Azobenzene residues

A large number of azobenzene-based amorphous and liquid crystalline polymers, particularly polyacrylates and polymethacrylates with chiral azobenzene pendants, have been prepared for the development of data storage and photonic devices [1-3,11-14]. For instance, the introduction of optically active mesogenic azobenzene residues into the side groups of the polymers produces chiral nematic and cholesteric phases, which are regulated by photoisomerization of the azobenzene units [10,14]. In most cases, however, the optical activity and chiroptical... 

Polysaccharide phenylcarbamate derivatives have a great advantage for the easy preparation of a film (membrane), which can be used as a new device for the rapid separation of enantiomers through enantioselective adsorption [105]. Photoresponsive cellulose and amylose membranes bearing azobenzene residues... 

More recently, a similar photocfaromic behavior was observed by Eisenbadi for the cis-trans isomeriration of azobenzene residues attached as side groups to copolymers when examined below their ass transition temperature. These copolymers were obtained by copolymerization of the mtmomers 4-methacryloylaminoazobenzene (TV) and 4-(3-methacryloyloxybutylcarbonylamino)azobenzene (V) with various alkyl acrylates and methacrylates. 

Fujimaki, M., Matsuzawa, Y., Flayashi, Y., and Ichimura, K. Monolayers of Calixl4 resor-cinarenes with Azobenzene Residues Exhibiting Efficient Photoisomerizability. Chem. Lett., 165 99S). 

Irie, M., Hirano, Y., Flashimoto, S., and Hayashi, K. Photoresponsive Polymers. Reversible Solution Viscosity Change of Polyamides Having Azobenzene Residues in the Main Chain. Macromolecules 14, 262 (1981). 

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

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. 

Fig. 5. Changes in ( ) content of the trans azobenzene residues in polyamide (6) backbone and (O) viscosity of the polyamide in V,V-dimethylacetamide on alternate irradiation with ultraviolet (410 nm > A > 350 nm) and visible (A > 470 nm) light at 20 °C [14], Polymer concentration was 0.9 g/dl...

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

Polyamide (6) was irradiated with a single 20 ns flash (530 nm) in A. iV-dimethyl-ac tamide. The kinetics of the cis to trans isomerization of backbone azobenzene residues was followed by time resolved optical absorption, and the subsequent conformation change of the total polymer chain by time resolved light scattering. Before each laser e eriment, the polymer was brought to a compact conformation by eontiimous ultraviolet irradiation, and then the unfolding proce was traced by the laser flash photolysis method. 

The large difference in the response times for optical absorption and light scattering suggests a two step mechanism for the photostimulated unfolding process. During the isomerization of the backbone azobenzene residues, the total chain conformation maintains in the initial compact conformation. After the isomerization is completed, the conformation relaxes to a more stable extended conformation in 1 ms. 

This scheme implies that the compact conformation having tram azobenzene residues is a constrained form capable of storing a certain amount of strain energy. The strain energy causes coil expansion, and it is released during the unfolding process. 

Fig. 16. Photostimuiated changes in the swelling degree of PHEMA having pendant azobenzene residues in water at 2S °C [40] UV A = 350 + 50 ntn Vis A > 470 run...

Fig. 25. Permeation profiles of proteins through the PHEMA membrane having pendant azobenzene residues at 30 °C [58]. The arrows represent the UV irradiation to the membrane (O) insulin, ( ) lysozyme, (A) chymotripsin, (A) albumin...

The transition state is then still in equilibrium with the ground state and incorporation of the azobenzene residue Into a polymer chain has no effect on Its rate of isomerization. 

The behavior of the polyamides described above could not be studied In bulk because the polymer Is crystalline and Is not transparent. However, with a methyl methacrylate copolymer carrying a small number of azobenzene residues in side chains... 

A comparison of the photochemical behavior of azobenzene residues In the backbone and the side chains of polymers Is of special Interest since the short lifetime of the excited state makes It more difficult for the chain to undergo long-range conformational changes favoring the trans-cls Isomerization process. 

The photoselection process resulting in the preferential distribution of azo-chromophores in their perpendicular orientation has been mainly performed by irradiation with visible (blue) light. It was confirmed by the attenuated total reflection method that polarized pumping UV light also caused the formation of the Z-isomer, again with perpendicular orientation, in a thin film of poly-(L-glutamate) with azobenzene residues in the side chains.86... 

D. Tabak and H. Morawetz, Rates of conformational transitions in solutions of randomly coiled polymers. III. Cis-trans isomerization of azobenzene residues in the backbone of polyamides, Macromolecules 3, 403 410 (1970). 

A typical example of the polymer having azobenzene residues is (4,5) ... 

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