Tetra-alkoxy substituted PPV derivatives: a new class of highly soluble liquid crystalline conjugated polymers

Two first examples of highly soluble tetra-alkoxy substituted PPV derivatives, poly(2,3,5,6-tetrahexyloxy-1,4-phenylene vinylene) (TH-PPV) and poly[2,3,5,6-tetra(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] TEH-PPV, are presented. Both polymers have been fully characterized and the solubility and UV-Vis absorption characteristics have been studied in various organic solvents. Due to the symmetric nature of the repeating unit, TH-PPV and TEH-PPV have an inherently regioregular nature, which is unusual for PPV-type polymers. Observations from polarized light microscopy and differential scanning calorimetry indicate that TH-PPV exhibits thermally induced order resulting in a liquid crystalline mesophase. In contrast, due to steric hindrance of the branched alkyl side chains, TEH-PPV is rather disordered. As a first demonstration of the charge transporting properties of TH-PPV, a simple solution processed electroluminescent device has been prepared. The electroluminescence spectrum shows a detailed structure similar to that observed from the solid-state photoluminescence spectrum and shows one of the highest energy emissions observed for PPVs. It is speculated that the enhanced order in TH-PPV will benefit the usage of this new class of polymers in optoelectronic applications, in which the improved charge carrier mobility is desirable.