Chiral smectic A phase

Note 6 A smectic A-phase containing a chiral molecule or dopant, can be called a chiral smectic A-phase. The recommended symbol is SmA wherein the () indicates that the macroscopic structure of the mesophase is chiral. 

Demikhov, E., Kozlovsky, M. V, Amorphous chiral smectic A phase of side-chain copolymers. Liquid Crystals 18, 911 (1995). 

The structure of the smectic A phase when it is composed of optically active material (i.e., smectic A ) remains the same as that for the achiral phase. The molecules are arranged in diffuse disordered layers, and there is no long-range periodic order. However, because of the molecular chirality, the environmental symmetry is reduced to [10]. As a consequence, when an electric field is applied to a chiral smectic A= phase there will be a coupling of the electroclinic susceptibility to the field and the long axes of the molecules will tilt with respect to the layer planes. The tilt angle, for relatively low applied fields, varies linearly with the field. This linear electrooptic phenomenon is called the electroclinic effect. 

This is a chiral smectic A with symmetry Dqo. Its properties are similar to those of the achiral SmA. However, close to the transition to the smectic C phase, the chiral smectic A phase shows interesting pretransitional phenomena in the dielectric and electrooptical effects (the so-caUed soft dielectric mode and electroclinic effect). They will be discussed in Chapter 13. 

Closely related to the chiral smectic-C phase is the so-called chiral smectic-A phase (sa, see Fig. 14b) (for an overview see [24, 25,119,142,143]). Without the presence of external electric fields, the Sa phase is identical to the smectic-A (sa)... 

Fig. 14 Ferroelectricity in LCs. (a) In the smectic-C phase an electric field (E) can reverse the tilt and thereby the direction of the polar axis perpendicular to the tilt direction, (b) Application of an electric field to a chiral smectic-A phase induces a tilt of the molecules and thereby a polar structure, (c) In an aligned bent-core nematic phase a dipole is present in the direction of the kink. [Ill, 112]...

Mono- and dicyclopalladation of mesogenic pyridazines was achieved and the subsequent reaction with /3-diketones gave metallomesogens 125-128 (Scheme 19). Monometallated derivatives have a flat central core, dimetallated derivatives have a sterically induced twist in the molecule rendering them chiral. Smectic A phases are exhibited, with transition temperatures in the region of 100/300 °G. 

The combination of liquid crystalline chirality and surface polarization is also the origin of the so-called surface electroclinic effect in chiral smectic A phases if the smectic layers are oriented perpendicular to the surface, the surface electric field tilts the layer normal away from the surface molecular orientation [108-110] (Fig. 7). 

S. Nishiyama. Y. Ouchi, H. lUceme, and A. Fukuila. Giant electroclinic effect in chiral smectic A phase of ferroelectric liquid crystals, Jpn. J. AppL Phys. 26 L1787 (1967). 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Recently, we also found that several types of chiral smectic C phases composed of achiral and chiral aromatic esters exhibit a large shift of their pitch band CD, depend-... 

A good deal in synthesis effort has been devoted to chiral liquid crystals, especially those w ith chiral smectic C phases. The chiral smectic C phase is ferroelectric. w hich gives it properties quite useful lor applications. Perhaps the most important properly of these phases is that a lateral dipnle can produce a spontaneous polarization... 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

Miyachi K, Matsushima J, Ishikawa K, Takezoe H, Fukuda A (1995) Spontaneous polarization parallel to the tilt plane in the antiferroelectric chiral smectic-CA phase of liquid-crystals as observed by polarized infrared-spectroscopy. Phys Rev E 52 R2153-R2156... 

Earlier work on nanoparticle-doped chiral smectic-A (SmA ) and chiral smec-tic-C (SmC ) phases including some intriguing electro-optic effects in ferroelectric SmC phases were summarized in two earlier reviews [1, 2],... 

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... 

Chiral lc-polymers can be prepared by a proper functionalization of lc-polymers with chiral and reactive groups. These elastomers are interesting, because they combine the mechanical orientability of achiral lc-elastomers with the properties of chiral lc-phases, e.g. the ferroelectric properties of the chiral smectic C phase. The synthesis of these elastomers was very complicated so far, but the use of lc-polymers, which are functionalized with hydroxyl-groups, has opened an easy access to these systems. Also photocrosslinkable chiral lc-polymers can be prepared via this route. 

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