New oxadiazole derivatives as promising electron transport materials: Synthesis and characterization of thermal, optical and electrochemical properties

2,5-bis-(4-biphenyl)-yl-1,3,4-oxadiazole (1a), 2,5-bis-(4-(6,8-difluoro)-biphenyl)-yl-1,3,4-oxadiazole (1b) and 2,5-bis-(4-(spiro-fluorenyl)-phenyl)-yl-1,3,4-oxadiazole (1c) were designed, synthesized and characterized. 1a-c were easily obtained from Suzuki reactions between 2,5-bis-(4-bromo-phynyl)-[1,3,4]oxadiazole (2) and aromatic boronic acids (3). They were characterized by ¹H-NMR, D SC, TGA, UV-Vis, photoluminescence (PL) spectrometry and CV. The melting temperatures (T_m) of 1a-c are 237, 208 and 370 °C, respectively, much higher than that of 2-tert -butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD, T_m = 136 °C). The oxidation potentials of 1a-c are 1.86, 1.94 and 1.18 V, and their reduction potentials are -2.31, -2.22 and -2.27 V, respectively, indicating that the introduction of electronegative oxadiazole unit lowers the electron density in molecules and enhances their stabilities. The LUMO/HOMO energy levels of 1a-c are as low as -2.39/-6.56, -2.48/-6.69 and -2.43/-5.88 eV, respectively. The good thermal stabilities and low orbital levels of 1a-c make them promising electron-transporting or hole-blocking materials for organic optoelectronic devices.