Highly efficient green organic light emitting diodes with phenanthroimidazole-based thermally activated delayed fluorescence emitters

Here, a phenanthroimidazole moiety was firstly introduced into the construction of thermally activated delayed fluorescence (TADF) emitters, and two novel TADF emitters consisting of phenanthroimidazole as the acceptor group and phenazine as the donor group, 1-(4-(tert-butyl)phenyl)-2-(4-(10-phenylphenazin-5(10H)-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (PPZTPI) and 1-phenyl-2-(4-(10-phenylphenazin-5(10H)-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (PPZPPI) were designed and synthesized. The highly twisted conformation between phenazine and phenanthroimidazole in the molecules results in effective spatial separation of the HOMO and LUMO and small singlet-triplet splittings. Both compounds possess obvious TADF features. Their crystallographic properties, electronic structures, thermal stabilities, photophysical properties, and energy levels are studied systematically. Organic light-emitting diodes (OLEDs) using these two green TADF emitters demonstrate high external quantum efficiencies of 21.06% for PPZPPI and 20.52% for PPZTPI, respectively, which is comparable to most of the previously reported TADF OLEDs.