Modulating the acceptor structure of dicyanopyridine based TADF emitters : Nearly 30% external quantum efficiency and suppression on efficiency roll-off in OLED

As the blooming of thermally activated delayed fluorescent (TADF) materials, organic light-emitting diodes (OLEDs) possessing high external quantum efficiency (EQE) and low efficiency roll-off are under eager pursuit. Herein, three molecules, 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (Ph-DMAC), 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-di(naphthalen-1-yl)pyridine-3,5-dicarbonitrile (Na-DMAC) and 4′-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-[3,2′:6′,3″-terpyridine]-3′,5′-dicarbonitrile (3Py-DMAC) with a twisted donor–acceptor structure comprising 3,5-dicynanopryidine with different 2,6-substutents as acceptor and 9,10-dimethylacridan as donor were designed and synthesized. With the highly twisted conformations, effective TADF is achieved in these molecules. Ph-DMAC delivers a high photoluminescent quantum yield (PLQY) of 89% ascribed to its rigid acceptor geometry. Na-DMAC and 3Py-DMAC suffer a slight decrease in efficiency due to excess vibration induced by their naphthyl and pyridinyl moieties. A Ph-DMAC based OLED achieves a very high maximum EQE of 29.1% due to its high PLQY and efficient TADF. With a relatively short delayed lifetime, 3Py-DMAC based OLEDs show only mild efficiency roll-off.