Mesomorphic polycatenars

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. 

While it is in discotic liquid crystals that the formation of columnar phases is most readily recognized supra), there exists a family of non-disk-like mesogens, the polycatenar liquid crystals, where these mesophases are also formed extensively. As will be seen from their shape, this observation is of some interest, but it is the fact that certain polycatenar materials can, within a homologous series, show mesomorphism characteristic of both rod-like and disklike mesogens that makes them particularly interesting. 

Some of the simplest systems reported raise a possible point of semantics as to whether these systems are, in fact, polycatenar in nature or perhaps ought to be described as discotic. The first of these 77 was based simply on the coordination of 3,4,5-trialkoxybenzonitriles to Pd(ll), following a motif for the formation of simple, calamitic systems "" described some years earlier. Only two compounds with different chain lengths were studied ( =10, 18). The ligands were themselves not mesomorphic, but on complexation, a Col), phase was observed over small temperature ranges, i.e., 73-91 °C for the complex with six tridecyloxy chains, and between 58 and 80 °C for that with six octadecyloxy chains. 

The ( -arene)tricarbonylchromium complexes 55 were reported by De-schenaux et al. [117]. The uncomplexed ligands showed a mesomorphism characteristic of polycatenar mesogens, progressing from N and Sc phases at shorter chain lengths to Colob and Colh phases at longer chain lengths. 

Results were also described from studies which introduced lateral substituents into these complexes and in one paper the effect of introducing lateral alkanoate groups was reported (113). " The use of lateral alkanoates had been described previously by Maitlis and co-workers in palladium complexes of 4-alkoxystilbazoles (Section 7.9.14.3.6(iv)) and had been shown to induce the formation of liquid crystal nematic phases. In the present situation where the complex was polycatenar, it was not clear whether the chains would act to increase the volume of the core of the polycatenar mesogen or simply as extra chains, so rendering the complex more classically discotic. In fact, the answer was neither and at all chain lengths (n and m) and degrees of substitution (R = H or OC H2 +i) where mesomorphism was observed, a nematic phase was seen at or near room temperature. This behavior was somewhat puzzling and is so far without explanation. 

Dianionic platinum(II) complexes ((119) n =10, 14, 18) with sulfur-rich dithiolato ligands based on tetrathiafulvalene have been reported.These potentially interesting complexes for their unique electronic properties, having a polycatenar structure unfortunately did not form a mesophase. The absence of mesomorphism may be simply due to the bulkiness of the counter ion. 

This series of compounds did not strictly behave as true polycatenar mesogens in the sense that no columnar mesomorphism was observed, despite their relatively low aspect ratio. Increasing the number of aromatic rings (144) to elongate the mesogenic core modified the thermal behavior quite drastically. Thus, complex (144) now displayed a CoIq phase at high temperature (Cr 198 Colo 215... 

More recently, a series of polycatenar salicylaldimine complexes (147) has been reported with M = Pd, Cu, Ni and Fe—Cl. For the two chain lengths reported (n = m= 10,16) it was found that neither of the Ni complexes, nor the shorter-chain Cu complex, was mesomorphic, but all the others showed a Col phase, identified from its characteristic optical texture. Clearing points were in the range 102 11°C for the mesomorphic Pd and Cu complexes, while the chloroiron(III) complexes cleared at 172°C (n = m= 10) and 160°C (n = m= 16). 

Up to now, in most cases, polycatenars such as phasmids and biforked mesogens have been symmetrical and have not contained any polar substituents such as Br, CN, or NO2. In order to study the influence of a lateral polarity on the mesomorphic properties, some new polar and non-synunetrical poly-catenar mesogens have been prepared. 

As a general rule, polycatenars display Sc mesomorphism, except in the case of smec-togenic A tetra- and pentacatenars with polar co-cyano chains (Sec. 4.2.2 of this chapter). 

Oblique and Rectangular Phases The first example of an oblique columnar phase has been observed on the first phasmid described (5, = 7, Table 1). A central terephthalylidene ring fitted with two ben-zoyloxyphenylimines seems to favor phases (Tables 1, 5 and 9). Regarding the rectangular columnar mesophase, chain-rich polycatenars such as phasmids do not display this mesomorphic symmetry, which so far has appeared only in the biforked tetraester series 19. 

Phasmids and polycatenar mesogens are just ten years old. Their intermediate shape between classical rod-like and disc-like liquid crystals allows practically all the mesomorphic symmetries (0, Cub,... 

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