Structure Engineering of Acridine Donor to Optimize Color Purity of Blue Thermally Activated Delayed Fluorescence Emitters

9,9‐Dimethyl‐9,10‐dihydroacridine (DMAC) is one of the most widely used electron donor for constructing high‐performance thermally activated delayed fluorescence (TADF) emitters. However, DMAC‐based emitters often suffer from the imperfect color purity, particularly in blue emitters, due to its strong electron‐donating capability. To modulate donor strength, 2,7‐F‐Ph‐DMAC and 2,7‐CF3‐Ph‐DMAC were designed by introducing the electron‐withdrawing 2‐fluorophenyl and 2‐(trifluoromethyl)phenyl at the 2,7‐positions of DMAC. These donors were used, in combination with 2,4,6‐triphenyl‐1,3,5‐triazine (TRZ) acceptor, to develop novel TADF emitters 2,7‐F‐Ph‐DMAC‐TRZ and 2,7‐CF3‐Ph‐DMAC‐TRZ. Compared to the F‐ or CF3‐free reference emitter, both two emitters showed hypsochromic effect in fluorescence and comparable photoluminescence quantum yields without sacrificing the reverse intersystem crossing rate constants. In particular, 2,7‐CF3‐Ph‐DMAC‐TRZ based OLED exhibited a blue shift by up to 39 nm and significantly improved Commission International de l'Éclairage (CIE) coordinates from (0.36, 0.55) to (0.22, 0.41), while the external quantum efficiency kept stable at about 22.5%. This donor engineering strategy should be valid for improving the color purity of large amount of acridine based TADF emitters. It can be predicted that pure blue TADF emitters should be feasible if these F‐ or CF3‐modifed acridine donors are combined with other weaker electron acceptors.