Photoswitchable Polymers

The synthesis of a photoswitchable polymer by grafting an azobenzene dye to methacrylate followed by polymerization is presented. The azobenzene dye undergoes a trans-cis photoisomerization that causes a persistent change in the refractive index of cast polymer films. This novel polymer was incorporated into superlattices prepared by spin casting and the optical activity of the polymer was maintained. A modifted coextruder that allows the rapidproduction of soft matter superlattices was designed and fabricated. 

Fig. 19. A pyridine-functionalized porphyrin that can be photoswitched between a trivial open polymer and a cooperative closed dimer.

Lim S-J, An B-K, Jung S-D, Chung M-A, Park SY (2004) Photoswitchable organic nanoparticles and a polymer film employing multifunctional molecules with enhanced fluorescence emission and bistable photochromism. Angew Chem Int Ed 43 6346-6350... 

Nagamura describes in Chapter 9 the fabrication and properties of supramolecular and polymeric systems that display unique linear and nonlinear optical response. Such materials may find application in high-density information processing systems of the future. Chapter 10, by Shinkai and James, describes the properties of supramolecular photoswitchable ion receptors. Finally, in Chapter 11, Ishikawa and Ye discuss the application of state-of-the art fluorescence methods to explore the properties of polymers with nm-scale resolution. 

The photochromic chromophores can be classified into two categories, depending on the thermal stability of the photogenerated isomers. When photogenerated isomers are unstable and revert thermally to their initial isomer state in the dark, the chromophores are classified as T-type (thermally reversible type). Most photochromic chromophores belong to this type. The photogenerated blue color of 6-nitro-l, 3, 3 -trimethylspiro-[2H-l-benzopyran-2,2 -indoline], for example, disappears in less than half an hour even in high Tg polymer matrices.181 Such thermally unstable photochromic chromophores cannot be applied in photoswitchable molecular systems, because the switched states are unstable. For those applications, the characteristic of persistence, or in other words thermal irreversibility, is indispensable. 

Scheme 6 Photoswitching of a-chymotrypsin through its immobilization in photoisomerizable polymers.

Photoswitchable antigen/antibody (substrate/ receptor) complexes 1. Reversible immunosensors 2. Patterning of surfaces with biomaterials using antigen/antibody-biomaterial conjugates (Design of biosensor arrays, biochips) 1. Immobilization of systems on electronic transducers (electrodes, piezoelectric crystals, FET) or the assembly of biomaterials on inert supports by non-covalent interactions (eg. glass, polymers)... 

The occurrence of two different structures that can be interconverted through the agency of an external light stimulus can be the basis of a molecular switch. Moreover, when photochromic molecules are incorporated into polymeric compounds, their photoisomerization can affect the structure and the physical properties of the attached macromolecules. Therefore, photochromic polymers may be highly promising materials for application in optical technologies, as well as in the design of photoswitchable devices. 

Fig. 4 Photoswitching in molecularly imprinted polymers incorporating azo dyes. Upon UV irradiation, the affinity of the MIP decreases. The extent of the effect is dependent on the ligand. The most significant effect was observed with his(TBA)-/V-Z-l,-glutamate (squares) and iV-Z-L-melhylesterglutamate (triangles). On the other hand, only a weak effect was observed with triamterene (crosses). Reprinted, with permission, from [69]. Copyright (2009) American Chemical Society...

PHOTOASSISTED POLING AND PHOTOSWITCHING OF NLO PROPERTIES OF SPIROPYRANS AND OTHER PHOTOCHROMIC MOLECULES IN POLYMERS AND CRYSTALS... 

Disperse Red 1 (DRl) used for photoassisted poling in polymers, the back thermal reaction advantageously allows one to end up with materials containing the most stable fmws-isomers. In any case, the back reaction can be induced by light and plays an important role in both orientation and disorientation processes. Photoassisted poling does not necessarily require a stable photoisomer, as shown for azo compounds, which are very efficient. On the contrary, photoswitching of second-order NLO properties makes sense only when the photoisomer is thermally stable. 

The merit of photochromes is that conversion between both photoisomcrs is chemically reversible, hence enabling photoswitching of various properties going from one molecular species to the other.As pointed out in Section 10.2, the two molecular species in a photochromic couple can exhibit vlifferent molecular NLO properties. Attempts to extend this property to an organized system (monolayer) and to a material (polymer, crystal) are reviewed in this section. 

Photoswitching for single molecules in a polymer, Poisson kinetics [36,47]... 

Fulgides and related diarylethenes have been investigated extensively because of the long-term thermal stability of their photocyclized colored forms, which could lead to this application in erasable optical recording materials and photoswitchable optical elements. The entrapment of these photochromic molecules in polymer films is necessary for these practical applications. Picosecond laser photolysis was employed to study the electrocyclic reaction of a furylfulgide (18, Figure 8) in polymer solids and revealed that the colored structure was formed with a time constant of ca. lOps irrespective of the nature of the polymer matrix.49... 

Several approaches to the photoregulation of proteins have been employed, including photoisomerizable ligands and inhibitors, casting the protein inside a photoisomerizable polymer, and chemical modification of the protein itself [136], Several azobenzene protein effectors have been reported to act as photoregulators of enzymes and receptors. Photoregulation of cysteine and serine proteases was carried out using simple photoswitchable inhibitors which block the active site in their trans form only [137]. Conversely, the cis isomers of a-ketoester-based inhibitors were more potent inhibitors of a-chymotrypsin than the trans forms... 

The variety of the examples listed above documents that dendrimer chemistry has attained increasing interest. In this compilation of recent results the trend towards functional and application-oriented molecules including biochemically active, photoswitchable, and polymerlike dendrimers is particularly apparent. Dendrimers cross the boundaries of classical organic chemistry and as new materials will penetrate deeper into the topical fields of nanostructures , supramolecules and polymers in the future. Increasing industrial research on dendrimers and the commercial availability of PAMAM and polyamine dendrimers should stimulate further investigations in this field. 

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