Azobenzene applications

These considerations show that caution must be exercised in applying of the Hammett equation to systems which are known to involve tautomeric equilibria, e.g. the pyridinecarboxylic acids or y-hydroxypyridines. If in such systems the experimental p L "values are linear in a, there is at least a strong suggestion that Krp is insensitive to the nature of the substituents. Some applications of these ideas in the azobenzene series have proved of considerable interest. ... 

Table 2.4 shows a comparison of the experimental and PPP-MO calculated electronic spectral data for azobenzene and the three isomeric monoamino derivatives. It is noteworthy that the ortho isomer is observed to be most bathochromic, while the para isomer is least bathoch-romic. From a consideration of the principles of the application of the valence-bond approach to colour described in the previous section, it might have been expected that the ortho and para isomers would be most bathochromic with the meta isomer least bathochromic. In contrast, the data contained in Table 2.4 demonstrate that the PPP-MO method is capable of correctly accounting for the relative bathochromicities of the amino isomers. It is clear, at least in this case, that the valence-bond method is inferior to the molecular orbital approach. An explanation for the failure of the valence-bond method to predict the order of bathochromicities of the o-, m- and p-aminoazobenzenes emerges from a consideration of the changes in 7r-electron charge densities on excitation calculated by the PPP-MO method, as illustrated in Figure 2.14. 

The compound exists normally as the trans or ( )-isomer 21a. This molecule is essentially planar both in the solid state and in solution, although in the gas phase there is evidence that it deviates from planarity. When irradiated with UY light, the ( )-isomer undergoes conversion substantially into the cis or (Z)-isomer 21b which may be isolated as a pure compound. In darkness, the (Z)-isomer reverts thermally to the (F)-isomer which is thermodynamically more stable because of reduced steric congestion. Some early disperse dyes, which were relatively simple azobenzene derivatives introduced commercially initially for application to cellulose acetate fibres, were found to be prone to photochromism (formerly referred to as phototropy), a reversible light-induced colour change. C. I. Disperse Red 1 (22) is an example of a dye which has been observed, under certain circumstances, to give rise to this phenomenon. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

In the presence of an oxidant, e.g., chlorate or bromate ions, the electrode reaction is transposed into an adsorption coupled regenerative catalytic mechanism. Figure 2.85 depicts the dependence of the azobenzene net peak current with the concentration of the chlorate ions used as an oxidant. Different curves in Fig. 2.85 correspond to different adsorption strength of the redox couple that is controlled by the content of acetonitrile in the aqueous electrolyte. In most of the cases, parabolic curves have been obtained, in agreement with the theoretically predicted effect for the surface catalytic reaction shown in Fig. 2.81. In a medium containing 50% (v/v) acetonitrile (curve 5 in Fig. 2.85) the current dramatically increases, confirming that moderate adsorption provides the best conditions for analytical application. 

A summary of the in situ use of the azobenzene probases is given in Table 2. Apart from the generation of ylid, referred to above, the main applications have been for N- and C-alkylation of weak nitrogen and carbon acids, for the promotion of condensation and substitution reactions involving carbanions such as the cyano-methyl anion, for an interesting carboxylation reaction (entries 4 and 17), and for base-promoted cyclisations (entries 5 and 6). 

The calculations predict that azobenzene derivatives have nearly identical dipole moments and molecular hyperpolarizabilities as the stilbenes. Selection of compounds for use in specific applications can therefore, be based on linear optical properties (absorption) and photochemical stability requirements without sacrifice of nonlinear optical response. 

The qualitative application is illustrated by the approximate location of E°w for the azobenzene/aniline couple on redox ladders constructed by Schwarzenbach et al. (e.g., Figure K.3 in Reference 120). The estimate, around -0.1 V vs. NEIE, comes from electrochemical studies that report non-Nemstian dependence of E1/2 on pEI and additional evidence for the non-reversibility of this reaction (8,121). 

Scheme 17 Reversible photostimulation of alcohol dehydrogenase (AlcDH) by the application of the photoisomerizable azobenzene-modified NAD+ (32a) in the presence ofthe Z1 HOI monoclonal antibody. Disphorase (Dl) is used to regenerate the oxidized cofactor.

These azobenzene LCs display the liquid crystalline phase only when the azobenzene moiety is in the trans form, and no liquid crystalline phase at any temperature when the azobenzene moiety is in the cis form. In these azobenzene LC system, it was predicted that phase transition should be induced on essentially the same time-scale as the photochemical reaction of the photoresponsive moiety in each mesogen, if the photochemical reactions of a large number of mesogens were induced simultaneously by the use of a short laser pulse (Figure 7).1391 On the basis of such a new concept, the photoresponse of azobenzene LCs with the laser pulse was examined, and it was found that the N to I phase transition was induced in 200 xsJ39 40 This fast response, on the microsecond timescale, had been demonstrated for the first time in NLCs. From the viewpoint of application of LCs to photonic devices, such a fast response is quite encouraging. 

Figure 5.23 Schematic illustration of the organization of the molecular train monolayer assembly on a gold electrode and its photoinduced translocation. Reprinted from /. Electroanal. Cheat., 497, I. Willner, V. Pardo-Yissar, E. Katz and K. T. Ranjit, A photoactive "molecular train" for optoelectronic applications light-simulated translocation of a /3-cyclodextrin receptor within a stoppered azobenzene-alkyl chain supramolecular monolayer assembly on an Au-electrode, 172-177, Copyright (2001), with permission of Elsevier Science...

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