Synthesis and characterization of poly(p-phenylenevinylene) based alternating copolymers for light emitting diodes

A novel series of para-phenylenevinylene and aromatic amine segments based alternating copolymers, poly(2,5-dihexyl-1,4-phenylenevinylene-alt-N-phenyl-4′,4′′-diphenylene vinylene) (polymer-I) and poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene-alt-N-phenyl-4′′,4′′′-diphenylenevinylene) (polymer-II) were synthesized by Horner–Wittig–Emmons reaction. The resulting copolymers are soluble in organic solvents and easily spin-cast onto indium–tin oxide (ITO) substrate without any defects. The resulting copolymers are shown strong absorption bands at 418 and 443 nm with alkyl and alkyloxy substituents, which is due to the π–π* transition of the conjugated backbone of the corresponding copolymers. The phenylenevinylene part acts as the emission role and the aromatic amine portion impart the hole transporting mobility and increase the thermal stability up to 425 °C of the resulting copolymers. Light emitting diodes (LEDs) were fabricated by sandwitching the emissive copolymers between ITO and calcium electrodes. The forward bias turn-on voltage for the LEDs is 4.4 V for polymer-I and 2.6 V for polymer-II. The emission colors could be tuned from 488 to 506 nm by introducing the alkyl and alkyloxy substituents into the phenylenevinylene ring with applied electric field.