Bipolar deep-blue phenanthroimidazole derivatives: Structure, photophysical and electroluminescent properties

Two efficient fluorescence molecules, named 1-(4-trifluoromethylphenyl)-2-(4'-(1,4,5-triphenyl-1H-imidazole-2yl)-[1,1'-biphenyl]-4-yl)-1H-phenanthro[ 9,10-d] imidazole (PPI-PIM-1) and 1-(4-trifluoromethylphenyl)-2-(4'( 2,4,5-triphenyl-1H-imidazole-1-yl)-[ 1,1'-biphenyl]-4-yl)-1H-phenanthro[ 9,10-d] imidazole (PPI-PIM-2) were designed and synthesized, in which the electron-withdrawing trifluoromethyl group was introduced firstly to N1-position benzene of phenanthroimidazole to improve the electron transporting ability and tune the charge transfer (CT) component. In addition, compared to biphenanthroimidazole derivatives, here, we break one rigid plane of phenanthroimidazole to inhibit aggregation-induced-quenching (ACQ) and change the position from C1 to N1 to restrict the conjugation length simultaneously. The devices based on PPI-PIM-1 and PPI-PIM-2 exhibited stable deep-blue emission with CIE coordinates of (0.15, 0.08) and (0.15, 0.07) and achieved maximum external quantum efficiencies of 4.3% and 4.5% respectively. More importantly, the corresponding exciton utilization efficiencies are as high as 27.0% and 30.3%.