On the polymorphism of the smectic A phases of highly polar compounds

The smectic A phase is characterized by a layered arrangement of rod-like molecules, with the average orientation of the molecules normal to the layers and a liquid-like organization within the layers. When the molecules have a strongly polar end group (like C=N, -NO2 etc.,), neighbouring molecules prefer an antiparallel orientation. This results in a bilayer structure, with a layer spacing much larger than the molecular length. In many compounds, the structure of the bilayer is sensitive to temperature (or pressure). This results in some interesting phenomena: (a) reentrant phases, as for example, the appearance of a nematic phase below the temperature range of existence of the A phase; (b) a number of polymorphic forms of smectic A; and (c) an 'induced smectic A' phase when the compounds with the highly polar terminal groups are mixed with weakly polar compounds even when neither compound by itself exhibits the A phase. This article gives a brief summary of recent results on all these aspects