Poly photochromic reactions

Fig. 3 Photochromic reactions of poly(L-glutamic acid) containing azobenzene units in the side chains (III).

Fig. 13 Reverse photochromic reactions of spiropyran-modified poly(L-lysine) (XVI) in hexafluoro-2-propanol.

When azobenzenes are attached to polypeptides, photochromic reactions of azobenzenes can induce the change in helical properties of the polypeptides, which may be detected by CD spectrum as well as optical rotation. For 4-phenylazophen-ylamine-condensed poly(y-glutamic acid) A-9 containing up to 80 mol% of 4-phenylazophenylamide side chain, UV irradiation in organic solvents, such as... 

The analogy of these reversible processes with those observed in spiropyran-modified poly(Glu) suggests the occurrence of similar photochromic reactions. Accordin y, HFP stabilizes the colored ionic merocyanine structure, while irradiation gives the colorless spiro structure. 

Photochromic reactions in polymers also depend on the polarity of the polymer matrix. For example, when dispersed in or covalently attached to polymer matrices, the stability of polar merocyanine (MC) isomers in a polar matrix will be enhanced. Consequently, the reverse reaction rates in polar matrices will be reduced. Interactions between polar polymer matrices and photochromic l,3,3-trimethylindoline-6 -nitrobenzopyrylospiran (SP) molecules resulted in a blueshift of the absorption spectrum of SP. As shown in Figure 9.1, for the poly(n.-butyl methacrylate) (PnBMA) matrix with a lower Tg, larger blueshifts are observed compared to that of PMMA and styrene-butadiene-styrene (SBS) matrices. This behavior was attributed to polar and ionic interactions between PnBMA matrices and SP [27], the magnitude of these interactions though wiU vary depending upon the Tg of the matrix. 

The reaction can be used to introduce chromophores onto the surface of polymeric films containing hydroxy groups. When a poly(2-hydroxyethylmethacrylate) (PHEMA) film 14 was immersed in an acetonitrile solution of dimethyl terephthalate (DMTP) and l-methyl-2-arylcy-clopropane 13 (where the aryl group is 1-pyrenyl or 9-phenanthryl) and then irradiated at 313nm, a chemically modified film 15 was obtained that gives absorption and fluorescence spectra similar to 13, and therefore may be useful as a photocatalyst, photochromic device etc. - ... 

As with all photochromic polymers, the polymer matrix effect is very important in fulgides and fulgimides as the polymer free volume affects rate of photocoloration and photobleaching. For example, Bahajaj and Asiri recently reported that the rate of photochemical reactions of fulgide doped in a variety of polymer matrices decreased in the order polystyrene > poly(methyl methacrylate) > epoxy resin [37]. Polystyrene possesses a larger free volume than poly(methyl methacrylate) and epoxy resin... 

Photochromism is the phenomenon where the absorption speetrum of a molecule or crystal changes reversibly when the sample is irradiated by light of a eertain wavelength. When such a photoisomerization reaction is carried out in a polymer matrix instead of in solution, a strong decrease in the reaction rate (coloration = decoloration) is observed." The poly-... 

The CD spectra of the dark-adapted samples of poly(Glu bearing 85 mol% photochromic units are those of random coil polypeptides. CD bands of small intensity are also present in the near UV-visible region, in correspondence with the merocyanine electronic transitions. The solutions bleached after exposure to visible Ught display the typical pattern of the a-helix, with the two minima at 222 and 208 nm, thus indicating that the isomerization of the side chains produces spiralization of the main chain. The back reaction in the dark is accompanied by a progressive decrease of the helix content and restoration of the original disordered conformation (Figure 14.10). 

In the poly(vinyl chloride)-FeCl3 complex, the photochromic effect has only been observed in the presence of oxygen [1671], which participates, probably in the following reactions ... 

Photochromic changes and photoinduced reaction involving a flourene-based TPA chromophore given in Figure 39(a) are used for recording 3D data on a dye-doped poly(methyl methacrylate) film. The chromophore produces a... 

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