Liquid crystals mesomorphic complexes

Depending on the R group, this reaction could lead to the formation of gold(I) isocyanide complexes that behave as liquid crystals. Thus, complexes [Au(C6F5) (C = N(QH4)OCioH2i-p)] and [Au(QF5)(C = N(QH4)OCnH2n + rP)] [64] where n = 4, 6, 8, 10 and 12 show this behavior. All of these complexes are mesomorphic and behave as liquid crystals showing a nematic (N) phase when the isocyanide has a... 

However, the hydrogen-bonded mesogens that are of most interest in the context of this article are those elaborated initially by Kato and Frechet in the early 1990s [24-33]. In this approach, a pyridine, which may or may not have liquid crystal properties, was hydrogen bonded with a 4-substituted benzoic acid to form a new species with its own, distinct mesomorphism. For example, complex 9 shows a SmA phase that persists to 238 °C (n = 2, m = 4), while its free component pyridine is nematic to 213 °C the component benzoic acid is also nematic (as the H-bonded dimer) to 147 °C (although note that the notional monomer would not be liquid crystalline). 

Another approach to get new liquid crystals is the lateral fluorination of the stilbazole ligands,337 which is a common and highly effective tool to exert control over mesomorphism, crystal phase stability, and physical properties. Other modifications include the use of more alkoxy substituents and other alkyl sulfate anions.338-344 Ionic silver amino complexes also display liquid crystalline behavior at rather low temperatures they are of the form [Ag(NH2 -CJl +OJX (X = N03, n = 6,8,10,12,14 X = BF4, = 8,10,12,14).345... 

Nonmesomorphic as well as mesomorphic F-shaped acids with a structure similar to banana liquid crystals, have been complexed to a 2,4,6-triamino-l,3,5-triazine derivative <06JA4487>. 

When a liquid crystal is a metal-containing material, it is called a metallomesogen. It is commonly accepted that the first report on metallomesogens was by Vorlander in 1923, who reported a series of mercury mesomorphic complexes (Figure 7.4) [2]. The subject was not very popular until the end of the 1970s, then the number of reports increased regularly until the 1990s and then stabilized. 

Complexes with the simplest alkoxyphenylisocyanide and several halides are prepared by metathetical reactions of [AuCl(CNR)] with KX salts (Figure 7.19) [17]. The chloro-derivatives (n > 4) andthebromo-complexes (n > 6) display SmA phases. However, the ligands and the iodo-complexes are not liquid crystals. The transition temperatures decrease in the order Cl > Br > I, according to the decrease in polarity of the Au—X bond. It is important to note that the coordination of a very simple non-mesomorphic isocyanide (only one alkoxy chain and one aromatic ring) to Au—Cl allows the formation of a quite ordered and stable smectic mesophase. 

The corresponding pyrazole mononuclear derivatives [AuCl(pzH)] have been prepared with the same ligands (Figure 7.41) [38]. Only the monosubstituted non-mesomorphic pyrazole leads to a liquid crystal, not the disubstituted mesomorphic ligand. The complex displays a SmA mesophase on cooling to 69.2 °C, although this is mixed with some crystallization. Moreover, this derivative is luminescent at 77 in the solid state. In addition, the corresponding trinuclear and mononuclear derivatives, namely [ Au(pz) 3] and [Au(Hpz)2]+, which could be expected to be liquid crystals are non-mesomorphic. 

Gitsov presented a series of poly(benzyl ether) monodendrimers capable of cation complexation lacking alkyloxy side chains which were non-mesomorphic [93], Laschat followed a different route to obtain disk-like liquid crystals equipped with crown ether moieties. The crown ether was not attached to the mesogenic... 

The transition regions in liquid crystalline polymers or mesogenic polymers is much more complex. These transitions are referred to as mesomorphic transitions, and occur when one goes from a crystal to a liquid crystal, from a liquid crystal to another liquid... 

Supramolecular chemistry can be used to create the bent cores that give rise to the symmetry breaking in this family of liquid crystals [139]. The formation of a complex between a calamitic benzoic acid derivative and a bent core terminated with a pyridyl group—neither of which display mesomorphic behaviour—gave rise to a material which displayed SmCP mesophases. The achiral bent cores can also give rise to symmetry breaking when they are attached to flexible polymeric chains, such as poly(siloxane) [140]. 

Having found that gold triflates were mesomorphic, we returned to silver and decided to look at other anions. In so doing, we made complexes of silver trifiate and nitrate, each of which exhibited similar patterns of liquid-crystal behavior namely, the formation of Sc and Sa phases at rather elevated temperatures (the phase diagram for the nitrates is reproduced in Fig. 40) (54). While neither of these systems showed a cubic phase, we did find a nematic phase in the triflates. 

Having synthesized a number of complexes of monoalkoxystilbazoles, we explored the effect of using various poly(alkoxy)stilbazoles, given the interesting mesomorphism found in polycatenar mesogens (Section VI,A,3). This work is discussed in detail elsewhere 24). Here, we give an overview of the work to enable the reader to see the overall pattern of liquid-crystal behavior and the issues that arise. 

Metal-containing liquid crystals opened the way to new geometries and new topologies, in comparison with wholly organic materials, and allowed further investigation of the relationship between structure and mesomorphic properties. Mononuclear (Ni, Cu, Rh, Pd, Pt, V, Ag, Au, Zn, rare earths) and dinuclear (Rh, Mo, Ru, Ni, Cu and Pd) coordination complexes containing liquid crystals were prepared and studied during the last decade [5]. 

We designed and studied [27] a family of unsymmetrically l,l -disubstituted ferrocene derivatives obtained by combining the organic units A and B (above), used to prepare the families 13 and 14, respectively within the same molecular framework. Structures 25 (Fig. 9-21) led to remarkable mesomorphic properties. All derivatives exhibited liquid crystal properties. Compound 25 (n = 11) gave rise to an enantiotropic smectic A phase. Complex 25 (n = 12) showed an enantiotropic... 

As an example, it was reported that the TfO salt of the tetracoordinated silver(l) complex with 2,2 -bipyridine-4,4 -disubstituted ligands 33 yielded a novel ionic liquid crystal displaying columnar mesomorphism [69]. The coordination and the ionic bonds drive the supramolecular assembly because the ligand is nonmesogenic or smectic, but when coordinated to the metal, it gives columnar mesomorphism... 

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