Binding photoresponsive

Other recent developments include crown ethers which contain potential switch mechanisms for complexation and transport. When photoresponsive chromophores are linked to crown ethers, ion binding can change on photoirradiation. The crown (120) is formed as its tram isomer with no alkali metal affinity, but photoirradiation gives a cis isomer capable of alkali metal complexation 500 this is an example of a reversible all or none ion-binding capability. A... 

Self-supported photoresponsive MIP membranes synthesised using a photosensitive monomer, p-phenylazo-acrylanilide, and dansylamine as template, have been applied as recognition element in sensors [207, 208]. The binding capacity of the membrane for dansylamide could be reversibly modified by changing the illumination of membrane. Other examples are NADH and NADPH imprinted membranes... 

The first example of a photoresponsive [2]rotaxane, published in 1997 by Nakashima and co-workers, is one of those cases [61]. Molecular shuttle E/Z-224+ consists of an a-cyclodextrin macrocycle, and a tetracationic thread containing an azobiphenoxy moiety, very closely related to azobenzene, and two bipyridinium stations. The well-known E-Z isomerizations of azobenzenes and the ability of cyclodextrins to bind lipophylic compounds in water are exploited in this system to achieve shuttling. When the azobiphenoxy station is in its trans form, E-224+, the cyclodextrin encapsulates it preferentially over the more hydrophilic bipyridinium station (Scheme 12). 

Photoisomerization of an azobenzene function located in a complex molecule is often accompanied by conformational changes. This approach has again been employed in the construction of photoresponsive crown ethers, a topic which has been the subject of a recent review. Cylindrical ionophores in which two diaza-crown ethers are linked by two photoresponsive azobenzene groups change their ability to bind polymethylene-diammonium salts on irradi-... 

It is usually assumed that the association-dissociation reactions occur at the membrane interfaces. Selectivity of this facilitated transfer is based on the different affinity of the carrier for the components of the source phase. In artificial liquid membrane systems, crown ethers are widely used as carriers to separate metal ions. Most of the ethers are photoresponsive and their structure and resulting metal-binding ability can be altered under irradiation. 

The various examples of photoresponsive supramolecular systems that have been described in this chapter illustrate how these systems can be characterized by steady-state and time-resolved spectroscopic techniques based on either absorption or emission of light. Pertinent use of steady-state methods can provide important information in a simple vay stoichiometry and stability constant(s) of host-guest complexes, evidence for the existence of photoinduced processes such as electron transfer, energy transfer, excimer formation, etc. Investigation of the dynamics of these processes and characterization of reaction intermediates requires in most cases time-resolved techniques. Time-resolved fluorometry and transient absorption spectroscopy are frequently complementary, as illustrated by the study of photoinduced electron transfer processes. Time-resolved fluorometry is restricted to phenomena whose duration is of the same order of magnitude as the lifetime of the excited state of the fluorophores, whereas transient absorption spectroscopy allows one to monitor longer processes such as diffusion-controlled binding. 

New photoresponsive crown ethers incorporating an azobenzene moiety have been prepared. The binding properties of the crown ether (3) containing an intraannular 4-methoxyphenylazo substituent... 

Figure 7 A photoresponsive thioindigocrown ether for metal ion binding [17]...

The aim of the design of a photoresponsive crown ether is to control the cation binding capacity of the ligand via a photochemical reaction. Expressed in another way, the photoresponsive crown ether exists in two forms a basic (stable) one, and a second one which is obtained by light irradiation... 

Shinkai S, Minami T, Kusano Y, Manabe O. 1983. Photoresponsive crown ethers. 8. Azobenzenophane type switched on crown ethers which exhibit an all or nothing change in ion binding ability. J Am Chem Soc 105(7) 1851 1856. 

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. 

ABSTRACT. Modified Y-cyclodextrins appended by an aromatic moiety show unique binding behavior, including a guest molecule together with the appended moiety in their Y-cyclodextrin cavities. Excimer formation, photoresponsive binding, association with 3-cyclodextrin and enhanced catalytic activity have been explained on this basis. 

The cis-trans isomerism of azobenzene can be used as a photoresponsive "switch" to regulate binding ability of CD when an azobenzene moiety is attached to CD. The photoresponsive CD reported first is azobenzene-capped g-CD in which the cap structure changes reversibly enlarging the hydrophobic cavity around substrate in its cis form. ... 

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