Photoresponsive polymers conformation change

Photoresponsive polymers are quite special polymers, able to respond to light and dark conditions and thus giving rise to reversible variations in their structure and conformation. These photoinduced structural changes may in turn be accompanied by reversible changes in the physical and chemical properties of the polymeric materials. 

Some fine review articles dealing with various aspects of photochromic polymers are reported in the literature,11-91 and several photoresponse effects have been described. These include light-induced conformational changes, photostimulated variations of viscosity and solubility, photocontrol of membrane functions, and photomechanical effects. Here we provide an overview of the photoresponse effects... 

The conformation of polymers governs their various physico-chemical properties in solution. To begin with, we outline the guiding principle for designing polymers which change the conformation reversibly by photoirradiation, since they are relevant as a general model of photoresponsive polymers. 

It is of particular interest to know how fast long polymer chains change their conformation in response to a short laser pulse. The dynamics of this process can be studied in photoresponsive polymers which undergo photostimulated conformation changes [25]. The isomerization of photochromic chromophores can be induced in less than 10 s with a short laser pulse. The conformation change subsequent to it can be followed with a time resolved light scattering system combined with the short laser pulse source (Fig. 8). 

Synthetic polymers containing photochromic units can undergo reversible changes of their physical and chemical characteristics. (For recent reviews, see Refs. 24-26). Recent examples involving photochromic compounds other than spiropyrans are reported in Refs. 27-31. Spiropyran-containing polymers such as polyacrylates have also been prepared and were found to show photoinduced variations of their viscosity.18- 0 The change in the viscosity of the polymers partly reflects the polymer conformation. Thus, spiropyran-attached poly(L-tyro-sine) and poly(L-lysine) were synthesized by Vandewyer and Smets at the University of Louvain in 1970.32 33 No photoresponsiveness was observed however, for these modified peptides. 

Most of the work on photoresponsive azobenzene-containing materials is based on polymer matrices. Numerous chemistries have been utilized to graft azobenzene ligands to various polymer chains. The azobenzene chromophores transfer light energy into conformational changes upon photoirradiation, which can be used to control chemical and physical properties of the materials, such as viscosity, conductivity, pH, solubility, wettability, permeability, transport properties, mechanical properties, and structural properties. 

Irie, M. and Schnabel, W. (1981) Photoresponsive polymers - on the dynamics of conformational-changes of polyamides with backbone azobenzene groups. Macromolecules, 14, 1246-1249. 

In 1966, Lovrien was the first to propose photoresponsive polymers in which photochromic units, such as azobenzene chromophores, are parts of the polymers. He envisioned that isomerization of the photoresponsive units in the polymer by light energy may influence the conformation of the polymers, thereby resulting in a reversible change of the polymers in solution. He found that the viscosity of an aqueous solution of copolymers of methacrylic acid and N-(2,2 -dimethoxyazobenzene)acrylamide increased upon UV light irradiation. The photoisomerization of the azobenzene units to the Z-configu-ration led the polymer coil to expand due to a decrease in the hydrophobic interaction between the azobenzene chromophores. 

Since the 1970s a great number of photoresponsive polymers have been designed and prepared. The physical and chemical properties of these polymers in conjunction with their reversible conformational change upon photoirradiation have been examined. This work was aimed at the reversible control of their properties. 

It is indispensable to know the responsive rate, when constructing photore-sponsive devices using the photoresponsive polymers described in the foregoing. The most convenient way to find the response time is to measure the conformational change of the polymer chain in solution. Direct measurement of the conformational change was carried out using a flash photolysis method... 

Other more complex systems have been obtained, consisting by a photoresponsive crown ethers which combines within the same molecule both a crown ether and a photoresponsive chromophore. The polymer changes its conformation in response to irradiation, which in turn induces a change in the complexation ability with suitable alkali metal ions, for instance Cs" [95] ... 

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