Host-guest redox-controlled

In host-guest systems based on electron donor/ acceptor interactions, association/dissociation can be driven by redox processes so that it is possible to design electrochemical switches than can be used to control energy- and electron-transfer processes. 

The host guest chemistry of crown ethers continues to be exploited for the development of new ionophores for the selective recovery of Hg(II). A novel crown ring system containing a redox switchable trithiadiazapentalene/trithiotriuret unit (17/18) allows control of Hg(II) in solvent extraction experiments between chloroform and water. The thiocar-bonyl sulfur donor sites outside the macrocycUc cavity of (18) are responsible for strong complexation and extractabiUty into chloroform. 

Additional experiments were aimed at investigating other Ti-electron-rich site types that might prove suitable for incorporation into systems capable of undergoing controlled switching. Thus both benzidine 34 and 4,4 -biphenol 35 were demonstrated to form stable host-guest complexes in acetonitrile with the eyclo-bis(paraquat-/7-phenylene) tetracation. Although these hosts are structurally related, they have different redox properties, with 34 being much more readily oxidised than 35. 

Gonzalez, B, Cuadrado I, Alonso B, Casado CM, Moran M, Kaifer AE (2002) Mixed cobaltocenium-ferrocene heterobimetaUic complexes and their binding interactions with 1-cyclodextrin. A three-state, host-guest system under redox control. Organometallics 21 3544-3551... 

Jeon WS, Kim H-J, Lee C, Kim K (2002) Control of the stoichiometry in host-guest com-plexation by redox chemistry of guests inclusion of methylviologen in cucurbit[8]uril. Chem Commun 1828-1829... 

Another approach to the organization of integrated optoelectronic switches is schematically detailed in Fig. 23, and involves the organization of a photoisomerizable command interface on the solid support [86]. The command surface controls the interfacial electron transfer to a solution-state redox species. In one photoisomeric state, electron transfer to a redox probe solubilized in the electrolyte solution is prohibited (e.g. by repulsive interactions), whereas in the complementary state of the monolayer the interfacial electron transfer is allowed (e.g. because of associative interactions). Various interactions, such as electrostatic interactions, host-guest or donor-acceptor interactions, contribute to the selective contacting of the redox probe to one state of the photoisomerizable monolayer. 

The field of supramolecular chemistry is concerned with a large number of systems ranging from simple host-guest complexes to more complicated solution assemblies, as well as two-dimensional (organized monolayers) and three-dimensional assemblies (crystalline solids). Nonco-valent interactions play an important role in the kinetic assembly and thermodynamic stabilization of all these systems and constitute their most distinctive feature. Electron-transfer reactions can obviously be affected by supramolecular structures, but the reverse is also true. It is possible to alter the structure and the thermodynamic stability of supramolecular assemblies using electrochemical (redox) conversions. In other words, electron-transfer reactions can be utilized to exert some degree of control on supramolecular aggregates. Provided in this article is an overview of the interplay between supramolecular structure and electron-transfer reactions. 

Scheme 5 Redox control on the guest properties of TTF in the presence of two possible hosts (2 and 3).

Wang, Y. Alvarez, J. Kaifer, A.E. Redox control of host-guest recognition A case of host selection determined by the oxidation state of the guest. Chem. Commun. 1998. 1457. 

Host-Guest Complexation Under Redox Control ... 

HOST-GUEST COMPLEXATION UNDER REDOX CONTROL THERMODYNAMIC LIMITS... 

A comprehensive summary of the syntheses and applications of the ferrocene polymer brushes has been presented. Ferrocene-functionalized polymer brushes covalently bonded or physisorbed on solid substrate surfaces can be readily prepared via well-known surface modification techniques, such as surface-initiated polymerizations, reaction of the end-functional groups of polymer chains with substrate surfaces, and host-guest interaction. These electroactive polymer brushes have found applications in biosensors, anion-exchange chromatography, and redox-controlled modification of surface wetting properties. 

Control of self-assembly by switchable redox processes in complexes between dendrimers with re-donor and re-acceptor units permitted specific prediction of the space occupied by dendritic aggregates. Use of cucurbit[8]uril (see Fig. 6.9) as donor-host compound for viologen guests (dendronised 4,4 -bipyridines as acceptors) promotes formation of such charge-transfer complexes [24]. 

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