Palladium complexes metallomesogens

Recent progress in material science, notably with the development of new materials exhibiting blue phases, has generated a renewed interest in the incorporation of the functional properties with the unique structure of frustrated phases. Synthesis of a monosubstituted ferrocene-based chiral Schiff s base derivative which exhibits TGBA and blue phases has been reported [17] (Fig. 9). Other metallomesogens leading to blue phases have been found for palladium complexes [18] (Fig. 10). Optically active materials incorporating... 

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. 

Metallomesogens have been shown to form helical supramolecular organisations in their mesophases [95]. Chiral oxazoline complexes with various metal ions and six alkyl chains did not show LC behaviour, but when mixed with trinitrofluorenone form achiral smectic A phases [96]. Furthermore, when a branch was included in the structure of the ligands (Fig. 12) the corresponding complexes with copper(II) and palladium(II) form columnar mesophases which have a helical organisation [97]. The presence of the stereogenic centre near the central metal ion in these complexes (Fig. 12) is enough to cause the parallel molecules to stack in a tilted manner with... 

The first report of a ferroelectric effect by a metallomesogen was for an open-book palladium(II) complexes (Figure 44) of type discussed above, where the chirality was introduced in the bridging carboxylate [14a]. The H NMR spectra identified that the complex was a mix of isomers trans-AR,R (34%), trans-AR,R (34%) and cw-R,R (32%). Ferroelectric switching was demonstrated, although at a much slower rate ( 1 s) than in organic liquid crystal systems, due to the high viscosity of the material. 

We have pursued the study of polycatenar metallomesogens using poly(alkoxy) stilbazoles [177], and have recently imdertaken a systematic study of the complexes of series of these ligands with palladium(ll), platinum(II) and silveifl) ions. 

Refractive index studies allow measurements of the birefringence, but also of the polarisability. The mean polarisability (a) can be determined using an isotropic liquid solution, whereas the polarisability anisotropy (Aa) is measured in the mesophase. Polarisability studies have been made on palladium, platinum, iridium and silver metallomesogens, and show a higher polarisability for the metal complexes than for their parent ligands. ... 

Several palladium(II)-containing metallomesogens show luminescence in the solid state. For instance, the palladium(Il) complexes of the type shown in Figure 2.28 are photoluminescent at The dinuclear... 

In group 10 we find probably the largest collection of metallomesogens, mainly due to the large collection of ortho-metallated complexes of palladium(ii), which have been exploited by several scientists. 

Starting from his initial work in 1989 on organometaUic compounds with a ferroelectric phase, Espinet developed the chemistry of metallomesogens based on dimeric palladium derivatives of chiral Schiff bases (Pd2(/r-Cl)2L 2 where L represents a chiral Schiff base ligand.Several examples of these complexes (6.21-6.23) are shown in Figure 6.14... 

The first systematic study of d-block metallomesogens were carried out by Giroud and Mueller-Westerhoff [11] and marked, in 1977, the beginning of interest in metal-containing systems. They were dithiolene complexes of nickel, palladium and platinum with two p-alkyl phenyl substituents (see structure 3). Nickel and platinum complexes showed nematic and smectic phases, depending on the chain length. Palladium analogs do not possess any mesomorphic properties [81]. 

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