Thermotropic Liquid-crystalline Properties of Azomethine Dimers: Synthesis and Characterisation

Azomethine is one of the most important function groups that can be used in preparing many functional materials including liquid-crystalline compounds as well as polymers and semiconducting materials. This key functional group is the basis of forming a large number of rod-like and banana-shaped mesogens forming thermotropic liquid-crystalline (LC) compounds. Herein, a series of azomethine dimers 1-14 that were prepared by condensation reactions of benzaldehyde, biphenyl carboxaldehyde and 9-anthraldehyde with various aromatic diamines of varying flexibility in ethanol in the presence of tosic acid, was described. Their chemical structures were determined by Fourier transform infrared and 1H and 13C nuclear magnetic resonance (NMR) spectroscopies, as well as elemental analysis. Their thermal properties were also examined by using several experimental techniques, including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarising optical microscopy (POM) and variable temperature X-ray diffraction (VTXRD). IR spectra were recorded with a Shimadzu Fourier transform infrared (FTIR) analyser with neat films of azomethine compounds on KBr pellets. The 1H and 13C NMR spectra of several azomethine dimers at both room temperature and high temperature, usually at 95 °C whenever possible, were recorded with a Varian nuclear magnetic resonance (VNMR) 400 spectrometer with three radio frequency (RF) channels operating at 400 and 100 MHz, respectively, in d6- DMSO using tetramethylsilane (TMS) as an internal standard, since their solubility in the chosen deuterated solvent was rather low at room temperature. Azomethine dimers 1-2, prepared from benzaldehyde and 1,9-bis(4-aminophenoxy)nonane, exhibited a monotropic, nematic liquid-crystalline (LC) phase. The majority of the azomethine dimers containing biphenyl moieties 3-8 exhibited enantiotropic, nematic LC phase on melting at relatively low temperatures since they developed typical Schlieren, threaded or marbled textures in their LC phase. They also had accessible isotropisation temperatures well below their decomposition temperatures. Azomethine dimers containing anthracene moieties 9-14 did not exhibit LC properties but exhibited polymorphism as determined by POM and VTXRD in two cases. All these azomethine dimers 1-14 had excellent thermal stability that was in the broad range of temperatures of 307–400°C depending on their degrees of aromaticity index. In future, studies could focus on designing stimuli-responsive dimers of these functional groups for advanced optoelectronic applications or incorporating them into polymer networks for high-performance materials.