Azobenzene, photochemical isomerization

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]. 

A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the ji-ji transition between 355nm and 325nm attributed to the trans and cis isomers, respectively. 

The number of chemical reactions that have been examined with the heat flow microcalorimeter of figure 10.2 is still fairly small. We have selected reaction 10.17, the photochemical isomerization of trans- to cz s-azobenzene, to illustrate the method. 

Hong, J.-D., Park, E.-S., and Jung, B.-D. (1999). Studies on photochemical isomerization of azobenzene in self-assembled boloamphiphilic monolayers. Mol. Cryst. Liq. Cryst.. Sci. Technol. Sect. A, 327, 119. 

The compact conformation returns to the initial extended conformation by the thermal or photochemical isomerization of the chromophores from the cis to trans form. Polyamides with azobenzene groups in the polymer backbone are among the earliest in which trans-cis isomerizable chromophores are used to regulate the conformation of polymer chains(4.5). 

Hong JD, Park ES, Park AL.1999. Effects of added salt on photochemical isomerization of azobenzene in alternate multilayer assemblies bipolar amphiphile polyelectrol5he. Langmuir 15 6515 6521. 

H. Akiyama, A. Tanaka, H. Hiramatsu, J. Nagasawa, N. Tamaoki, Reflection colour changes in cholesteric liquid crystals after the addition and photochemical isomerization of mesogenic azobenzenes tethered to sugar alcohols. J. Mater. Chem. 19, 5956-5963 (2009)... 

Azobenzenes are well-known photochromic dyes with trans-cis isomerization (Fig. 24), and their photochromic reactions in various heterogeneous systems have been reported. Photochemical isomerization of azobenzenes intercalated in the hydrophobic interlayer space of alkylammonium-montmorillonite and TSMs has been investigated (212,213). Intercalation compounds were obtained by a solid-solid reaction between the organophilic silicates and azobenzene. The intercalated azobenzene showed reversible trans-to-cis photoisomerization upon UV irradiation and subsequent thermal treatment (or visible light irradiation). The hydrophobic interlayer spaces of the alkylammonium silicates were proved to serve as media for the immobilization and photochemical isomerization of the azo dye. 

Ramamurthy and Jayaraman employed the cavity of poly(aIkyl aryl ether) dendrimers of the third generation with various n-alkyl linker (ethyl to n-pentyl) connecting the branch points (C2G3-C5G3) for photochemical isomerizations of azobenzene and stilbene. Isomerization was carried out in aqueous dendrimer solutions. Irradiation of solutions for 6 h furnished products. The results of isomerization of stilbene in the presence of various dendrimer solutions and in... 

Natarajan B, et al. Dynamic internal cavities of dendrimers as constrained media a study of photochemical isomerizations of stilbene and azobenzene using poly(alkyl aryl ether) dendrimers. J Org Chem 2012 77 2219-24. 

Both the hindered rotation around the amide bond and the thermal cis-trans isomerization of azobenzene residues are characterized by high energy barriers in the neighborhood of 20 kcal/ mole. Some years ago, Malkin and Fischer studied the photochemical isomerization of azobenzene and found that the quantum yield is temperature-dependent they interpreted their data as reflecting an energy barrier of 2-3 kcal/mole between the excited cis and trans species. A comparison of the quantum yields for the photoisomerization of azobenzene residues in the backbone of polymer chains and in low molecular weight analogs should, therefore, indicate whether conformational transitions characterized by low activation... 

In the following section, the UVA IS spectroscopy of azobenzenes, the knowledge of which is a prerequisite for photochemical isomerization reactions, is described. Conditions and quantum yields of the photoisomerization will be dealt with in Section 89.3 for the azobenzene molecule and some of its derivatives. The mechanism of these simple light-induced unimolecular reactions (i.e., the dynamics of molecular changes) has only recently been studied directly by means of experimental laser spectroscopy methods. The results obtained thus far are summarized and compared with conclusions from quantum chemical calculations in Section 89.4. Thermal Z—isomerization reactions influence the concentration ratio of E- and Z-isomers in the photostationary state on irradiation of azobenzenes. They are described separately in Section 89.5. Finally, a selection of interesting prospective applications of the photoisomerism of azobenzenes is presented in Section 89.6. 

SAMs containing an azobenzene moiety have also been used to modify the electrochemical properties of electrodes by several investigators. Thus, Uosaki and collaborators constructed a SAM by depositing an alkyl thiol molecule containing ferrocene and azobenzene moieties on gold electrode surface (Scheme 65). They found that the charge-transfer rate of the ferrocene group in the SAM could be reversibly controlled by electro- and photochemical isomerization between ,Z-forms of the azobenzene moiety in the SAM. 

Meanwhile, a self-assembled alternating multilayer film, composed of a bipolar amphiphile, 4,4 -bis[(12-(trimethylammonio)dodecyl)oxy]azobenzene dibromide 29 and an anionic polyelectrolyte poly(vinyl sulfate, potassium salt) on a pre-coated fused silica substrate, was prepared. The effects of added salt on the photochemical isomerization of the azobenzene unit in the self-assembled multilayer films were investigated. ... 

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. 

We now report the photochromic reaction of an azobenzene in the nanoporous silica film. Since the photochromic behavior is environmentally sensitive, photochromism of organic substances in solid matrices has been investigated to understand as well as to modify the photochromic behavior.[21] Photochromism of azobenzene and its derivatives due to cis-trans isomerization (Scheme I) has widely been investigated. Photocontrol of chemical and physical functions of various supramolecular systems has vigorously been studied by using photochemical configurational change of azobenzene derivatives.[22,23]... 

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

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