Synthesis, Properties, and Applications of Tetraphenylmethane-Based Molecular Materials for Light-Emitting Devices

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-trifluoromethylphenyl))-2-oxadiazolyl)phenyl)methane (CF3OXD), tetrakis(4-(5-(4-diphenylaminophenyl)-2-oxadiazolyl)phenyl)methane ( p -TPAOXD), and tetrakis(3-(5-(4-diphenylaminophenyl)-2-oxadiazolyl)phenyl)methane ( m -TPAOXD), 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 p -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/m 2, and a photometric efficiency more than 0.8 cd/A can be achieved with p -TPAOXD-based devices.