Synthesis and Electroluminescence Properties of Novel Main Chain Poly(p-phenylenevinylene)s Possessing Pendant Phenylazomethine Dendrons as Metal Ligation Sites

Novel poly(p-phenylenevinylene)s with a dendritic phenylazomethine (DPA-PPVs) were synthesized via the Heck reaction by initially complexing with rare-earth-metal ions, by filling up the coordination trap site of dendritic phenylazomethine (DPA) as the catalyst. The physical properties of the DPA-PPVs were evaluated. The solution, thermal, and optical properties affected by the various contents and generation of the DPA were investigated. DPA-PPV with the lower content and a smaller generation number showed the longest absorption and fluorescence maximum wavelength based on a relatively planar structure and a longer conjugation length. These polymers successfully assemble metal ions on their imine site supported by the spectral change similar to their monomeric model compounds. Two types of electroluminescent (EL) devices were fabricated:  (I) ITO/PEDOT/DPA-PPV/Alq/CsF/Al; (II) ITO/PEDOT/DPA-PPV/TAZ/Alq/CsF/Al, where Alq (tris-8-hydroxyquinoline aluminum) acts as an electron transport layer (ETL) and TAZ (3,5-bis(tert-butylphenyl)-4-phenyltriazole) acts as a hole-blocking layer. In both devices, the luminescence from DPA-PPV was observed with a lower generation and a smaller amount of the DPA unit (3a), as explained by its physical properties. For the type II device with 3a, the emission from DPA-PPV, the lower driving voltage and enhanced efficiency were followed by simply complexing with a very small amount of SnCl2. On the other hand, for 3b, the emission from Alq, not from DPA-PPV, was enhanced by the complexation, and 3b-SnCl2 mainly acts as a hole transport layer with the enhanced hole injection and recombination into the Alq layer, which could not be completely blocked because of the enhanced hole transport property of the DPA unit.