Short chain copper(II) n-alkanoate liquid crystals

Abstract

Four short chain members of the copper(II) n-alkanoate series Cu(Cn)₂, from propanoate to hexanoate, have been synthesized, purified and characterized by means of optical microscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), FTIR and Raman spectroscopy. The TGA study shows that decomposition starts on heating above 420–470 K (depending on the sample) in a nitrogen atmosphere. In addition, thermal decomposition was investigated by DSC using special high pressure pans and endo- and exc-thermic processes were found which have not been reported previously. All but one of the compounds melt to a liquid crystal phase, which decomposes before the clearing point. The exception is the propanoate homologue, which decomposes directly from the solid state. Despite this problem of sample decomposition, the identification by optical microscopy of the tetragonal (butanoate) and hexagonal (pentanoate and hexanoate) discotic columnar phases was, for first time, possible by the addition of small amounts of the corresponding acid to the samples. These results are in agreement with the X-ray diffraction study performed using swelled mixtures of these salts with hydrocarbon solvents. Two solid–solid transitions, not previously reported in the literature, were found for the butanoate homologue at 395.9 and 422.9 K with ΔH=8.27 and 1.37 kJ mol⁻¹, respectively. The solid and liquid crystalline phases were investigated using variable temperature FTIR spectroscopy.