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

PIRO, OSCAR ENRIQUE; ECHEVERRÍA, GUSTAVO ALBERTO; CUKIERNIK, FABIO DANIEL

Crystallography Reviews

Taylor & Francis

Año: 2018 vol. 24 p. 3 - 21

0889-311X

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) studiesmake a substantial contribution to the field of LC. Focusing thisreview on smectic (Sm, lamellar) LC, we first describe how extremelycareful XRD studies performed on mono-domain samples in theLC phase helped to elucidate the molecular structure of orderedSm phases. Then, we describe selected examples in which singlecrystal(SC) XRD on the solid-state phase of the mesogens providedinformation about their supra-molecular organization in the Smphase. Finally, we present a different approach to this problem inthe case of a thermo-tropic Schiff base (SB) which undergoes crystal↔LC↔isotropic liquid phase transformations. By combined SCand variable-temperature powder XRD, we show that the SB LC is ahexatic smectic B phase that derives from the crystal phase by relativelysmall topological changes promoted by the set-in of thermalrotational disorder around the long SB molecular axis.