Photostimulated conformational changes

Fig. 2. Schematic illustration of photostimulated conformation changes of polymer chains...

Photostimulated conformation changes observed for polypeptides with pendant azobenzene residues may also be classified as the mechanism (5) [23, 24]. 

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). 

The photostimulated conformation change of polystyrene with pendant azobenzene groups in cyclohexane has also been studied by the time-resolved light scattering technique [26]. 

It seems possible to amplify the photostimulated conformational changes in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. The first proposal to use the structural changes at the molecular level for direct conversion of photon energy into mechanical work has been made by Merian (13.) in 1966. Since then, many materials, most of which contained azobenzene chromophores, have been reported to show photostimulated deformation(JM). Till now, however, the reported deformations were limited to less than 10%. In addition, Matejka et. al. have pointed out that in many cases photo-heating effect instead of photochemical reaction plays a dominant role in the deformation(15,16). 

In 1981, self-diffraction was observed in nematic MBBA [64] under circumstances where director reorientation could not occur. The measured nonlinear susceptibility was as large as the giant nonlinearity associated with director reorientation the mechanism proposed was photostimulated conformational changes of the liquid crystal molecules. Since the linear polarizability of the excited conformer differs from that of the molecule in the ground state, there is a direct contribution to the bulk susceptibility. The metastable conformers also act as... 

Figure 1. Schematic illustrations of photostimulated conformational changes of polymer chains.

Poly(methacrylic acid) with pendant spirobenzopyran groups also showed photostimulated conformational changes in methanol.Visible light irradiation of the solution increased the viscosity while ultraviolet light irradiation caused a decrease in the solution viscosity. 

The photostimulated conformational change is interpreted by the change of balance between polymer - polymer and polymer - solvent interactions due to the formation and disappearance of polar merocyanine groups. Alternate irradiation of visible and ultraviolet light brings about reversible viscosity changes of as much as 40 % and the increase/decrease cycles of the viscosity can be repeated many times without any noticeable fatigue. 

It seems possible to amplify the photostimulated conformational change of polymer chains in solution at the molecular level into the shape change of polymer gels or solids at the macrosize level. A proposal to... 

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... 

From this point of view, polypeptides containing photochromic units in the side chains are quite special polymers. They can exist in ordered or disordered conformations, and photoisomerization of their photochromic side chains can produce order = disorder conformational changes. These photostimulated structural variations, such as random coil a-helix, take place as highly cooperative transitions therefore photochromic polypeptides actually work as amplifiers and transducers of the primary photochemical events occurring in the photosensitive side chains. 

In due consideration of these results, we have decided to employ electrostatic forces to achieve a large reversible deformation of gels. The electrostatic force is expected to be a more effective driving force for the conformational changes of polymer chains than trans-cis geometrical isomerization of unsaturated linkages. The second part describes the photostimulated dilation of polymer gels. 

Figure 2. Photostimulated (A) conformational change and (B) viscosity change of polyCN,N-dimethylacrylamide) having pendant triphenylmethane leucohydroxide groups 9.1 mol%) in methanol at 30°C. Polymer concentration was 0.06 g/dl. A >270 nm. (Reproduced with permission from Ref.12. Copyright 1985, Huethig Wepf Verlag.)...

The photostimulated dilation of the gel is well understood by the osmotic pressure mechanism. Although the electrostatic repulsive forces between the positive ions play an important role in the conformational changes in solution, they are considered to make a minor contribution to the gel dilation. 

The concept of photostimulated phase separation can be applied to construct chemical-induced phase transition systems, which change the conformation reversibly in response to special chemicals. For the systems, host molecules are used as the receptor groups instead of photoisomerizable chromophores. Host molecules, such as crown ethers or cyclodextrins, are known to change the property by capturing guest chemicals in their cavity [16]. We employed benzo[l 8]crown-6 as the receptor molecule and incorporated it into the pendant groups of PNIPAM. 

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