-definition organic light-emitting diode (OLED) displays. To achieve efficient pure-red OLEDs with excellent color purity, we report three novel MR-TADF emitters: PhCzBN, PhBCzBN, and BCzBN, which integrate a dibenzo[<i>c</i>,<i>g</i>]carbazole segment into a para-boron/oxygen-embedded framework, resulting in the progressive extension of the molecular conjugation. This extension of the π-conjugated skeleton enhances frontier molecular orbital (FMO) delocalization and red-shifts the emission, yielding pure-red emission in toluene with satisfactory peak positions and narrow linewidths. The sensitized OLEDs incorporating PhCzBN, PhBCzBN, and BCzBN exhibit maximum external quantum efficiencies of 31.5%, 33.6%, and 33.8%, respectively. The current efficiencies of these devices reach as high as 42.5 cd A^-1, which is higher than reported pure-red emitters with comparable CIE coordinates. Notably, the devices based on BCzBN demonstrate an emission peak at 636 nm and superior CIE coordinates of (0.700, 0.300), closely aligning with the BT.2020 requirements for the red gamut. This work presents a straightforward yet effective approach for developing high-performance pure-red MR-TADF OLEDs, marking a substantial advancement in wide-color gamut display technologies.