Asymmetric discotic liquid crystals based on rufigallol

Raja, K. S. ; Ramakrishnan, S. ; Raghunathan, V. A. (1997) Asymmetric discotic liquid crystals based on rufigallol Chemistry of Materials, 9 (7). pp. 1630-1637. ISSN 0897-4756

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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 (Dh) as evidenced from X-ray diffraction and polarized light microscopic studies. Differential scanning calorimetric (DSC) studies suggest that the temperatures of isotropization (TD-i) and the molar isotropization entropy (ΔSi) fall with increase in asymmetry. The fall in ΔSi 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 (Cn) present in a molecule. Furthermore, from the linear variation of the disk area, πD2/4, versus Cn (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.

Item Type:Article
Source:Copyright of this article belongs to American Chemical Society.
ID Code:67249
Deposited On:29 Oct 2011 12:03
Last Modified:06 Jul 2012 05:21

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