Gold liquid crystals

In summary, it appears that gold(I) will continue to be a very rich source of inspiration and experimentation, not only in liquid crystal research but in research using gold liquid crystals at the interface of gold chemistry and the material... 

Bayon, R., Coco, S. and Espinet, P. (2005) Gold Liquid Crystals Displaying Luminescence in the Mesophase and Short F... F Interactions in the Solid State. Chemistry - A European Journal, 11, 1079-1085. 

However, the main research result from those years was the discovery of the room-temperature single-electron phenomenon. In the 1990s, STM experiments on liquid crystal had shown a very weak staircase (Nejoh 1991) only one year later, the clear observations of the coulomb blockade and the coulomb staircase were demonstrated on gold nanoparticles (Shonenberger et al. 1992a) and the role of system symmetry on the appearance of these two phenomena was outlined (Shonenberger et al. 1992b). 

Depending on the R group, this reaction could lead to the formation of gold(I) isocyanide complexes that behave as liquid crystals. Thus, complexes [Au(C6F5) (C = N(QH4)OCioH2i-p)] and [Au(QF5)(C = N(QH4)OCnH2n + rP)] [64] where n = 4, 6, 8, 10 and 12 show this behavior. All of these complexes are mesomorphic and behave as liquid crystals showing a nematic (N) phase when the isocyanide has a... 

It is interesting to note the influence of the counteranions on the thermal behavior. Irrespective of the isocyanide used, all the nitrate gold derivatives show low thermal stability and undergo extensive decomposition at relatively low temperatures (only the low melting trialkoxyphenyl derivative shows liquid crystal behavior). In contrast. 

The preparation and study of metal nanoparticles constitutes an important area of current research. Such materials display fascinating chemical and physical properties due to their size [62, 63]. In order to prevent aggregation, metal nanoparticles are often synthesized in the presence of ligands, functionalized polymers and surfactants. In this regard, much effort has focused on the properties of nanoparticles dispersed into LCs. In contrast, the number of nanoparticles reported that display liquid crystal behavior themselves is low. Most of them are based on alkanethiolate stabilized gold nanoparticles. 

Liquid crystal gold nanopartides that exhibit a thermotropic nematic phase in the bulk have been reported recently (Figure 8.28). 

The nematic nanoparticies have been prepared by a two step synthetic process. First, gold nanopartides are covered with an alkylthiol monolayer (hexyl- and dodecylthiol) in a second step, the alkylthiol-nanoparticles are reacted with the functionalized thiol mesogen in dichloromethane at room temperature to obtain the monolayer-protected liquid crystal gold nanopartides. These materials are chemically stable and display a nematic mesophase at room temperature [67, 68]. Other examples include liquid crystal gold nanopartides functionalized by hexaalkoxy-substituted triphenylene [69]. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

Figure 8.29 Compounds used to prepare liquid crystal mixtures containing gold nanoparticles.

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

Besides this, the remarkable properties of gold(I) compounds, which often give rise to aurophilic interactions and/or to luminescence, are of interest when these properties are transported into the liquid crystal field. Although there is much still to be studied, it is already clear that luminescence can survive in the condensed but mobile state of a mesophase, and even in the isotropic liquid state of a molten gold compound. It also seems that aurophilicity can contribute in some cases to the formation of mesophases. 

Omenat, A., Serrano, J.L., Sierra, T., Amabilino, D.B., Minguet, M., Ramos, E. and Veciana, J. (1999) Chiral linear isocyanide palladium(II) and gold(I) complexes as ferroelectric liquid crystals. Journal of Materials Chemistry, 9, 2301-2305. 

Espinet, P. (1999) Liquid crystal made of gold. Cold Bulletin, 32, 127-134. 

Dobbs, W., Suisse, J.-M., Douce, L. and Welter, R. (2006) Electrodeposition of Silver Partides and Gold Nanopartides from Ionic Liquid-Crystal Precursors. Angewandte Chemie (International Edition in English), 45, 4179-4182. 

Qi, H. and Hegmann, T. (2006) Formation of periodic stripe patterns in nematic liquid crystals doped with functionalized gold nanopartides. Journal of Materials Chemistry, 16, 4197-4205. 

Edward Tiekink and gold complexes in liquid crystals by Pablo Espinet. Homogenous catalysis by gold complexes is not included here since a book dedicated especially to this topic edited by Stephen Hashmi will also appear in 2009 from this publishing house. 

Cyclic trinuclear gold(I) complexes provide a novel and productive strategy for achieving supramolecular structures. While molecules of this type have been known for more than twenty years, some of their remarkable properties have only been recognized recently. Some can form liquid crystals at room temperature [41], while others lead to luminescent materials with surprising properties. We will now summarize some selected examples to illustrate the behavior of these trinuclear systems. 

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. 

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