Hyperbranched polymer for light-emitting applications

A light-emitting partially conjugated hyperbranched polymer (2,5-dimethoxy-substituted hyperbranched poly(p-phenylene vinylene), MOHPV) based on rigid fluorescent conjugated segments, 2,5-dimethoxy-substituted distyrylbenzene (a derivative of oligo-poly(p-phenylene vinylene)), and flexible non-conjugated spacers, trioxymethylpropane, was synthesized via an A(2) + B-3 approach. The weight-average molecular weight was 2.48 x 10(4) g mol(-1). The introduction of two methoxy groups into central rings of the oligo-poly(p-phenylene vinylene) imparted to MOHPV better solubility in common organic solvents and processability than its analogues reported in our previous work, especially the fully conjugated hyperbranched polymers. The effect of the molar ratio of monomer A2 to monomer B3 on the molecular weight and molecular weight distribution was investigated. A single-layer light-emitting diode was fabricated employing MOHPV as an emitter. A double-layer light-emitting diode was also fabricated by doping an electron transport material, 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole, into the emitting layer and inserting a thin layer of tri(8-hydroxyquinoline)aluminium as electron-transporting/hole-blocking layer. A maximum luminance of 1500 cd m(-2) at 12 V and a maximum electroluminescence efficiency of 1.38 cd A(-1) at 14 mA cm(-3), which are approximately 43.5 and 12.9 times greater, respectively, than those of the single-layer device, were achieved. The synthetic simplicity, excellent solubility and solution processability, and less of a propensity to aggregation make MOHPV a novel type of emitter for polymer light-emitting displays. (C) 2010 Society of Chemical Industry