Managing Orientation of Nitrogens in Bipyrimidine-Based Thermally Activated Delayed Fluorescent Emitters to Suppress Nonradiative Mechanisms

Three novel green thermally activated delayed fluorescence (TADF) materials, (5,5′-bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,2′-bipyrimidine (22bpmAc), 10,10′-([2,5′-bipyrimidine]-2′,5-diylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine) (25bpmAc), and 2,2′-bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-5,5′-bipyrimidine (55bpmAc)) based on bipyrimidine cores as the new electron-accepting units and acridine as the electron-donating unit were designed, synthesized, and applied as green emitters in TADF devices. The TADF emitters were designed to have different nitrogen orientations in the bipyrimidine core. Two emitters, 25bpmAc and 55bpmAc, showed higher quantum efficiencies and narrower emission spectra relative to 22bpmAc. The former emitters have restricted geometry by hydrogen-bonding interactions on a central core, whereas the latter emitter does not have hydrogen bonding. The intramolecular hydrogen-bonding interaction suppressed a nonradiative mechanism in the 25bpmAc and 55bpmAc TADF emitters. The TADF device fabricated with 55bpmAc showed a maximum external quantum efficiency of 24.9% for the color coordinates (0.22, 0.46). This work demonstrates the importance of intramolecular hydrogen-bonding interactions for designing highly efficient TADF emitters to suppress nonradiative mechanism..