Stacking discotics

Warman JM, de Haas MP, Dicker G, Grozema FC, Ptris J, Debije MG (2004) Charge mobilities in organic semiconducting materials determined by pulse-radiolysis time-resolved microwave conductivity 7c-bond-conjugated polymers versus tc—tc-stacked discotics. Chem Mater 16 4600... 

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. 

Figure 8.27 Stacking of two gold pyrazolate trimers into a discotic dimer of trimers.

A nematic phase of discotic molecules exists where the short molecular axes are correlated directionally but this phase is still rather rare. By far and away the most common behaviour is for the molecules to stack in columns, which are then arranged in a particular way with respect to one another [7]. Examples are given in Fig. 4. 

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... 

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes layers of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between layers is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (ND) or in columns that are internally ordered (D ) or disordered (Dd) and may be stacked vertically,... 

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. 

Figure 2. Stack-type arrangement of a variety of discotic liquid crystals.

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... 

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