Photochromic polymers-azobenzene

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

In the following, a number of examples are presented with emphasis on how such amplified imaging device can be built in self-supporting polymer materials. The present photochromic compounds (azobenzene and spiropyran derivatives) are rather common and have been used many times in molecular assemblies without touching upon the concept of image amplification. The most recent references are given(4. - 6). 

A photochromic polymer containing azobenzene units has also been prepared by modification of a naturally occurring microbial poly(E-L-lysine) (Scheme 5, Structure IX), and investigated by means of absorption and circular dichroism spectroscopy.1431 The structure of this polymer, however, does not correspond to those of polypeptides, which are poly(amide)s of a-amino acids, and therefore the results cannot be discussed in terms of the typical polypeptide structures (a-helix, P-structure, random coil) and their standard CD spectra. 

H. Kamogawa, M. Kato, and H. Sugiyama, Syntheses and properties of photochromic polymer of azobenzene and thiazine series, J. Polym. Sci. Part A-l, 6, 2967-2981 (1968). 

A similar but much more detailed interpretation was recently given by Eisen-bach who showed the general validity of the WLE-equation in the interpretation of the photochromism of azobenzenes and spirobenzopyrans linked to amorphous bulk polymers. Eisenbach used directly the WLF-equation ... 

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

PHOTOCHROMIC POLYMERS FOR OPTICAL DATA STORAGE AZOBENZENES AND PHOTODIMERS... 

We shall now embark on the main focus of this chapter photochromic azobenzene polymers and photochromic materials capable of undergoing photodimerization for potential use in optical data storage applications. Herein, the photochromic polymers described are those in which the photochromic molecule is covalently linked, either as part of the main chain or as a side-chain, to the polymeric system. 

Matejka et al. synthesized several types of photochromic polymers based on a copolymer of maleic anhydride and styrene with azobenzene moieties both in the side chains and cross-links of the polymer network (Matejka et al., 1981 Matejka and Dusek, 1981 Matejka et al., 1979). The photomechanical effect was enhanced by an increase in the content of photochromic groups, and the photoinduced contraction of the sample amounted to 1% for a polymer with 5.4mol% azobenzene moieties. 

Azobenzene-subsdtuted polythiophenes has led to the development of novel dual-photochromic polymers, where the isomerization of the photo-active side-chains. 

Zhao X.Y, Wang M. and Liu H.J. (2008), Structure dependence of photochromism of azobenzene-functionalized polythiophene derivatives./owrna/ of Applied Polymer Science, 108 pp. 863-869. 

Blair et al. [9] have carried out film balance measurements on monolayers of photochromic polymers (4) and (5). Figure 7 shows an example of the pressure-area curve for Structure (4). A significant reduction in area per monomer unit was observed on changing from dark to light conditions this was interpreted as being the result of trans-io-cis isomerization of azobenzene chromophores the more extended conformation of the fr 5-isomer would produce a large apparent area. The difference between the pressure-area curves of the meta- and /jaro-polyamide was interpreted based on the assumption that the mcto-azo-polymer chain may have a helical structure, whereas the /jara-azo-polymer has a linear structure. 

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. 

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