FTIR and X-ray investigation of triphenylene based discotic liquid crystals.
Novel disc like molecules based on hexa-n-alkoxy benzoates of triphenylene were synthesised at Hull university. The compounds exhibited thermotropic liquid crystalline behaviour. The compounds differed chemically based upon the number and position of methyl additions to the ester benzoate linkage. Unsymmetrical compounds based on hexa-n-alkoxy triphenylenes were also examined. A number of techniques were employed to observe and measure the physical properties of these compounds. Polarising optical microscopy was used to observe and record the phase behaviour. Typical schlieren nematic textures were often observed in the liquid crystalline phase. The transition temperatures of the phase transitions were recorded to within +/-0.1 °C. Methyl additions to the ester benzoate linkage plays a major role in determining transition temperatures and also the ranges of liquid crystal phase. X-ray diffraction investigations allowed the molecular planar spacings to be measured, use of a heating stage enabled measurements to be taken in the liquid crystalline phase. All the samples produced a diffuse broad diffraction ring in the liquid crystalline phase, indicating that the samples are not highly ordered and that they are likely to have adopted a hexagonal packing arrangement. Planar spacings measured were in the range 22 - 30A, only one sample, DB26, showed a diffraction ring corresponding to a planar spacing of 4.1A, indicating that molecular columns or partial columns were able to form from molecules stacking one on top of another. Thus methyl groups on the ester benzoate linkage disrupt the formation of columns, in turn reducing transition temperatures. A number of methods of successfully aligning the discotic materials using surface treatments are presented. Rubbed PVA and HTAB layers aligned the samples hometropically, while SiO deposited layers aligned the samples homogeneously. (The SiO deposition used an evaporation angle of 45°, a deposition angle of 5° is commonly used to obtain homotropic alignment of calamitic materials.) Methods that produced homogeonous alignment of calamitic materials produced hometropically aligned discotic materials and vice-versa. FTIR allows conformational information about a molecule to be determined. The CH[2] wagging region was investigated to determine conformational information relating to the alkyl arms. Combination of FTIR and aligned samples allowed the alignment process to be investigated, it was determined that the alignment of the molecules occurs in a series of stages, the triphenylene cores align first on cooling, followed by the ester benzoate linkages and finally the alkyl arms. Computer modelling simulations allowed various molecular conformations to be observed, combination with X-ray diffraction data allowed molecular structures to be generated. The software allowed various molecular dimensions to be easily measured and the effect and extent of interdigitation of molecular arms to be observed. The molecular dynamics calculations were only able to calculate energy minimisations for crystalline structures, but the crystalline results offered valuable insights into the liquid crystalline structures and behaviour.
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Sheffield Hallam University (United Kingdom).