4- amino-4 nitro-azobenzene

Figure 4.2 Photo-induced trans is isomerization of 4-(4-nitrophenylazo)aniline (4-amino-4 -nitro-azobenzene, Disperse orange 3). The back reaction to the more stable trans isomer may be photo-induced or thermally induced...

Fermenting yeast is able to reduce added nitrobenzene to the corresponding amine, aniline, to quite a considerable extent. Part of the added nitrobenzene remains unattacked, but 70% of it could be converted to aniline. Since a direct reduction of the nitro group to the amino group is improbable, Neuberg and Welde tried phytochemical treatment of the possible intermediaries, namely, nitrosobenzene and phenylhydroxylamine on the one hand and azoxybenzene and azobenzene on the other hand. 

Six membered heterocyclics can be synthesized in various ways. Nitrogen analogues of stilbene, like the anilides of benzaldehyde or acetophenone sometimes form azaphenanthrenes 19). Similarly the plasma reaction of azobenzene yields a small amount of diazaphenanthrene 19>. Better results are observed in the cyclization of hydroxy, amino or nitro compounds. Suitable starting materials are molecules with two phenyl groups linked by a carbon or hetero atom. 

Many substituted azobenzenes belong to the azobenzene type, as well. Substitution may shift the n —> it band from 440 nm (azobenzene), to 465 nm (hexamethylazobenzene), to 480 nm (2,2 -dimethyl-4,6,4 ,6 -tetra-tert.butylazobenzene), and even to 520 nm (hexaphenylazobenzene). Hydrocarbon, halogen, nitro, carboxy, acetyl, hydroxy, m-amino" and even 2,2 4,4 ,6,6 hexaphenyl substitution— influences the (7i,7t )-(n,7i ) state energy gap, but not so much as to shift the molecule into the aminoazobenzene group. In the context of this book, it is important that the long-chain and polymeric molecules containing azobenzene units coupled by means of hydroxy and carboxy substituents are of the azobenzene type. 

The dependence of the photoisomerization process on the polarity of the environment varies greatly for differently substituted compounds. King et al. found that p-nitro- or cyanosubstituted azobenzenes had photostation-ary states vv ith similar E/Z ratios, but that an additional p -amino function stopped photoisomerization in acetonitrile. This was not true, however, in methylcyclohexane, where lifetimes of Z-isomers were determined to be in the order of seconds at room temperature. These findings are retained at -35°C, which is taken as proof that it is not the fast thermal Z —> E isomerization that fakes the lack of photoisomerization. This conclusion may be questionable, however, considering the weak temperature dependence for azobenzene-type molecules (Figure 1.10). 

Dinitro-4, 4 -diamino-azobenzene, H2N.(02N).C(5H3.N N.C6H3(N02).NH2 red plates or orn ndls (from phenol + ethanol), rap 340-41° was obtd by hydrolysis with H2SO4 of 4, 4 -diacetamido-3, -dinitro-azobenzene, itself obtd by coupling of 2-nitro-4-nitrosoacetanilide 4-amino-2-nitroacetanilide in acetic acid (Ref 2)... 

Similar conclusions concerning the relation between the relative molecular orientation and the first hyperpolarizability have been drawn by Hamada [13] in the case of the MNA dimer as well as by Okuno et al. [53] in a study on linear and parallel dimers of 4-dimethylamino-4 -carboxyazobenzene and 4-dimethyl-amino-2 -nitro-4 -carboxyazobenzene. The impetus was the desire for an explanation for the large second-order nonlinear optical responses of cone-shaped azobenzene dendrimers. Okono et al. also assessed the adequacy of three classical electrostatic models (Section IV). 

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