The synthesis and properties of polyether substituted oligothiophenes : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand
A number of novel dialkoxystyryl-substituted terthiophenes were synthesised as precursors to form conducting polymers. These compounds contained either crown ethers or polyether chains designed to complex metal cations, and polymerisable terthiophene moieties. Two isomeric cross-linked bis(terthiophene) crown ethers were also synthesised as monomers for conducting polymer synthesis, but could not be investigated further due to their insolubility. The solubility issue was circumvented by the formation of hemicrown compounds, containing two styryl-terthiophene units linked by a polyether chain. Thiophene analogues of the crown ether, open-chain ether, bis(terthiophene) crown ether and hemicrown compounds were also successfully synthesised and characterised. The response of the terthiophene crown compounds, open-chain compounds and hemicrowns to a large range of metal cations was investigated by UV and fluorescence spectroscopy. The results obtained from this work were consistent with complexation based on size-fit and charge density of ions, and with hard-soft-acid-base theory. Chemical polymerisation of the terthiophene crown monomers and open-chain ether terthiophene compounds was carried out using FeCl3. This led to the isolation of dimeric sexithiophene compounds in high yield. Characterisation of the pure sexithiophene derivatives showed that they were the product of regioselective dimerisation, caused by the asymmetric reactivity of the terthiophene-based monomers. This is believed to be due to uneven electron spin-density distribution, and theoretical calculations on the radical cation support this view. Producing dialkoxystyryl-substituted sexithiophenes by this synthetic route gave excellent yields of isomerically-pure product. Chemical oxidation of terthiophene compounds using Cu(ClO4)2 was observed with UV/VIS/NIR spectroscopy. This allowed the observation and identification of absorption bands due to oxidised species. Reduction of these species led to sexithiophene dimers, as seen for chemical polymerisation using FeCl3. Electrochemical polymerisations of the terthiophene, thiophene and sexithiophene compounds were carried out by cyclic voltammetry. Those that formed adherent films were analysed by UV/VIS/NIR spectroscopy in both the neutral and oxidised form. The electrochemical and spectroscopic evidence again pointed to the formation of dimers as the primary product of oxidation from terthiophene-based monomers. The surface morphology of the films was investigated by scanning electron microscopy, and showed a variety of morphologies.