A Red Thermally Activated Delayed Fluorescence Emitter Simultaneously Having High Photoluminescence Quantum Efficiency and Preferentially Horizontal Emitting Dipole Orientation

The development of red thermally activated delayed fluorescence (TADF) emitters having excellent optoelectronic properties and satisfactory electroluminescence efficiency is full of challenges due to strict molecular design principles. Two red TADF molecules, 3‐(9,9‐dimethylacridin‐10(9H)‐yl)acenaphtho[1,2‐b]quinoxaline‐9,10‐dicarbonitrile and 3‐(2,7‐dimethyl‐10H‐spiro[acridine‐9,9′‐fluoren]‐10‐yl)acenaphtho[1,2‐b]quinoxaline‐9,10‐dicarbonitrile, are developed by adopting a donor–acceptor molecular architecture bearing an electron‐accepting acenaphtho[1,2‐b]quinoxaline‐9,10‐dicarbonitrile (ANQDC) moiety and a 9,9‐dimethyl‐9,10‐dihydroacridine or 2,7‐dimethyl‐10H‐spiro[acridine‐9,9′‐fluorene] electron donor. The combined effects of rigid and planar D/A moieties and highly steric hindrance between D and A groups endow both molecules with high rigidity to suppress nonradiative decay processes, resulting in high photoluminescence quantum efficiencies (Φ_PLs) of up to 95%. Attributed to the linear and planar acceptor motif and rod‐like molecular configuration, both emitters achieve high horizontal ratios of emitting dipole orientation of ≈80%. The organic light‐emitting diodes (OLEDs) based on both emitters exhibit red emissions peaking at ≈615 nm and successfully afford ultrahigh electroluminescence performance with an external quantum efficiency of nearly 28% and a power efficiency of above 50 lm W^?1, on par with the state‐of‐the‐art device efficiency for red TADF OLEDs. This presents a feasible design strategy for excellent TADF emitters simultaneously possessing high Φ_PLs and horizontally aligned emitting dipoles.