Synthesis, properties, and applications of tetraphenylmethane-based molecular materials for light-emitting devices

Abstract

Tetraphenylmethane-based compounds, tetrakis(4-(5-(3,5-di-tert-butylphenyl)-2-oxadiazolyl)phenyl)methane (TBUOXD), tetrakis(4-(5-(3,4-dimethoxyphenyl)-2-oxadiazolyl)phenyl)methane (OMEOXD), tetrakis(4-(5-(3-(α,α,α-trifiuoromethylphenyl))- 2-oxadiazolyl)phenyl)methane (CF3OXD), tetrakis(4-(5-(4-diphenylaminophenyl)-2-oxadiazolyl)phenyl)methane (ρTPAOXD), and tetrakis(3-(5-(4-diphenylaminophenyl)-2-oxadiazolyl)phenyl)methane (mTPAOXD), were synthesized. These compounds were characterized by NMR, microanalysis, UV-visible spectroscopy, fluorescence spectroscopy, cyclic voltammetry, DSC, and TGA. TBUOXD, OMEOXD, and CF3OXD showed rather similar electrochemical and spectroscopic characteristics to those of the hole-blocking material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (PBD). However, TBUOXD, OMEOXD, and CF3OXD all crystallize well above 200 °C, much higher than the 70-90 °C observed for PBD. In addition, high onset glass transition temperatures of 97-175 °C were detected for the compounds. Both CF3OXD and PBD were adopted as a hole-blocking layer in organic light-emitting devices (OLEDs). A comparable performance with an external quantum efficiency of ∼1% was obtained for devices based on both CF3OXD and PBD, although a much lower allowed current density was found for the CF3OXD-based device. The bipolar blue fluorescent isomers ρ-TPAOXD and m-TPAOXD are both amorphous, showing no melting and crystallization transition. Only glass transitions at 187 and 149 °C were observed respectively. Single-layer OLEDs based on these two compounds were also fabricated by varied fabrication conditions (solvent, cathode material, film thickness, polymer blending). A turn-on voltage as low as 4 V, blue electroluminescence with an intensity of 1700 cd/m2, and a photometric efficiency more than 0.8 cd/A can be achieved with p-TPAOXD-based devices.