Synthesis and liquid crystalline properties of Rufigallol based discotic materials

Abstract

In 1977 Chandrashekar et al. discovered the discotic liquid crystalline phase. A typical discotic molecule has an aromatic core to which are attached via ester or ether links six to eight alkyl chains, for example benzene hexa-n-alkanoates. Since then, a variety of discotic liquid crystals have been reported in literature. The field of discotic liquid crystals has grown rapidly with over 1000 discotic compounds known. The area of discotic liquid crystals is reviewed in Chapter I; the types of mesophases exhibited along with typical examples from literature, physical properties, methods of characterization, potential applications, and various classes of discotic compounds viz. thermotropic small molecules, hydrogen-bonded liquid crystals, polymers, and lyotropic liquid crystals are the topics covered in this review. Chapter II deals with the synthesis of 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). It has been demonstrated that a variety of hexaethers of rufigallol can be purified from their corresponding partially etherified counterparts very efficiently by using basic alumina as the stationary phase in column chromatography; in all cases, the pure yellow hexaether elutes out while the burgundy partially etherified products with acidic phenolic residues remain stuck at the top of the column. 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 (T_D-i) and the molar isotropization entropy (?S_i) fall with increase in asymmetry. The fall in ?S_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 suggest that the inter-columnar 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 disc area, ?D²/4, versus C_n (which was shown to be system-independent), it was possible to calculate the volume occupied by a single alkyl chain methylene (CH?) unit in the columnar mesophase. This value is significantly smaller than the one estimated in liquid n-alkanes, suggesting that the alkyl chains are not truly liquid-like in such discotic columnar 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. In Chapter III, the synthesis of two series of thermotropic main-chain discotic liquid crystalline polyethers, PR4m-n, based on rufigallol which were prepared starting from the symmetric tetraethers of rufigallol, R4m; ‘m’ and ‘n’ represent the number of carbon atoms in the side chain and spacer segment, respectively, is discussed. The symmetric tetraethers were in turn readily prepared by selective alkylation of rufigallol under controlled phase-transfer conditions. GPC analysis of the polymers suggested that they were all of moderate molecular weights, with M_n varying between 5400 to 17,000. The length of the spacer segment ‘n’ in these polyethers was systematically varied and its effect on the phase transition temperatures and the mesophase structure was examined using DSC, polarized light microscopy, and X-ray diffraction. It is noticed that when the spacer lengths are relatively long (n ? 2m), the isotropization temperature (T_D-i) decreases as the spacer length ‘n’ increases; an observation that is in accordance with those previously made. However, when the spacer lengths are relatively small (n < 2m), the dependence of T_D-i is quite the opposite; T_D-i actually increases with increase in spacer length. Furthermore, X-ray diffraction studies indicate that, in the discotic columnar mesophases that are formed, the columns pack in a hexagonal manner when n ? 2m, while they do so in a rectangular lattice when n < 2m, leading to the formation of Dh and Dr mesophases, respectively. Finally, comparison of the discotic polyethers with their low molar mass analogues confirms the role of polymerization in stabilizing the mesophase; while all the polymers exhibit columnar mesophases, some of their low molar mass analogues are not liquid crystalline. The synthesis of chromophoric discotic amphiphiles based on rufigallol, 1,2,3,5,6,7-hexa-(o-methoxy-ethoxy-ethoxy)-9,10-anthraquinone, hexa-(o-methoxy-ethoxy-ethoxy-ethoxy)-9,10-anthraquinone and 1,2,3,5,6,7-hexa-(undecylinic acid-11-oxy)-9,10-anthraquinone, 4.2.1h, 4.2.1i, and 4.2.1g is presented in Chapter IV. The UV-Visible spectra of the amphiphile 4.2.1g was monitored over a range of concentrations; the plot of absorbance versus concentration was shown to follow the Beer-Lambert relationship. The CAC of the amphiphile was determined by fluorescence spectroscopy; the monomer fluorescence was quenched beyond the CAC suggesting the formation of cofacial aggregates. Transmission electron microscopic studies of the amphiphile 4.2.1g at post-CAC concentrations showed the presence of uniform worm-like micelles having a diameter of 33 Å which is close to the diameter of the disk-like molecule, 4.2.1g, obtained from molecular modeling. The TEM observations thus support the fluorescence data which suggests cofacial aggregation of the molecules in the micelles. At higher concentration, the amphiphile 4.2.1g in aqueous K?CO? forms a lyotropic liquid crystalline gel which exhibits a birefringent pattern when viewed under a polarizing light microscope. The non-ionic discotic amphiphiles 4.2.1h and 4.2.1i did not exhibit any birefringent lyotropic textures when examined by polarizing light microscopy using the penetration technique; the hydrophilic-lipophilic balance (HLB) of these molecules is such that they are extremely water-soluble, i.e., they are not sufficiently amphiphilic to exhibit lyotropic mesophases. Preliminary studies of the behavior of amphiphiles 4.2.1g, 4.2.1h, and 4.2.1i at the air-water interface were carried out using a Langmuir trough. The observed molecular area for the amphiphile 4.2.1g of (124 Ų) is much closer to the projected area of the compressed conformation of the molecule and probably the one adopted by the molecule in the Langmuir film. The amphiphile 4.2.1g does not exhibit thermotropic liquid crystallinity. It seems that the COOH groups at the termini of the side chains of 4.2.1g introduce hydrogen bonding interactions between discs thus pushing up the melting temperature of the alkyl chains which destabilizes and in this case destroys the discotic mesophase.