Unusual thermal behaviour of mesogens containing an N,N-dialkylaminomethylene lateral substituent

An N,N-dialkylaminomethylene lateral substituent can be used to reduce the melting temperatures of nematogens having a long rigid core. However, compa red with a single lateral alkoxy chain, this lateral substituent does not e nhance the thermodynamic stability of the mesophase. DSC curves and optical microscopy show that after melting, a new solid appears in the liquid crys tal phase or in the isotropic phase of these compounds. NMR spectra of the nematic phase and of the solid phase show that a thermal reaction releases dialkylamine into the medium and produces a disubstituted 2H-indazole in ra ther good yield. Further heating of each individual mixture leads to a seco nd wide range enantiotropic nematic phase. The purified disubstituted 2H-in dazole has a larger nematic range and the nematic phase is stable at high t emperature, indicating that the 2H-indazole motif can be used to build new mesogenic structures. The temperature dependence of the C-13 chemical shift s of one of the initial compounds was obtained. The results indicate that t he two chains on the nitrogen atom are equivalent and more or less folded b ack in the same way along the mesogenic core.