Rigid Heterocyclic Modification Promoted Nitrogen/Carbonyl Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters for High‐Efficiency Electroluminescence

Abstract Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters, exhibiting narrow emission spectra and high photoluminescence quantum yield (PLQY), have garnered substantial interest in organic light‐emitting diodes (OLEDs). Nevertheless, nitrogen/carbonyl (N/C═O)‐based MR‐TADF emitters, seldom demonstrate high external quantum efficiency (EQE) and narrow emission without sensitizer concurrently, hindering the progress within this system. In this study, two N/C═O‐based MR‐TADF emitters, tQAO‐2DBT and tQAO‐2DBF are reported, wherein the two rigid aromatic heterocycles, dibenzothiophene and dibenzofuran, are bonded to quinolino [3,2,1‐de]acridine‐5,9‐dione (QAO) core at the C4‐position, respectively. The structural feature not only constructs their hybrid excited state of long‐range charge transfer and short‐range charge transfer, but also is enabled to protect the QAO core from adjacent chromophores, thereby partially mitigating aggregation‐induced quenching. Consequently, sky‐blue OLED devices employing these emitters achieve maximum EQE of 28.1% and 28.9%, with full width at half maximum of 36 and 37 nm respectively. These findings provide insight into the balance of high‐efficiency and narrow band emission in N/C═O‐based MR‐TADF emitters.