Poly , azobenzene groups

Polymers of poly(L-ornithine) possessing varying contents of azobenzene groups, from 20% up to almost 100% [Scheme 5, VIII (n= 3)], were found to be essentially a-helical in HFP when the samples were kept in the dark. The CD spectra also exhibited a couplet of bands centered at about 320 nm, attributed to electronic interactions between the azo side chains in the trans configuration. Irradiation at 360 nm and the consequent trans—xis photoisomerization, abolished the side chain CD... 

Poly(propyleneamine) dendrimers of generations 1 and 4 (89) functionalized with azobenzene groups were investigated as hosts for eosin Y (eosin = 2, 4, 5, 7 -tetrabromofluorescein dianion) in DMF solution [159]. The peripheral azobenzene groups can be switched by light excitation from the E to the Z form. The fluorescent excited state of eosin is reductively quenehed by the tertiary amine units present in the dendrimer structure. This electron transfer quenching takes place with a static... 

Fig. 30. Transmittance changes at 750 nm of a 1 % aqueous solution of poly(7V-isopropylacrylamide) with pendant azobenzene groups (2.7 mol%) when heated at a rate of 2°C/min. (O) before photoirradiation ( ) under photostationary state with ultraviolet irradiation (410 nm > A > 350 nm)...

Inoue Y, Kuad P, Okumura Y, Takashima Y, Yamaguchi H, Harada A. 2007. Thermal and photochemical switching of conformation of poly(ethylene glycol) substituted cyclo dextrin with an azobenzene group at the chain end. J Am Chem Soc 129(20) 6396 6397. 

In the case of an azobenzene-modified poly(arylether ketone amide) (see Chart 5.2), a pronounced volume contraction due to photo-induced trans-cis isomerization of the azobenzene groups was evidenced by means of size-exclusion chromatography (SEC) [25]. When irradiated in dilute N,JV-diethylacetamide solution, this polymer underwent a reduction in its hydrodynamic radius by a factor of 2.7, corresponding to a contraction of the hydrodynamic volume by a factor of about 20. This pronounced shrinkage effect is believed to be due to a large number of conformationally restricted backbone segments, because other more flexible polyamides and polyurea polymers exhibit much weaker contraction effects. 

Fig. 5.5 Photoresponsive behavior of membranes of an azo-modified poly(L-glutamic acid) containing 12-14 mol% azobenzene groups at 60°C. (a) Membrane potential,...

Chart 5.12 Chemical structure of a poly(L-glutamate) with in-chain azobenzene groups. 

Chart 5.13 Chemical structure of a hairy-rod-type poly(gluta-mate) bearing pendant azobenzene groups. 

This enzymatically synthesized azophenol polymer has an extremely high dye content (nearly 100%) and is soluble in most polar organic solvents. It forms good optical quality thin films. Polymer solutions show reversible trans to cis photoisomerizations of the azobenzene groups with long relaxation time. The poly(azophenol) films also exhibit photoinduced absorption dichroism and large photoinduced birefringence with unusual relaxation behavior. 

By UV irradiation the stable trans form passes into cis form, which is reflected in modification of some important properties of the system, such as the increase of hydrophobicity, the decay of viscosity, and the weakness of intermolecular interactions. On the initial stage, due to the trans conformation of the azobenzene groups, the dye is disposed alongside of the extended macromolecular chains the cis form, obtained by irradiation, limits the dye-polymer interactions and poly(methacrylic acid) adopts a coiled conformation. 

A second mechanism is based on the modifications caused by the effect of UV waves on the intramolecular interactions between the pendant azobenzene groups bonded on a poly(styrene-co-maleic anhydride) copolymer. 

A detailed investigation of photochromism, conformation and CD properties was carried out in recent years on poly(L-glutamic acid) having various contents of azobenzene groups in the side chains. In trimethylphosphate (TMP), the azo-polypeptides exhibited the a-helix CD pattern. In water at acid pH, the polymers containing 16 mol % and 21 mol % azobenzene groups adopted the a-helix structure while a P-structure was present in a 36 mol % azo-polypeptide. 

Poly(azobenzene) and its derivatives have applications in optical devices [17]. A novel polyaniline containing azo groups was synthesized by the HRP-catalyzed oxidative coupling of 4,4 -diaminoazobenzene (Scheme 1). The polymerization was carried out at pH 6.0 in tris buffer with 70% yield. The polymer analyzed by GPC (80 000, PD = 4.8) was soluble in DMSO and DMF. Azo groups were detected in the main chains as well as in the side chains. Photoexcitation studies indicated that cis-trans isomerization of the chromophore may be the result of structmal constraints in the polymer [ 18). Photodynamic properties of the azo functionahzed polyaniline in their relaxed or constrained conformations were quite different. 

Dark-adapted (all trans azo groups) poly(L-lysine) bearing 43 mol % of azobenzene groups, in a medium of hexafluoroisopropanol/water/sodium dodecyl sulfate, shows a CD spectrum which can be attributed to the presence of a -form. Irradiation at 340 nm causes the disruption of the -structure and promotes the formation of an a-helix (helix content % 50%), as revealed by the appearance of the typical CD pattern. Upon irradiation at 450 nm, the spectrum reverses again... 

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