Crystallography and the liquid crystal phase: a new approach to structural studies on a thermo-tropic smectic Schiff base

Abstract

In spite of the apparent contradiction between ‘liquid crystals’ (LC, materials exhibiting some degree of disorder) and ‘crystallography’ (a paradigmatic ordered kingdom), X-ray diffraction (XRD) studies make a substantial contribution to the field of LC. Focusing this review on smectic (Sm, lamellar) LC, we first describe how extremely careful XRD studies performed on mono-domain samples in the LC phase helped to elucidate the molecular structure of ordered Sm phases. Then, we describe selected examples in which single-crystal (SC) XRD on the solid-state phase of the mesogens provided information about their supra-molecular organization in the Sm phase. Finally, we present a different approach to this problem in the case of a thermo-tropic Schiff base (SB) which undergoes crystal ↔ LC ↔ isotropic liquid phase transformations. By combined SC and variable-temperature powder XRD, we show that the SB LC is a hexatic smectic B phase that derives from the crystal phase by relatively small topological changes promoted by the set-in of thermal rotational disorder around the long SB molecular axis.