Supramolecular photoswitchable

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. 

Amide groups with long hydrophobic chains have been attached to the thiophene rings in order to promote the formation of supramolecular assemblies in solution. The reversible photoswitching of 13 from the open form to the closed form causes a change in the extent of aggregation this in turn causes a decrease in viscosity. Thus the viscosity could be modulated by appropriate illumination <2004CSR85>. 

Fig. 11A-E. Some switchable units for supramolecular chemistry. A A chemically switchable guest 25 based on an amine. B A photoswitchable host for saccharides based on conformational change. C A pH-switchable guest for cucubituril. D A redox-switchable cyclophane. E A photoswitchable azobenzene-based guest for the cyclophane 22...

Supramolecular chemistry typically involves the complexes based on molecular recognition, which include crown ethers, catenates, rotaxanes, etc. Azobenzene derivatives are often used as structural elements in such supramolecular assembly. Already in 1980 (Shinkai et ah, 1980), azobenzene moiety had been introduced into crown ether. Other recent examples include azobenzene-cyclodextrin complexes (Callari et ah, 2006), azobenzene-attached nanotube-cyclodextrin complexes (Descalzo et al., 2006), and molecular recognition complexes with DNA (Haruta et al., 2008). Numerous examples of photoswitch-able azobenzene supramolecular systems in solutions can be found (Yagai et al., 2005 Balzani et al., 2002). But there are no investigations in these systems concerning photoorientation and mass transport. 

Archut A, et al. 1998. Azobenzene functionalized cascade molecules photoswitchable supramolecular systems. Chem Eur J 4(4) 699 706. 

Photoswitchable supramolecular dendrimer-like structures were constructed starting from azobenzene-based and CD-based building blocks. For example a bis-azobenzene with a dipyridyl linker trans-Azo dimer) and a p-CD trimer formed an hyperbranched structure whose shape could be controlled by light (Fig. 11). AFM evidenced branched structures of several microns turning to disordered particles upon UV irradiation. Moreover a supramolecular dendrimer was recently built up in water starting from an hydrophilic hyperbranched polyglycerol with a-CD apex (CD-g-HPG) and an hydrophobic hyperbranched poly(3-ethyl-3-oxetane-methanol) with azobenzene apex (AZO-g-HPBO) (Fig. 12). The two components in 1 1 molar ratio selfassembled into a Janus-like dendrimer (JHBP, HPBO-6-HPG) of diameter ca. 5.1 nm, due to inclusion complexation of the azobenzene apex into the CD cavity of the partner... 

Figure 34.13 Photoswitchable supramolecular polymer developed by Takeshita et a. (Reprinted with permission from Ref [58] 2005, Royal Society of Chemistry.)... 

Figure 10. Example of a supramolecular species where energy transfer from D to A can be photoswitched. For more details see text...

Photoresponsive receptors for binding and releasing anions 13JP079. Photoswitching of chiral supramolecular environments and photoin-duced lower critical solution temperature transitions in aqueous media following a supramolecular approach 130BC873. 

Xiao W, Chen W-H, Zhang J, Li C, Zhuo R-X, Zhang X-Z (2011) Design of a photoswitchable hollow microcapsular drug delivery system by using a supramolecular drugloading approach. J Phys Chem B 115 13796-13802... 

All these examples demonstrate the possibiHty of manipulating self-assembHng structures of amphiphiles and artificial supramolecular structures, respectively, by reversible photoswitching between E- and Z-azobenzene structures. Advancements in the direction of real practical appHcations will depend on demonstrating the usefulness and stability of such systems. 

Archut, A., VSgtle, R, De Cola, L., Azzellini, G.C., Balzani, V., Ramanujam, RS., and Berg, R.H., Azobenzene-functionalized cascade molecules photoswitchable supramolecular systems, Chem.-Eur. 4, 699,1998. 

Photoswitchable materials molecular entities or supramolecular assemblies, which use light to toggle reversibly between distinct states of the system with markedly different properties [1, 2]. As a consequence of the changes produced at the molecular scale, a significant modification of some of the macroscopic properties of the material are induced. On this basis, many photochromic [3-5], fluorescence [6-8], chirality [9-11], and halochromic [12-14] switches have been reported during the past decades. 

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