Liquid-crystalline properties of unsaturated piperazine-2,5-dione derivatives

The synthesis, characterization, and mesomorphic properties of a new type o f liquid-crystalline compounds, (3Z,6Z)-3,6-bis(3,4-dialkoxybenzylidene)pip erazine-2,5-diones are reported. These compounds were derived from unsatura ted piperazine-2,5-dione as the core group, and were prepared by condensati on reactions of 1,4-diacetylpiperazine-2,5-dione and 3,4-dialkoxybenzaldehy des. The products were characterized by H-1- and C-13-NMR spectroscopy, and elemental analysis, and the phase behavior of these compounds was characte rized and studied by differential scanning calorimetry (DSC) and polarizati on microscopy. The results indicate that these rod-like compounds exhibit s mectic C (S-c) phases. However, for the derivatives with two flexible alkox y side chains, highly ordered smectic G (S-G) phases were also formed and c onfirmed by X-ray powder diffraction. The liquid crystallinity of these mol ecules was attributed to the presence of intermolecular hydrogen bonds invo lving the NH groups of the heterocyclic rings. The correlation of phase beh avior and molecular shape is also discussed.