Formation of binary liquid crystal mixtures from a thermal cyclization process

Abstract

An unusual process, the change of an isotropic phase into a nematic phase at a constant temperature, is reported. The conversion is the result of the thermal cyclization of a single compound, which is either isotropic or nematic, to form two compounds, both of which are enantiotropic liquid crystals. The starting material is a class of compounds having a diazo link between two para-substituted phenyl rings and a 4-n-alkoxybenzoyloxymethylene lateral group in the ortho position with respect to the diazo link. The DSC curves and optical microscopy show that, after melting, the compound undergoes an exothermic reaction to form a mixture that has liquid crystalline properties over a wide temperature range. The mixture is composed of two mesogenic compounds: a disubstituted 2H-indazole and a 4-n-alkoxybenzoic acid. The liquid crystalline properties are reproducible in further heating-cooling cycles. The time-dependent ¹³C NMR spectra at a constant temperature slightly above the melting temperature indicate that a small extent of reaction is sufficient to induce liquid crystalline properties in the melt. Optical measurement of the appearance of the nematic phase at different constant temperatures gives the activation energy of the reaction as 71.5±0.8 and 81.8±1.2 kJ mol^-1 for the two series of compounds studied. The X-ray structure of a parent compound shows the coplanarity and the rigidness of the central core containing the 2H-indazole fragment and the attached aromatic ring. This central core is slightly bent with an angle of 157° between the terminal bonds.