Photoisomerization in polymers

PHOTOISOMERIZATION IN POLYMER FILMS IN THE PRESENCE OF ELECTROSTATIC AND OPTICAL FIELDS... 

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

Angeloni, Chiellini, and their co-workers [116-118] reported the synthesis and characterization of both MCLC and SCLC polymers containing the 4,4 -bisalkoxyazobenzene chromophore. These interesting materials photoisomerized in solution but their photochemistry in an LC mesophase was not reported. 

Azobenzenes have been utilized to measure the free volume in polymers and the speed of polymeric segmental motion [42, 43], Azobenzenes that are covalently bonded to a polymer backbone may influence various properties of the macromolecule. Photoisomerization of such substances will cause changes in wettability [44], viscosity [45], solubility [46], membrane properties [47], and swelling properties [48]. 

Luminescence properties of and phenomena in polymer systems continues to be widely researched in connection with mechanisms of polymer degradation and stabilization, molecular dynamics, solubility, blend miscibility, and solar energy harnessing. A number of interesting reviews have appeared. Molecular dynamics of polymers in solution and in the solid state have been covered, as has excimer formation,photoresponsive polymers,behaviour of polymer gels, and photochromic phenomena. Photoisomerization of enzymes and model compounds has also been discussed in depth, with particular emphasis on proteins and synthetic polymers containing azo-compounds or spirobenzopyrans. ... 

Photoresponsive membranes of cellulose-2,4-diacetate incorporating 6-nitro-l, 3, 3 -trimethylspiro-(2H-l-benzopyran-2,2 -indoline) have been prepared and chiroptical and fluorescence properties of optically active co-polymers of acenaphthalene with methyl acrylate/methacrylate have been investigated. Marked optical activity is induced in the aromatic units only for the co-polymer with methyl methacrylate. This difference in behaviour is associated with an overall higher main-chain flexibility and conformational freedom in acrylates compared with methacrylates. Photoisomerization in polyurethanes containing azo-links has been found to be dependent upon the thermal history of the polymer, and photoisomerization of linoleic acid and... 

Rearrangements.- E-Photoisomerization occurs readily in imines and in azo compounds. The syn-isomer (1), for example, is the major product of irradiation of nitrofurazone (2) in solution and is formed together with the corresponding azine on exposure to laboratory illumination. The photoisomerization of azobenzene derivatives in solution, in membranes, in host-guest complexes of cyclodextrins, and in polymers continues to attract attention. The reversibility of E-photoisomerization of azobenzene in cyclo-hexane solution has been established, and the E/ -ratios generated by irradiation of various azobenzene derivatives adsorbed on... 

Naito, T, Horie, K., and Mita, I. (1991). Photochemistry in polymer solids. 11. Effects of the size of reaction groups and the mode of photoisomerization on photochromic reactions in polycarbonate film. Macromolecules 24, 2907-2911. 

These examples and investigations on azobenzene moieties in polymers show that the photochromic behavior is mainly Gontrolled by the free volume distribution around the chromophore. Tb obtain LBK films in which azobenzene moieties can undergo photoisomerization, therefore, the free volume around the azobenzene chromophore must be controlled precisely to allow for the molecular rearrangement inherent in the reversible tran to cis photo-isomerization. This is possible by (1) mixing with other amphiphiles, (2) adjusting the architecture of the amphiphile, or (3) attaching the chromophore to a polymer either by coulomb interaction or covalently. 

The corresponding fatty acid could not be photoisomerized in the LBK film. By attaching the azobenzene chromophore to the hydrophilic backbone, however, the free volume in LBK films was increased and photoisomerization was possible (i.e., 50 to 70% cts-isomer compared to 0% for the nontethered azobenzene amphiphile and 90% cis-isomer in solution). However, concomitant with the increased free volume, there is a decrease in the orientational order of the chromophores. These polymers have been widely used as command surfaces to control the orientation of liquid crystals and to investigate the photomechanical effect. ... 

In conclusion regarding the results obtained with polymer-bound azobenzene chromophores, establishing liquid crystalline phases that combine order and flexibility in the side chain region of the polymer seems to be the best way to obtain materials with a strong photoresponse. This is because a high density of chromophores can be combined with sufficient flexibility that the chromophores still can be photoisomerized in the dense packing of an LBK film. When a low density of chromophores is acceptable, however, the chromophores can be diluted along the polymer chain to reduce their interaction and secure sufficient free volume. 

The polymer environment plays an important role in the mobility of isomers. In this chapter, it was shown that second harmonic generation can provide detailed information of photoisomerization in a polymer film in the presence of a large electrostatic field. Photoisomerization can control the optical non-linearities of materials that have applications in optical switching and optical storage applications. 

Even when the azobenzene chromophores are incorporated into the polymer backbone, the dipole moment increase of azobenzene residues by photoirradiation can also induce a change in polymer chain conformation. The solution viscosity of poly(dimethylsiloxane) containing azobenzene residues in the main chain decreased upon ultraviolet irradiation, and the effect was attributed to the trans to cis photoisomerization [9]. The photodecrease of the viscosity depended on the polarity of the solvent. It was 24% in non-polar heptane, but negligible in polar dichloroethane. 

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