Photochemical azobenzene

As just mentioned, phosphorus porphyrins have unique photochemical properties. Their photophysics is also interesting. Emitter-quencher assemblies based on porphyrin building blocks have attracted attention due to their potential to serve as models in photosynthetic research (see [90] for an example) or for the development of photoswitches that could be used for the fabrication of molecular electronic/optical devices. In this context, Maiya and coworkers constructed a P(VI) porphyrin system 59b with two switchable azobenzene groups positioned in the apical positions of the pseudo-octahedral phosphorus atom [92]. Photoswitch ability (luminescence on/off) was demonstrated as... 

Spectral Transparence Starting from 230 nm 4-Hydroxy-Azobenzene Presence of a Thermally and Photochemically Isomerizable Azo Group Trans-Cis Photoisomerization Photochromism ... 

Photoresponsive systems incorporating an azobenzene moiety. The capped crown ether (196), shown as the (E) isomer, was synthesized initially by a high-dilution condensation between diaza-18-crown-6 and 3,3 -bis(chlorocarbonyl)azobenzene (Shinkai et al., 1980). Extraction patterns for the alkali metals differed between the (E) and (Z) isomers giving a clear example of photochemical control of the complexation behaviour. Subsequently, the analogue (197) was synthesized in which 2,2 -azopyridine was used for the cap (Shinkai Manabe, 1984). Photo-... 

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. 

Photochemical switching of the phase transition is also found in the polyion complex film. Figure 29 shows reversible cycles of the absorption at 370nm by the coupling of the thermal and photoinduced phase transition of the complex film with carboxymethylcellulose 8. In conclusion, we indicate that the immobilized bilayer membranes containing the azobenzene chromophore are available to the erasable memory materials based on the phase transition triggered by thermal and photochemical processes. The polyion complex technique is clearly shown to be a very useful method for materialization of the immobilized bilayer membranes. 

We have demonstrated that a chiral vesicle composed of di-palmitoyl-L- a-phosphatidylcholine(l-DPPC) doped with the azobenzene containing amphiphiles shown in Scheme 2 is a subject to photochemically triggered phase transition and exhibits a non-linear photoresponse in terms of ICD appearing at the absorption band of azobenzene. 

Photochemical response of these liposomes is different from each other. With progression of trans - cis photoisomerization of azobenzene, ICD at the absorption band of the trans isomer decreases. As shown in Figure 4, depression in ICD is almost proportional to the amount of photoisomerization for the phase separated system. Photoisomerization in the domain of azobenzene aggregate proceeds independently from the rest of DPPC aggregate so that the depression in ICD corresponds to the concentration of remaining transazobenzene. When the two components are molecularly mixed, change of... 

Photochemically-driven dethreading and rethreading of an azobenzene-based pseudorotaxane in acetonitrile occurs when (5) threads with (6) (Figure 12.17). 

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. 

The experimental procedure adopted in the thermochemical study of reaction 10.17 was fairly simple. First, an electrical calibration was made. Then, after balancing the light input to the cells, 2.7 cm3 of a 7 x 10-3 mol dm-3 solution of hrms-azobenzene in heptane was added to the photochemical reactor. This solution was irradiated for a certain period (1.5-3.8 h) with 436 nm light, and the thermogram was recorded. The area of this thermogram multiplied by the calibration constant (e) gives A0b H. 

The 1,2,3-diazasilacyclopentane (229) is formed in a photochemical reaction between trans azobenzene and hexamethylsilacyclopropane (Equation (18)) <840M579>. 1,3,4-Diazasilacyclopentane (230) is formed in a formally analogous manner (Scheme 24) in high yields <930M529>. The much... 

As an example of compounds containing —N=N— double bonds, photoisomerization of azobenzene has been extensively studied. Irradiation of azobenzene in neutral solutions is known to result in cis trans photoequilibration.102 Similar photochemical cis - trans isom-... 

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]... 

Photochemomechanical systems have also been studied using gels and photochemical reactions such as photochromism of spiropyrans [20], trans-ds transition of azobenzene [21] and photo-dissociation of triphenylmethane leuco cyanide [22]. It is an attractive approach to utilize a molecular level conformational change. 

Other 1,2-cycloadditions have been accomplishedphotochemically. The photolytic decomposition of diazoketones in the presence of imines to give azetidinones [see, for example, Eq. (80)] is sometimes preferable310 to the direct chemical addition of ketene to imine. Diazetidinones of general structure (291) can be prepared311 either by thermal or photochemical addition of ketenes to azobenzenes, or by photolysis of diazoketones in azobenzene. 

Photochemical switching of the electrical properties of conductive Langmuir-Blodgett films results from photoinduced conformational changes produced by an azobenzene unit [8.253]. 

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. 

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. 

Chahidi C, Giraud M, Aubailly M, Valla A, Santus R. 2,4-Diamino-6-pteridinecarboxyaldehyde and an azobenzene derivative are produced by UV photodegradation of methotrexate. Photochem Photobiol 1986 44 ... 

Corma [208] and Dutta [209] and their coworkers reported the photocyclization of azobenzene in zeolite HY and microporous aluminophosphate, respectively. They also demonstrated that the acid groups in these media directly intervene in the photochemical reactions of azobenzene. 

It is known from literature that several reversible photochemical reactions, such as geometric isomerism of azobenzene [7], electrocyclic reaction of dihydroindolizines, fulgides and diarylethylenes with heterocyclic groups [8-10], dimerization of anthracene [11], and photochromic reaction of spirocompounds [12] have been also employed to provide photocontrol over metal-ion binding ability of crown ethers. 

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