Asymmetric discotic liquid crystals based on rufigallol

Abstract

Two series of asymmetric hexaethers (asymmetric by virtue of having alkyl chains of unequal lengths) of rufigallol, viz., R442n and R482n (1,5-dialkoxy-2,3,6,7-tetrabutoxy-9,10-anthraquinone and 1,5-dialkoxy-2,3,6,7-tetraoctyloxy-9,10-anthraquinone, respectively) were synthesized. These hexaethers exhibit a typical hexagonal columnar mesophase (D_h) as evidenced from X-ray diffraction and polarized light microscopic studies. Differential scanning calorimetric (DSC) studies suggest that the temperatures of isotropization (T_D-i) and the molar isotropization entropy (ΔS_i) fall with increase in asymmetry. The fall in ΔS_i with increase in asymmetry is significantly larger than what may be expected merely from increase in the relative alkyl chain content, suggesting that asymmetry causes a significant decrease in the extent of ordering in the columnar mesophase. However, X-ray diffraction studies suggests that the intercolumnar spacing (D) is not affected by the presence of asymmetry but depends only on the total number of alkyl chain carbon atoms (C_n) present in a molecule. Furthermore, from the linear variation of the disk area, πD²/4, versus C_n (which was shown to be system independent), it was possible to calculate the volume occupied by a single alkyl chain methylene (CH₂) unit in the columnar mesophase. This value is significantly smaller than that estimated in liquid n-alkanes, suggesting that the alkyl chains are not truely liquidlike in such discotic coulmnar mesophases. A further interesting observation in these systems is that when the alkyl chains are very long, an additional ordering of the side chains suggestive of side-chain crystallization is observed.