Metallomesogens ferroelectricity

Finally, ferroelectricity has been shown for columnar metallomesogens.35 Serrano and co-workers have shown that metal ft-diketonates, provided with chiral side chains (e.g., 32), form helical columns (vide supra), which can also be switched under an alternating electric field. 

The first report of a ferroelectric effect by a metallomesogen was for an open-book palladium(II) complexes (Figure 44) of type discussed above, where the chirality was introduced in the bridging carboxylate [14a]. The H NMR spectra identified that the complex was a mix of isomers trans-AR,R (34%), trans-AR,R (34%) and cw-R,R (32%). Ferroelectric switching was demonstrated, although at a much slower rate ( 1 s) than in organic liquid crystal systems, due to the high viscosity of the material. 

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... 

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). ... 

Finally, the first example of a ferroelectric metallomesogen was obtained with a chiral derivative of 26 [43]. A derivative was synthesised by ligand exchange where the carboxylate was (R)-2-chloropropionate (instead of m 2m+i)y nd a mixture was produced whose composition was evaluated to be anfi-AR,R 34% antz-AR,R 34% syn-R,R 32% by NMR. The resulting mixture showed a phase (Crys 102 Sc 119 S 149T) which was found to switch at a speed of 330 ms at a square wave voltage of 17 V and 0.5 Hz and a cell thickness of 11 mm. 

The results reported above clearly indicate that the butadiene iron tricarbonyl moiety can be inserted into mesogenic structures, to give, when suitably designed, ferroelectric properties. This opens up a new and fascinating area of research in which metallomesogens can be used in electro-optical devices. However, little information is yet available on the structure of the mesophases. 

Starting from his initial work in 1989 on organometaUic compounds with a ferroelectric phase, Espinet developed the chemistry of metallomesogens based on dimeric palladium derivatives of chiral Schiff bases (Pd2(/r-Cl)2L 2 where L represents a chiral Schiff base ligand.Several examples of these complexes (6.21-6.23) are shown in Figure 6.14... 

The synthesis of nonchiral smectic liquid crystals is a broad topic for discussion, however, it can be divided into subsections in two different ways. For example, smectic systems can be split into metallomesogens and nonmetallomesogens, alternatively, they can be divided into materials for (1) meso-phase structure elucidation and classification [ 1 ], (2) property-structure correlations [2] and (3) host systems for ferroelectric and antiferroelectric mixtures. In the following sections template structures used for the synthesis of smectic materials will be described, followed by discussions of the syntheses of materials that have extensive histories in the elucidation of smectic phase structures, and finally of the syntheses of smectogens that are useful in applications. 

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)... 

Some of these materials display the potentially chiral ferroelectric SmC phase in addition to a SmA or cholesteric mesophase. Pyzuk [108] found for copper and nickel complexes some blue phase or novel type of amorphous phase between a tightly twisted chiral nematic phase and the isotropic liquid. These ferroelectric metallomesogens are interesting as they can be aligned par-... 

I phases, smectics 16 Idemitsu polymer FLC 651 irainomethyl, spacers 846 iminopyridine complexes, metallomesogens 905 improper ferroelectrics 537 f improved Alt-Pleshko addressing technique (lAPT) 207... 

M ribbons, chromonics 996 Mach-Zehnder interferometer 132 macrocyeles, hydrocarbon cores 719 f macrocyclic complexes, metallomesogens 926 macroscopic flow, chiral nematics 379 macroscopic polarization, ferroelectrics 541 ff macroscopic properties... 

Neumann principles, ferroelectrics 541 ff, 609, 615 nickel, ligands 914 f, 924 nitration, hydrocarbon cores 709 nitrile ligands, metallomesogens 902 nitro derivatives, polycatenars 878 nitrobenzene derivatives, chaige transfer 948 nitrobiphenylcarboxylic acids, synthesis 433 nitroester, phasmids 866 nomenclature... 

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