Stacking columnar discotics

Turning now to those molecules whose shape can be approximated by oblate spheroids, one arrives at the discotic phase. Here the average of the normals to planes of the molecules corresponds to the director. A fluid phase in which these normals point in roughly the same direction over a macroscopic distance is said to be discotic. If this factor is the only degree of order, the material is said to be in the nematic discotic phase. If, in addition, the discs stack in regular columns, the material is said to be in the columnar discotic phase. Such structures have been discussed in Section 4.5.1. 

A new class of hexabenzocoronene derivatives, e.g. compounds (Scheme 7.13) can self-organize into liquid crystalline phases composed of molecular stacks that orient spontaneously parallel to the surface [177, 178]. Field effect transistors based on these materials show high charge-carrier mobilities, high on/off ratios, and low tum-on voltages. So far compound 44 exhibits the best field effect transistor properties achieved for a columnar discotic material. Polarized fight microscopy revealed that these materials tend to orient their columns parallel to the surface upon thermal annealing. 

We start by reminding ourselves that columnar discotic liquid crystals are comprised of disordered stacks (1-dimensional fluids) of disc-shaped molecules arranged on a two-dimensional lattice (Fig. 1) [1]. This structure imparts novel properties to these materials from which applications are likely to stem. One such property is the transport of charge along the individual molecular stacks [2-7]. The separation between the aromatic cores in, for example, the hexa-alkoxytriphenylenes (HATn), the archetypal columnar discotic mesogen, is of the order of 0.35 nm, so that considerable overlap of n orbitals of adjacent aromatic rings is... 

Lead forms a columnar discotic liquid crys-wit t e metal sitting out of the plane of e mo ecule, 75. X-ray data led to the suggestion that the molecules were stacked antiferroelectrically in a tilted stack [164]. 

Other liquid crystal phases composed of molecules having unusual molecular shapes are also known. Some compounds form a cone or bowl shape. These materials are often referred to as bowlic, and some typical examples are hexasubstituted tribenzocyclonones and aza-crowns (17), molecules of which stack on top of each other to form an ordered column which may have anomalous electrical conductivity. When a disklike molecule is divided by a rod-shaped spacer, the molecule (18) is referred to as a phasmid. It exhibits phases bearing some resemblance to columnar discotics. 

As mentioned above, LLCs, which are columnar stacks of discotic molecules, can be photoaligned, thus forming a thin polarizing layer [61-63], However, the thermotropic discotic LC can also be ordered by a photoalignment method [64-66], A thin film of 25 nm thickness has been prepared on a fused silica surface from a photosensitive polymer with a side-chain p-cyanoazobenzene photosensitive group [64] (Figure 4,22),... 

Figure C2.2.7. Schematic illustrating tire classification and nomenclature of discotic liquid crystal phases. For tire columnar phases, tire subscripts are usually used in combination witli each otlier. For example, denotes a rectangular lattice of columns in which tire molecules are stacked in a disordered manner (after [33])...

Most polymeric Hquid crystals are based on stiff rod-like molecular units which are called calamitic mesogens. There are some unusual polymers (which are not discussed here) that contain flat disk-like molecular units called discotic mesogens in which the disks form columnar arrays like stacks of poker chips. 

The mesophases of thermotropic liquid crystals are described as calamitic if the constituent molecules are rod-like and columnar, if the constituent molecules, which often have a disc like shape(discotic), stack into columns. 

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. 

Nuckolls and Katz have synthesized discotic liquid crystalline molecules in which the core is a helix in its own right.37 Nonracemic helicene 33 was found to assemble into a columnar mesophase in which the helicenes stack on top of each other. CD spectroscopy showed a strong increase of the Cotton effect upon going from the molecularly dissolved state to the aggregated state, exhibiting an amplification of chirality. These helical columns give rise to a strong expression of chirality because the intrinsic shape of the helicenes... 

Just as chiral induction can be realised in discotic liquid crystals, it can also be realised in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical trisamides (Fig. 15), which actually exhibit discotic liquid crystalline phases, also form chiral columnar stacks through it-it interactions when dissolved in apolar solvents, which are depicted schematically in Fig. 15 [121]. An achiral compound of this type (15) exhibits no optical activity in dodecane, but when the compound is dissolved in the chiral CR)-(-)-2,6-dimelhyloctanc significant Cotton effects (only slightly less intense than those observed in a chiral derivative) are detected. The chiral disc-like trisamide 16 can also be used as a dopant at concentrations as low as 2.5% to induce supramolecular chirality in the stacks of achiral compound. In this case, the presence of the additional hydrogen... 

The formation of liquid crystals (LC) from suitably functionalized acetylenic SPMs has often been the motivation for their synthesis. A number of groups have sought to capitalize on the planar or nearly planar disk-like geometry of SPMs to promote the formation one-dimensional columnar stacks based on a combination of n-n stacking, van der Waals, dipole, and hydrophobic interactions. For example, Heiney, Moore, and coworkers have reported the high-resolution X-ray diffraction analysis (XRD) of a tubular, discotic LC based on SPM 133, which has been synthesized via a Sonogashira reaction [106]. These studies reveal an unanticipated distortion and... 

In calamitic systems it is the long axis of the molecules that is correlated in the mesophases but in discotic systems it is the short axis and different types of organization are seen, although disk-like molecules also form a nematic phase in which a unique axis is orientationally correlated (Fig. 25). However, below the nematic phase is a series of columnar phases (Col) in which the disks are stacked up into columns, which are themselves arranged according to some symmetric pattern. Typical... 

The most extensive series of experiments using the PR-TRMC technique in recent years has been directed towards an understanding of the charge transport properties of discotic materials. In these materials the individual molecules, consisting of an aromatic core with several peripherally substituted alkyl chains, columnarly stack with the columns packed in a well-organized, two-dimensional lattice. The types of aromatic cores investigated are illustrated in Fig. 5. 

In discotics, the interdisk stacking interaction is several orders of magnitude stronger than the inter-columnar interactions, because of the phase separation induced by the side-chains whose van der Waals interactions are much weaker. Generally, long polymers are obtained via discotics that have strong and specific intermolecular interactions, either via a large aromatic core or via additional specific intermolecular... 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

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