Polymer containing azobenzene residues

Azobenzene derivatives may be photoisomeriz-ed from the stable trans form to the cis isomer and the reverse reaction takes place at a convenient rate by a thermal mechanism in the neighborhood of ambient temperatures. Since the isomerization is accompanied by a large shift in the ultraviolet spectrum, the kinetics of both the photochemical and the thermal process may be followed by UV spectroscopy. We have utilized this technique for comparing rate constants of the thermal reaction, 27 and the photochemical quantum yields S in polymers containing azobenzene residues and in their low molecular weight analogs. 

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. 

Blair, H. S., and McArdle, C. B. Photoresponsive Polymers 2. The Monolayer Behaviour of Photochromic Polymers Containing Aromatic Azobenzene Residues. Polymer 25. 1347 (1984). 

We reported photoinduced orientation on a polymer containing a Disperse Red 1 residue and a veiy short (2 methylene units) spacer between the azobenzene group and the main chain > The structure of this first polymer is shown - together with other polymers investigated in our laboratory in Scheme 1. 

Also, when copolymers of polystyrene and 4-(methacryloyl-amino)azobenzene containing 2.2-6.5% of the latter are irradiated in a cyclohexane soluticMi with 15 flashes of 347 nm of Ught. The polymeric chains contract [221], This occurs at a high rate per second as a consequence of isomerization. At a later stage, several hundred seconds after the flash, there is evidence of polymer aggregation and precipitation [221], In addition, when azobenzene residues are introduced into the main chain of poly(dimethylsiloxane), reversible solution viscosity changes can be obtained by irradiation with ultraviolet light [221],... 

The experimental results revealed no significant difference in the rate of photoisomerization of azobenzene residues in the backbone of polyamides and in low molecular weight analogous azobenzene derivatives when both were studied in dilute solution.28 However, while the photochemical reactivity of the small species was relatively insensitive to the concentration of added polymer, the quantum yield for the photoisomerization of the azobenzene residues in the polymer backbone dropped precipitously with increasing concentration. In a glassy polymer film containing 8% DMSO plasticizer, the quantum yield for the isomerization of the polymer was reduced by a factor of 2500 while it was reduced only by a factor of 5 for the small molecule (Figure 3). 

Here the quantum yield of a small number of azobenzene residues built into the backbone of a polyamide was compared with the quantum yield of an analogous low molecular weight azobenzene derivative in a series of polyamide - DMSO mixtures. As expected, the quantum yield for the azobenzene residues in the polymer backbones was very small in systems containing a high polymer concentration, since here conformational transitions of the polymer chains would necessarily be cooperative, while the photoisomerization of the small molecule was relatively insensitive to the polymer concentration. However, in highly dilute solution there was absolutely no difference between the photoisomerization efficiency of azobenzene residues built into the polymer chains and the small azobenzene derivatives. This identity in the behavior of the polymer and its analog in dilute solution shows that the isomerization of azobenzenes in the chain backbone can be accomplished without a simultaneous hindered rotation in another portion of the chain. 

In contrast to the first-order kinetics of the conventional azobenzene isomerization in solution, a slight deviation from first-order kinetics was observed even in aqueous solutions for polyelectrolytic azobenzene polymers substituted with hydrophobic alkyl residues (14, Fig. 4).43 In an aqueous solution of the amphiphilic polysulfonates containing a small amount of azobenzene moiety, the extent of E-to-Z photoisomerization under UV irradiation was reduced as a result of the compartmentalization of the azo chromophores in a hydrophobic core, which resulted in motion of their restriction. 

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