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-(alpha,alpha,alpha -trifluoromethylphenyl))-2-oxadiazolyl)phenyl)methane (CF3OXD), tetrakis(4 -(5-(4-diphenylaminophenyl)-2-oxadiazolyl)phenyl)methane (p-TPAOXD), and te trakis(3-(5-(4-diphenylaminophenyl)-2-oxadiazolyl)phenyl)methane (m-TPAOXD) , were synthesized. These compounds were characterized by NMR, microanalysi s, UV-visible spectroscopy, fluorescence spectroscopy, cyclic voltammetry, DSC, and TGA. TBUOXD, OMEOXD, and CF3OXD showed rather similar electrochemi cal and spectroscopic characteristics to those of the hole-blocking materia l 2-(4-biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (PBD). However, T BUOXD, OMEOXD, and CF3OXD all crystallize well above 200 degreesC, much hig her than the 70-90 degreesC observed for PBD. In addition, high onset glass transition temperatures of 97-175 degreesC were detected for the compounds . Both CF3OXD and PBD were adopted as a hole-blocking layer in organic ligh t-emitting devices (OLEDs). A comparable performance with an external quant um efficiency of similar to1% was obtained for devices based on both CF3OXD and PBD, although a much lower allowed current density was found for the C F3OXD-based device. The bipolar blue fluorescent isomers p-TPAOXD and m-TPA OXD are both amorphous, showing no melting and crystallization transition. Only glass transitions at 187 and 149 degreesC were observed respectively. Single-layer OLEDs based on these two compounds were also fabricated by var ied fabrication conditions (solvent, cathode material, film thickness, poly mer blending). A turn-on voltage as low as 4 V, blue electroluminescence wi th 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.