Molecule swallow-tailed

Fig. 2a-f. Mesogenic molecules with differing degrees of polar and sterical asymmetry a symmetric molecule with rigid core and two hydrocarbon tails b terminally polar molecule (arrow indicates the permanent dipole) c swallow-tailed (hiforked) molecule d hanana shaped molecule e terminally fluorinated molecule f polyphilic molecule (hatched areas correspond to the fluorinated fragment)... 

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. 

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. 

Fig. 10. Typical example of the double-swallow-tailed molecules...

Fig. 3.1 Different forms of mesogenic molecules rods (a), lath-like (b), discs (c), swallow-tail (d), bowls (e) double swallow-tails (f), main-chain (g) and comb-like (b) polymers, propellers (i) and spirals (j)...

Swallow-tailed mesogen - a mesogen composed of molecules with an elongated rigid... 

There are also compounds with swallow tails on both ends of the molecule ( bi-swal-low-tailed compounds) (e.g. 13 [201]). 

Because of the very bulky swallow tails, the packing bi-swallow-tailed compounds in layers of smectic A phases would lead to large gaps between the cores. Therefore in smectic C layers the packing in which the molecules are shifted somewhat in the Ion-... 

Figure 10. Proposed model of smectic A phases of swallow-tailed compounds with strings of interdigitated molecules. (By courtesy of Diele et al. [91]).

As shown by Pelzl et al. [43, 130-132], whether or not smectic A phases are induced, depends on the shape and length of the rod-like mixing components. This phenomenon can be explained by a filling of the gaps between the aromatic parts of the double-swallow-tailed molecules. Therefore, such induced smectic A phases are called filled smectic A phases [130]. It is legitimate to discuss these filled phases as a new type of guest-host system. Dielectric measurements in pure substances as well as... 

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 first group includes rod-like mesogens substituted by long-chain groups at any position, e.g. laterally alkyl substituted and swallow-tailed mesogens. In such compounds the liquid crystalline behaviour is largely determined by the repulsion forces of the surrounding molecules, which press... 

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