Azobenzene, interconversion

Rotational Raman spectra of N2 and O2 Resonance fluorescence spectrum of Br2 Vibrational-rotational spectra of CD3H and CH3D Spectrophotometric study of stabihty of metal ion-EDTA complexes Kinetics of the H2 + I2 = 2HI reaction in the gas phase Weak-acid catalysis of BH4 decomposition Photochemistry of the cis-trans azobenzene interconversion Isotope effect on reaction-rate corrstants... 

Three azobenzeneophane-type crown ethers in which the 4,4 positions of azobenzene are joined by a polyoxyethylene chain have been synthesized (Shinkai, Minami, Kusano Manabe, 1983). On irradiation with UV light, the ( ) (or trans) form (198) is isomerized to the (Z) (or cis) isomer (199). The ( ) isomer may be regenerated by heating, or by irradiation with visible light the interconversion is completely reversible. 

In prior work, related molecules had been obtained in which a polyoxyethylene chain linked the 2,2 -positions of the azobenzene moiety (Shiga,Takagi Ueno, 1980). Howeverforsuchsystems,theds trans interconversion is accompanied by gradual photodegradation probably resulting from the presence of steric strain in the cis isomers. 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

Poly(L-lysine) containing azobenzene units linked to the side chains by means of a sulfonamide function (Scheme 4, Structure VI), was obtained by treating poly(L-lysine) with p-phenylazobenzenesulfonyl chloride. The poly(a-amino acid) was modified quantitatively conversion to the azo-lysine units of VI was effectively 100%. The azo-modified polypeptide was soluble in HFP, in which it exhibited an intense photochromism attributed to the trans-cis photoisomerization of the azobenzene units. Like other sulfonated azobenzene compounds, 33 azosulfonyl-modified polymers of L-lysine were found to be very stable in their tis form, and no thermal decay was observed at room temperature over periods of times as long as several weeks. Interconversion between the two forms at room temperature could only be effected by irradiation at appropriate wavelengths. This behavior allowed the authors to purify the trans and cis forms of the model compound NE-azobenzenesulfonyl-L-lysine (VII) by chromatography, and to measure the absorption spectra of the two pure photoisomers. 

Fig. 9 Reversible cell adhesion by photochemical control of azobenzene SAMs on gold. Interconversion of Z and E configurations (top). Cell adhesive peptide, RGD, is displayed when SAMs adopt the E configuration at 450-490 nm, and cells adhere and grow (bottom left and right). At 340-380 nm, azobenzene converts to the Z conformation, masking RGD, and cell adhesion is prevented (center bottom). Reproduced from [148] with permission. Copyright Wiley, 2009...

One concludes from these facts that pseudo-stilbenes are not suitable for persistent switching of the molecular form. Any information based on E-Z isomerization is quickly lost. If, however, fast interconversion of E- and Z-forms is the aim, as it is in the alignment of the higher-order polarizability tensor of donor/acceptor azobenzenes, then thermal isomerization supports the photoisomerization process. 

In contrast, in the parent system (128), the cis isomer does not associate, allowing regulation by light of the interconversion between monomer (cis) and dimer (trans) [355]. An azobenzene-appended y-CD shows enhanced binding ability upon UV irradiation for guests such as (l )-fenchone and... 

Azohenzene derivatives constitute a family of dye molecules well known for their photochromic properties, which are due to the reversible cis-trans photoisomerization.Figure 16.11 shows a photoisomerization process of azobenzene. Azohenzene derivatives have two geometric isomers the trans and the cis forms. The isomerization reaction is a light- or heat-induced interconversion of the two isomers. Because the trans form is generally more stable, the thermal isomerization is generally in the direction of from cis to trans. Light induces transformations in both directions. 

Fig. 15 Interconversion of trans- and cis-azobenzene derivatives upon UV and visible light irradiation. Adapted from [156]...

The Hght-driven interconversion of trans to cis isomers of azobenzene was used to control the heterogeneous ET of a SAM consisting of a ferrocene-alkanethiol that was substituted by azobenzene. Electrochemical investigation of a mixed SAM containing 20% of the above mentioned cis azobenzene-ferrocene derivative and 80% of the trans isomer can be converted to pure trans isomer SAM by electroreduction and converted back to the 20/80 mixture by UV irradiation [18]. 

Compound 421 results from interconversion of the monoprotonated azoxyben-zenes 418 and 419. When the reactions were carried out using chlorosulfonic acid, the same mixture of products 420-422 was isolated, except that it contained relatively more of the o-hydroxyazobenzene 421 as compared with the /7-isomer 422. /7-Methylazoxybenzene 423 rearranges on treatment with chlorosulfonic acid at low temperature (—10 °C) to yield a mixture of the azobenzenes 424-427 (Equation 130). 

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