Effect of Substituents with the Different Electron-Donating Abilities on Optoelectronic Properties of Bipolar Thioxanthone Derivatives

The synthesis and optoelectronic properties of four simple-structure thioxanthone derivatives employing thioxanthone as an acceptor unit, coupled with moieties having very different electron-donating abilities such as phenoxazine, 3,6-di-tert- butylcarbazole, 3,7-di-tert-butylphenothiazine, or 2,7-di-tert-butyl-9,9-dimethylacridane, are reported. The compounds form molec-ular glasses with glass transition temperatures reaching 116 degrees C. Ionization potentials of the compounds estimated by photo-electron emission method range from 5.42 to 5.74 eV. Thioxanthone derivatives containing 3,6-tert-butylcarbazole or 2,7-di-tert- butyl-9,9-dimethylacridane moieties with weak electron-donating strengths were characterized by bipolar charge transport with relatively close hole and electron mobility values of 6.8 x 10-5/2.4 x 10-5 and 3.1 x 10-5/4.6 x 10-6 cm2/(V s) recorded at 3.6 x 105 V/cm. The other compounds demonstrated hole-transporting properties. The films of thioxanthones containing phenoxazine or 2,7-di-tert-butyl-9,9-dimethylacridane moieties showed efficient thermally activated delayed fluorescence with a photoluminescence quantum yield of up to 50% due to the solid-state luminescence enhancement. Organic-light-emitting diodes containing the synthesized compounds as emitters showed very different external quantum efficiencies (0.9-10.3%) and blue, sky blue, green, or yellow electroluminescence colors, thus reflecting the effects of donor substituents.