Triphenylenes discotic cores

There are relatively few reports of liquid crystal dimers containing discotic mesogenic units [105-112] and this has been due in large part to the difficulties in preparing monofunctionalised discotic precursors. Without such monofunctional precursors the preparation of discotic dimers often involves laborious separation procedures involving several isomers of the discotic core. In recent years, however, there have been many advances in the synthesis of triphenylene-... 

Discotic architectures that use a central triphenylene core with six peripheral triphenylene units also exhibit colunmar mesophases, and these are commonly called star-like liquid crystals. Compoimd 28 is a very large molecule that uses flexible spacers to attach peripheral triphenylene units to a central discotic core in a star-like manner. The hexagonal columnar phase of compound 28 has been identified as hexagonal. Such stractmes are ohgomeric and could almost be considered polymeric (see Chapter 5). 

Triphenylene (TP) derivatives have been described as the work horses in the field of DLCs [11]. It is the most studied discotic core system [20, 21]. TP (1), see Scheme 4.1, belongs to the polycyclic aromatic hydrocarbon (PAH) group and has been known in the chemical literature for more than a century. This trimer of benzene was isolated from the pyrolytic products of benzene by Schultz who named it as triphenylene [22]. It can also be isolated from coal tar. Trimerization of cyclohexanone followed by dehydrogenation has been used to generate TP in the early twentieth century and its various chemical and physical properties were studied [22]. 

To some extent, the design criteria for discotic mesogens are somewhat simpler than those for their calamitic counterparts. In many cases, it is possible to choose a favorite disk-like molecule, then add 6-8 peripheral alkyl chains to generate a mesomorphic (i.e., hquid-crystalline) material. Thus, phthalocyanines, triphenylenes, truxenes, and many other systems will generate mesogenic (i.e., liquid-crystal-like) materials with appropriate substitution these examples are shown in Fig. 24. Note that it is not necessary for the mesogen to have a planar core. 

Triphenylenes provided with nonionic di(ethylene oxide) side-chains (25f)132 134 or with ionic alkyl chains (25g)135 form supramolecular polymers in water.136 The arene—arene interactions of the aromatic cores allow for the formation of columnar micelles . At low concentrations the columns are relatively short, and the solutions are isotropic. At higher concentrations the longer columns interact and lyotropic mesophases are formed.133 Computer simulations showed that in the isotropic solution the polymerization of the discotics is driven by solute-solute attraction and follows the theory of isodesmic linear aggregation the association constants for dimerization, trimerization, and etc., are equal and the DP of the column thus can easily be tuned by concentration and temperature.137 138 At higher concentrations the sizes of the columns are influenced by their neighbors, the columns align, and the DP rises rapidly. 

Kumar S, Pal SK (2005) Ionic discotic liquid crystals synthesis and characterization of pyridinium bromides containing a triphenylene core. Tetrahedron Lett 46 4127-4130... 

Deuterium NMR spectroscopy of the discotic phase of hexa-n-hexyloxy triphenylene has led to similar conclusions. Spectra of two selectively deuterated isotopic species, one in which all aromatic positions are substituted and the other in which only the a-carbon side chains are substituted, bring out the difference between the order parameters of the cores and the tails. Fig. 6.1.3 gives the quadrupole splittings of the aromatic and the a-aliphatic deuterons versus temperature in the meso-phase region. It is seen that the rigid core is highly ordered, the orientational order parameter s ranging from 0.95 to 0.90, whereas the a-aliphatic chains are in a disordered state. 

Till now only few examples of discotic liquid crystalline polymers are known. In the first example described in literature (1),hexasubstituted triphenylene cores as discotic sidegroups, are linked to a polysiloxane backbone via a flexible alkyl spacer. Discotic main chain polymers (2,3) posses as disclike core benzene or tripenylene derivatives with the same high degree of substitution, i.e. six. 

Most discotic liquid crystals are based on a few core structures, e.g., benzene, triphenylene and traxene. Suitable benzene starting materials are commercially available e.g, hexabromobenzene and hexahydroxybenzene). Hexahydroxytriphenylene (163) and hexabromotriphenylene (166) can be synthesised according to Schemes 39 and 40, respectively. The peripheral moieties, all six of them, are attached to the core units in the same marmer as for calamitic liquid crystals e.g., direct bond, ester-linked and acetylene-linked). 

Discotic liquid crystals are a prototypical self-assembled columnar system [9-13]. This class of liquid-crystalline compounds is relatively new, discovered in 1977 by Chandrasekar and coworkers [14,15]. The assembly motif, shown in Figure 2, for this class of compounds has an aromatic core that is surrounded by hydrocarbon chains. These disk-shaped molecules then stack to form columns. These one-dimensional stacks aggregate to form arrayed columns. When the columns have a circular cross-section they typically stack into a hexagonal arrangement as shown in Figure 2(a). Some of the original discotics were hexa-substituted phthalocyanines (1), benzenes (2), and triphenylenes (3), shown in Figme 2(b). The self-assembly of classical discotics will not be presented in depth here because it has been a focus of several comprehensive reviews [9-15]. 

Since their discovery as discotic liquid crystals, both triphenylenes and phthalocyanines have been studied as chiral columns [74]. There are no chirooptic signatures such as circular dichroism or optical rotation changes for the triphenylenes that are substituted with chiral side chains. For the phthalocyanines, Nolte and coworkers have shown that when their exterior is substituted with chiral side chains the phthalocyanine core shows an altered CD spectrum in aggregates, in thin films and in concentrated solutions [75]. These helical stacks, wrapped like ropes, can be seen with electron... 

The experiment was performed on a glass-forming discotic liquid crystal (2S, 3S)-2-chloro-3-methyl-pentanoyloxi-pentakis (pentyloxi)-triphenylene [9.7]. The triphenylene cores are stacked to form a discotic columnar phase. Several mechanisms can lead to axial motions around the columnar axis and 2D-exchange NMR can be used to distinguish these mechanisms. Figure 9.6... 

Until 1977, it was believed that only rod-like molecules, having a considerably higher length than width, could show liquid crystalline properties (calamitic mesogens). However, in 1977, Chandrasekhar realized that not only rod-like molecules, but also compounds with a disc-like molecular shape are capable of forming mesophases (Chandrasekhar et al. 1977 Sabine et al. 2007) and these are referred to as discotic liquid crystals (DLCs). The first examples of this kind of mesomorphism were observed in hexa-alkanoyloxybenzenes, hexaalkoxy- and -aUcanoyloxy-triphenylenes (Fig. 16.2), and were established by X-ray studies (Kumar 2010). Generally, the disc shaped molecules have central core as benzene or polyaromatic, such as triphenylene or phthalocyanine. 

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