Mesogenic properties

The introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

Coco, S Espinet, P Marb n-Alvarez, J.M. and Levelut, A.M. (1997) Effects of Isonitrile Substituents on Mesogenic Properties of Halogoldiisonitrile Complexes Calamitic and Discotic Liquid Crystals. Journal of Materials Chemistry, 7, 19-23. 

Arias, J., Bardaji, M. and Espinet, P. (2008) Luminescence and Mesogenic Properties in Crown-Ether-Isocyanide or Carbene Gold(I) Complexes Luminescence in Solution, in the Solid, in the Mesophase, and in the Isotropic Liquid State. Inorganic Chemistry, 47, 3559-3567. 

Compounds presented in Scheme 9 have provided evidence for the influence of the otherwise rarely investigated />-anilino-ethynyl group and a variety of auxiliary ligands on the luminescence, NLO, and mesogenic properties of this type of complexes.98... 

Similar work was carried out with ligands featuring different flexible or rigid spacers (X = -C(0)0-, -C=C-, -CH2-CH2-) and terminal alkoxy groups where n = 9-12. These compounds and the carbene complexes derived therefrom by addition of alcohols also show mesogenic properties (Scheme 49).211 212... 

This polymer suffered thermal degradation only above 400 C, before melting. For molecular weights higher than about 20,000, it showed a lyotropic liquid-crystal behaviour in N-metnypyrrolidone containing LiCl. This is the first reported example of a furanic polymer with mesogenic properties. 

Scheme 18 Synthesis and mesogenic properties of crown ethers 33 with a lateral p-terphenyl group...

In cases where the substituents X and Y are long-chain alkyl esters or amides, liquid crystalline behaviour is observed (Section 13.3). While the individual components have no mesogenic properties, the aggregate forms a metastable columnar hexagonal type structure. 

In conclusion the ferrocene unit is a poor component of a mesogen but can be tolerated if the moleeule is long enough to compensate for the reduetion of mesogenic properties induced by its inclusion. This statement is in agreement with the findings obtained for the 1,3-disubstituted ferroeene derivatives (compare 15 (Ijd = 8.3) and 16 l/d = 7.2) with 17 l/d = 4.6)). 

The strong effect on the mesophase sequence of lateral mono- and difluorination at different sites of an aromatic mesogenic core structure is illustrated by the data in Table 4.5 [40]. The relatively moderate mesogenic properties of material 14 were dramatically improved by introduction of two lateral fluorine substituents in the liquid crystal 22. Unfortunately, this target was achieved only by a purely empirical trial-and-error process requiring a tremendous amount of synthetic work. 

The example of DH4T/Ag(lll) also demonstrates that even the structure of the ordered phase is affected by the mesogenic property of the molecules [83]. 

A systematic study of numerous homo PEIs derived from 27b and various diphenols was conducted by the author [21,52,53]. This study revealed that the dicarboxylic acid has good mesogenic properties despite its lack of symmetry, and some unexpected structure property relationships were detected. The PEIs of unsubstituted hydroquinone was not prepared, because a Tm above 450 °C was expected. Even the smallest substituent (e.g. Cl or CH3) reduces the Tm to values below 400 °C (33a, c, see Table 4). In fact the PEIs of all monosubstituted hydroquinones (33a-i) were thermotropic with broad nematic phases. Remarkable is the reluctance to crystallize in the case of 33g. Three days of annealing were required to obtain a crystallinity below 20%. Also other more or less bulky substitutes, such as those of 33d, h and i reduce the tendency to crystallize. As discussed below, (and in Sect. 4), this is an interesting aspect for the synthesis of... 

Neither polyimides based on pyromellitic dianhydride nor on biphenyltet-racarboxylic anhydride with aliphatic spacers have yielded LC polyimides [77-79]. The need for an extra ring to obtain mesogenic properties was evident. Employment of a terphenyl tetracarboxylic anhydride has been reported to produce a LC polyimide free of other functional groups (19). 

A very recent synthesis of a thermotropic copolyimide contains an AT-(car-boxyphenyl) trimellitimide ester unit linked to a p-aminobenzoic acid via an amide linkage, which in turn is linked by an alkane diol unit back to the trimellitimide unit [86]. This is probably the first example of a copoly(ester-amide-im-ide) with mesogenic properties identified. Its MI score is 9.6. 

The five series of PEIs based on N-(4-carboxyphenyl) trimellitimide, cited above, clearly demonstrate that there is a critical value of the MI around 9.5 above which mesogenic properties are reported. This value is about half to one unit lower than observed with polyesters and polyamides. It is open to speculation whether this a real difference, possibly reflecting an inductive effect due to jr overlap of neighbouring chains, or whether it is just within the bounds of normal variation. 

A series of copolymers containing both imide and carbonate units have been reported by Sato et al. [84,85]. The structures are depicted in Table 26. Two copolymers have been synthesised and coreacted together in differing ratios above a 1 1 ratio thermotropic behaviour was observed. The first copolymer was constructed from pyromellitic dianhydride separated from carbonate groups by aliphatic spacers. This polymer was isotropic on its own. The second polymer was a thermotropic polycarbonate containing a biphenyl unit and spacers. The MI score=9.1 at the onset of mesogenic properties. This value is surprisingly low and is about half a unit lower than observed with PEIs. 

However, apart from the examples developed above, the study of the self-assembly, in relation to the mesogenic properties for these suspensions of nanotubes. 

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