Thermal cis-trans isomerizations

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... 

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. 

FIGURE 3.8 Dependence of the absorbance of linearly polarized probe light at 360 nm (squares) and 450 nm (circles) on the angle. between the probe and the UV light polarization.TMs behavior is fitted by a cos with an amplitude that decays with the cis trans thermal isomerization rate. After reference 22, redrawn by permission of ACS. 

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-... 

The effect of the structure of the polymer backbone on photo-orientation can be seen from the dynamic behavior as well as from the steady-state values of the photoinduced anisotropy in all azo-PURs. The photo-orientation dynamics of PUR-2 resemble but also contrast with those of PUR-1. In PUR-2, AbSi exceeds AbsQ, but not quite, as is the case for PUR-1, and the photostationary-state anisotropy is smaller than that of PUR-1, as can be seen in Figure 4.20. PUR-1 and PUR-2 exhibit exactly the same extinction coefficient at the analysis wavelength because they have the same azo chromophore furthermore, the rate of the cis- trans thermal isomerization is nearly the same in both polymers. The seemingly small difference into the... 

A comment must be made about the cis— trans thermal isomerization rate at pressure. At room temperature, the thermal back reaction of DRl-PMMA follows a complex, nonexponential recovery, most of which is completed after a few seconds with a rate of 0.25 s" and deviates from a single exponential decay after the first 10 seconds.Larger relaxation times at Tg -98°C include slow polymer motion coupled with the chromophores rotational diffusion. We confirmed that this behavior is true in the polymer... 

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). 

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