Monodisperse Starburst Oligofluorene-Functionalized 4,4′,4″-Tris(carbazol-9-yl)-triphenylamines : Their Synthesis and Deep-Blue Fluorescence Properties for Organic Light-Emitting Diode Applications

Four monodisperse starburst oligomers bearing a 4,4′,4″-tris(carbazol-9-yl)-triphenylamine (TCTA) core and six oligofluorene arms are synthesized and characterized. The lengths of oligofluorene arms vary from one to four fluorene units, giving the starburst oligomers molecular weights ranging from 3072 to 10 068 Da (1 Da = 1.66 × 10^-27 kg). All of the starburst oligomers have good film-forming capabilities, and display bright, deep-blue fluorescence (λ_max = 395-416 nm) both in solution and in the solid state with the quantum efficiencies of the films (Φ_PL) varying between 27 and 88 %. Electrochemical studies demonstrate that these materials have large energy gaps, and are stable for both p-doping and n-doping processes. Electroluminescent devices are successfully fabricated using these materials as hole-transporting emitters, and emit deep-blue light. Devices with luminance values up to 1025 cd m^-2 at 11 V and luminous efficiencies of 0.47 cd A^-1 at 100 cd m^-2 have been produced, which translates to an external quantum efficiency of 1.4 %. In addition, these large-energy-gap starburst oligomers are good host materials for red electrophosphorescence. The luminance of the red electrophosphorescent devices is as high as 4452 cd m^-2, with a luminous efficiency of 4.31 cd A^-1 at 15 mA cm^-2: This value is much higher than those obtained from the commonly used holetransporting materials, such as poly(vinyl carbazole) (PVK) (1.10 cd A^-1 at 16 mA cm^-2).