Achiral polyphilics, polar

Dantlgraber, G. Eremin, A. Diele, S. Hauser, A. Kresse, H. Pelzl, G. Tschierske, C. Chirality and macroscopic polar order in a ferroelectric smectic liquid-crystalline phase formed by achiral polyphilic bent-core molecules. Angew. Chem. Int. Ed. 2002, 41, 2408-2412. 

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

Finally, we should mention that the asymmetry of molecular shape, polyphilic effects and conformational constraints are the dominant factors in the stabilization of polar ordering in achiral mesogens. The examples presented above are, therefore, highly significant. They show that many liquid crystalline structures are intrinsically polar and may be effectively stabilized by suitable design of the mesogenic molecules. 

Actually, such bowl phases are still to be found. However, polar achiral phases have been observed in the so-called polyphilic compounds [8]. The rod-like molecules of these compounds consist of distinctly different chemical parts, a hydrophilic rigid core (a biphenyl moiety) and hydrophobic perlluoroalkyl- and alkyl-chains at opposite edges. Such molecules form polar blocks that, in turn, form a polar phase manifesting pyroelectric and piezoelectric properties with a field-induced hysteresis characteristic of ferroelectric phases. 

In principle, there is no reason to exclude the polar ordering of dipoles in the smectic A phase (Fig. 2b) along the molecular axis. No examples of such phases have been observed for low-molar-mass liquid crystals or for LC polymers. Tour-nilhac et al. [101] suggested an approach to ferroelectricity in the phase of polyphilic molecules consisting of aromatic, aliphatic, and perfluorinated parts they realized their approach successfully for an achiral smectic C phase only. An attempt to extend that approach to polyphilic LC copolymers [102] indicated antiferroelec-tric behavior of the synthesized materials. 

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