Ferroelectricity columnar phases

A homologous dibenzopyrene compound 51, which differs from 48 only in side-chain length, was studied by the same authors and shown to have two ferroelectric phases.58,59 The coexistence of the two columnar phases depended... 

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). ... 

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. 

Between the polarized nematic and soHd phases a density region seems to exist where dipolar hard and soft spheres form ferroelectric columns with square ordering in the plane perpendicular to the coliunns [102,134], The stabihty and density range of this phase needs further investigation. Such a columnar phase has also been identifled in a system of extended dipoles where the coliunns order on a hexagonal lattice [ 146]. 

For (b), two models are plausible One may assume either an antiferro-electric pattern of tilted columns for which small fields cause a deviation of the tilt directions and for which a larger threshold field changes the pattern of tilt directions into a ferroelectric structure. Alternatively, a helical tilt may be distorted at lower fields and unwound at higher fields. As the helix pitch in columnar phases is usually much shorter than the wavelength of visible light, no selective reflection of light is expected and one may best distinguish between the two cases by X-ray or circular dichroism measurements. Only... 

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. 

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... 

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

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)... 

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. ... 

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