Double-Swallow-Tailed Mesogens

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-... 

Lengthening the aromatic part of double-swallow-tailed mesogens causes a lasting... 

Figure 12. Phase diagram of a homologous series of six-ring double-swallow-tailed mesogens [139] (D=Col).

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). 

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. 

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)...

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). 

Swallow-tailed and double-swallowtailed compounds, as well as laterally alkyl and aryl substituted mesogens, often show behaviours that lie at the boundary to another field, e.g. many of the derivatives exhibit the phenomenon of a glassy state on supercooling [123]. The special molecular... 

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