Mesomorphic stilbazole complexes

C. Mesomorphic Stilbazole Complexes of Groups 10 and 11 Metals Hydrogen-Bonded Stilbazole Mesogens... 

The behavior is typical of relatively simple dipolar materials where a N phase was found at short chain lengths, giving way to a SmA phase in higher homologs. While the iridium complexes could be cycled in and out of the isotropic phase with few problems of decomposition, the rhodium complexes began to decompose rapidly on melting in air. Lateral fluorination of air-iridium stilbazole complexes strongly destabilized the mesophase the 2-fluorinated 4-alkoxystilbazole systems exhibited only monotropic N and SmA phases, whereas the structural isomeric 3-fluori-nated compounds were not mesomorphic. " ... 

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. 

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... 

Having synthesized a number of complexes of monoalkoxystilbazoles, we explored the effect of using various poly(alkoxy)stilbazoles, given the interesting mesomorphism found in polycatenar mesogens (Section VI,A,3). This work is discussed in detail elsewhere 24). Here, we give an overview of the work to enable the reader to see the overall pattern of liquid-crystal behavior and the issues that arise. 

Two homologs of 78a (M = Pt, X = Cl, = 4, 6) were investigated in the formation of LB fdms. "" It was found that better fdms were obtained with the homolog that was mesomorphic at room temperature (n = 6). In the absence of more data, it is difficult to know how strongly this is correlated with the ambient liquid-crystal behavior of the material, or whether it results from a much lower degree of amphipathy due to the shorter chains. For example, well-ordered LB films of Ir(l) stilbazoles (71, see Section X.11.2) have been formed where the mesophase of the complex is found above 85 °C. 

The relationship between the length of the alkyl end group and mesomorphic properties for complex 10 derived from fran5-4-alkoxy-4 -stilbazole and 4-hexyloxybenzoic acid (2, n = 6) is shown in Fig. 5 [33, 34]. 

The interaction between phenol and pyridine has also been used to form mesomorphic complexes 18-20 [55-57]. A 1 1 mixture of 4-cyanophenol and a 4-alkoxy-4 -stilbazole shows a monotropic nematic phase, while the stilbazole exhibits a very narrow temperature range of ordered smectic phases [58] and 4-cyanophenol is nonmesogenic. Complexes 19, based on... 

The polymeric complexes derived from 4-nitro- and cyanostilbazoles also show a smectic A phase up to about 200 °C [78a]. Polysiloxane complexes 32 also exhibit thermally stable smectic A or C mesophases [79-81]. These carboxyl-functionalized polymers and stilbazoles are miscible in a whole range of composition and show stable mesomorphic behavior [26, 79]. The introduction of the chiral stilbazole for the formation of a mesogenic complex leads to the induction of ferroelectricity [80]. Polymeric complex 33 exhibits a chiral smectic C phase, while no ferroelectricity is observed for each of single components. The value of spontaneous polarization for 33 x — 0.43, n = 5) is 21.0 nC/cm at 112 °C. The hydrogen bonding between the carboxylic acid and... 

The use of hydrogen bonding for mesogenic network formation has been expected for the formation of dynamic mesomorphic states due to the induction of dynamic properties, e.g. reversibility and fast exchange of the H-bonding [108-111]. Supramolecular liquid-crystalline networks have been prepared by the self-assembly of a polyacrylate with a benzoic acid moiety and 4,4 -bipyridine [108]. Complexes 46 form dynamically cross-linked structures obtained by the complexation of a carboxyl-functionalized polyacrylate and a mixture of a stilbazole, which has the single H-bond acceptor, and a bipyridine, which has the two acceptors. 

Of those investigated, the tetracatenar complexes of Pd and Pt with 3,4-disubstituted stilbazoles, 40, one complex 41 (M=Pd, n=14), and the hexacatenar complexes of Pd with 3,4,5- and 2,3,4-trisubstituted ligands (43 and 44, respectively) were mesomorphic. The two other series of complexes... 

Hydrogen bonding has also offered a convenient and versatile approach to the design of bent-core mesomorphism. For example, the use of carboxylic acids and stilbazoles 9 and 10, which act as H-donor and H-acceptor, respectively, has afforded bent-shaped complexes. The SmCP phases (Fig. 5.3) represented by such... 

Figure 58 Mesomorphism of hexacatenar stilbazole ester complexes (111).

The mesomorphism of silver(I) complexes of poly(alkoxy)stilbazoles has been reviewed recendy. However, it is appropriate to cover this work here in order to see the effect of an anion on mesomorphism in cationic complexes and to allow some comparison to be made with the complexes described in the preceding section. In these studies, there were two main variables, namely the substitution pattern of the stilbazole used and the nature of the anion. The former was varied in the same way as for the palladium and platinum complexes just described, while the latter was either triflate or an alkylsulfate whose chain length was varied. 

More recently, 4-alkoxy-2- and 4-alkoxy-3-stilbazoles have been reported (Figure 85) and their silver complexes with AgDOS and AgOTf have been synthesized. While the ligands themselves are non-mesomorphic, the eomplexes show only SmA phases, although complexes of the 2-stilbazoles do not form readily. While the new complexes are Uquid crystalline, the use of the isomeric stilbazoles suppresses the polymorphism of the 4-stilbazole isomers. 

In still another variation, a chiral or optically active stilbazole (6c), which by itself displays no mesophase, was complexed with the polysiloxane-based polymers [18]. Again, mesomorphicity was obtained, with the usual effects on the transition temperatures (which are, once more, broad and dependent on thermal history). No glass transitions were detected. X-ray data indicate that the mesophase is smectic C,... 

---