Efficient and Narrowband Pure‐Green Emission From MR‐TADF Emitter Designed With Para‐N‐π‐N Extended Conjugation and Twisted Xylene

ABSTRACT Achieving high Commission Internationale de L'Eclairage (CIE) y values in green emitters is essential for next‐generation display technologies. However, the number of green multiresonance thermally activated delayed fluorescence (MR‐TADF) emitters achieving pure emission with high CIEy values is still limited, hindering compliance with the Broadcast Service Television 2020 (BT.2020) standard. Here, we report a new MR‐TADF emitter , xy‐ BPhCz, rationally designed by modifying the well‐established blue DABNA core with phenylcarbazole, xylene, and t‐butyl substituents. These structural modifications extend the π‐framework, induce a red‐shift, while simultaneously suppressing molecular aggregation and isomer formation, ultimately giving ultrapure green emission. The newly designed xy‐ BPhCz exhibits a high photoluminescence quantum yield of 84.5%. An ultra‐narrow full width at half maximum (FWHM) of 16 nm in cyclohexane was recorded, corresponding to CIE coordinates of (0.194, 0.739) which is closely approaching the BT.2020 green standard. When implemented in a phosphorescence‐sensitized OLED, the device achieved an external quantum efficiency of 28.0%. Particularly, the TADF device showed a high CIEy of 0.714, with a small FWHM of 25.8 nm, underscoring its potential for next‐generation ultra‐high‐definition displays.