High‐Efficiency Thermally Activated Delayed Fluorescence Emitters with High Horizontal Orientation and Narrow Deep‐Blue Emission

Abstract Highly efficient 10‐(5,9‐dioxa‐13 b ‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐10 H ‐dispiro[acridine‐9,9′‐anthracene‐10′,9″‐fluorene] (OBOtSAc) and 10‐(2,12‐di‐ tert ‐butyl‐5,9‐dioxa‐13 b ‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐10 H ‐dispiro[acridine‐9,9′‐anthracene‐10′,9″‐fluorene] (tBuOBOtSAc) emitters, comprising almost perpendicularly linked rigid 5,9‐dioxa‐13 b ‐boranaphtho[3,2,1‐de]anthracene (OBO) electron acceptors and a rigid and linear tri‐spiral acridine electron donor, are reported here. OBOtSAc and tBuOBOtSAc show deep‐blue emission (λ max = 452 and 446 nm) with narrow full width at half maximum (FWHM) values of 50 and 48 nm, respectively. Due to the rigid and twisted structures, and the appropriate singlet and triplet energy levels of both emitters, 10 wt% doped films of OBOtSAc and tBuOBOtSAc in a bis[2‐(diphenylphosphino)phenyl]ether oxide (DPEPO) host show efficient thermally activated delayed fluorescence (TADF) emission and photoluminescence quantum yields (PLQYs) of 97% and 90%, respectively. Moreover, linear‐shaped OBOtSAc and tBuOBOtSAc lead to excellent horizontal emitting dipole orientations (88% and 90%, respectively). Consequently, organic light‐emitting diode (OLED) devices using OBOtSAc and tBuOBOtSAc exhibit maximum external quantum efficiencies (EQEs) of 31.2% and 28.2%, respectively, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.147, 0.092) and (0.149, 0.061), respectively.