Achiral smectics

Tbe purpose of tbe bydroxyl group is to acbieve some hydrogen bonding with the nearby carbonyl group and therefore hinder the motion of the chiral center. Another way to achieve the chiral smectic Cphase is to add a chiral dopant to a smectic Chquid crystal. In order to achieve a material with fast switching times, a chiral compound with high spontaneous polarization is sometimes added to a mixture of low viscosity achiral smectic C compounds. These dopants sometimes possess Hquid crystal phases in pure form and sometimes do not. 

In detailed studies of this mixture, strong evidence was obtained suggesting that the new achiral smectic phase is antiferroelectric. This consisted mainly of the observation of a double hysteresis loop in the polarization vs. applied electric field curve for the material.30 In addition, it was shown that the mixture is a... 

Note 4 Locally, the structure of the chiral smectic C mesophase is essentially the same as that of the achiral smectic C mesophase except that there is a precession of the tilt direction about a single axis. It has the symmetry C2 in the Schoenflies notation. 

The existence of the layers and director tilt in the achiral smectic C liquid crystal phase are experimental facts. Given these, the maximum possible symmetry of the phase would be Ci, with a C2 axis normal to the tilt plane, and a a plane congruent with the tilt plane. In fact, there is no fundamental reason why a given C phase must possess either of these symmetry elements. But, breaking of either of the symmetry elements would afford polar symmetry, and no C phase has ever been shown to possess any property associated with polar symmetry (e.g. pyroelectricity). Therefore, we can say that all known C phases indeed possess the maximum possible symmetry consistent with the layers and tilt. 

It should be stated that an electric field of < 10 V/pm was applied to the cell in order to unwind the FLC helix of 3, and the observed NLO behavior is a combination of the electric field induced SHG (EFISH) and that due to the spontaneous polar order in the phase. While other FLCs give much lower SHG efficiency with the same applied fields, and achiral smectic LC phenylbenzoates in our hands give unobservable SHG under identical conditions, we cannot completely rule out at this time the possibility that a significant amount of the response from compound 3 is due to the electrical poling. Control experiments to test for this (e.g. by SHG from compound 5 and/or racemic 3) are in progress, as are further experiments aimed at obtaining the phase-matched SHG efficiency for 3. 

Metallomesogens have been shown to form helical supramolecular organisations in their mesophases [95]. Chiral oxazoline complexes with various metal ions and six alkyl chains did not show LC behaviour, but when mixed with trinitrofluorenone form achiral smectic A phases [96]. Furthermore, when a branch was included in the structure of the ligands (Fig. 12) the corresponding complexes with copper(II) and palladium(II) form columnar mesophases which have a helical organisation [97]. The presence of the stereogenic centre near the central metal ion in these complexes (Fig. 12) is enough to cause the parallel molecules to stack in a tilted manner with... 

Plate 2. The contact regions between the (R)- and the (S)-l-methylheptyl 4 -(4-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates under a cover-slip on a microscope slide and between crossed polars (xlOO). The central region is where the two materials met and mixed. This region exhibits the normal focal-conic texture of the achiral smectic A phase. The left- and right-hand sides of the plate show the homeotropic and filamentary textures of the chiral TGBA phase... 

The chirality of molecules breaks the mirror symmetry C2h of the achiral smectic C phase. The only symmetry element left is a twofold rotation axis C2, and the point symmetry group becomes C2 instead of C2h- The structure of a single smectic C layer is shown in Fig. 4.34. As in achiral smectic C, the molecules in the layer obey head-to-tail symmetry, the director n coincides with average orientation of molecular axes and form angle 1 with the smectic normal h. 

Due to low symmetry (C2) of the chiral smectic C phase, its theoretical description is very complicated. Even description of the achiral smectic C phase is not at all simple. In the chiral SmC phase two new aspects are very important, the spatially modulated (helical) structure and the presence of spontaneous polarisation. The strict theory of the SmA -SmC transition developed by Pikin [10] is based on consideration of the two-component order parameter, represented by the c-director whose projections ( 1, 2) = are combinations of the director compo-... 

Achiral smectic materials with anticUnic molecular packing are very rare [40] and their antiferroelectric properties have unequivocally been demonstrated only in 1996 [41]. The antiferroelectilc properties have been observed in mixtures of two achiral components, although no one of the two manifested this behaviour. In different mixtures of a rod like mesogenic compound (monomer) with the polymer comprised by chemically same rod-like mesogenic molecules a characteristic antiferroelectric hysteresis of the pyroelectric coefficient proportional to the spontaneous polarization value has been observed for an example see Fig. 13.27a. Upon application of a low voltage the response is linear, at a higher field a field-induced AF-F transition occurs. 

V. Domenici, M. Geppi, C.A. Veracini, NMR in chiral and achiral smectic phases structure, orientational order and dynamics, Prog. Nucl. Magn. Reson. Spectrosc. 50 (2007) 1. 

To start with, the present chapter will address some fundamental concepts of liquid crystals to enable a thorough comprehension of the aims and scope of this thesis. The properties of and the discovery of the thermotropic SmC phase will be dealt with in more detail, as they are essential for understanding the significance of the thesis presented. Finally, examples of lyotropic analogs of the achiral smectic C (SmC) phase, which were known up to now, will be discussed in this introductory chapter. 

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. 

The incorporation of a chiral dopant into an achiral smectic C matrix results in the helical twisting of the structure of the mixture. In the simplest case, the wave vector of the helix is proportional to the concentration of a dopant. 

FIGURE 1.19. Temperature dependence of the spontaneous polarization induced by the chiral dipolar dopant L-4-decloxybenzylidene-4 - aunino-2-cyano-propylcinnamate in the achiral smectic C matrix of 4-nonyloxy-benzy-hdene-4 -amino-pentylcinnamate. The concentration of the dopant is indicated in weight percent. 

A special strategy has been developed for composing ferroelectric mixtures. Such materials are based on chiral dipolar compounds which form the smectic C phase. The chemical synthesis of such compounds is extremely difficult. However, the problem can be solved step by step an achiral smectic C matrix with a wide temperature range may be worked out separately, and then doped by a chiral dipolar additive (the chirality and dipole moment cannot be decoupled from each other, as was discussed in Section 1.2.5). 

The field-induced tilt in the achiral smectic A phase (see above) results in a change in symmetry from to C2h The induced tilt angle is proportional to the field squared and the factor (TcA/T-rc A) 7 where y= 1.3 [1]. Due to the tilt, optical biaxiality can be observed in the vicinity of the transition. 

In smectic A phases where the smectic layers are perpendicular to the molecules, the orientation of the whole structure is, in principle, fixed once the orientation of the molecules is defined by the interface. The surface orientation of achiral smectic A phases is then the same as that of the nematic phase [88, 89]. However, since splay deformations of smectic layers (director bend deformations) are forbidden and layer bend deformations (director splay deformations) require a lot of energy, smectic phases tend to adopt uniform configurations, even between two walls inducing two different orientations. In the latter case, the surface orientation of the smectic phase differs from that of the nematic phase, and depends on the layer configuration in the bulk [90, 91]. 

Generally the applications of achiral smectic liquid crystals fall into three categories as follows ... 

A. Findon and H.F. Gleeson, Elastic constants of an achiral smectic-C material, Ferroelectrics, 277, 35-45 (2002). 

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