Benzene derivatives, discotics

A few words of clarification about the nomenclature are relevant here. Columnar phases have been known for many years they were evident, for example, in the work of Spegt and Skoulios (28) on metal soaps, although these are not classical disk-shaped molecules. In 1977, however, a hexasubstituted benzene derivative was reported (29), this derivative was the first example of a properly disk-like mesogen, and the term discotic was coined to describe the mesophases it formed. Thus, for example, the discotic hexagonal phase was labeled Dh- The introduction of this nomenclature has actually caused confusion as disk-like molecules are not alone in their capacity to form columnar phases (indeed, some... 

Tetrasubstituted [292], 1,2,3,5-tetrasubstituted [293] and pentasubstituted [293] benzene derivatives can also be discotic. The most typical structure is the sixfold substituted benzene, which was used in the first discotic materials [7]. The substituents can be different in nature (31 [7]). 

This idea of the association of molecules resulting in discotic units was also used by Matsunaga et al. [198, 293] in order to explain the discotic mesomorphic properties of tri- and tetrasubstituted benzene derivatives. 

The mesophase-stabilizing effect of the amide group allows discotic benzene derivatives to be prepared that carry fewer long chains. The syntheses of such materials are shown in Schemes 4-6, and their meso-phase properties are given in Tables 4-6. 

A widely studied class of discotic benzene derivatives are the multiynes prepared by Praefcke and coworkers [24], Like the structurally similar naphthalenes, these materials are prepared by palladium-catalysed coupling of polybromobenzenes with acetylene derivatives (Scheme 8). Their mesophase behaviour is given in Table 8. The hexakisalkynylbenzenes themselves exhib-... 

Finally, a class of benzene derivatives that also form discotic mesophases is the hexa-... 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

Discotic liquid crystals The molecules possess a disc shape in many cases they are polysubstituted benzene or triphenyl derivatives with lateral extended aliphatic chains. These molecules arrange, e.g., in large columns a discotic nematic phase is known too. 

Supported FeCl3 will catalyse the oxidation of activated phenols and the coupling of aromatics such as that shown in Figure 4.1.56 It is interesting to note that FeCl3 is inactive in this reaction. Similarly FeC -alumina has been successfully used to synthesise several symmmetrical and unsymmetrical hexa-alkoxytriphenylenes (very important as discotic liquid crystals) from dialkoxy-benzenes and terphenyl derivatives.57... 

Till now only few examples of discotic liquid crystalline polymers are known. In the first example described in literature (1),hexasubstituted triphenylene cores as discotic sidegroups, are linked to a polysiloxane backbone via a flexible alkyl spacer. Discotic main chain polymers (2,3) posses as disclike core benzene or tripenylene derivatives with the same high degree of substitution, i.e. six. 

One further class of discotic benzenes comprises the hexakis[(alkoxyphenoxy)methyl] derivatives [28]. These compounds are simply prepared from hexakis(bromomethyl)-benzene and an excess of p-alkoxyphenox-ide (Scheme 9). The thermal behaviour of these materials is given in Table 9 [28], but it should be noted that the authors were un-... 

Triphenylene (TP) derivatives have been described as the work horses in the field of DLCs [11]. It is the most studied discotic core system [20, 21]. TP (1), see Scheme 4.1, belongs to the polycyclic aromatic hydrocarbon (PAH) group and has been known in the chemical literature for more than a century. This trimer of benzene was isolated from the pyrolytic products of benzene by Schultz who named it as triphenylene [22]. It can also be isolated from coal tar. Trimerization of cyclohexanone followed by dehydrogenation has been used to generate TP in the early twentieth century and its various chemical and physical properties were studied [22]. 

Hexaazatriphenylene (HAT) is an electron-deficient polycyclic heteroaromatic discotic core. Hexaazatriphenylene derivatives have received a considerable attention because of their easy synthetic accessibility, electron deficiency, coordination properties and n-complexation ability. Praefcke and coworkers prepared several hexaalkyl and hexaalkoxy derivatives of hexaazatriphenylene but none of these compounds were found to be liquid crystalline in nature [88]. Hexaazatriphenylene derivatives can be prepared via a one-step cyclocondensation of benzene hexamine 44 and various symmetrical 1,2-diketones 45 as shown in Scheme 4.12. An improved synthesis of the parent core by the condensation of glyoxal with freshly prepared hexaaminobenzene was reported by Rogers [89]. 

The time-of-flight electron mobility in a oxadiazole derivative namely, l,3,5-tris 5-[3,4,5-tris(octyloxy)phenyl]-l,3,4-oxadiazole-2-yl benzene,has recently been studied by Zhan et al. [211], The synthesis of this material is straightforward as shown in Scheme 4.38. Condensation of 1,3,5-benzenetricarbonyl trichloride with the hydrazide 182 yields the precursor 183, which can be converted to discotic oxadiazole derivative 184 by heating in POCI3 at 80 °C [211,212],... 

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