Synthesis and properties of poly(fluorene-alt-cyanophenylenevinylene)-based alternating copolymers for light-emitting diodes

The novel statistical alternating EL copolymers poly[bis{2-(4-phenylenevinylene)-2-cyanoethenyl}-9,9-dihexyl-9H-fluorene-2,7-yl-alt-1,4-phenylene vinylene] (polymer I), poly[bis{ 2-(4-phenylenevinylene)-2-cyanoethenyl}-9,9-dihexyl-9H-fluorene-2,7-yl-alt-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (polymer II), and poly[bis{2-(4-phenylenevinylene)-2-cyanoethenyl}-9,9-dihexyl-9H-fluorene-2,7-yl-alt-1,4-biphenylenevinylene] (polymer III) were synthesized using modified Heck coupling reactions, and their EL characteristics were investigated. These fluorene- and cyanophenylenevinylene-based alternating copolymers, which have strong electron-withdrawing CN groups on their vinylene units, were found to be completely soluble in common organic solvents and easily spin-coated onto indium-tin oxide (ITO) coated glass substrates. The HOMO and LUMO energy levels and the electrooptical properties of these copolymers can be tuned by varying the main-chain structures and the side-chain groups. These well-defined conjugated copolymers can be synthesized with or without dialkyloxy substituents on the phenylenevinylene or biphenylenevinylene units instead of the phenylenevinylene unit; these variations in composition give rise to different emission maxima, ranging from bright yellow to a yellowish-orange color. A double-layer polymer light-emitting diode (PLED) with an ITO/PEDOT/polymer II/Al configuration exhibited a maximum brightness and luminescence efficiency of 7500 cd/m(2) at 20 V and 0.21 lm/W at 6.7 V, respectively.