Photoresponsive spiropyrans

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

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). 

Photoresponsiveness of Poly(spiropyran-L-glutamate) under Acidic Conditions... 

Quite interesting photoresponsive behavior was observed when spiropyran-modified poly(L-glutamate) was dissolved in HFP and a small amount of trifluoroacetic acid... 

Photomechanical effects have been also observed in monolayers obtained from poly(L-glutamic acid) modified with carbocyanine1831 and spiropyran dyes.184 In the latter case, irradiation at 254 nm produced changes in the molecular conformation, which in turn caused photomodulation of the surface pressure and surface area of the films. From all these examples, it appears that photoresponsive monolayers are quite fascinating systems, which may eventually come to be regarded as a machine to transform light into mechanical energy . 81 ... 

Synthetic polymers containing photochromic units can undergo reversible changes of their physical and chemical characteristics. (For recent reviews, see Refs. 24-26). Recent examples involving photochromic compounds other than spiropyrans are reported in Refs. 27-31. Spiropyran-containing polymers such as polyacrylates have also been prepared and were found to show photoinduced variations of their viscosity.18- 0 The change in the viscosity of the polymers partly reflects the polymer conformation. Thus, spiropyran-attached poly(L-tyro-sine) and poly(L-lysine) were synthesized by Vandewyer and Smets at the University of Louvain in 1970.32 33 No photoresponsiveness was observed however, for these modified peptides. 

Now, application in 3D optical storage memoiy and lasers devices or infrared sensitive spiropyranes, are the focus of investigation in optical technology, which has raised interest in these photoresponsive materials. 

Moreover, a low molar mass spiropyran compound entrapped in a membrane consisting of plasticized poly(vinyl chloride) rendered the latter photoresponsive. A membrane potential change of more than 100 mV was induced by irradiation with light [27]. For further details and additional references, the reader is referred to the relevant reviews [11, 28]. 

Another typical photoresponsive material for preparation of switchable surfaces is the spiropyran-merocyanine system. The spiropyran isomerizes to zwitterionic merocyanine conformation by UV exposure, and the reverse reaction can be triggered by irradiation with visible light as well as azobenzene. The changes in hydrophilic/hydrophobic properties through the isomerization of spiropyran groups also enable the control of cell adhesion/ detachment. Edahiro et al. reported photoresponsive cell culture substrates grafted... 

Fissi, A. Pieroni, O. Ruggeri, G. CiardeUi, F. Photoresponsive polymers. Photomodulation of the macromolecular structure in poly(l-lysine) containing spiropyran units. Macromolecules 1995,28,302-309. 

Polymer films comprising photoresponsive molecules such as 2-nitrobenzyl [260, 261], spiropyran [262-264], azobenzene [265], cinnamoyl [266], and coumarin [267] represent other attractive candidates for controlling protein and cell adhesion on... 

Fissi, A., Pieroni, O., Ciardelli, F., Fabbri, D., Ruggeri, G., and Umezawa, K. (1993) Photoresponsive polypeptides photochromism and conformation of poly(L-glutamic acid) containing spiropyran units. Biopolymers, 33,1505-1517. 

B. Wang, K. Chen, R.-D. Yang, F. Yang, and J. Liu, Photoresponsive nanogels synthesized using spiropyrane-modified pullulan as potential drug carriers, J. Appl. Polym. Set aid., 131,40288, 2013. 

Alonso, M., Reboto, V., Guiscardo, L, San Martin, A., and Rodriguez-Gabello, J.C. (2000) Spiropyran derivative of an elastin-like bioelastic polymer photoresponsive molecular machine to convert sunlight into mechanical work. Macromolecules, 33, 9480-9482. 

In addition to electric potential, light can also be used to trigger switching properties of surfaces and polymers. Application of ultraviolet (UV) light to these materials may result in reversible changes in characteristics such as hydrophilic-ity/hydrophobicity, structural arrangement, and shape. Commonly utilized photoresponsive materials include azobenzene molecules, spiropyran molecules, and shape-memory polymers. 

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