Cyano‐Modified Multi‐Resonance Thermally Activated Delayed Fluorescent Emitters Towards Pure‐Green OLEDs with a CIE_y Value of 0.74

Abstract The development of pure‐green organic emitters with ideal emission peaks and ultra‐narrow full‐widths at half‐maximum (FWHMs) remains a formidable challenge. Herein, we report two new green emitters, CNBN and MCNBN , which achieve extremely narrow FWHMs by synergistic rigid π‐extension and cyano‐substitution of a sky‐blue multi‐resonance thermally activated delayed fluorescence (MR‐TADF) core. The introduction of cyano groups induces red‐shifts in the emission to the green region and dramatically minimizes the FWHMs. In toluene solution, CNBN and MCNBN exhibit narrowband emission with a maximum at 501 nm and 510 nm with ultra‐narrow FWHMs of 14 nm/0.066 eV and 15 nm/0.071 eV, respectively. Given the near‐unity photoluminescence quantum yields and almost 100 % horizontal dipole orientation, the electroluminescent (EL) devices based on CNBN and MCNBN deliver external quantum efficiencies (EQEs) exceeding 30 % with FWHMs of 16 nm/0.072 eV and 17 nm/0.080 eV, respectively. Notably, the MCNBN ‐based device achieves pure‐green emission with a maximum at 517 nm with Commission Internationale de l’Éclairage coordinates of (0.17, 0.74), closely aligning with the BT.2020 green standard.