Columnar oblique mesophase

Note 1 See Fig. 15d for an illustration of the molecular arrangement in a columnar oblique mesophase. 

Note 4 There also exist chiral columnar oblique mesophases, with the tilt directions of the columnar discs varying regularly along the columns. 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

Note Depending on the order in the molecular stacking in the columns and the two-dimensional lattice symmetry of the columnar packing, the columnar mesophases may be classified into three major classes hexagonal, rectangular and oblique (see Definitions 3.2.2.1. to 3.2.2.3). 

Columnar mesophase characterised by a liquid-like molecular order along the column, in which the columns are arranged with an oblique packing. 

Fig. 15. Plan views of the two-dimensional lattice of the columns in columnar rectangular (a) to (c) and oblique (d) mesophases. Ovals indicate the planes of the molecular discs.

Liquid crystalline phases can show not only long-range orientational order as nematic phases do but also long-range positional order. When this positional order is one-dimensional, the mesophase is called lamellar or smectic when it is two-dimensional, it is called columnar. The latter case is often found with thermotropic liquid-crystal disk-like molecules. Such molecules stack in columns that assemble on a 2-D lattice of hexagonal, rectangular, or oblique symmetry. The molecules in a given column only show 1-D liquid-Hke order and the uncorrelated columns are free to slide past each other, which ensures the mesophase fluidity [73]. 

Besides smectic mesophases, columnar mesophases generally present a more pronounced order, since they result from the self-assembly of columns of indefinite length according to a two-dimensional lattice [2]. The symmetry of the latter is found to be hexagonal, rectangular or oblique (Fig. 1) [3]. 

Fig. la-e. Different types of lattices found in columnar mesophases. Two-dimensional lattice with a) hexagonal symmetry, b) rectangular symmetry (P2i/a), c) rectangular symmetry (P2/ a), d) centred rectangular symmetry (C2/m), e) oblique (PI). (From Levelut AM (1983) J Chimie Physique 80 149)... 

Such molecules exhibit columnar mesophases and smectic C phases. In contrast to the corresponding tetracatenar compounds described above, the columnar phase is, in most of these swallow-tailed compounds, the low temperature phase with respect to the smectic C phase [58]. This inversion of thermotropic sequence with respect to the biforked compounds described just above has not found any explanation in relation with molecular parameters. The symmetry of the columnar two-dimensional lattice is found to be hexagonal, oblique and even rectangular [59] in some cases. For the latter case, the model proposed involves several molecules packed side by side to form the lattice unit, the different moieties being packed alternately in order to form a two-dimensional centred cell as shown in Fig. 11. 

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. 

New compounds containing two or even four palladium atoms and eight or twelve flexible side chains have been synthesized by Praefcke et al. [138]. The compound, 58, shows an enantiotropic discotic hexagonal columnar mesophase over a wide range of temperature and a lyotropic nematic phase in various organic solvents. X-ray diffraction studies reveal an oblique arrangement of disordered columns. 

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