Columnar discotic mesophase

H. Duran, B. Hartmann-Azanza, M. Steinhart, D. Gehrig, F. Laquai, X. Feng, K Mullen, H.-J. Butt, G. Floudas, Arrays of aligned supramolecular wires by macroscopic orientation of columnar discotic mesophases. ACS Nano 6, 9359-9365 (2012)... 

Supramolecular discotic liquid crystals may be generated via the initial formation of disk-like supermolecules. Thus, the tautomerism-induced self-assembly of three units of the lactam-lactim form of disubstituted derivatives of phthalhydrazide yields a disk-like trimeric supermolecule (Figure 3). Thereafter, these disks self-organize into a thermotropic, columnar discotic mesophase [13]. 

The discotic mesophases are classified in two types columnar, and nematic discotic. The structure of the nematic discotic mesophase (Np, Figure 8.3, left) is similar to that of rod-like molecules, but constituted by disk-like units. In columnar mesophases, the molecules are stacked in a columnar disposition and, depending on the type of columnar arrangement, several columnar mesophases are known. The most common lattices of the columnar phases are nematic discotic (No), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rectangular (Col,) mesophases. 

PAH chemistry is of practical as well as theoretical interest. PAHs can be regarded as well defined subunits of graphite, an important industrial material, which is so far not totally understood at the macroscopic level. In this context, it is our aim to delineate the molecular size at which the electronic properties of PAHs converge to those of graphite. Furthermore, alkyl substituted derivatives of hexabenzocoronene (HBC) form discotic mesophases and, therefore, provide opportunities for materials which allow one-dimensional transport processes along their columnar axis [83,84]. Their application for photovoltaics and Xerox processes is also of current interest. 

Cyclic hydrocarbons with six equivalent substituent chains emanating from several carbon atoms about the ring can again adopt conformations in which the molecular shape is disk-like, but the aspect ratio is much larger. If the chains are somewhat flexible and the molecules possess an appropriate balance between order and disorder and attractive and repulsive intermolecular forces, the creation of new mesophase types can be envisioned - and several have been found [102,104]. The two general classes of aggregation in liquid crystalline phases of discotic molecules are lenticular nematic (N ) and columnar discotic (D) [100-104]. Carbonaceous pitch mesophases, discussed in section 1.4.5, resemble phases. Only those discotic mesophases with benzene, cyclohexane and shape-related cores having primarily alkyl chains as substituents will be discussed here. 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

During the last 17 years an enormous amount of research into the synthesis of disc-like materials and into the stmcmral characterisation of the discotic mesophases generated has been performed by many research groups throughout the world. Two basic types of discotic mesophases have been widely recognised, these arc columnar and nematic. 

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

There are several polyols that exhibit discotic mesophases. Diisobutylsilanediol, which has been known since 1955 [342] but was not classified as hexagonal columnar until 1980 [343], associates in dimers [343] or oligomers [344] in order to form disk-like units (42). 

The liquid crystalline phase of dinuclear copper stearate has been known since 1938, and was reinvestigated in 1964 [38] but it was not until 1984 [39] that this mesophase was definitely characterized by Giroud-Godquin et al. as columnar discotic by X-ray diffraction. This was the first example of a thermotropic hexagonal discotic meso-... 

It has been shown that a j8-diketonate liquid crystal containing palladium(II) and oxo-vanadium(IV), analogous to known copper complexes (which display discotic lamellar and columnar mesophases), has been prepared and characterized palladium complexes are mesomorphic but their identification is difficult due to their decomposition in the isotropic phase the complex n = 10 seems to present a Nj phase on the basis of optical microscopy. The vanadium complexes are enantiotropic discotic [75], while the vanadium complex, 30, presented by Styring et al. [76] (n= 10), exhibited a short monotropic columnar discotic phase. 

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