Tetracatenar Compounds

Such molecules exhibit columnar mesophases and smectic C phases. In contrast to the corresponding tetracatenar compounds described above, the columnar phase is, in most of these swallow-tailed compounds, the low temperature phase with respect to the smectic C phase [58]. This inversion of thermotropic sequence with respect to the biforked compounds described just above has not found any explanation in relation with molecular parameters. The symmetry of the columnar two-dimensional lattice is found to be hexagonal, oblique and even rectangular [59] in some cases. For the latter case, the model proposed involves several molecules packed side by side to form the lattice unit, the different moieties being packed alternately in order to form a two-dimensional centred cell as shown in Fig. 11. 

FIGURE 10 Schematic representation of the transition from the SmC phase to a columnar meso-phase in tetracatenar compounds. Adapted from Donnio and Bruce (1997). 

Table 39. Transition temperatures of tetracatenar compounds (by courtesy of Malthete et al. [21 ]). R...

The simultaneous occurrence of nematic, cubic, layer and columnar structures in pure mesogens is of great importance to bridging the gap between calamitic and discotic molecules and the corresponding mesophases. Smectic, cubic and columnar phases have also been described in polycatenar compounds, and thus double-swallow-tailed mesogens can also be considered to be a special case of tetracatenar compounds (see Chap. XII of this volume). 

The two tetracatenar complexes ( = 5, 12) exhibited a columnar phase, between 73 and 125 °C for the short-chain-length compound, and from room temperature up to 106 °C for the longer-chain derivative. The hexacatenar material (n = S) displayed an unidentified phase between 70 and 122 °C that was probably columnar due to the structural similarity with the previous compounds. These mesophase assignments were performed by microscopy only, and exact phase symmetry was not determined. 

Even if they are not, strictly speaking, polycatenar molecules, double swallow-tailed compounds with long branches can be considered as a special case of tetracatenar mesogens, where the branches are not directly attached to the terminal rings [57], as shown in Fig. 10. 

The change from lamellar to columnar phases has been observed in several tetracatenar series [21]. Obviously, the predominant influence of the cores in the lower members stabilizes lamellar phases, while the increasing influence of the flexible chains in the higher members leads to the dominance of columnar phases in these compounds. Cyclohexane rings have also been introduced into tetracatenar com-... 

Tetracatenars 2mm-2mm Within the homologous series with a six-benzene-ring core 26 [24], only one type of mesophase has been detected by optical observations. The textures are fan-shaped, with developable domains in every way similar to those of the Oh phase of tetracatenars 2mp-2mp. As an example, for the compound with n = 10 K 76 °C Oh 110 °C I. Despite the unusual length of the central rigid part, the transition temperatures are surprisingly low (Table 10), a fact evidently connected with the meta position of all the alkyloxy chains. 

Tetracatenars Smpm-lp As seen in Sec. 4.1.4. of this chapter, a metastable cubic phase can be obtained with a four-ben-zene-ring core fitted with four paraffinic chains lp-3mpm (compound 10). When a perfluorinated stiff rod was grafted in place of the p-alkyloxy chain, a stable cubic phase... 

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