Isomerization from cis to trans

Isomerization from cis to trans does not occur with equal ease in all polymers. Qualitatively, the more conjugated poly-n-octylCOT isomerizes more quickly in solution than the less conjugated poly-TMSCOT. Differential scanning calorimetry was performed on all of the polymers (Table HI). 

In addition to the changes in double bond position, there is isomerization from cis to trans, and 90 percent of the peroxides formed may be in the trans configuration (Lundberg 1961). 

The kinetic study for the reaction outlined in Scheme 3.17 has demonstrated that the isomerization from cis to trans is signihcantly retarded hy the addition of a small amount of PPhs, implying prior dissociation of the coordinated PPhs, but the rate constant reaches a constant value at a certain concentration of PPhs, suggesting involvement of a concomitant process independent of PPha concentration. Although this isomerization proceeds via several different routes, Eyring plot of the total isomerization rate constants gives apparent kinetic parameters with negative entropy of activation = —21 J mol ), in which a mecha-... 

Fig. 1. Time-dependent FTIR spectrum, showing the thermal isomerization from cis to trans-polyacetylene (T = 145°C). One spectrum every two seconds.

Isomerization from cis to trans and back of azo groups, however, is not the only mechanism that can affect photo-viscosity change in polymeric solutions. Thus, reversible solution viscosity changes were also observed [222] in solutions of poly(dimethylacrylamide) with pendant triphenylmethane leucohydroxide in methanol. This can be illustrated as follows ... 

Figure 5 shows typical x-ray diffraction curves of two types of polyacetylene films. The usual Shirakawa-type film [1,23] gave a very sharp diffraction curve. The halfwidth of the x-ray diffraction peak at 2 of 23.2 (110 and 2(X) reflections) was small (A20 = 1.2 ). However, it increased to 1.8 after the heat treatment for thermal isomerization from cis to trans form, implying that the ordered crystal structure in the as-grown cis-rich film suffered some destruction in the trans film. On the other hand, the highly stretchable film synthesized by the SE method gave a little diffused diffraction curve, as shown in Fig. 5. As the mechanical stretching proceeds, the half-width of the x-ray diffraction peak (A20 = 2.0°) decreased drastically to 1.5...

Thin films of all the synthesized compounds also showed the expected photochemical behavior. For example, irradiation of a film of compound Iwith 350 nm light over time resulted in a decrease in absorption at 320 nm and an increase in absorption at 436 nm, again indicating formation of the cis isomer (Fig. 5a). After 15 minutes of irradiation, a photostationary state was reached and no further changes in absorption were observed. The lower relative change in absorption at 436 nm is due to the cis isomer having a lower extinction coefficient than the trans isomer at 320 nm. Exposure of the film to a visible lamp resulted in complete isomerization back to the trans form within 2.5 hours (Fig. 5b). The same transformation occurred within minutes when the cis film was exposed to sunlight. The thermal isomerization from cis to trans was also monitored in the dark at room temperature and the first order rate constant of 4.6 x 10-6 s-1 was... 

Fig. 28.2 (a) Time-dependent FTIR spectrum showing the thermal isomerization from cis- to trans-FA. T = 145°C. Spectra were recorded every 2 s. (b) Time dependence of CH out-of-plane wavenumber of cis-PA during thermal isomerization at 145°C. (c) Time dependence of CH out-of-plane wavenumber of trans-PA during termal isomerization at 145°C. (From Ref. 48.)... 

This photoremovable protecting group has been employed as a photocleavable linker to reagents bound to Au surfaces. 4-Hydroxy-stilbene was linked to 6-bromohexyl-3-nitro-4-bromomethylbenzoate in 51% yield, then thiolated by trimethylsilylthioxy dehalogenation in THF, followed by desilylation in situ. The self-assembled monolayers of long-chain alkyl thiolate on bulk polycrystalline gold were constructed. Upon irradiation at 350 nm, the Z, -photoisomerization attained a photostationary state within 25 min, while the dissociation took about 60 min however, sensitization with 1,4-dibromonaphthalene (Ep = 58.1 kcal/mol) produced a cleaner photoisomerization. The unidirectional isomerization, from cis to trans, by both direct irradiation and sensitization was followed by the release of a bound chain from the metal surface. 

---