Cyclohexanes discotics

The structure of the molecular association can be controlled by the substitution patterns of the hydroxyl groups on the cyclohexane rings, as shown in Fig. 22. When all the hydroxyl groups are modified by alkoxyl chains, the compound forms one column by one molecule (Fig. 22A), while the molecules that have two hydroxyl groups form one column by two molecules by hydrogen bonding (Fig. 22B). In the case of inositols with four hydroxyl groups, five molecules behave as one molecular discotic component (Fig. 22C). 

Cyclic hydrocarbons with six equivalent substituent chains emanating from several carbon atoms about the ring can again adopt conformations in which the molecular shape is disk-like, but the aspect ratio is much larger. If the chains are somewhat flexible and the molecules possess an appropriate balance between order and disorder and attractive and repulsive intermolecular forces, the creation of new mesophase types can be envisioned - and several have been found [102,104]. The two general classes of aggregation in liquid crystalline phases of discotic molecules are lenticular nematic (N ) and columnar discotic (D) [100-104]. Carbonaceous pitch mesophases, discussed in section 1.4.5, resemble phases. Only those discotic mesophases with benzene, cyclohexane and shape-related cores having primarily alkyl chains as substituents will be discussed here. 

The vast majority of discotic mesogens with cyclohexane cores and six substituent chains are based on ui- and s-IS. Pertinent to the discussion above concerning the relationship between the direction of projection of chains on 1,2,3,4,5,6-hexasubstituted cyclohexanes, the hexa(n-hexyl) ether of m-IS with one axial hexyloxy (6-O-m-IS) has been shown not to be liquid crystalline the corresponding s-IS hexaether with a more disk-like shape (6-O-s-IS) forms an enantiotropic columnar discotic phase... 

In some rare cases, e.g., when a calamitic phase consists of solely aliphatic compotmds or cyclohexane derivatives, its anisotropy xf" is very small and can even vanish. As to the discotic mesophases, they have, as a rule, negative diamagnetic anisotropy = Zjj" — itl" < ue to a considerably larger value of the susceptibility component perpendicular to the director (for discotics would be closer to zero line than Xn " in the plot similar to Fig. 7.2). 

The synthesis of discotic liquid crystalline cyclohexanes is relatively straightforward. Both classes of derivative (the scylloinosi-tol ethers and esters) are synthesized from commercially available scylloinositol (Scheme 60). The esters are prepared by treatment with an excess of acid chloride in trifluoroacetic acid [222], whereas the ether is synthesized using potassium hydroxide and hexyl bromide [223]. The mesophase... 

Table 54. Transition temperatures for cyclohexane (inositol) based discotic mesogens (see Scheme 60 for structures).

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