Mesomorphic complexes

Related materials 70 were prepared by Han et al All of the ligands showed monotropic phases, except for 70e, where the phases were enantiotropic. All showed a N phase with a SmC phase seen for 70b and 70e. On complexation, mesomorphism was suppressed in 70d and 70e, while 70a and 70c showed a N phase (Cr 101.5 N 142.4 I and Cr 114.6N 130.2 I) and 70b showed a SmA phase (Cr 118.4 SmA 128.5 I). Clearing points for the mesomorphic complexes were rather similar (between 128 and 142 °C), whereas for the free ligands the difference was greater (between 53 and 185 °C). 

Reaction of a sodium alkoxydithiobenzoate with zinc acetate in dilute acetic add led to complexes of the empirical formula [Zn( -odtb)2]. The as-obtained materials were analytically pure, orange powders which showed a complex mesomorphism, apparently involving more than one species. However, crystallization yielded red crystals which had a well-defined mesomorphism, giving mainly nematic and some smectic C mesophases. 

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... 

Three series of mesomorphic dithiobenzoatogold(III) complexes (2) have been described. They show a SmA mesophase in the range 150-200 °C [8, 9]. The thermal stability of these complexes is low and the clearing transitions occur with... 

Lateral monofluorination of the p-alkoxyphenyl isocyanide system [XAu (CNC6H40C H2 +i)] (X = C1, Br, I), in ortho-(3-F) (6a) and meta-(2-F) (6b) positions relative to the alkoxy chain was also studied [18]. None of the free fluorinated ligands is a LC, but their gold complexes display mesomorphic properties. This is a typical case of a promesogenic ligand which yields a mesogen upon coordination to a metal. 

Derivatives of aliphatic alkynes (14 and 15) are more thermally unstable than 12, but they show SmA and N phases at low temperatures (below 130 °C). The type of phase and the mesophase stability depend on the length of both the terminal and the lateral chains. When both chains are elongated, the mesomorphism becomes metastable and compounds 14 display monotropic N and SmA transitions. Complexes IS, which contains an ester group with an opposite direction to that of complexes 14, display less stable nematic mesophases. 

In summary, [Au(alkynyl)(CNR)j complexes also show mesomorphic behavior but, in contrast to the halide-isocyanide derivatives, they are thermally less stable and usually decompose before or at the clearing temperatures. This thermal instability is thought to be associated with the gold-(alkynyl) bond and makes these compounds... 

As indicated above in chiral mesophases, the introduction of a functional group in mesogenic stmctures offers the opportunity to achieve functional LCs. With this aim, mesomorphic crown-ether-isocyanide-gold(I) complexes (26) have been prepared recently [38]. The derivatives with one alkoxy chain show monotropic SmC mesophases at or close to room temperature. In contrast, the complexes with three alkoxy chains behave as monotropic (n = 4) or enantiotropic (n > 4) LCs. The structure of the mesophases could not be fully eluddated because X-ray diffraction studies in the mesophase were unsuccessful and mesophase characterization was made only on the basis of polarized optical microscopy. These complexes are luminescent not only in the solid state and in solution, but also in the mesophase and in the isotropic liquid state at moderate temperatures. The emission spectra of 26a with n=12 were... 

Synthesis and phase behaviour of mesomorphic transition-metal complexes of alkoxydithiobenzoates. Journal of Materials Chemistry, 1, 843-855. 

Bruce, D.W. and Hudson, S.A. (1994) Mesomorphic complexes of silver trifluoromethane sulfonate and silver dodecylsulfate with 2- and 3-fluoro-4-alkoxy-4-stilbazoles. Journal of Materials Chemistry, 4, 479-486. 

Coco, S., Fernandez-Mayordomo, C., Falagan, S. and Espinet, P. (2003) Effect of alkoxy chains on the mesomorphic properties of (p-alkoxy)tetrafluoroarylgold(I) isocyanide complexes. Inorganica Chimica Acta, 350, 366-370. 

Ballesteros, B., Coco, S. and Espinet, P. (2004) Mesomorphic Mixtures of Metal Isocyanide Complexes, Induding Smectic C Mesophases at room temperature and Liquid Crystalline Molecular Alloys . Chemistry of Materials, 16, 2062-2067. 

Zhang, S.-W., Ishii, R. and Takahashi, S. (1997) Syntheses and Mesomorphic Properties of Gold(I)-Carbene Complexes. Organometallics, 16, 20-26. 

However, the hydrogen-bonded mesogens that are of most interest in the context of this article are those elaborated initially by Kato and Frechet in the early 1990s [24-33]. In this approach, a pyridine, which may or may not have liquid crystal properties, was hydrogen bonded with a 4-substituted benzoic acid to form a new species with its own, distinct mesomorphism. For example, complex 9 shows a SmA phase that persists to 238 °C (n = 2, m = 4), while its free component pyridine is nematic to 213 °C the component benzoic acid is also nematic (as the H-bonded dimer) to 147 °C (although note that the notional monomer would not be liquid crystalline). 

The complex c/.v-IrCl(CO)2(OPhVpy), OPhVpy= /ra .v-4-alkoxy-4 -stilbazole), is mesomorphic and shows nematic and smectic phases below 140 °C.617,618... 

Another approach to get new liquid crystals is the lateral fluorination of the stilbazole ligands,337 which is a common and highly effective tool to exert control over mesomorphism, crystal phase stability, and physical properties. Other modifications include the use of more alkoxy substituents and other alkyl sulfate anions.338-344 Ionic silver amino complexes also display liquid crystalline behavior at rather low temperatures they are of the form [Ag(NH2 -CJl +OJX (X = N03, n = 6,8,10,12,14 X = BF4, = 8,10,12,14).345... 

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

Nonmesomorphic as well as mesomorphic F-shaped acids with a structure similar to banana liquid crystals, have been complexed to a 2,4,6-triamino-l,3,5-triazine derivative <06JA4487>. 

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