Deep‐Red Thermally Activated Delayed‐Fluorescence Emitters with Nearly 100% Horizontal Emitting‐Dipole‐Orientation Ratios Using a Dibenzo[ <i>a</i> , <i>c</i> ]phenazine‐11,12‐dicarbonitrile Acceptor

Abstract The horizontal emitting‐dipole‐orientation (EDO) in linear and planar small molecules is a widely recognized phenomenon that is critical to the light‐outcoupling efficiency and overall performance of organic light‐emitting diodes (OLEDs). To maximize the application potential of this phenomenon, two new thermally activated delayed fluorescence (TADF) emitters are designed and synthesized, 10,13‐bis(4‐(3,6‐di‐tert‐butyl‐9H‐carbazol‐9‐yl)phenyl)dibenzo[a,c]phenazine‐11,12‐dicarbonitrile ( t Cz‐BP2CN ) and 10,13‐bis(4‐(bis(4‐(tert‐butyl)phenyl)amino)phenyl)dibenzo[a,c]phenazine‐11,12‐dicarbonitrile ( t DPA‐BP2CN ), exhibiting ≈100% horizontal‐EDO ratios (98% and 99% for t Cz‐BP2CN and t DPA‐BP2CN , respectively). This high horizontal‐EDO ratio is realized by exploiting a para ‐directed linkage between donor moieties and a planar dibeno[a,c]phenazine acceptor unit. The employed T‐shaped geometry design strategy enhances molecular planarity, realizing near‐perfect EDO ratios of ≈100% in both emitters. Additionally, the adopted T‐shaped geometry induced a hydrogen bonding effect between the donor and acceptor units, thereby enhancing the photoluminescence quantum yield of the t DPA‐BP2CN emitter. Overall, OLEDs incorporating t DPA‐BP2CN as a TADF emitter exhibit a high external quantum efficiency, 18.5%, with pure‐red emission at 657 nm.