Synthesis and Characterisation of Novel Thiophene Based Azomethine Polymers and Study of Their Liquid Crystalline, Electrochemical and Optoelectronic Properties

Abstract

This work reports the synthesis, structural characterisation, liquid crystallinity, luminescence and electroluminescence of novel thiophene azomethine polymers. The polymers under study were prepared via oxidative polymerisation of four novel monomers at room temperature using iron (III) chloride. The chemical structures of the prepared monomers and polymers were confirmed by infrared and 1H and 13CNMR spectroscopy. Molecular masses were determined for monomers and polymers by gas/liquid chromatography-mass spectrometry (GC/LC-MS) and by gel-permeation (size exclusion) chromatography (SEC), respectively. Thermal stability studies of the prepared materials were achieved by thermogravimetric analysis (TGA), and the onset of weight loss To and the endset Tmax were calculated from the thermograms. Liquid crystalline mesophases and phase changes of the monomers and polymers were studied by differential scanning calorimetry (DSC) and polarised optical microscopy (POM), and the glass transition temperatures Tg of the polymers were determined from the DSC curves. The electrochemical band gaps, HOMO and LUMO energy levels were measured by cyclic voltammetry. UV-visible absorption-emission spectra (liquid and solid films) of the polymers were obtained at room temperature with different solvents. Optical band gaps were calculated from the absorption edges, and were in good agreement with those estimated from cyclic voltammetry. Mixing the polymers with lanthanide salts such as EuCl3 and YbCl3 gave modified fluorescence, and the light emitted was much more intense than that from the pure polymers. Polymer based light-emitting diodes (PLEDs) were fabricated by spin coating, and their current-voltage characteristics were measured. In preliminary work, the polymer devices were found to produce electroluminescent spectra similar to the PL spectra of the corresponding samples. Molecular modelling studies were performed both on polymer segments and monomer molecules; the absorption spectra of the prepared polymers, HOMO and LUMO energy levels were calculated with ZINDO using AMI geometry optimisation.