Metallomesogens

Most other fluorinated metallomesogens represent disc-like or board-like molecules (Fig. 44). The focus of these studies was on the cross-over between lamellar and columnar self-assembly for molecules with a disc-like core. Stabilization of SmA 

A unique temperature dependent phase sequence hexagonal columnar - reentrant isotropic (Isore) - smectic A was observed by Szydlowska et al. for some c/,v-enaininoketone Ni(II) and Cu(II) complexes combining two fluorinated and two hydrocarbon chains (compound 171). Related complexes with only one RF-chain 

Fused copper porphyrin dimers (compounds 156 and 177, see Figs. 42 and 47) with an orthorhombic phase and a rectangular column phase, respectively, were recently reported by Aida et al. [277]. Interestingly, the partly RF-substituted 

A completely different type of amphotropic materials is made up by various metal or-ganyls, such as the linear ones illustrated by the di-palladium organyl 45, the twinned rod-like organyl 47, or the disc-like orga-nyls 48 and 49 (Figs. 30 to 32). These and related metallomesogens exhibit thermo- 

Twin-like, bis-linear di-palladium meso-gens of type 47 which thermotropically exhibit smectic phases [159] form lyotrop-ically different variants of supramolecular packings depending on the length and type of their four substituents as well as on the chain length of the organic solvent [154, 155], In contrast to 45 such //-shaped metallomesogens are lyotropic liquid crystalline only without TNF. 

The formal addition of lateral substituents to the ehemical structure of 47 (ringed in the formula of 48 in Fig. 31) leads to disc- 

The lath-like tetrapalladium or -platinum liquid crystals of structure 49 form columnar (Colob) mesophases on heating. Usually, the range of the mesophase this class of compounds exhibits is quite broad, typically about 200-250 K wide [155]. 

At high concentrations of 49 in solvents, such as in long-chain alkanes, a viscous two dimensionally ordered columnar mesophase of a similar type as under only thermal conditions is observed. At lower concentrations the formation of either one or two different nematic columnar mesophases appears [147, 148, 150]. 

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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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