Swallow-Tailed Compounds

Fig, 18a,b, The polymorphic behaviour of the phasmidic-like mesogens a molecular model with six terminal chains b phase diagram for the binary mixture of double swallow-tailed compound (I) with conventional rod-like mesogen (II) (adapted from Letko et al. [123])... 

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. 

Let us first concentrate on the flexodomains. With the help of the general theory e — es can be determined from the measurements of Uc and qc of the flexodomains if the elastic coefficients K and and the anisotropy are known. In this way the value of ei — es = 5.7 pC/m was obtained for BMAOB. Note that this value has been confirmed for twisted cell geometry. In a later study BMAOB doped with a swallowtail compound in concentrations of up to 4.55 mole% was analysed. The value of ei — es for the mixture was found to slightly increase with increasing concentration of the swallow-tail compound (see Fig. 6 in Marinov et... 

The swallow-tailed compounds are a special case of compounds with branched terminal substituents, with unusually large branches. 

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-... 

Recently, bi-swallow-tailed compounds that show a very unusual polymorphism have been described. Weissflog and coworkers [205] have presented a series (Table 38) of compounds the lower members of which show classical smectic C and nemat-... 

Weissflog et al. [206] in another series of bi-swallow-tailed compounds, found very complex polymorphism (see 14). In the... 

Table 38. Bi-swallow-tailed compounds (data from Weissflog et al. [205]).

A re-entrant isotropic phase between the nematic phase and a cubic phase has been observed in a pure double-swallow-tailed compound [206]. Furthermore, another such compound was found to exhibit a reentrant SmC phase below an oblique columnar phase, with the sequence SmC-Col b-SmC-N-I upon heating [207]. 

There are strong tendencies to an antiparallel order in the short range produced by steric interactions as shown in Section 2.2.2 of this chapter. In such cases the addition of compounds with different molecular shape should destroy these forces. Classical examples for them are swallow-tailed compounds... 

Figure 35. Phase diagram of the swallow-tailed compound A with the laterally branched F [137],...

Figure 39. Static dielectric anisotropies of a mixture of the swallow-tailed compound A and the respective components 20 K below the beginning of the clearing process [165].

Monofluoro- and difluoromethylalkyl chains do not stabilize antiferroelectricity. Racemates do not exhibit antiferroelectricity, but they do frequently have the herringbone structure. Moreover, Nishiyama and coworkers [53, 54] have shown that chirality is not essential for the appearance of the herringbone structure the swallow-tailed compounds (e.g. 9),... 

If the lateral chain is attached at the 3-position, i.e. ortho to an other alkyloxy chain, the behaviour changes. The properties of these substances are comparable to those of the swallow-tailed compounds. However, mesogens bearing three alkyloxy chains linked directly to the outer aromatic rings are grouped as tricatenar substances, according to the definition given by Malthete et al. [42] (see Chap. XII of this volume). 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

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

In principle, the terminal branches of swallow-tailed compounds can be attached at both ends of a rod. However, only dialkyl benzylidenemalonates have been used extensively to prepare double-swallow-tailed mesogens. The minimum size of the basic mesogen is four cyclic units, these mostly being phenyl rings [85]. The clearing points in the series 45 decrease strongly with in-... 

Table 15. Variation of the middle segment A of double-swallow-tailed compounds [137] (D=Col).

Table 16. The influence of a bulky lateral substituent R on the mesophase behaviour of double-swallow-tailed compounds.

FIGURE 10 The dielectric absorption curves for a swallow-tailed compound from [52]. 

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