Synthesis, structure-properties of planar, end-substituted, light-emitting oligophenylenevinylenes

A novel homologous series of highly soluble, multiple alkoxy end-substituted oligophenylenevinylenes (OPV) containing up to six phenyl rings were synthesized. In contrast to the lateral substituted OPVs, our theoretical and experimental evidences have shown that the electron donating groups incorporated at the cnds of an oligomer do not affect co-planarity of the π-conjugated system. They can also promote a faster convergence of the HOMO-LUMO energy gap relative to those of unsubstituted OPVs suggesting a shorter effective conjugation length in this series. These end-substituted OPVs containing more than three phenyl-rings exhibit very high fluorescence quantum yields (more than 74%). Multi-layer LEDs using these end-substituted OPVs as emissive layers were fabricated and investigated. We have found that there is a bathochromic shift of the PL and EL maxima with an increase of chain length in this series. All these oligomer-based multi-layer LEDs show very low turn-on-voltage. In addition, the higher homologues of these oligomers suffered from severe interchain interaction leading to a dramatic decrease in the external quantum efficiency; however, this can be alleviated by doping the oligomer into the host matrix.