Azobenzene introduction

The introduction of a p-amino-substituent enhances the basicity of azobenzene very considerably. The value of p/iTa changes from —2-90 (Yeh and Jaffe, 1959a) to -i-2 82 (Bascombe and Bell, 1959). The controversy over the site of protonation of p-aminoazobenzene has been resolved in favour of a tautomeric mixture of the ammonium [48] and azonium [49] forms on the basis of an analysis of... 

Since it was observed that absorption ceased after 3.3 atoms of hydrogen were taken up per mole of nitrobenzene, Equation 14 is shown as producing 4 moles of cyanocobaltate(II) per mole of substrate via reaction of the latter with CoH. Since further absorption of hydrogen occurred only upon introduction of alkali, it is implied that an intermediate complex, X, is formed which is not subject to further reaction with CoH but may be decomposed by alkali. The stoichiometry of this equation requires formulation of complex X as [Co(CN)5(C6H5NH)]—3. However, since absorption ceased after two atoms of hydrogen had been absorbed per atom of cobalt present, it is implied that a binuclear complex is formed, perhaps involving phenylhydroxylamine, azobenzene, or some other reduction intermediate. 

The introduction of azobenzene units into the side chains of poly(L-lysine) has been achieved be means of various procedures and different azo reagents. The polymers described initially contained azobenzene units linked to the lysine side chains by means of an amide moiety 29-311 (Scheme 4, Structure V). More recently, Fissi et al. 321 have described azo-modified poly(L-lysine) in which the azobenzene units are linked to the Lys side chains by means of a sulfonamide function (Scheme 4, Structure VI). The two families of azo-modified poly(L-lysine) have been found to exhibit completely different conformational and photoresponsive behavior. 

A large number of azobenzene-based amorphous and liquid crystalline polymers, particularly polyacrylates and polymethacrylates with chiral azobenzene pendants, have been prepared for the development of data storage and photonic devices [1-3,11-14]. For instance, the introduction of optically active mesogenic azobenzene residues into the side groups of the polymers produces chiral nematic and cholesteric phases, which are regulated by photoisomerization of the azobenzene units [10,14]. In most cases, however, the optical activity and chiroptical... 

An interesting modification of the benzyl ester group is the introduction of chromo-phores that allow visual monitoring of the isolation and purification steps. Esters of 4-(dimethylamino)-4 -(hydroxymethyl)azobenzene additionally provide a basic substituent that enables attachment to ion-exchange resins and thus facilitates removal of excess starting materials, e.g. acylating agents, and possible byproducts.P ... 

The free volume around the azobenzene that is necessary for an unrestricted photolsomerization can be established by the introduction of bulky head groups or additional alkyl chains in the azobenzene amphiphile. An example of this approach has been published by Markava et al. The authors compared azobenzene amphiphiles with one (14) and two (15) alkyl chains as well as amphiphiles with small (16) and large (17) head groups. They found that photoisomerization takes place in LBK films of 15 and 17, where the areal requirements of the amphiphile are high, but not in the LBK films of 14 or 16, which can be packed more densely. 

This chapter discusses photochromic azobenzene polymers and photochromic molecules capable of undergoing photodimerization as materials for potential use in optical data storage applications. A brief introduction to photochromism, including recent examples of some photochromic systems, other than azobenzenes, based on variety of applications is presented at the outset. Thereafter, we shall focus on azobenzene photoaddressable polymers (PAPs) and [4 + 4]-photocycloaddition as materials for ultrahigh capacity optical data storage. 

Figure 5.49 shows the transport numbers of various anions (sulfate, fluoride, bromide and nitrate) relative to chloride ions for a membrane having an azobenzene moiety (M-2 membrane) before and after UV irradiation when 1 1 mixed salt solutions (concentration of sodium ions 0.150 N) were electrodialyzed. The transport numbers of all anions increase upon UV irradiation due to the increase in water content, which is based on the increase in dipole moment of the azobenzene moiety of the membrane by UV irradiation, and the increase in pore size of the membrane due to isomerization from the trans to the cis form.135 In particular the permeation of halide ions, (fluoride and bromide) increases remarkably. Fluoride ions permeate more selectively through the membrane than chloride ions.135 The increase in permeation of multi-atomic anions (sulfate and nitrate) is not large, which might be because they suffer steric hindrance. After amination of the M-2 membrane with trimethylamine (M-3 membrane), the permeation of sulfate and fluoride ions decreases and that of bromide and nitrate ions increases compared with the M-l membrane without UV irradiation. This is due to introduction of a bulky, hydrophobic group, the azobenzene moiety, into the membrane. However, the transport numbers of the measured anions (sulfate,... 

In sum, a larger repulsion between specific parts of the alkyl chains in a bilayer is reflected in a higher lateral pressure, a decrease in chain order and more area available for that part of the tails. Such changes can be induced by, for example, a higher temperature or introduction of a cis C=C bond into the tail or via trans-cis isomerization of double-tailed, azobenzene-substituted amphihiles. ... 

Bioengineering has more recently been broadened by expanding the natural protein alphabet with artificial amino acids. This enables novel and nonnatural protein sequences to be created, while still exploiting the highly efficient natural synthesis machinery. Chiral azobenzene amino acids have been synthesized and incorporated into protein sequences (Wang and Schultz, 2004). The introduction of artificial photoactive residues opens the possibility of... 

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