Columnar mesophases ferroelectricity

Fig. 19. Antiferroelectric to ferroelectric transition in a columnar mesophase made of molecules with a cone-shaped core...

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). ... 

The vanadyl group is attractive since it exhibits a square pyramidal structure and has a large directional dipole associated with the V=0 bond. The axial polarity should result from the head-to-tail alignment of these groups in a linear chain structure -Y-0-Y-0-. Even if these materials have the molecular requirements necessary to yield columnar mesophases with an axial polarity, unfortunately it was not possible to define the existence of a spontaneous polarization. However, a very recent paper [126] seems to indicate that ferroelectric columnar mesophases with the polarization parallel... 

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. 

Reddy, R. A. Raghunathan, V. A. Sadashiva, B. K. Novel ferroelectric and antiferroelec-tric smectic and columnar mesophases in fluorinated symmetrical bent-core compounds. Chem. Mater. 2005,17, 274—283. 

More recently, chiral discotic materials that exhibit tilted columnar mesophases have attracted attention because of their potential for ferroelectric switching. A spontaneous polarisation is generated because of the restriction in rotation of the peripheral chiral chains. 

In addition, structures with an iron complex in place of the central ring in series such as 17,18, and 19 may also exhibit nematic, SmC, and columnar mesophases. These new metallomesogens are of interest because they are easily prepared in optically pure forms and therefore can lead to ferroelectric properties in the chiral SmC form. For example, complex 21 with n = 11 exhibits SmC and cholesteric properties (Cr 52.5°C SmC 111°C N 119 I), while only a hexagonal columnar phase is observed with n = 12 (Cr60.5°C 112.5 I)... 

In the early development of liquid crystals, for the most part, the study of small molecular systems dominated the field because of the close link between molecular design and commercial applications. However, it is only in the last 20 years that materials with unusual, and often hybrid structures have been investigated for their liquid-crystalline behavior. As noted, phasmidic materials, which have molecular structures that are part-disc part-rod, were found to exhibit both columnar and smectic phases. More recently, molecular systems having bent-rod-like structures have been investigated and found to exhibit a wide range of novel phases, many of which were found to be ferroelectric or antiferroelectric (without molecular chirality) due to the reduced symmetry of their mesophase structures. 

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