Liquid crystal glycolipids - Carbohydrate Chemistry: Chemical and Biological Approaches - page 334

Consider now joining together two molecules that have one head and
two tails. They can have the tails of one molecule joined to the tails of one
other to give a macrocyclic system, or they can have just one tail-to-tail
join leaving the other two to be conformationally free. Where the two tails
are joined together the macrocyclic system will be not be con-
formationally free, and so the system will have self-organising properties
similar to those of a bolaphile, i.e. tending to support the formation of
lamellar phases if the two head groups are the same. If on the other hand
there is a single join the shape associated with the aliphatic chains can be
broader due to the conformational freedom of the other two chains, and
hence the structure could resemble two back-to-back wedge shaped
monomer units. In this case one might expect to observe the formation of
columnar phases. Compound 22 is an example of such a glycolipid, it was
prepared as a mimic of membrane components of archael bacteria;
several other examples have recently been reported that have similar
molecular architectures. 29,33 For the material shown the free aliphatic
chains are branched, which broadens the aliphatic region of the mol-
ecule. Investigations of the phase behaviour of glycolipid 22 shows that it
exhibits a columnar mesophase, which was characterised from its clas-
sical defect textures as being a disordered columnar hexagonal phase. 34
The fan-like domains of the phase exhibited no elliptical/hyperbolic de-
fects, negating the presence of a lamellar phase, and were smooth indi-
cating that the positions of the molecules in the phase are disordered.
Moreover, as with the materials described in section 4, once melted it did
not return easily into the solid state even upon cooling to 25 1C, instead
it glassified, and when heated returned to the liquid crystal state without
change in the microscopic texture, indicating that it readily self-heals.
The term ''bimesogenic'' infers that there are two mesogenic units
within a molecular structure, and that they are the same, dimesogens
similarly have two mesogenic units tied together, but that they do not
necessarily have the same structure. Compound 23 has a supermolecular
architecture that combines two mesogenic units together, in this case a
glycolipid and a cholesterol unit. The overall structure is dichotomous,
and with two units that may have very different properties. 35