Discotic compounds

Figure 9 Fraction helical bonds as a function of the temperature T of the discotic compound 1 of Figure 6 in the solvent n-butanol. Symbols indicate experiments done in the range of concentrations of 10 6-10 2 M (van der Schoot et al., 2000). The temperature has been scaled to the helical transition temperature Th and a temperature Tc = Th(AcBTh/ /Jh )exp(//ArBTh) that depends on the frustration energy j and the excess enthalpy of the helical bond h. Remarkably, data taken over four orders in concentration collapse onto the theoretical curve, indicated by the drawn line.

Ordered thin layers can also been obtained from dilute solutions of discotic compounds onto gold or silicon surfaces. For example, triphenylene derivatives with a single long (o-alkylthiol chain have been shown to self-assemble into monolayers, in which the two types of orientation, face-on and... 

The first liquid crystals of disc-shaped molecules, now generally referred to as discotic liquid crystals, were prepared and identified in 1977. Since then a large number of discotic compounds have been synthesized and a variety of mesophases discovered. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase in its simplest form consists of discs stacked one on top of the other aperiodically to form liquid-like columns, the different columns constituting a two-dimensional lattice (fig. 1.1.8 (a)). The structure is somewhat similar to that of the hexagonal phase of soap-water and other lyotropic... 

DLCs generally exhibit two types of phases nematic phase and columnar phase (Fig. 7.1). Discotic nematic phases are rarely observed while columnar phases are commonly found. In the nematic phase, the molecules possess only orientational ordering however in the columnar phase, the disc-like molecules spontaneously self-assemble into infinite one dimensional (ID) stacks, which in turn self-organize to various two dimensional (2D) liquid crystalline lattices. The hexagonal columnar phase is the most commonly encountered phase in DLCs. Some discotic compounds are also known to exhibit lamellar and cubic phases. 

Scheme 7.4 Chemical structures of some discotic compounds aligned by chemical structure...

Fig. 7.8 Magnetic alignment of discotic liquid crystals, a Molecular structure of a discotic compound and b an arrangement of columnar stacks perpendicular to the magnetic field on a substrate. Reproduced with permission from [142]. Copyright 2006 Elsevier...

Scheme 7.10 Structure of discotic compounds which fcmn self-assembled monolayers...

SCHEME 3.7 Chemical structure of discotic compounds aligned by zone crystallization technique. 

SCHEME 3.8 Chemical structure of discotic compounds which have been aligned by using the LB technique. 

SCHEME 3.10 Chemical structures of discotic compounds aligned by the application of magnetic fields. 

Figure 8.4 Structure of the WV film and the employed discotic compound (PDM = polymerized discotic material). Mori 2005. Reproduced with permission from IEEE.

Large discotic compounds such as 28 are a reeent development, and this type of architecture offers much possibility for future development. 

Rgure 2 Examples of EPs to which the theory of Section II applies r lindrical micelles of surfactant molecules, self-assembled stacks of discotic compounds, and chains of bifunclional molecules. The chains grafted to the core of the discotics enhance their solubility. 

Also, multiple coupling reactions are known to proceed in high yields and have been used in the preparation of discotic compounds [15, 94, 95]. 

Vorlander also had the idea to look for mesogenic properties in the cases of star-like or cross-like molecules, however, his coworker did not have any success in this area [5, 6]. About 1977, Chandrasekhar et al. [7] and Billard et al. [8] were, independently, able to prove that disk-like molecules can in fact form mesophases, called columnar phases. Since then, several hundred discotic compounds have been synthesized [9-14]. 

Calamitic and discotic compounds can be described theoretically using the same basic model, i.e. the cylinder, by elongation or compression, respectively, of the central axis of the cylinder [23]. Of course, the cylinder model is a oversimplification of real mesogenic molecules. As most of the compounds contain ring systems, which are somewhat flat, it could be proposed that parallelepipeds provide better models. In fact, such models have been used for molecular-statistical theories, and these have allowed the description of, in addition to calamitic and discotic molecules, flat elongated (board-like, sanidic) molecules [24]. 

Most of the discotic compounds consist of a central core to which are attached 3-12 substituents, which may contain ring systems and/or flexible chains. The most simple central core is the benzene ring, which was used for the synthesis of the first disco-tic compounds [7]. 

The cores of many discotic compounds are relatively flat. In contrast to this, there are derivatives of macrocycles that deviate substantially from the flat shape, being rather cone-shaped [333, 334]. These compounds are called pyramidic or bowlic compounds. 

Many unconventional and discotic compounds, especially those with large molecules, show a pronounced tendency towards the glassy state [196,319f, 319g, 352-354, 358-365]. Glassy states are well known and common in polymers, and liquid crystalline oligomers show phase behaviours that are intermediate between those of low molecu-... 

In low molar mass liquid crystals, conformational dynamics is typically too fast to be studied by 2D exchange NMR or line shape analysis. Dynamical studies of small molecules belong therefore almost exclusively to the realm of relaxation experiments. There are, however, cases of slow motions even in low molar mass systems, which can be investigated by 2D exchange spectroscopy. A recently investigated example is a discotic tetrabenzocyclophane derivative, which shows very slow motions in the columnar phase compared to other discotic compounds [97]. As illustrated in Fig. 6, deute-... 

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