Metallomesogenes

Metallocomplexes Metalloenzymes Metalloid peroxides Metalloimmunoassays Metallomesogens... 

Metallomesogens. It is also possible to synthesize compounds based on metal atoms which possess Hquid crystal phases. The series based on dithiolene complexes (1), where M = Ni, Pd, or Pt, contains a number of compounds which show the Hquid crystal phases typical of rod-like molecules (13,14). 

Disk-shaped molecules based on a metal atom possess discotic Hquid crystal phases. An example is octasubstituted metaHophthalocyanine. FiaaHy, metallomesogens which combine both rod-like and disk-like features iato a single molecule adopt the biaxial nematic phase. In addition to there being a preferred direction for orientation of the longest molecular axis as is tme for the nematic phase, perpendicular to this direction is another preferred direction for orientation of the shortest molecular axis (12). NonmetaHomesogens which combine both rod- and disk-like features iato a single molecule also adopt a biaxial nematic phase, but at least ia one case the amount of biaxiaHty is very small (15). 

To date, the crystal structures of more than 200 mesogenic compounds are known. In this review, we wish to present a general overview of the crystal structures of mesogenic compounds up to the end of 1997. Unfortunately, it is not possible to consider the crystal structure determinations of carbohydrate liquid crystals [13, 14], metallomesogens [15-18], phasmid and biforked mesogens [19-22], perfluorinated mesogenic compounds [23-27], benzoic acids [6, 28-31], cinnamic acids [7, 32, 33], dicarboxylic acids [34, 35], cinnamate compounds [8, 36-40], and discotic liquid crystals [41-43] due to the lack of space. 

Serrano JL (ed) (1996) Metallomesogens synthesis, properties, and applications. VCH, Weinheim... 

Zeng, H., Lai, C.K andSwager,TM. (1994) Transition Metals in Highly Correlated Discotic Phases Designing Metallomesogens with Selected Intermolecular Organizations. Chemistry of Materials, 6, 101-103. 

A different mechanism is that of charge transfer, of which there are many examples based, in particular, on metallomesogens [14]. In these cases, mesophases can be induced by adding the electron-poor TNF (2,4,7-trinitro-9-fluorenone) and Fig. 9 shows Pd mesogen 4, which shows a particular type of SmA phase when TNF is added [15]. 

Liquid-crystalline complexes (metallomesogens) containing platinum(II) are new types of materials that have been the subject of several studies. These have largely included complexes of the type trans-[PtX2L2] (X = C1, L = cyanobiphenyls 229 X = C1, carboxylate, L = 4-alkoxy-4 stilbazoles 230 X = C1, L = 2,4-, 3,4-, or 3,5-dialkoxystilbazoles, 2,3,4-, 2,4,5-, or 3,4,5-trialkoxystilbazoles).231,232 Their liquid-crystalline properties have also been reported. 

Metallomesogens are unique compounds which combine the properties of liquid crystals with that of metal complexes to generate novel electric or... 

Trzaska and co-workers showed a similar propeller mechanism for the formation of helical columns from disclike metallomesogens (29-31).34 These metallomesogens also have C3 symmetry and 30 and 31 are provided with chiral side chains. In the hexagonal columnar mesophase these chiral side chains induce a Cotton effect in the chromophore of the helically arranged core. Heating the mesophase to the isotropic liquid results in the disappearance of the Cotton effect because of the loss of helical order. This effect illustrates the need for the molecules to be positionally ordered in order for the side-chain chirality to be transferred to the supramolecular column. 

Finally, ferroelectricity has been shown for columnar metallomesogens.35 Serrano and co-workers have shown that metal ft-diketonates, provided with chiral side chains (e.g., 32), form helical columns (vide supra), which can also be switched under an alternating electric field. 

Thermotropic liquid crystals, 15 86-98 bent-core, 15 98 discotic phases of, 15 96 frustrated phases of, 15 94-96 metallomesogens, 15 97 nematic liquid crystals, 15 86-92 smectic liquid crystals, 15 92-94 Thermotropic mesophases, 20 79 Thermotropic polycarbonates, 19 804 Thermotropic polyesters, liquid-crystalline, 20 34... 

Increasing the concentration of surfactants in water to a level above the CMC leads to the formation of rod-like micelles and, subsequently, liquid crystals [251]. Both liquid crystals and liquid-crystalline polymers [252] have been used as media for small particle generation [253, 254] and have also acted as piezoelectric devices [255]. Of particular interest are metallomesogens, the metal complexes of organic ligands which exhibit liquid crystalline behavior [255],... 

Metallomesogens, Synthesis, Properties, and Applications -, Ed. Serrano, J. L. VCH, Weinheim New York, Basel, Cambridge, Tokyo, 1996. 

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