Azobenzene isomerization

The incorporation of a cationic azobenzene derivative, p-( a> -dimethyl-ethanolammonioethoxyj-azobenzene bromide, into nanoporous silica films and the photochemical reactions of the adsorbed dye were investigated. The nanoporous silica films were prepared from tetramethoxysilane and octadecyltrimethyl-ammonium chloride by the rapid solvent evaporation method which we have reported previously. The adsorption of the cationic azo dye was conducted by casting an ethanol solution of the dye onto the nanoporous silica films. Upon UV light irradiation, trans-azobenzene isomerized photochemically to the c/s-form and photochemically formed c/ s-form turned back to the frans-form upon visible light irradiation. The nanoporous silica films were proved to be an excellent reaction media to immobilize organic photocromic species. 

Azobenzene has also been added to the internal surface of mesoporous films and powders through postsynthesis grafting.58,60-62 The effect of oligomer size on azobenzene isomerization kinetics was studied for branched oligomer-modified azobenzene derivatives in solution and covalently grafted to nanoporous silica 61 the size of the dendrimer had little effect on the thermal cis to trans isomerization rate in solution... 

Bortolus and Monti have determined the quantum yields of azobenzene isomerization in different solvents.They found an increase of E Z isomerization but a decrease of Z E isomerization when solvents with high dielectric constant are used. This phenomenon is independent of the irradiation wavelength. Table 1.2 shows the special feature of wavelength-dependent quantum yields of azobenzene. 

Gegiou et found only a very slight viscosity effect, both in the n-Ti and in the jt-jc absorption bands on the isomerization quantum yield. They used glycerol as a viscous solvent, but the result may also be transferred to polymer matrices. In solid matrices, several photoisomerization modes are observed (see the preceding section on the influence of temperature), A com parison between azobenzene isomerization in liquid methylmethacrylate and the slow mode in poly (methylmethacrylate) showed that the difference in quantum yields on Si (0.17) and S2 excitation (0.03) is retained in the solid matrix. The fast process is not observed in n —> n excitation. These data are important in relation to the use of the azobenzene isomerization method for the determination of the free volume in a polymer. 

Contrary to the Z-forms of protonated azobenzenes, the Z-forms of donor/acceptor di-substituted azobenzenes isomerize very quickly at room temperature. To investigate this reaction, the Z-form is produced by flash excitation. The isomerization of these pseudo-stilbenes is strongly dependent on the polarity of the solvent. " For instance, 4-diethyIamino-4 -nitroazobenzene isomerizes with k g = 0.007 s in hexane, but with kz, g = 600 s in N-methylformamide. 4-Anilino-4 -nitroazobenzene in cyclohexane obeys the relation In kz, E = 22( 3) - 72( 9)T0V(8.314 T). Schanze et al. " have established two linear relationships between the free-activation enthalpy of isomerization and the Kosower Z values of aprotic and protic solvents. Sanchez and de Rossi report a strong pH sensitivity of... 

Isomerization needs some extra sweep volume. The volumes for the two mechanisms of azobenzene should be quite different—ca. 0.25 nm for rotation and ca. 0.12 nm for in version. This bears out in restricted spaces. In some zeolites azobenzene can isomerize whereas stilbene does not. Kuriyama and Oishi found that there are two separate AH versus AS lines for azobenzenes isomerizing by inversion (azobenzene type) and rotation (pseudo-stilbene type). ... 

Biswas, N., and Umapathy, S. (1997). Early time dynamics of trans-azobenzene isomerization in solution from resonance Raman intensity analysis. /. Chem. Phys. 107, 7849-7858. 

In contrast to the first-order kinetics of the conventional azobenzene isomerization in solution, a slight deviation from first-order kinetics was observed even in aqueous solutions for polyelectrolytic azobenzene polymers substituted with hydrophobic alkyl residues (14, Fig. 4).43 In an aqueous solution of the amphiphilic polysulfonates containing a small amount of azobenzene moiety, the extent of E-to-Z photoisomerization under UV irradiation was reduced as a result of the compartmentalization of the azo chromophores in a hydrophobic core, which resulted in motion of their restriction. 

Fig. 8A-C. Molecular switches based on the binding ability of crown ethers. A A photo-switchable binding event based on conformational change upon azobenzene isomerization. B A photoswitchable binding event based on both conformational and electrostatic changes. C A redox-switchable calix[4]arene-crown ether...

Judging from the Eaf/EarsT ratios, the correlation between the solvent and chemical coordinate in the isomerization of the carbocyanine cations was of the same order as that in the azobenzene isomerization in MPD. 

Figure 1.4. The mechanism of azobenzene isomerization proceeds either via rotation or inversion. The cis state has the phenyl rings tilted at 90° with respect to the CNNC plane.

This Study has shown that small changes in the conformation/orientation of the peptide by the azobenzene isomerization can modulate the availability and potency of active sites for cell surface receptors. 

Photoresponsive gels based on the photon mode work by cis-trans photoisomerism of photochromic molecules like azobenzene or by photoionization of triphenylmethanelqyco. Ishihara et al. found that poly(2-hydroxyethylmethacrylate) gel shrinks upon ultraviolet irradiation after equilibrium swelling is reached in water [33]. They fiirther found that this gel swells again upon irradiation by visible light. Azobenzene isomerizes... 

Figure 42.9 Schematic depiction of iight-trig- azobenzene isomerism transformation, lead-gered drug release from p-CD-modified hoiiow ing to the detachment from p-CD-functional-MSNPs capped with an azo-containing amphi- ized MSNPs and activating drug release [172]. philic copolymer. UV light causes trans-to-cis...

It is well known that azobenzenes isomerize from trans to cis and from cis to trans when subjected to light of appropriate... 

Azobenzene isomerization normally is a very clean reaction (Scheme 1). In the presence of oxygen, slow photooxidation to azobenzene occurs. ... 

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