Efficient Blue Carbonyl‐Nitrogen Multi‐Resonance Molecules for High‐Performance Hyperfluorescence OLEDs

Abstract Efficient blue multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials are highly desired for the fabrication of organic light‐emitting diodes (OLEDs) with high color purity. But only very limited carbonyl‐nitrogen (CO‐N) blue MR‐TADF are well‐developed. Herein, two new blue CO‐N MR‐TADF molecules ( t BuPQCZ and 2 t BuPQCZ) are explored by introducing bulky substituents to a planar CO‐N MR‐TADF core constructed by fusing carbonyl groups with carbazole, and their thermal stability, electronic structures, and photophysical properties are investigated. They exhibit blue photoluminescence (PL) peaks at 451 and 447 nm, and narrow full width at half maximums (FWHMs) of 26 and 25 nm in toluene. In doped films, they exhibit strong blue emissions with PL peaks at 474 and 469 nm, FWHMs of 51 and 49 nm, and satisfactory PL quantum yields of 80% and 72%. Moreover, blue OLEDs using them as emitters exhibit excellent external quantum efficiencies ( EQE s) of 20.4% and 21.5%, and the hyperfluorescence OLEDs based on them provide state‐of‐the‐art EQE s of 30.1% and 29.0% with small FWHMs of 34 and 32 nm and Commission Internationale de I'Eclairage coordinates of (0.136, 0.134) and (0.141, 0.105). The insights gained in this work should be valuble for the development of efficient blue CO‐N MR‐TADF materials.