Mesogens metallomesogens

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. 

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]. 

There has been a series of previous reviews on metallomesogens [3, 4] and even a book [5]. There is a also specific review of gold mesogens, published in 1999 [6]. 

Gold(III) usually displays a square planar geometry, typically observed in d8 metallic complexes such as palladium(II), platinum(II), rhodium(I) and iridium (I), for which an enormous number of liquid crystals have been described [3-5], mainly as orthometallated compounds. However, only a gold(III) metallomesogen has been published. Since the first gold mesogen was reported in 1986, many other compounds have been described. 

The term metallomesogen is often used when a (transition) metal is complexed (by coordinative bonds between the crown heteroatoms and the metal center) by a crown ether. These compounds were discussed in the previous sections as complexed crown ethers (instead of metallomesogens ) together with their uncomplexed precursors. In this section, mesogens with a covalent bond between... 

The colour of metallomesogens was a property that attracted the attention of the earliest workers in this field. Giroud and Mueller-Westerhoff reported that mesogenic nickel(II) dithiolene complexes formed dark, highly reflective crystals and were green in solution. The corresponding... 

Whereas in non-metal mesogens there is often a requirement to increase intermolecular interaction energies in order to cause mesomorphism, in metallomesogens the object is often to reduce the intermolecular interaction energies in order to reduce the transition temperatures to below the regime of thermal decomposition. 

Other metallomesogens containing the optically active butadiene iron tricarbonyl moiety, incorporated into chiral nematic mesogens, have recently been reported [116]. Both the diastereoisomers exhibited a chiral nematic and a monotropic smectic phase, the chiral nematic phase being monotropic for one diastereoisomer, and enantiotropic for the other. 

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