Penta-substituted arylbenzene derivatives as efficient hole-transporting materials

Pentaarylbenzene was selected as an acceptor for the design and synthesis of a series of hole-transporting materials. Carbazole with and without tert-butyl groups and diphenylamine were used as the donor units. All the compounds of the series exhibit high thermal stability with 5 % weight loss temperatures higher than 400 °C. The compound containing diphenylamino groups forms amorphous films while carbazolyl-containing hole-transporters exhibit crystallinity. The photoluminescence spectra of the solutions of the carbazole derivatives show two peaks assigned to the local excited (LE) and charge transfer (CT) state emission. Meanwhile the spectrum of the compound with diphenylamino groups shows a single CT peak manifested by positive solvatochromism. The attachment of tert-butyl groups to carbazole moieties results in the elevated energy level of the highest occupied molecular orbital and enhanced drift mobility values of charge carriers up to 4 × 10⁻³ cm²/V‧s at an electric field of ca. 1.6 × 10⁵ V/cm. Cabrazole-based hole-transporters exhibit higher triplet energy of ca. 2.88 eV than the derivative of diphenylamine (2.8 eV). Nevertheless, due to the amorphous nature of the layers, the diphenylamino-containing compound shows good performance in organic light-emitting diodes utilizing an emitter exhibiting thermally activated delayed fluorescence. The device shows external quantum efficiency of 22 % at 1000 cd/m² and 19 % at 3000 cd/m².