Oligomers discotic

Meier et al. have also contributed to the field of cyclic PAV oligomers with the synthesis of cyclic dl -trans (all- ) trimers (e.g. 113) containing alkoxy-substituted 1,7-naphthylene and 1,9-phenanthrylene building blocks, via a Siegrist-type trimerizing olefination [134,135]. Suitable substitution at the periphery of the cyclic trimers allows for the formation of stable, thermotropic discotic mesophases [134]. 

Figure 23 Synthesis of twin discotic dimer and branched oligomers from CTTV-7 mesogens connected with a hexadecane spacer. (From Ref. 76.)...

Cellulose and oligocellulose derivatives have been studied and a variety of thermotropic LC phases established, i.e., cholesteric and colunmar structures for cellulose derivatives, discotic columnar and smectic-type for the oligomers, depending on the side group and the main-chain lengths. A list of compounds exhibiting thermotropic me-sophases is presented in Table 5. 

Scheme 10a in Fig. 5 represents the formation of low-MW closed assemblies such as host-guest complexes, supermolecules, and cyclic oligomers. Examples include the supermolecules 11 and 12 and the cyclic folic acid tetramer (3 in Fig. 3). These compounds aroused interest due to their pronounced thermotropic behaviors compared to the unbound components. The enhancement of liquid crystallinity for Complex 11 is explainable by general consideration of soft interactions in low-MW mesogens. " More complex is the case of Complexes 12 and 3 that, due to discotic shape, can form columnar stacks. A close relationship between liquid crystallinity and columnar growth was anticipated.However, recent analysis showed that columnar growth of the folate tetramer receives only a minor contribution from contact forces. The large excluded volume effect of thin disks is the main driving force for liquid crystallinity and self-assembly.

Aida et al. have shown that o-phenylene octamers can be used as surface modifiers to obtain homeotropic ordering of a variety of DLCs [116]. The octa-meric o-phenylenes are known to fold helically into a cylindrical architecture that resembles to a pi-stacked column of DLCs. It has been found that the oligomers adhere to the glass substrate with its cylindrical axis orthogonal to the surface. Therefore it is speculated that this face-on orientation of the octamer likely nucleates the homeotropic ordering of the liquid crystalline discotic materials. 

There are several polyols that exhibit discotic mesophases. Diisobutylsilanediol, which has been known since 1955 [342] but was not classified as hexagonal columnar until 1980 [343], associates in dimers [343] or oligomers [344] in order to form disk-like units (42). 

Many unconventional and discotic compounds, especially those with large molecules, show a pronounced tendency towards the glassy state [196,319f, 319g, 352-354, 358-365]. Glassy states are well known and common in polymers, and liquid crystalline oligomers show phase behaviours that are intermediate between those of low molecu-... 

Discotic liquid crystals of phenylacetylene macrocycles 29 were first reported by Zhang and Moore in 1994 [136]. Their synthesis involves the stepwise build-up of phenylacetylene hexamers [137] followed by a macrocyclization step [138]. The key reactions of the synthesis (Scheme 33) are the palladium-catalysed coupling of aryl iodides and acetylenes, and the selective deprotection steps allowing the ends of the growing oligomer to be distinguished. The synthesis shown is just one of the many possible sequences that can be used to prepare the macrocycles. The mesophase behaviour of the discotic derivatives prepared to date is shown in Table 35 [136]. In some ways... 

Mukkamala, R., Burns, C.L., Jr., Catchings, R.M., III, and Weiss, R.G., Photopolymerization of carbohydrate-based discotic mesogens. Syntheses and phase properties of l,2,3,4,6-penta-o-(trans-3,4-dialkoxycinnamoyl)-(d)-glucopyranoses and their oligomers, J.Am. Chem. Soc, 118,9498,1996. 

The hydroxy-functionalized TPs are very valuable precursors for the synthesis of many other discotics such as TP dimers, oligomers, polymers, networks, mixed tail derivatives and lower and higher degree substituted TP derivatives. Although a number of hydroxyl-alkokxy-TPs are possible and many of them have also been prepared, monohydroxy-pentaalkoxy-TPs are of particular importance. Accordingly, several methods have been developed to prepare these materials [68-74]. 

It may be noted that a large number of other discotic oligomers and polymers have been realized but their conducting properties have not been evaluated and, therefore, their synthesis has also not been covered here. Interested readers may... 

Kumar, S. Triphenylene-based discotic liquid crystal dimers, oligomers and polymers, liq. Cryst. 32, 1089-1113 (2005)... 

Boden, N., Bushby, R.J., Cammidge, A.N., Martin, P.S. Glass-forming discotic liquid-crystalline oligomers. J. Mater. Chem. 5, 1857-1860 (1995)... 

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