Smectic C phase chiral

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. 

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. 

Note 5 This chiral smectic C phase is also known as the ferroelectric chiral smectic C phase. 

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... 

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. 

Figure 2. Schematic representation of the cholesteric and chiral smectic C phase. The repeating distance along the helical axis (pitch) is between 200 nm to some pm.

The first chiral combined lc polymers prepared for this purpose showed the desired cholesteric and chiral smectic C phases only at high temperatures (8) (the melting point was always above 100°C). By using lateral substituents (see Figure 3) it is possible however to suppress the melting temperature and to obtain polymers with a glass transition temperature of about room temperature, without losing the cholesteric and chiral smectic C phases (9). 

First X-ray measurements show that the helical superstructure of the cholesteric and chiral smectic C phase can be untwisted by stretching the elastomer (5). High strains of 300% are necessary for this purpose (compared to 20% for the achiral elastomers). Nevertheless these results show that the chiral lc elastomers have the potential to act as mechano-optical couplers (cholesteric phase) or as piezo-elements (chiral smectic C phase) (5), because the mechanically induced change of the helical superstructure has to change the optical transmission or reflection properties or the spontaneous polarization. Both effects however have not yet been measured directly. 

Starting from these polymers it is possible to introduce the chiral acids known from low molar mass liquid crystals (12) and to obtain the chiral homopolymers presented in Scheme III and Table III. These polymers show a high spontaneous polarization in the chiral smectic C phase (14) (see polymer 7, Table III) and selective reflection of visible light in the cholesteric phase (see polymer 9, Table III) (13). 

Calamitic metallomesogens forming a chiral smectic C phase (SmC ) are ferroelectric materials. Due to the low symmetry of this phase when the helix is unwound (C2) the molecular dipoles are aUgned within the layers of the SmC phase, giving rise to ferroelectric order in the layers. Because the SmC phase has a helical structure, there is no net macroscopic dipole moment for the bulk phase. However, it is possible to unwind the helix by application of an external electric field or by surface anchoring in thin cells. Under such conditions, a well-aligned film of the ferroelectric liquid crystal can exhibit a net polarisation, called the spontaneous polarisation (Ps). Ferroelectric liquid crystals are of interest for display applications because the macroscopic polarisation can be switched very fast by an... 

Almost all the smectic phases, in which the molecules are arranged in layers and are tilted with respect to the layers, have counterpart chiral phases. The most important one of this class is the chiral smectic C phase — Sc phase. In these chiral liquid crystal phases, the molecules are tilted at a constant angle with respect to the layer normal but the tilt azimuthal rotates uniformly along the chiral axis and forms a helical structure. 

The ease of forming the smectic mesophase by this class of side-group type liquid crystalline polymers has rendered a great possibility in synthesizing polymeric chiral smectic materials useful in non-linear optics, transducers, pyroelectric detectors and display devices (Chapter 6). The first polymer forming a chiral smectic-C phase was synthesized by Shibaev et al. (1984). It has a polymethacrylate main chain, a long polymethylene spacer, and a mesogenic unit attached at the end with a chiral moiety (polymer (3.60)). Since then, a lot of polymers with chiral mesophases have been synthesized and studied (Le Barny and Dubois, 1989). 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

Figure 6.38. The chiral smectic C phase (a and b) and the ferroelectric liquid crystal display (c and d).

The use of chiral H-bonding components yields stable chiral phases by molecular association [35-37]. For example, a ferroelectric phase derived from a chiral smectic C phase can be induced by the self-assembly of nonferro-electric molecules [35-37]. Complex 11 (n = 8) exhibits a chiral smectic phase from 109 to 123 °C [35]. In this case, the H-bonded mesogen consists of nonmesomorphic chiral benzoic acid and a stilhazole that shows a very narrow temperature range of ordered smectic phases. The value of spontaneous polarization for 11 at 115 °C is 33.0 nC/cm. When the oxygen in the alkoxy... 

The polymeric complexes derived from 4-nitro- and cyanostilbazoles also show a smectic A phase up to about 200 °C [78a]. Polysiloxane complexes 32 also exhibit thermally stable smectic A or C mesophases [79-81]. These carboxyl-functionalized polymers and stilbazoles are miscible in a whole range of composition and show stable mesomorphic behavior [26, 79]. The introduction of the chiral stilbazole for the formation of a mesogenic complex leads to the induction of ferroelectricity [80]. Polymeric complex 33 exhibits a chiral smectic C phase, while no ferroelectricity is observed for each of single components. The value of spontaneous polarization for 33 x — 0.43, n = 5) is 21.0 nC/cm at 112 °C. The hydrogen bonding between the carboxylic acid and... 

Because of the interest in chiral LC phases in general, which give rise to selective reflection of light (cholesteric phase) or fer-roelectricity (chiral smectic C phase), chiral combined LC polymers were prepared quite early on [4]. Polymers with cholesteric and chiral smectic C phases could be prepared easily. As these polymers were synthesized using to the polycondensation process shown in Scheme 1, the chiral groups had to be selected carefully in order to prevent racemization during polycondensation [4, 7, 12, 13]. 

X-ray measurements on LC elastomers have shown [6-8] that the reversible transition between a chiral smectic C phase with and without a helical superstructure can be induced mechanically. The helix untwisted state corresponds in this case to a polar ferroelectric monodomain. The piezoelectricity arising from this deformation of the helical superstructure (which does not require a complete untwisting) has been demonstrated [9] for polymers cross-linked by polymerization of pendant acrylate groups (Figure 15). 

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

Preceding the reports on elastomers, piezoelectricity in chiral smectic C phases of low-molar weight molecules or of polymers has usually been observed. The special property is that the system possesses macroscopic electrical polarization without an external field, so it is classified as ferroelectric. 

Experimentally, piezoelectricity in cholesteric and chiral smectic C phases was reported for liquid-crystalline networks [140-147]. Multidomain lightly cross-linked systems were synthesized, then the orientation is obtained by mechanical strain [140] or by poling [147]. In other samples this orientation is performed prior to the crosslinking process [144, 146]. Macro-scopically oriented samples were subjected to either a static or a periodically varying strain. Open circuit voltages across the samples were measured that are linear functions of the applied strain [140-142, 144, 145],... 

Alam MZ, Yoshioka T, Ogata T, Nonaka T, Kurihara S. 2007. The influence of molecular structure on helical twisting power of chiral azobenzene compounds. Liq Cryst 470 63 70. Bahr C, Escher C, Fliegner D, Heppke G, Molsen H. 1991. Behavior of helical pitch in cholesteric and chiral smectic C phases. Ber Bunsen Ges 95(10) 1233 1237. 

---