Efficient pure-red multiple resonance emitter based on a donor planarization strategy

Efficient pure-red multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters compatible with vacuum thermal evaporation remain scarce, which hampers the development of wide color gamuts for ultra-high-definition displays. The prevailing approach connecting one or more donors to the highest occupied molecular orbital positions of the MR core via single bonds suffers from inherent torsional flexibility, which results in the limited red shift, large molecular weight and broadened emission bandwidth. Herein, we present a donor planarization strategy in which the donor is intramolecularly fused with an MR core to generate a planar structural framework. This strategy not only enhances π-conjugation and the long-range charge transfer effect to enable a significant red shift in emission, but also improves molecular planarity to narrow the emission bandwidth. The proof-of-concept emitter, RBN-ICz, achieves a 96 nm red shift (from 519 to 615 nm) and a reduced emission bandwidth from 0.18 to 0.15 eV compared to parent molecule m-Cz-BNCz. Notably, RBN-ICz exhibits the lowest molecular weight among all reported red MR-TADF materials. The RBN-ICz-based organic light-emitting diode (OLED) achieves an external quantum efficiency (EQE) of 34.1% with Commission Internationale de l'Éclairage (CIE) coordinates of (0.67, 0.33), fully meeting the National Television System Committee (NTSC) red standard. This work offers an effective route for the development of high-efficiency, narrowband red MR-TADF emitters.