Thermal isomerization azobenzenes

Effect of /3-cyclodc trin on cis trans isomerization of azobenzenes was studied by Sanchez and de Rossi [26], It was found that the cis-trans thermal isomerization of / -Mcthvl Red, o-Methyl Red and Methyl Orange is inhibited by fi-CD at constant pH. The isomerization rate decreases 4, 8, and 1.67 times, respectively, in a solution containing 0.01 M /J-CD. This effect was attributed to the formation of inclusion complexes hindering rotation of the -N=N- bond. Isomerization of Methyl Yellow and naphthalene-l-azo-[4 -(dimethylamino)benzene] requiring mixed organic-aqueous... 

Azobenzenophanes, possessing 2, 3 or 4-azobenzene moieties connected at the meta-positions by methylene linkers, have been constructed the effects of photochemical and thermal isomerizations of the azo-linkages were investigated <03JOC8291>. Treatment of a DMSO solution of 1,4-dibenzyl-l,4-diazacyclododec-8-ene-6,10-diyne 11 in the presence of 100-fold... 

The pressure dependence of obtained above was converted to its dependence on the solvent viscosity t] determined at each pressure. It is shown in Fig.6. We see therein that ib has indeed the 7) dependence of eq.(6) and it is satisfied for t) variation extending over 10 times. We see thus that thermal isomerization of substituted azobenzenes andiV-benzylideneanilines in solvents has rate constants which can well be rationalized in the framework of the Sumi-Marcus model which gives eqs.(5) and (6). More detailed discussions can be found in Ref. 14. 

The photoinduced and thermal isomerization reactions are nearly perfectly reversible, and side reactions are virtually absent. In de-aerated hydrocarbon solution, azobenzene can be irradiated for days with near UV or visible radiation without any change of absorbance after the photostationary state is established. Under air, the only side reaction is a very slow oxidation to azoxybenzene. This can be checked without much effort by Mauser diagnostics (Section 1.2.2.3). For most azobenzenes, the application of absorbance diagrams gives perfectly straight lines, indicating that the isomerization is the only reaction (Figure 1.4). This fact warrants the use of azobenzene as a convenient actinometer. ... 

One-electron reduction of azobenzene yields the azobenzene radical anion. Its Z-form thermally isomerizes fast. This explains the cleavage/recom-bination mechanism reported for azosulfides and the reduction/oxidation mechanism of azobenzene derivatives in Langmuir-Blodgett monlayers... 

Pressure dependence was thoroughly investigated by Asano and his group. It turns out that the partial volumes of the Z-forms of 4-dimethylamino-4 nitorazobenzene and related molecules are ca. 250 cm moP in all solvents. Those of the E-forms are smaller and solvent-dependent. Thermal isomerization rates are weakly dependent on pressure in nonpolar solvents, but contrary to azobenzene- and aminoazobenzene-type compounds, they are strongly dependent in polar solvents in hexane 10%, in acetone 475% for 2100 bar (AV = -0.7 and -25.3 em mol, respectively). This has implications for the discussion of the mechanism of isomerization (Section 1.6). 

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. 

For pseudo-stilbene-type molecules, the question of the mechanism of thermal isomerization was taken up again in the early eighties by Whitten et al. and later by Kobayashi et al., " who, on the basis of their isomerization experiments with donor/acceptor-substituted azobenzenes in polar solvents, postulate rotation. Asano and coworkers infer from the isomeriza-... 

Ortruba III, j. P., and Weiss, R. G. (1983). Liquid crystalline solvents as mechanistic probes. 11. The syn —> anti thermal isomerization mechanism of some low- bipolarity azobenzenes. /. Org. Chem. 48, 3448-3453. 

Kobayashi, S., Yokoyama, H, and Kamei, H. (1987). Substituent and solvent effects on electronic absorption spectra and thermal isomerization of pull-push-substituted cis-azobenzenes. Chem, Phys. Lett. 138, 333-338. 

Kuriyama, Y., and Oishi, S. (1999). Mechanism of thermal isomerization of azobenzene in zeolite cavities. Chem. Lett. 1045-1046. 

Equation was derived without approximations. It is noteworthy that these solutions do not couple tensorial components of different orders and that they confirm that rotational diffusion and cis—>trans thermal isomerization are isotropic processes that do not favor any spatial direction. In Section 3.4, I discuss, through the example of azobenzene, how Equation 3.11 can be used to study reorientation processes during cis—>trans thermal isomerization after the end of irradiation. The next subsection gives analytical expressions at the early-time evolution and steady-state of photo-orientation, for the full quantification of coupled photo-orientation and photoisomerization in A<- B photoisomerizable systems where B is unknown. 

Equation 3.22. This type of experiment will be discussed eventually for spiropyran and diarylethene chromophores in films of PMMA. Next, I compare reorientation observations after cis—>trans thermal isomerization of azobenzene to the theoretical developments in Section 3.2.3.2. 

The process of reorientation during cis—>trans thermal isomerization can be seen at the value of in Equation 3.11, which shows that the cis anisotropy does not contribute to the trans anisotropy if the trans isomer loses total memory of the orientation in the cis isomer Q2 = 0). It is informative to note that in the realistic physical case—i.e., the case of the azobenzene molecule chemically attached to a polymer, where the cis and trans diffusion rates are negligible in comparison to the cis— trans isomerization rate—the relaxation of the cis and trans anisotropy, AA and can be written respectively in the form ... 

For all Azo-PURs, the quantum yields of the forth, i.e., trans—>cis, are small compared to those of the back, i.e., cis—>trans, isomerization—a feature that shows that the azo-chromophore is often in the trans form during trans<->cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every 1000 photons absorbed, and once in the cis, it isomerizes back to the trans for about 2 absorbed photons. In addition, the rate of cis—>trans thermal isomerization is quite high 0.45 s Q 1 shows that upon isomerization, the azo-chromophore rotates in a manner that maximizes molecular nonpolar orientation during isomerization in other words, it maximizes the second-order Legendre polynomial, i.e., the second moment, of the distribution of the isomeric reorientation. Q 1 also shows that the chromophore retains full memory of its orientation before isomerization and does not shake indiscriminately before it relaxes otherwise, it would be Q 0. The fact that the azo-chromophore moves, i.e., rotates, and retains full orientational memory after isomerization dictates that it reorients only by a well-defined, discrete angle upon isomerization. Next, I discuss photo-orientation processes in chromophores that isomerize by cyclization, a process that differs from the isomeric shape change of azobenzene derivatives. 

Polarized light absorption orients both isomers of photisomerizahle chromo-phores, and quantified photo-orientation both reveals the symmetrical nature of the isomers photochemical transitions and shows how chromophores move upon isomerization. Photo-orientation theory has matured by merging optics and photochemistry, and it now provides analytical means for powerful characterization of photo-orientation by photoisomerization. In azobenzenes, it was found that the photochemical quantum yields and the rate of the cis—>trans thermal isomerization strongly influence photo-... 

Azobenzene 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. Azobenzene derivatives have two geometric isomers the trans and the cis forms. The isomerization reaction is a light- or heat-induced inter-conversion 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. 

Since the early study by Paik and Morawetz24 on the E-Z thermal isomerization of azobenzenes attached as side groups in polymers, a number of papers... 

The photochromism of azobenzene probe molecules tells us furthermore that bulk materials differ distinctively from silica films.59 The pores in the sol-gel bulk materials were larger than those in the corresponding sol-gel films. The first-order kinetics of the thermal isomerization of the photogenerated Z-isomer revealed that the steric effects restricting the molecular motion increased in the order sol-gel... 

C. Barret, A. Natansohn, and P. Rochon, Cis-trans thermal isomerization rates of bound and doped azobenzenes in a series of polymers, Chem. Mater. 7, 899-903 (1995). 

T. Sasaki, T. Ikeda, and K. Ichimura, Photo- and thermal isomerization behaviors of azobenzene derivatives in liquid crystalline polymer matrices, Macromolecules 26, 151-154 (1993). 

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