Synthesis and characterization of quinoline-based copolymers for light emitting diodes

Two new electroluminescent copolymers containing biquinolines and 2,2-diphenylhexafluoropropane (F-PQ) or pyridine (Py-PQE) moieties were prepared. They possess excellent thermal stability (decomposition temperature greater than 500 °C), good electrochemical reversibility in reduction reactions, and high electron affinity. The energy levels for HOMO and LUMO determined by cyclic voltammetry were -5.80 and -2.89 eV for F-PQ, and -5.88 and -2.66 eV for Py-PQE, respectively. Electrical characterization of a double layer light emitting diode (LED) based on the structure of ITO/copper phthalocyanine (CuPc)/F-PQ/A1 showed good performance (a rectification ratio greater than 10⁵ and a low turn-on voltage of 6.2 V). A single layer LED fabricated with Py-PQE as an emitting layer and air-stable aluminium as a cathode exhibited a balanced injection/transport of hole and electron. A luminance of 94.0 cd m^-2 was observed from a double layer LED of ITO/CuPc/Py-PQE/A1 at a current density of 141.4 mA cm^-2.