Hexa discotic cores

Polycyclic aromatic hydrocarbons (PAHs), e.g. hexa-perz-hexabenzocoro-nene HBC (Figure 4.1a), are disk-shaped molecules, the so called discotics, that contain an aromatic core which provides an extended delocalised zi-orbital necessary for charge carrier transport [15]. Hence, this type of material is ge-... 

Deuterium NMR spectroscopy of the discotic phase of hexa-n-hexyloxy triphenylene has led to similar conclusions. Spectra of two selectively deuterated isotopic species, one in which all aromatic positions are substituted and the other in which only the a-carbon side chains are substituted, bring out the difference between the order parameters of the cores and the tails. Fig. 6.1.3 gives the quadrupole splittings of the aromatic and the a-aliphatic deuterons versus temperature in the meso-phase region. It is seen that the rigid core is highly ordered, the orientational order parameter s ranging from 0.95 to 0.90, whereas the a-aliphatic chains are in a disordered state. 

The vast majority of discotic mesogens with cyclohexane cores and six substituent chains are based on ui- and s-IS. Pertinent to the discussion above concerning the relationship between the direction of projection of chains on 1,2,3,4,5,6-hexasubstituted cyclohexanes, the hexa(n-hexyl) ether of m-IS with one axial hexyloxy (6-O-m-IS) has been shown not to be liquid crystalline the corresponding s-IS hexaether with a more disk-like shape (6-O-s-IS) forms an enantiotropic columnar discotic phase... 

Disc-shaped molecules forming mesophases are called discotic . An example for such a discotic mesogen is benzene-hexa-n-octanoate (BH8) [4] which is depicted in the upper right comer of Fig. 3.1. Again, aromatic cores lead to a... 

Compound 18 exhibits solely a chiral nematic discotic phase (N ) phase because the steric effect of the branched chains at the chiral centre disrupts the ability of the molecules to pack in columns. The large size of the planar aromatic core ensures a high clearing point, but the liquid crystal tendency depends critically on the type of chiral peripheral chain. Hexa-substituted phenylacetylenes were discussed in chapter 3 and exhibited the Np phase. Perhaps not surprisingly, when one of the peripheral acetylene units is chiral, the N phase is exhibited (compound 19). 

Discotic liquid crystals are a prototypical self-assembled columnar system [9-13]. This class of liquid-crystalline compounds is relatively new, discovered in 1977 by Chandrasekar and coworkers [14,15]. The assembly motif, shown in Figure 2, for this class of compounds has an aromatic core that is surrounded by hydrocarbon chains. These disk-shaped molecules then stack to form columns. These one-dimensional stacks aggregate to form arrayed columns. When the columns have a circular cross-section they typically stack into a hexagonal arrangement as shown in Figure 2(a). Some of the original discotics were hexa-substituted phthalocyanines (1), benzenes (2), and triphenylenes (3), shown in Figme 2(b). The self-assembly of classical discotics will not be presented in depth here because it has been a focus of several comprehensive reviews [9-15]. 

The first examples of thermotropic mesomorphism in discotic compounds were observed in the hexa-alkanoyloxy benzenes (1, R = C H2 +i) [1] and the hexa-alkoxy-and hexa-alkanoyloxytriphenylenes (2, R=C H2 +iO and C H2 +iCO O) [2, 3], and it was established by X-ray studies [ 1, 7] that the mesophases of these compounds have a columnar structure. About 1500 discotic mesogens are known to date [8] and a variety of new mesophase structures has been identified. The simplest of these compounds have flat or nearly flat cores surrounded by six or eight, or sometimes four,... 

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... 

Until 1977, it was believed that only rod-like molecules, having a considerably higher length than width, could show liquid crystalline properties (calamitic mesogens). However, in 1977, Chandrasekhar realized that not only rod-like molecules, but also compounds with a disc-like molecular shape are capable of forming mesophases (Chandrasekhar et al. 1977 Sabine et al. 2007) and these are referred to as discotic liquid crystals (DLCs). The first examples of this kind of mesomorphism were observed in hexa-alkanoyloxybenzenes, hexaalkoxy- and -aUcanoyloxy-triphenylenes (Fig. 16.2), and were established by X-ray studies (Kumar 2010). Generally, the disc shaped molecules have central core as benzene or polyaromatic, such as triphenylene or phthalocyanine. 

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