Chiral Tilted Columnar Phase

Actually, it turned out that the material where the first ferroelectric switching was observed has a ferroelectric structure, where the neighbor polarization directors make a 60° angle with each other. The columns have elliptical cross sections, and they form quasi-hexagonal lattices (each column has six neighbors). Under the electric field the polarization directions have to rotate 

Structure of tilted columnar liquid crystals of chiral disc-shape molecules, and their behavior in electric fields in planar geometry. 

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Whilst tilted smectic phases of chiral materials are in general electro-optically switchable, many chiral tilted columnar systems show no optical effect whatsoever when an applied voltage is reversed (Figure 11.12). This resilience to the reorientation of dipoles by an electrical field may in some cases be due to the more crystalline than liquid crystalline nature of the systems, but even in the quite conventional high-temperature tilted phase of the first chiral material (triphenylene-2,3,6,7,10,ll-hexayl ( S )-3-... 

We see from Figure 8.13 that a number of liquid crystal phases, SmC of chiral rod-shape and the tilted columnar phase of chiral disc shape molecules, as well as the SmCP of achiral bent-core, and the tilted bowl-shape molecules all have C2 symmetry with eight independent piezoelectric coefficients. The direct and converse i- piezoelectric effects have been mostly studied in the fluid SmC liquid crystals. 

Spontaneous polarisation can also be observed for chiral discotic metallomesogens forming columnar mesophases, when the chiral molecules are tilted with respect to the column axis. The tilt induces a dipole moment within the plane of the molecule. A net macroscopic polarisation can be obtained for rectangular columnar phases with Cz or P2i symmetry. Serrano and Sierra reported on ferroelectric switching in the columnar mesophase for chiral /3-diketonate complexes (Figure 2.52). ... 

In both cases, the loss of mirror symmetry may be caused by a helix either the orientation of the molecular director or the position of the molecules may spiral within the column, or the column lattice is helically distorted. Within the column, mirror symmetry may also be lost by the introduction of tilt and polarization. Similarly to a surface-stabilized (thus nonhelical) smectic C phase of a chiral material, a nonhelical columnar phase of chiral molecules may itself be termed chiral if the optical axis of the molecules is inclined with respect to the column axis the vector of the tilt-induced po-... 

Most columnar phases are formed by more or less disk-shaped molecules whose two faces are either identical or not sufficiently different to induce polar packing (heads up, tails down) with respect to the column axis. If, in addition, the optical axis of the molecules is on average parallel to the column axis, then the phase has mirror symmetry as long as no helix is present. In most cases we can assume that the disks are essentially free to rotate around their short optical axis. But, as in smectic phases, the optical axis does not have to coincide with the column axis. Such tilted phases, which may be considered the columnar analogues of the smectic C phase(s), show a tilt-induced dipole moment if made up by chiral molecules and are thus pyroelectric, at least locally. 

The occurrence of a net dipole moment in tilted columnar (and smectic) phases can be most conveniently visualized and explained with a schematic molecule that consists of a rigid disk- (or rod-) shaped core, elongated flexible chains attached in two (or one) long direction(s), and chiral polar groups at the chain-core junctions (Figure 11.2). If the core and chains are of the same thickness, the optical axis of the molecules will tend to align along the column axis (or the layer normal). But if the core and chain are of different thicknesses, the formation of a kink between core and chain is favored. If the (e.g., acyloxy) chains are thicker than the (e.g., condensed aromatic) core, space is more efficiently used and the void between cores is reduced, if the cores (but not the chains) are tilted toward the column axis (or the layer normal). Thus the optical axis tilts, and a kink forms between the core and... 

Apart from the triphenylene mentioned in Section 11.2, one other case has been reported where a chiral columnar phase structure manifests itself in an asymmetric texture 2,3,9,10,16,17,23,24-octakis-(5-3,7-dimethyloctyloxy)-phthalocyanine exhibits two columnar phases above room temperature [8], [14]. The one at the higher temperature has a rectangular column lattice, the other a hexagonal one. When cooled quickly from the isotropic liquid, left-handed spirals appear in the flower-like texture of the highly ordered room temperature phase (Figure 11.8). X-ray and circular dichroism measurements indicate a helical superstructure with a pitch of 55 A (a 16 molecules). It seems very probable that in this case the molecules are weakly tilted and that the tilt direction spirals around the column axis, especially because the phase at higher temperatures is rectangular and therefore most probably tilted as well. 

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

Columnar phases can be ferroelectric if the discotic molecules are chiral and tilted with respect to the column axis [74]. From sym-... 

Discotic and bowl-shaped molecules that form columnar phases, (a) Hexa-alkanoate [R=CH3-(CH2) -2C00-] of triphenylene (b) hexa-noyloxy-tribenzocyclononene, which forms a polar pyramidlc phase (c) l,2,5,6,8,9,12,13-octakis-((S)-2-heptyloxy) dlbenzo[e,l]pyrene, which has chiral therminal chains and forms tilted ferroelectric columnar liquid crystal. ... 

Lyotropic nematic phases (see Section A) can also be produced by preparing, for instance, binary or ternary mixtures of organic disc-like compounds in suitable solvents such as hydrocarbons [20]. In linear saturated [20,21] or, as found recently [21], even better in cyclic saturated hydrocarbons, preferably cyclohexane [21], almie or in such a solvent plus an achiral or a chiral electron acceptor compound, induction of lyotropic Ncd or N coi phases, respectively, can occur. Sometimes, an Ncd phase can be formed in addition to a columnar phase [21]. Furthermore, it has also been observed that even two different No>i phases can be induced in diat way in the same system [22,23] showing a nematic-nematic phase transition [22-24] due to a diffa ence in the construction of their columns. In one of these Ncoi phases the constituent discs of the columns spontaneously formed are tilted with respect to the column axis, but in the second, parallel Ncoi phase they are untilted [22,23]. However, reliable data about the length of the columns in Ncoi phases do not yet seem to exist... 

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... 

Similarly to the molecular engineering of calamitic molecules to produce ferroelectric smectic C phases [129], disk-like molecules with chiral peripheral chains tilted with respect to the columnar axis were predicted to lead to ferroelectric columnar mesophases [130]. Indeed, as it is the case with all flat disk-shaped mesogenic molecules, the tilt is mainly associated with the flat rigid aromatic cores of the molecules, the side-chains being in a disordered state around the columnar core. Thus, the nearest part of the chains from the cores makes an angle with the plane of the tilted aromatic part of the molecules. If the chiral centre and the dipole moment are located close to the core, then each column possesses a non-zero time averaged dipole moment, and therefore a spontaneous polarization. For reasons of symmetry, this polarization must be, on average, perpendicular to both the columnar axis and to the tilt direction in other words, the polarization is parallel to the axis about which the disk-shaped molecules rotate when they tilt as shown in Fig. 29. 

Figure 11.2. The origin of pyroelectricity in tilted chiral columnar (and smectic) phases side view on disks (or rods) Y — polar group the column axis is vertical, the optical axis is tilted, and the vector of the macroscopic dipole is perpendicular to the plane of the paper. (Reprinted with permission from J. Mater. Chem. 5, 417 1995, The Royal Society of Chemistry [1].)...

Discotic Ferroelectrics. Columnar meso-phases with chiral disk-shaped molecules whose normals are tilted with respect to the column axis also manifest ferroelectric switching [217]. In such phases the direction of the spontaneous polarization is perpendicular to both the column axis and to the normal to the molecular disks. The tilt of the disk forms a helix with its axis oriented along the columns. In an amphiphilic... 

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