Liquid crystal twins

Molecules of liquid crystals can be built from a variety of structural elements. For instance, in 1927 Vorlander synthesized the first liquid crystal twins, now called tail-to-tail twins. A Siamese twin mesogen is a compound in which two independently mesogenic parts are joined in a single molecule. While earlier twins were ligated by carbon-containing groups such as methylene, Dehne et aV have shown that sulfur, sulfinyl or sulfonyl groups can perform a similar role. Anatomy of MLCs from the point of view of possible structures was reviewed in detail by Demus. ... 

Note 2 A liquid-erystal dimer or mesogenic dimer is sometimes known as a twin mesogen. Use of the terms dimesogenic compounds and Siamese-twin mesogen for liquid-crystal dimer or mesogenic dimer is not recommended. 

Holographic optical elements can also be made by the preparation of polymer-dispersed liquid crystals using twin lasers in transmission holographic photopolymerisation (see section 5.4.2). They have also been made using photorefractive composites of polymer dispersed liquid crystals (see section 5.6.3). 

The neutral parent amido species [In(NH2)3] was synthesized by reaction of KNH2 with Ini, in liquid ammonia.Its manner of polymerization in the solid is unknown because diffraction studies were inconclusive as a result of crystal twinning. It was shown to decompose cleanly and quantitatively at moderate temperature to yield InN with liberation... 

This is not a method commonly used for coordination compounds, which do not often melt without decomposition. Where applicable, it can be used by cooling from a high-temperature melt to room temperature, or by cooling a room-temperature liquid to a lower temperature. The latter is a specialized technique, usually carried out in situ on a diffractometer, with monitoring of the crystal growth by optical and X-ray methods the sample is contained in a sealed capillary tube, and selective heating may be applied by an infra-red laser to develop a single crystal. Twins and multiple crystals often result from these methods. 

Twin liquid crystals containing either Fc 54 and 55 or terephthalate 56 and 57 as the connecting unit between mesomorphic rods were reported by Tschierske Comparison of their thermal and liquid-crystalline properties... 

Twins consist of two entities typical for liquid crystals that are connected either directly or indirectly by a spacer, which can be rigid or flexible. VorlSnder s group have synthesized compounds which, according to the modem nomenclature, are called twins (e.g. [214] see compounds 3767-3769 in [34]). In Siamese twins [215] two exactly equal moieties are attached to one another. The bond between the entities can occur in different positions and be composed of different linking groups. A survey of the twin types known up until 1988, is given in [2]. Some twins and oligomers are shown schematically in Table 43. 

Compound 28 [285] and many other metallomesogens can be considered as twins. In many cases (e.g. 29 [286] the ligands are themselves liquid crystals. 

Twins possibly occur in liquid crystals that have a structure close to that of solid crystals, with more or less extended three-dimensional order, possibly in a smectic E phase (see Sec. 7.4.4.6). 

Iridium complexes of dimeric stilbazole, 9, have been synthesized recently. These materials show a very laige odd-even effect in their melting behavior, and are called metal-containing Siamese Twin liquid crystals [18]. 

Shiota A, Ober CK (1996) Synthesis and curing of novel LC twin epoxy monomers Jot liquid crystal thermosets. J Polym Sci Polym Chem 34 1291-1303 Shiota A, Ober CK (1997) Analysis of smectic structure formation in liquid crystalline thennosets. 

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