New Series of Blue-Emitting and Electron-Transporting Copolymers Based on Cyanostilbene

Abstract

A novel series of conjugated copolymers having fluorene and binaphthyl moieties in the main chain and based on cyanostilbene were synthesized in good yields by a palladium-catalyzed Suzuki coupling reaction, a new approach different from the traditional Knoevenagel condensation polymerization. Through controllable modification of the main chain structures, not only were the thermal, electronic, and optical properties of the polymers tuned, but also the structure−property relationships were studied. All these polymers possess excellent thermal stability with glass-transition temperatures of 60−159 °C and onset decomposition temperatures of 411−417 °C. Cyclic voltammetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from −2.92 to −3.08 eV and low-lying HOMO energy levels ranging from −6.01 to −6.13 eV, and they may be promising candidates for electron-transporting or hole-blocking materials in light-emitting diodes (LEDs). The polymers in thin films emit strong blue luminance around 457−489 nm with narrow bandwidth upon photoexcitation. The single-layer LED fabricated with a copolymer F−CN composed of fluorene and cyanostilbene units using an air-stable aluminum electrode emits blue light with an external quantum efficiency of 0.006%. A double-layer LED, fabricated using a blend of poly(N-vinylcarbazole) and F−CN as emitting layer and tris(8-hydroxyquinolinato)aluminum as electron transporting layer, shows an external quantum efficiency of 0.2%. Preliminary electroluminescent results show that these polymers are new candidates for blue emissive materials in polymeric LEDs.