Swallow-tailed mesogens

Note A sketch of the structure of a swallow-tailed mesogen is... 

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

Swallow-tailed mesogens are characterized by a bulky alkyl branch connected to a terminal ring by a functional group or spacer. Thus such mesogens contain at one terminal end two long chains, and the molecular shape is therefore wedge-like and should be considered as a steric dipole. Most exam-... 

Table 12. Swallow-tailed mesogens containing a di-n-alkyl benzylidenemalonate segment. COOC H2 +i COOC H2n+i...

Table 13. Further chemical structures of terminal branches of 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). 

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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