Switching columnar phases

In summary, phthalocyanines modified with crown ethers are interesting synthetic targets as they are prone to form columnar phases. Their electron conductivity and complexation properties make them interesting candidates for the design of sensor materials or supramolecular switches. 

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). ... 

A possible model for the structure of the columnar phases is sketched below. Since two optical isomers (A and A) are present in a 50 50 ratio, they must be randomly distributed in a column. Also, it is possible to consider a stacking of the complexes in such a way that a net polarisation is induced in the column direction (Fig. 6a stacking of a same enantiomer). In columnar phases, the symmetry is such that the net dipole of every column cannot be cancelled out by the one of the neighbouring column, that is a perfectly antiferroelectric arrangement of the columns is impossible the system is frustrated (Fig. 6b). Thus, a net polarisation must result in the columnar phase whose direction it should be possible to switch. 

For example, for n = 12, two transitions within the liquid crystalline region are observed from a nematic columnar phase (Ncoi) to a hexagonal columnar lattice (Dh) and then finally to a rectangular lattice (Dr). The X-ray diffraction data on benzimi-dazolium salts have also been reported [40], and indicate a switch from a lamellar... 

It is important to note that also nonchiral molecules are capable of forming chiral mesophases. In particular, molecules with a bent core ( bananashaped molecules) can build polar, and even chiral liquid crystal structures [75]-[78]. Bent-core molecules form a variety of new phases (B1-B7, Table 1.3) which differ from the usual smectic and columnar phases (see also Chapter 8). As a consequence of the polar arrangement, antiferroelectric-like switching was observed in the B2 phase formed by bent-core molecules, and second harmonic generation was found in both the B2 phase and the B4 phase. The latter phase is probably a solid crystal. It consists of two domains showing selective reflection with opposite handedness. In the liquid crystalline B2 phase, the effective nonlinear susceptibility can be modulated by an external dc field [79] (Figure 1.15). 

The chirality of the columnar phase may manifest itself, as in other liquid crystal phases, in a pronounced circular dichroism, in an amplified specific rotation, in a piezoelectric response, and in electro-optical switching effects. The columnar organization of concentrated DNA is one of the most prominent examples of the occurrence of liquid crystal phases in chiral natural systems. 

Figure 11.12. Electro-optical switching of tilted columnar phases. (Reprinted with permission from J. Mater. Chem. 5, 417 1995, Royal Society of Chemistry [1].)...

Cases (c) and (d) both represent truly ferroelectric behavior, which can be found in smectic C phases of chiral molecules only if the helix is suppressed by the boundary conditions at the liquid crystal glass interfaces. Because it is nearly impossible to align columnar phases uniformly with rubbed or otherwise anisotropic polymer surfaces (the switching is generally studied in polydomain or sheared samples), and because the ferroelectric behavior remains present in thick cells, surface induction of ferroelectricity can be excluded. It must be assumed that the two-dimensional column lattice suppresses helix formation in these materials. 

Polar Columnar (ColP) Phases In calamitic rod-shaped LCs, the frustration occurring in the layer organization of molecules due to steric and/or polar effects leads to form a variety of 2D density structures such as undulated layers, modulated layers (ribbons), and Cols. The situation for bent-core molecules with an ability to form macroscopic polar order is much more complex, and different types of modulated smectic and Col phases form. Since their 2D X-ray patterns, structural models, and nomenclature have been in great detail described in the two previous reviews [29, 32], in this section, the field-induced switching properties of polar columnar (ColP) phases are focused on. 

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... 

Keith C, Reddy RA, Tschitaske C (2005) The first example of a liquid crystalline side-chain polymer with bent-cme mesogenic units ferroelectric switching and spontaneous achiral symmetry breaking in an achiral polymer. Chem Commun 7 871-873 Keith C, Dantlgraber G, Amaranatha Reddy R, Baumeister U, Tschierske C (2007) Ferroelectric and antiferroelectric smectic and columnar liquid crystalline phases framed by silylated and non-silylated molecules with ilurainated bent Cotes. Chtan Mater 19 694-710... 

This book does not intend to cover the whole field of thermotropic liquid crystalline (TLC) materials as it is extremely difficult to cover within a single book. Instead it presents a collection of Chapters written by experts on various exciting topics in the field. Properties of recently developed TLCs (such as banana-type, thiophene-based, and columnar TLCs), phase biaxiaUty, and novel polymeric TLCs are discussed In detail. Solid-state NMR studies to obtain atomistic-level structural and geometrical information of TLCs are presented. Synthesis of liquid crystalUne conjugated polymers, fast switching of nematic materials by an electric field, and photoconducting discotic systems are also presented. 

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