ASYMMETRIC DISCOTIC LIQUID-CRYSTALS BASED ON RUFIGALLOL

Two series of asymmetric hexaethers (asymmetric by virtue of having al kyl chains of unequal lengths) of rufigallol, viz., R442n and R482n ,5 -dialkoxy-2,3,6,7-tetrabutoxy-9,10-anthraquinone and dialkoxy-2,3,6,7- tetraoctyloxy-9,10-anthraquinone, respectively) were synthesized. Thes e hexaethers exhibit a typical hexagonal columnar mesophase (D-h) as e videnced from X-ray diffraction and polarized light microscopic studie s. Differential scanning calorimetric (DSC) studies suggest that the t emperatures of isotropization (TD-i) and the molar isotropization entr opy (Delta S-i) fall with increase in asymmetry. The fall in Delta S-i with increase in asymmetry is significantly larger than what may be e xpected merely from increase in the relative alkyl chain content, sugg esting that asymmetry causes a significant decrease in the extent of o rdering in the columnar mesophase. However, X-ray diffraction studies suggests that the intercolumnar spacing (D) is not affected by the pre sence of asymmetry but depends only on the total number of alkyl chain carbon atoms (C-n) present in a molecule. Furthermore, from the linea r variation of the disk area, pi D-2/4, versus C-n (which was shown to be system independent), it was possible to calculate the volume occup ied by a single alkyl chain methylene (CH2) unit in the columnar mesop hase. This value is significantly smaller than that estimated in liqui d n-alkanes, suggesting that the alkyl chains are not truely liquidlik e in such discotic coulmnar mesophases. A further interesting observat ion in these systems is that when the alkyl chains are very long, an a dditional ordering of the side chains suggestive of side-chain crystal lization is observed.