Polymers photochromic behavior

Linear polarization, and NLO properties, 12, 102 Linear polymers, siloxanes, synthesis, 3, 660 Linkage isomerism, for photochromic behavior, 1, 245 Linked cages, metallacarboranes, 3, 245 Linkers, traceless, chromium carbonyls as, 5, 251 Lipids, in bioorganometallic chemistry, 1, 904 Liquefied noble gases, in low-temperature infrared studies, 1, 264... 

The analogous polymer obtained from L-a,P-diaminopropanoic acid [Scheme 5, VIII(n= 1)] displayed photochromic behavior similar to that observed for the other homologues. In this case, however, irradiation at 360 nm produced variations of the CD spectra in the peptide region, and associated irreversible structural changes in the macromolecules.142 ... 

Poly(L-glutamate)s containing various molar percentages of spiropyran units in the side chains (XV) have been prepared by treating poly(L-glutamic acid) with N-(2-hy-droxyethyl) -spiropyran in the presence of dicyclohexylcarbodiimide and 4-pyrrolidino-pyridine.[60 62] The structure and photochromic behavior of the modified polymers are shown in Figure 8. 

Polymers of L-lysine containing spiropyran units in the side chains (XVI) (Figure 13) were found to show photochromic behavior in HFP analogous to that already... 

Because spiropyrans (and especially BIPS) are relatively easy to prepare, a variety of compounds are available from which to deduce correlations (usually semiempirical Hammett-like equations) between structure and photochromic parameters in dilute fluid solutions. The photochromic behavior of a single dye in a variety of solvents can also be correlated with a solvent parameter such as. In applications, however, the dyes are almost always relatively concentrated in a bulk polymer or a polymeric binder film, and various other additives such as antioxidants, surfactants, fire retardants, plasticizers, and other colorants are also present. Under these conditions, the observed photochromic behavior is often very different from that predicted by the dilute fluid solution correlations. 

Where possible, photochromic behavior (spectrokinetic parameters, photostability) in solution or in polymer matrices, and potential applications are described. 

The most intensively developed materials, however, are the azobenzene-containing side chain LC polymers, which show a unique combination of liquid crystallinity and photochromic behavior in a single macromolecule, whereas copolymerization of comonomers with different functionality allows fine tuning of phase behavior, photo-optical properties, and other parameters to the requirements of a particular application. Starting from the first publications of Eich et work in that area has been done by several research... 

These examples and investigations on azobenzene moieties in polymers show that the photochromic behavior is mainly Gontrolled by the free volume distribution around the chromophore. Tb obtain LBK films in which azobenzene moieties can undergo photoisomerization, therefore, the free volume around the azobenzene chromophore must be controlled precisely to allow for the molecular rearrangement inherent in the reversible tran to cis photo-isomerization. This is possible by (1) mixing with other amphiphiles, (2) adjusting the architecture of the amphiphile, or (3) attaching the chromophore to a polymer either by coulomb interaction or covalently. 

There have been many papers concerning liquid-crystalline polymers having azobenzenes in their side chains (Figure 14) because of the mesogenic nature of the azo chromophore. Liquid-crystalline polymers incorporating azobenzene moieties in their main chains (30) have also been prepared.102 A family of poly(acrylates) substituted with azobenzenes having chiral alkyl groups (31) showed smectic A as well as cholesteric phases.103 The photochromic behavior... 

The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

Yang, X., Yang, B., Liu, Y., Zhu, H., 2012. Microwave-assisted synthesis of novel spiroox-azines and their photochromic behaviors in polymer matrices. Optoelectronics and Advanced Materials-Rapid Communications 6 (11—12), 1146—1152. 

An alternative approach to develop cross-linked photoresponsive polymers for use as photochromic coatings (47) relies on the quatemization of the amine groups of IV-methyldiethanolamine polyterephthalate by a mixture of bis(2,5-dimethyl-4-chloromethyl-3-thienyl)-l,3-dioxol-2-one and p- or m-xylylene dichloride in different ratios [92]. The coatings obtained using this method exhibit photochromic behavior, although some examples of the ring-closed forms did bleach back to the ring-open isomers when stored in the dark. 

In pure HFP, the CD spectra are consistent with those of random coil polypeptide chains, and the photoisomerization reaction does not affect the polymer conformation at all. Addition of triethylamine to the HFP solution induces the coil- helix transition, but the amount of base necessary to induce the transition is different for the dark-adapted sample (15% v/v) than for the irradiated one (30% v/v). Thus, at any composition in the range 3-15% v/v of NEtj, exposure to sunlight produces reversible variations of the helix content. Combination of the effects due to the photochromic behavior with appropriate amounts of NEtj allows modulation of the extent of the photoresponse. 

CiardelK and co-workers, on the other hand, synthesized polymers of L-glutamic acid containing azobenzene or stilbene groups in the side chain (Scheme 62) and studied the relationship between their secondary structure and the photochromic behavior. ... 

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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