Azo linking group

The use of the azo-linking group (-N = N -) in liquid crystal chemistry is now well documented, and hundreds of compounds of this class have been cited in the chemical literature [47] the linkage is formed by an azo coupling reaction between a substituted aryl diazonium salt and a suitably activated substituted benzenoid compound [50]. 

Another interesting variation on the linkage holding together two benzocrowns was reported by Shinkai and coworkers In their case, the benzo groups on the crowns were also part of a photolabile azobenzene system. The azo-linked bis-crown was prepared as follows (see Eq. 3.34). 4 -Nitrobenzo-l 5-crown-5 was reductively dimerized... 

In the Chemical Abstracts method (also accepted by IUPAC Rule C-912), the naming of monoazo compounds with radicals R derived from two identical parent hydrocarbon hydrides is the same as mentioned above (for an exception see C-912.2). If the azo group links groups that are different when unsubstituted (R-N2-R ), a parent molecule RH is treated as substituted by R -N2- (Rule C-912.4) thus, 1.8 is called 4-(2-hydroxy-l-naphthylazo)benzenesulfonic acid or 4-(2-hydroxy-... 

LC polyesters belong to the class of thermotropic main-chain LCPs, which also comprises polymers such as polycarbonates, polyethers, polyphenylenes, polyester-imides, polymers containing azo- or azo V-oxide linking groups, some cellulose derivatives, and polypeptides such as po 1 y (y - be n zy 1 -1. - g 1 u tamate). Both from the academic and industrial points of view, polyesters are by far the most important representatives of this class of polymers. 

Azo dyes contain an azo link between two aromatic rings possessing a orthohydroxy group. The most important reagents include PAN, PAR, and Arsenazo III and generally they offer high sensitivity for the majority of transition metals. 

Bayle presented liquid crystal 34 (Scheme 19) bearing four aromatic units linked by ester and azo functional groups [56]. Two butyloxy groups are attached at the ends of the molecule and the crown ether is bound at the side of the molecule. The nematic phase exhibited by 34 is quite broad (AT = 96 K). Upon complexation with LiBF4, the nematic range diminishes with increasing amounts of added salt and disappears completely at 0.5 equiv. of added LiBF4 which is most likely due to the formation of a 2 1 crown lithium complex. From 0.2 equiv. of salt, a smectic... 

In the poly(ethyl acrylate)rubbers described above spirobenzopyran and aromatic azo chromophores were incorporated in the cross-links between the polyacrylic chains. In contrast, Matejika and al. studied cross-linked systems with azo side groups they considered especially the relative importance of the thermal effects occurring during photomechanical conversion. Most interesting are their results on copolymers ccmtaining low concentrations of chromophore (for which heat effects are less important), namely copolymers of )8-hydroxyethyl methacrylate (0.99) and azonaphthol methacrylate (XII) (0.01) swollen in water... 

Structurally similar polymers with azo links, Polymers 9 and 10, formed no meso-phases when the rigid aromatic unit contained terminal carbonyl groups, but these formed nematic mesophases when the ester link was reversed The transition temperatures of the azo and azoxy polymers with the same spacer were quite close to each other. Substitution of methyl groups on the phenylene rings of these polymers reduced melting temperatures without adversely affecting nematic mesophase stability... 

Synthesis of l-Phenylazo-Imidazolidin-2-ones. Substituent electronic effects are transmitted by the azo link, -N=N-, about 1.5 times more effectively than by a carbon-carbon double bond (23). This difference between azo and ethylenic links may be due to enhanced electronegativity of the nitrogen containing group. For instance, the standard reaction of N-aminoimidazolidin-2-one with aromatic diazonium salts was unsuccessful under a variety of reaction conditions (Table II) none of the desired l-phenylazo-imidazolidin-2-one could be isolated. 

A new method was found to prepare 2,3,4,4a-tetrahydro-l,2,3-benzotriazines following the rare C-C-N-N-N-C mode (a). The azo link in l-(2,6-dimethylphenyl)-3,3-di(primary alkyl)triazenes activates the a-methylene groups at N-3 sufficiently to undergo lithiation and, at the same time, most likely complexes the Li-atom brought in by metalation with BuLi. The carbanionic carbon in turn attacks a phenyl carbon atom ortAo to N-1, which is novel, since otherwise functionalized carbon atoms such as CN or a carbonyl group at C-2 are attacked (Equation (102) Section 9.01.9). No predictions regarding a wider applicability, though, are possible at present. 

It will be seen that all the direct dyes so far discussed are extended, linear molecules—a characteristic of the class. It has been shown that the distance between the two azo groups in a substantive dye is 1.08 nm and that of the polymer unit length in cellulose is 1.03 nm, so that the operation of van der Waals forces is facilitated, though there is clear proof that this is not the only factor present since substantivity disappears in CA and CT fibres. H-bonding is invoked as a source of attachment, the hydrogen atoms of—OH groups in the cellulose unit bonding with N of an azo link... 

Examples of cyclic units are 1,4-phenylene, 2,6-naphthalene and fr s-1,4-cyclohexylene rings. The linking group connects the cyclic units on each side to give the chain a linear character. The preferred confirmation is planar (linear). A number of different groups fulfil these requirements direct bond, ester, imino, azo, azoxy, and methylol groups (semiflexible). 

The properties required of ferroelectric hosts therefore means that materials such as esters, Schiff s bases, and azo compounds are unsuitable because of their slow response times and poor stability. The best materials discovered so far, rely on removal of any functional or linking groups which might increase the viscosity or lower the stability. Figure 39 shows a variety of families of host materials [31,33,48-51 ]. These materials have a number of attributes in common. First, they are devoid of linking groups, thus the aromatic or heterocyclic rings in the core are directly linked. Second, only alkyl or alkoxy terminal chains are used in order to maintain as low a viscosity as possible. Third, some materials carry lateral fluoro substituents in order to increase... 

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