Mesophases hydrogen bonding

The interest in the structures of simple R2Si(OH)2 compounds lies in the fact that one of them, Bu 2Si(OH)2, forms a discotic liquid crystalline phase (308,309). Despite many attempts, it has not proved possible to obtain crystals of Bu 2Si(OH)2 suitable for a crystallographic study, the material obtained from various solvents usually being of a fine fibrous nature. The discotic phase of Bu 2Si(OH)2 has been proposed (309) to be due to the formation of dimeric disks of molecules which remain on breaking the interdimer hydrogen bonds in a structure of type 65 at the transition between crystal and mesophase. As has been described, structure type 65 is found for several diols similar to Bu 2Si(OH)2, and it is thus quite likely that Bu 2Si(OH)2 does indeed have the proposed structure. 

Fig. 10 The molecules involved in the formation of hydrogen-bonded nematic and columnar phases (upper part) and a representation of the arrangement of the molecules in the mesophases (lower part)...

Note 3 A cubic mesophase may be formed by rod-like molecules with strong, specific intermolecular interactions, such as hydrogen bonding, between them. However, they are also found in polycatenary compounds where there are no specific, strong interactions. 

RF-substituted benzoic acids (compounds 44—49 in Fig. 16) are somewhat distinct from the other taper shaped amphiphiles as they tend to form discrete cyclic hydrogen bonded dimers, which provide some rigidity to the core region [126], This gives rise to some interesting effects on mesophase structure. For example, the single... 

Hydrogen bonded dimers of 4-RF-substituted benzoic acids [119, 120, 122, 126], /rcyclohexane carboxylic acids [120], and hydrogen bonded complexes of benzoic acids with 4,4 -bipyridine and stilbazole [166, 243-246] can be regarded as supramolecular rod-like mesogens with two fluorinated chains. Figures 33 and 34 show comparisons of selected benzoic acid dimers and their bipyridine and stilbazole complexes. Comparison of the benzoic acid dimers 122 and 123 with -alkyl chains and F-terminated -alkyl chains, respectively, indicates that a single F-atom at the end of the alkyl chain reduces the mesophase stability... 

In addition, taper-shaped and dendritic units were connected either covalently (at the apex or at the periphery) [133,135, 383,405] or by means of self assembly (e.g., hydrogen bonding [399]) to a polymer backbone. For example, polymers based on poly[4-(3-methacryloylpropyl]pyridine] form hydrogen bonded complexes with partially fluorinated mono-, di-, or trisubstituted benzoic acids (see complex 221 in Fig. 72). The type of mesophase formed by these complexes varies from smectic via columnar to cubic, most probably micellar cubic, with rising... 

Two component hydrogen bonded mixtures of dendrimers and T-shaped branched amphiphiles are also a rich source of mesophases. Figure 13.21 shows a polarised optical microscope image of the contact region between the dendrimer 13.18 and the T-shaped amphiphile 13.19. A wide variety of mesophases co-exist as one component diffuses into another. The majority of these have been identified and studied.13... 

Kato, T., Frechet, J. M. J., New approach to mesophase stabilization through hydrogen-bonding molecular-interactions in binary-mixtures. J. Am. Chem. Soc. 1989, 111, 8533-8534. 

The valency of the hydrogen bond donors and acceptors can be varied to produce chiral mesophases [85]. Mixtures of the divalent 4,4 -bipyridine with 4-[(S)-2-methylbutoxy]benzoic acid in ratios between 1 9 and 4 6 show LC behaviour, but the chiral mesophases are exhibited for only a small range of compositions [86]. Here, the cholesteric and a blue LC phase were observed. The association of the acid to the bipyridine was confirmed by a crystal structure of the 1 2 complex. 

The preparation of a cubic phase with supramolecular chirality was achieved using a branched folic acid derivative incorporating glutamic acid residues (Fig. 11) as the source of chirality [93]. The pterin rings of folic acid residues are able to form a cyclic tetramer as a result of two hydrogen bonds between the components. Depending on the number of carbon atoms in the alkyl substituents, the compounds form columnar phases over a wide temperature range, and for 8 and 9 form cubic phases at temperatures above 130 °C. Addition of sodium triflate stabilises the cubic phase for 7, and the salt is incorporated into the other mesophases. It was implied that the cation resides between stacked tetramers. Supramolecular chirality is expressed for both the columnar and the cubic phases, as revealed by vari-... 

The greatest amount of work we have carried out with alkoxystil-bazoles is in the field of liquid crystals indeed, this is where our interest in stilbazoles started. After a brief and rather general introduction to liquid crystals, we will consider various types of complex that form liquid-crystal mesophases when complexed to stilbazoles, emphasizing patterns of behavior without delving into the subtleties. A more detailed discussion of the silver systems may be found elsewhere 24). Finally, although this article appears in a series that concentrates on inorganic chemistry, we offer an overview of some of our work with stilbazoles in hydrogen-bonded liquid crystals. 

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