INFLUENCE OF STERIC INTERACTIONS AND RANDOM SIDE-CHAIN VARIATIONS ON THE MESOMORPHIC PROPERTIES OF BOWLIC MESOGENS

The effect of structural variations on the mesomorphic nature of colum nar liquid crystals of general structure I is studied. X-ray crystal s tructure analyses of compounds 1a and 2a reveal the columnar organizat ion of the macrocyclic cores in the solid state. The up and down asymm etry of the cores is resolved in an alternating sequence of up-up and down-down intermolecular contacts. Specific interactions between the R substituents are present, which influence the stacking distance betwe en the cores in the down-down intermolecular contacts. Statistical inc orporation of different side chains on the same macrocyclic core produ ces, when the difference in length between the alkyl chains is appropr iate, random mixtures having wide mesomorphic ranges. The increase of disorder associated with the statistical side chain distribution on th e core and with the structural heterogeneity of each component of the mixture have the effect of depressing mainly the crystal-mesophase tra nsition temperature.