Metallomesogens chiral

Trzaska and co-workers showed a similar propeller mechanism for the formation of helical columns from disclike metallomesogens (29-31).34 These metallomesogens also have C3 symmetry and 30 and 31 are provided with chiral side chains. In the hexagonal columnar mesophase these chiral side chains induce a Cotton effect in the chromophore of the helically arranged core. Heating the mesophase to the isotropic liquid results in the disappearance of the Cotton effect because of the loss of helical order. This effect illustrates the need for the molecules to be positionally ordered in order for the side-chain chirality to be transferred to the supramolecular column. 

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. 

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... 

Fig. 12 Chiral metallomesogen based on a chiral oxazoline core and representations of its helical organisation from above (top right) and side-on (bottom right) to the helix axis [97]...

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. 

Figure 2.41 Room, temperature chiral silver(I)-containing metallomesogens (X = TfO, DOS)...

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). ... 

Recent progress in material science, notably with the development of new materials exhibiting blue phases, has generated a renewed interest in the incorporation of the functional properties with the unique structure of frustrated phases. Synthesis of a monosubstituted ferrocene-based chiral Schiff s base derivative which exhibits TGBA and blue phases has been reported [17] (Fig. 9). Other metallomesogens leading to blue phases have been found for palladium complexes [18] (Fig. 10). Optically active materials incorporating... 

Other metallomesogens containing the optically active butadienetricarbonyliron(O) moiety, incorporated into promesogenic chiral nematic ligands derived from cholesteryl, have been reported. " Both the diastereoisomers exhibited a chiral nematic phase and a monotropic smectic phase, the chiral nematic phase being monotropic for the (-l-)-isomer (Cr 193 (S 93 N 131) I), and enantiotropic for the (—(-isomer (Cr 117 (S 99) N 133 I). 

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. 

Metallomesogens with the Chiral Element Attached Directly to the Metal... 

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... 

Metallomesogens based on dimeric palladium (II) derivatives of enantiopure Schiff bases Pd2(lt-C1)2L 2 (6.21-6.23) where L represents a chiral Schiff base ligand. 

Metallomesogens formed by hexacatenar chirals have also been synthesized by Serrano, based on tetracoordinated complexes of Pd, Ni, Cu and Zn (6.28-29) (Figure 6.17). 

Metallomesogens Where the Metal and Ligands Generate Helical Chirality... 

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