Azobenzene, photoisomerization

Ingerman, L. A. Waters, M. L. Photoswitchable dynamic combinatorial libraries Coupling azobenzene photoisomerization with hydrazone exchange. J. Org. Chem. 2009, 74, 111-117. 

Azobenzene photoisomerization Two states and two relaxation pathways explain the violation of Kasha s rule. 

Switching devices that are reversible and work on the molecular level are essential features of nanomachinery. Control of the access to capsules, the transport of molecules in and out of the cavities, is desirable and we examined a well-established system that uses light as a switching device the cis-trans photoisomerization of azobenzenes [58, 59]. The azobenzenes have been applied in the supramolecular chemistry of crown ethers [60-62], cyclodextrins [63,64], and even proteins [65, 66]. The photoisomerization changes the shape in a predictable way and we used azobenzene photoisomerization in an indirect sense to control reversible encapsulation. 

Z- E isomerization yield is nearly temperature-independent (Figure 1.10) or increases at low temperature, with only a small difference for excitation to the two lowest-excited states. So obviously, the E —> Z photoisomerization— after irradiation to the (n,7C ) state as well as the Z —> E isomerization— proceeds even at low temperature and in frozen solvents. In solid matrices, fast and slowly isomerizing molecules are observed on it —> it excitation. The fast process has a quantum yield of < = 0.14 that is temperature independent down to 4 K. With strong lasers, photoisomerization in the E —> Z direction have been exploited, even at 4 K in hole burning experiments. Thus, azobenzene photoisomerization cannot be frozen out. 

Organized azo-molecular assemblies allow for the study of photoisomerization and photo-orientation in sterically and orientationally well-defined media, such as LBK multilayers, the molecular-interactions-based order of which can be altered by photoisomerization of the azo units.In this chapter, we also discuss the photoisomerization-induced changes in the structural and optical properties of highly organized organic films containing azobenzenes. Photoisomerization and photo-orientation of azobenzenes is compared in amorphous spin-cast films, in LBK supramolecular assemblies, and in self-assembled monolayers. 

These photoresponsive peptides form stable monolayers at the air/water interface,the structure of which depends on the isomerization of the azobenzene. Photoisomerization, therefore, induces changes in the structure, and with that an increase in the areal requirements at constant surface pressure. As confirmed by UV-vis spectroscopy, LBK films of these peptide mono-layers deposited on quartz glass also retain the ability to isomerize. 

FIGURE 6.13 Azobenzene photoisomerization and interaction with polarized light. 

The kinetics of spiropyran and azobenzene photoisomerization deviate from first order when these dyes are entrapped in a solid matrix below Tg.24-34 This behavior has been attributed to the presence of a distribution of free volume within the matrix, as shown in Table 3.11 .35 When the probe is located in sites of free volume Vf greater than the critical volume for isomerization Vfc, the reaction proceeds at the same rate as in solution. For sites of Vf < Vfc, the reaction is retarded, since it becomes controlled by the matrix molecular motions. At low temperature, the local molecular motions are frozen and fluctuations of local free volume become increasingly small as the temperature decreases. Consequently the fraction of sites where Vf < Vk increases. 

Schultz T, Quenneville J, Levine B, Toniolo A, Martinez TJ, Lochbrunner S, Schmitt M, Shaffer JP, Zgierski MZ, Stolow A. 2003. Mechanism and dynamics of azobenzene photoisomerization. J Am Chem Soc 125 8098 8099. 

Colloidal silica nanoparticles of 5-15 nm functionalized on the surface either with p-CD- or with (phenylazo)benzoic acid self-assembled into large NPs when both components were mixed in water. Such severe aggregation was mediated by host-guest complexation. Deaggregation was induced by trans-to-cis azobenzene photoisomerization and cis decomplexation. 

Although the concept of molecular machines was contemplated by Richard Feynman in 1959 in his famous literature There s Plenty of Rooms at the Bottom, it was not realized until 1981 when a Japanese scientist, Seiji Shinkai, reported the first synthetic molecular machine based on azobenzene photoisomerization. From then on, more and more artificial molecular machines with various functions have been synthesized. 

Since the beginning of the debate on the mechanism of azobenzene photoisomerization, two Umiting cases were discussed rotation around the azo bond with a reduced bond order in analogy to stilbenes, or inversion in the plane due to rehybridization of one azo-nitrogen with small changes of the azo Jt-bond (Figure 89.4). 

The change in the absorption spectrum and the molecular structure on photoisomerization of azobenzenes can be used for practical applications. The literature on possible applications in photoresponsive materials was reviewed up to 1988 by Rau. Important contributions by Japanese workers, for example, on light-driven potassium ion transport through membranes by means of crown ethers with a photofunctional azobenzene cap, were recently summarized by Shinkai. Moreover, light-manipulation of other super molecules based on azobenzene photoisomerization is described in the book by Feringa. " The E— Z photoisomerization for the preparation of thermally unstable Z-isomers for use as kinetic probes in microheterogeneous media was described in Section 89.5. 

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