Theory of Photo-Orientation

In the following discussion, A and B refer to the trans and cis isomers, respectively. We assume that A- B photoconversion occurs upon excitation of a purely polarized transition with light linearly polarized along the laboratory axis Z, and we define a site-fixed, right-handed orthogonal system of axes for each of the isomers A and B in which the molecule can exist, such that the angle between the and Zb axes is x- In isomers A and B, the electric dipole moments and responsible for excitation of a photochemical 

This model alleviates the concept of the somewhat ambiguous molecular anisometry that is based on an arbitrary choice of fixed molecular axes. So, for each of the A and B isomers, the isotropic absorbance Aa,b = Abs// + lAhsf yS, the anisotropy AA g = Absj - Absf and the optical order parameter g = AA ySA gare given by  

FIGUMi 3.4 (X, Y, Z) indicate the laboratory coordinates axes, and (Xa.b Ya.b indicate die isomers fixed molecular coordinates axes. The angl 6, p, x, and (O, and die transition electric dipole moments Al, and Mb , are as defined in the text. 

When Legendre formalism is used, the variations of the cis and trans orientational distributions are given by the variations of their expansion parameters, i.e., Indeed, by substituting Equations 3.4 

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. 

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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. 

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-... 

In this chapter, I discuss the phenomenon of molecular orientation induced by photoisomerization whereby experiment merges with theory to assess molecular movement during isomerization. The theory unifies photochemistry with optics, and it provides rigourous analytical tools for powerful quantification of coupled photoisomerization and photo-orientation. Experiments on spectrally distinguishable isomers detail the mechanisms of chromophore reorientation during photo- and thermal isomerization. In particular, I... 

Together with PAP, photo-induced depoling (PID) is another interesting phenomenon at the interface of photochemistry and organic nonlinear optics. Indeed, PID of poled polymers occurs when NLO chromophores, which are oriented in a polar manner, undergo photoisomerization without applied dc field. The chromophores lose their initial polar orientation after photoisomerization and reorientation in azimuthal directions around the initial polar axis, thereby erasing FID has been observed both by photo-induced destruction of EO Pockels and by second harmonic generation. The first published PID experiments have been reported for DRl in PMMA, and the theory of PID is discussed in detail in reference 25. 

Photoisomerization was studied from a purely photochemical point of view in which photo-orientation effects can be disregarded. While this feature can be true in low viscosity solutions where photo-induced molecular orientation can be overcome by molecular rotational diffusion, in polymeric environments, especially in thin solid film configurations, spontaneous molecular mobility can be strongly hindered and photo-orientation effects arc appreciable. The theory that coupled photoisomerization and photo-orientation processes was also recently developed, based on the formalism of Legendre Polynomials, and more recent further theoretical developments have helped quantify coupled photoisomerization and photo-orientation processes in films of polymer. 

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