Blue emitting OLED with star-shaped multifunctional oligomer

A fluorescent star-shaped oligomer with a nitrogen atom as a core and both a hole transporting arylamine and an electron transporting 1,3,4-oxadiazole moiety, tri(4-(5-phenyl-1, 3, 4-oxadiazol-2-yl)phenyl)amine (TPOPA), has been designed and synthesized using a 5-step reaction procedure. The synthesized compound was characterized by elemental analysis, ¹H-NMR and mass spectroscopy. Thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis show that TPOPA exhibits high thermal stability (T_d, 373°C) and high glass-transition temperature (T_g, 116 °C). Photoluminescence measurements indicate that the star-shaped oligomer shows intense blue emission peaked at 445 nm with a high quantum yield of 0.68 under near UV light excitation. The HOMO value of TPOPA is -5.64 eV and the LUMO is -2.58 eV based on the electro-chemical determinations. Reversible anodic oxidation results suggest that the hole-transporting is predominant for TPOPA. Two single layered devices were fabricated by vacuum evaporation with configurations of ITO / CuPC (15 nm) / TPOPA (95 nm) / Ca(30 nm) / Al(100 nm) (device 1) and ITO / CuPC(15 nm) / TPOPA (175 nm) / Ca(30 nm) / Al(100 nm) (device 2), where TPOPA was used both as emitter and carrier - transporting material, CuPC as a hole-injection and electron block material. The devices show blue wide-band emission peaked at 438 nm with a maximum luminance of 650 cd/m² and 512 cd/m² under an operating voltage of 12 V, respectively.