Unusual mesogenic compounds; molecular geometry and stability of ordered fluid mixtures

Abstract

The most known mesogenic molecules have the following structure ▪ (calamitic molecules) conducting to nematic and smectic mesophases. With this general scheme and particular terminal groups pure compounds exhibiting stable nematic or smectic mesophases and particular physical properties can be obtained. For example with stable free radicals we obtain mesomorphoric spin labels for the structural studies by ESR technique. With a side group incorporating an appropriate heavy metal atom we obtain derivatives suitable for Mössbauer measurements. Other mesophases can be obtained with non extended molecules; plastic crystals with globular molecules and discotic mesophases with disk-like molecules. A simple way to study the respective stabilities of the mesomorphic and isotropic binary mixtures versus temperature and concentration is the selting of isobaric please diagrams. Generally two mesogenic molecules having a similar geometrical form are totally miscible in the mesomorphic states. Contrarily, two mesogenic molecules exhibiting dissimilar forms (calamitic and globular, calamitic and disk-like, globular and discotic) are totally miscible in the liquid phase, but the common miscibilities in the partially ordered states are very small.