Azobenzene polymers photoisomerization

Yamaguchi, I. Osakada, K. and Yamamoto, T. (2000) Pseudopolyrotaxane composed of an azobenzene polymer and y-cyclodextrin. Reversible and irreversible photoisomerization of the azobenzene groups in the polymer chain, Chem. Commun. 1335-1336. 

Monolayers representing two-dimensional arrays of membrane-mimetic assemblies, consisting of azobenzene (poly-L-lysine) with 43 % loading of the photoisomerizable units, were prepared.1441 The compressed trans-azobenzene polymer mono-layer exhibited a surface pressure of 7 mN m 1, whereas photoisomerization of the monolayer to the cis-azobenzene state by UV light decreased the surface pressure to... 

Under illumination, azobenzene undergoes photoisomerization. If the azobenzene is bound to a polymer chain, the consequence of this repeated photo-... 

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. 

Liquid crystalline azobenzene polymers can undergo photoisomerization on irradiation at 360 or 450 nm on gold films. This permits changing the refractive index of the film depending upon the wavelength used136. A study has examined the photopolymerization of p-styrene sulphonate counteranions associated with monolayers of disulphide amphiphiles as self-assembled monolayers on gold137. 

It should be emphasized here that the grating schematized in Fig. 11.1 is a volume grating, that is, the index change occurs throughout the thickness direction of the film. Upon exposure to an interference pattern with no mask, a remarkable property of azobenzene polymers is the formation of surface relief grating (SRG) as a result of a mass transport triggered by the trans-cis photoisomerization (Kim et ah, 1995 Rochon et al., 1995). As reviewed in Chapters 4 and 8 there has been a... 

LC in nonirradiated areas and can be further polymerized upon flood irradiation (photomask removed). Basically, the grating is formed by alternating polymer-rich and LC-rich regions. As shown in Fig. 11.24, the grating can not only be switched by an electric field because of different responses of the two regions but is also sensitive to UV and visible light by virtue of the photoisomerization of the small amount of azobenzene polymer in nonirradiated areas. 

Azobenzene-containing polymers are reported in the literature at a rate of about 150 papers/year for the last five years. There are two main directions of interest the first relates to the donor-acceptor substituted azobenzenes as the structure generating second-order polarizability and - by noncentrosymmetrical alignment - second order nonlinear optical properties in poled films. The second direction of interest is related to the photoinduced isomerization between the trans (more stable) and cis configurations of the azobenzene. This photoisomerization has a wealth of unexpected and useful consequences which were noted only in the last decade. Both these directions appeared because of the availability of lasers, which on the one hand allowed the observation of the nonlinear optical properties and on the other hand... 

In crosslinked liquid-crystalline polymers and elastomers, and more recently also in azobenzene crystals, photoisomerization can lead to reversible macroscopic shape changes and complex 3D movements [34] as illustrated in Fig. le. The key for achieving large-scale photomechanical response is an efficient control over molecular alignment and, as mentioned earlier, liquid-crystalline materials are particularly useful in this respect. The optical-to-mechanical energy conversion provides a pathway to the use of light to fuel photomechanical oscillators [35], plastic micromotors [36], robotic-arm movements [37], etc. 

T. G. Pedersen, P. S. Ramanujam, P. M. Johansen, and S. Hvilsted, Quantum theory and experimental studies of absorption spectra and photoisomerization of azobenzene polymers. Journal of the Optical Society of America 15, 2721 (1998). 

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

Poly(L-lysine) containing azobenzene units linked to the side chains by means of a sulfonamide function (Scheme 4, Structure VI), was obtained by treating poly(L-lysine) with p-phenylazobenzenesulfonyl chloride. The poly(a-amino acid) was modified quantitatively conversion to the azo-lysine units of VI was effectively 100%. The azo-modified polypeptide was soluble in HFP, in which it exhibited an intense photochromism attributed to the trans-cis photoisomerization of the azobenzene units. Like other sulfonated azobenzene compounds, 33 azosulfonyl-modified polymers of L-lysine were found to be very stable in their tis form, and no thermal decay was observed at room temperature over periods of times as long as several weeks. Interconversion between the two forms at room temperature could only be effected by irradiation at appropriate wavelengths. This behavior allowed the authors to purify the trans and cis forms of the model compound NE-azobenzenesulfonyl-L-lysine (VII) by chromatography, and to measure the absorption spectra of the two pure photoisomers. 

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... 

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