Photo-orientation by photoisomerization

FIGURE 3.3 Variation of the absorbance. O.D., of a film of DR I -doped PMMA (2.5% w/w) with (A) low, 0.28 mW/cm and (B) high, 28 mW/cm irradiation (488 nm Ar laser) intensity. The analysis light was the 514 nm from the same Ar laser, and both A// and are measured. A represents the isotropic absorbance (A//+2AjJ/3. (C) Variation of the absorbance—parallel, perpendicular, and isotropic—with the irradiation light intensity.The data were taken 3 minutes after the irradiation was turned on. At low irradiation intensity, the cis population is small and Aj increases as a consequence of the orientational distribution. At high irradiation intensity, saturation of the orientational hole-burning process tends to equalize A// and A. (After reference 20, by permission.) 

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Section 3.2 of this chapter recalls the pure photochemical point of view of photoisomerization of azobenzene derivatives. Section 3.3 discusses the theory of photo-orientation by photoisomerization and gives analytical expressions for the measurement of coupled photoisomerization and photo-orientation parameters. Sections 3.4 and 3.5 review observations of photo-orientation in azobenzene and push-pull azobenzene derivatives, respectively. Among other things, these sections address photo-orientation in both cis and trans isomers and discuss the effect of trans<->cis cycling, i.e., the photochemical quantum yields, on photo-orientation. Section 3.6 discusses the effect of the symmetry of photochemical transitions on photo-orientation in spiropyran and diarylethene-type chromophores. Finally, I make some concluding observations in Section 3.7. 

This system of equations shows, through even orders, that polarized light irradiation creates anisotropy and photo-orientation by photoisomerization. A solution to the time evolution of the cis and trans expansion parameters cannot be found without approximations this is when physics comes into play. Approximate numerical simulations are possible. 1 will show that for detailed and precise comparison of experimental data with the photo-orientation theory, it is not necessary to have a solution for the dynamics, even in the most general case where there is not enough room for approximations, i.e., that of push-pull azo dyes, such as DRl, because of the strong overlap of the linear absorption spectra of the cis and trans isomers of such chromophores. Rigorous analytical expressions of the steady-state behavior and the early time evolution provide the necessary tool for a full characterization of photo-orientation by photoisomerization. 

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-... 

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