Biphenyls nematics

The main factor in determining the handedness of the cholesterics induced by bridged 1,1 -binaphtliyls is the helicity (P or M) of the solute, and this observation is the basis of many configurational studies of chiral binaphthyls. All the homochiral (aP)-binaphthyls 15-19 have an M helicity of the core, and all induce, in biphenyl nematics, M cholesterics.65,75 By systematic structural variations of the covalent bridge, it is possible to obtain I J -binaphthalenes with dihedral angles ranging from 60° to 96° (see series 20-24) the handedness of the cholesteric phase always matches the helicity... 

Fig. 19 Phase transition behaviour of the mixture of 22 and the LC 4-pentyl-4 -(cyano)biphenyl (nematic 24-35 °C) as a function of their molar ratios and proposed structure for the molecular aggregates...

Figure 16, A typical cyano-biphenyl nematic liquid crystal molecule.

The compound in Fig. 3b exhibits two smectic phases (Sm and Sm ) in addition to nematic, whereas the compound in Fig. 3a exhibits only a nematic phase. The substitution of an alkoxy for an alkyl tail is known to shift phase transition temperatures considerably. In the cyano-biphenyls (Fig. 4), substitution of an alkoxy tail raises the melting point from 24 to 48 °C and T from 35 to 68 °C [22]. 

In this section, we will describe the crystal structures of the mesogenic A -n-alkoxy-4-cyano-biphenyls (Fig. 1) [53-57]. The compounds have the phase sequences crystal-nematic-isotropic for CB05—CB07 and monotropic nematic for CB01-CB04 crystal-smectic A-nematic-isotropic for CB08. The crystal and molecular data of the investigated compounds CBOn are summarised in Table 2. 

Apart from the parent compound 1 and its very simple alkyl derivatives, 1,3,4-oxadiazoles are solids. Solid oxadiazoles containing biphenyl or triphenyl substituents exhibit interesting properties upon heating. The symmetric 2,5-bisbiphenyl-4-yl-l,3,4-oxadiazole 38 melts into an isotropic phase showing small monotropic mesophase. By contrast, the asymmetric (hockey stick-shaped) mesogen 2-terphenyl-4-yl-5-phenyl-l,3,4-oxadiazole 39 exhibits a more stable enantiotropic liquid crystalline phase (a smectic phase as well as a nematic phase) <2001PCB8845>. 

Fig. 16. Structure of a banana molecule 4,6-dichloro-l,3-phenylene bis[4 -(9-deceny-loxy)biphenyl] carboxylate and 13C NMR spectra in the nematic (top) and isotropic (bottom) phase.

By the same procedure are obtained the corresponding biphenyl isocyanide derivatives (Figure 7.20) [18]. Now, the free isonitriles are already liquid crystals displaying nematic and SmA phases with a short range of existence at moderate temperatures (40-85 °C), while the complexes show a marked increase in the melting points and also an expansion ofthe range of existence of the mesophase (up to 140 °C N and SmA phases). The exception is the shortest iodo-derivative, which is not a mesogen. Most of the complexes decompose into the isotropic state (above 220 °C). The biphenyl moiety increases the polarizability anisotropy compared to the phenyl and hence facilitates liquid crystal behavior. 

Upon addition of a terminal crown ether to the calamitic core unit, nematic phases were observed for 2, 3 in agreement with previous studies on substituted cyano biphenyls [25, 26]. 

A. di Matteo, S. M. Todd, G. GottareUi, G. Solladie, V. E. Williams and R. P. Lemieux, Correlation between molecular structure and helicity of induced chiral nematics in terms of short-range and electrostatic-induction interactions. The case of chiral biphenyls, J. Am. Chem. Soc., 123 (2001) 7842-7851. 

Complexes of these stilbazoles were prepared with some rather simple benzoic acids, such as 4-nitrobenzoic acid, 4-cyanobenzoic acid, and biphenyl-4-carboxyhc acid (Fig. 59) (71). Here, none of the components is liquid crystalline indeed none possesses a flexible chain. Thus one would expect that complexes between them would do nothing but melt. How wrong The data (Fig. 60) reveal that five of the complexes exhibit a nematic phase Remarkably, some these materials have clearing points around 220° C, indicating that the hydrogen bonds are stable to these temperatures. Whoever said hydrogen bonds are not strong ... 

Samples were approximately 400 mg sealed in 6 mm glass tubing after degassing by repeated freezing and evacuation. All observations are in the nematic phase. Synthesis of 4-cyano-4 -[ H]upentyl-biphenyl was by the procedure of Gray and Mosley. ... 

Ferrocene derivative 16 showed an enantiotropic nematic phase between 204 °C and 235 °C. This result indicated that a biphenyl system, when associated with the ferrocenyl moiety substituted in the 1,3-positions, was also propitious to liquid crystal formation (compare 16 with its l,T-isomeric analogue 7 (n = 6)). 

The shorter biphenyl group has been used with alkyl spacers, Polymers 15 and 16 of Table 1, to give both smectic and nematic polymers depending on the direction of the linking ester group 20,22,26) found that if the mesogen is... 

A molecule with three cholesterol units and one biphenyl unit according to Scheme 1 connected to a tetrasiloxane shows an elongated structure (Fig. 3). Molecules of this shape can be arranged in nematic, smectic, or cholesteric phases. 

In the series of complexes shown as B in Figure 37, the effect of fluorination of the biphenyl ring has been studied [77b]. The effects are similar to those of fluorination discussed above, namely where the fluorine points away fi om the core, that is substitution in the 2, 3 or 4 -position then Sa phases are observed whereas fluorination at the 2 -position produces a nematic complex. Fluorines at both the 2 and 2 -position completely destabilizes the mesophases. 

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