Electrochemical switching

A Redox and Chemically Controllable Bistable Neutral [2]Rotaxane 8.4.3.1 Electrochemical Switching 

Pml unit from the crown ether. Regeneration of the neutral Npl unit by an oxidative process restores the bistable, neutral [2]rotaxane to its ground state. 

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Electrochemical polymeriza tion of heterocycles is useful in the preparation of conducting composite materials. One technique employed involves the electro-polymerization of pyrrole into a swollen polymer previously deposited on the electrode surface (148—153). This method allows variation of the physical properties of the material by control of the amount of conducting polymer incorporated into the matrix film. If the matrix polymer is an ionomer such as Nation (154—158) it contributes the dopant ion for the oxidized conducting polymer and acts as an effective medium for ion transport during electrochemical switching of the material. 

Figure 35. Schematic representation of the reversible variation of volume associated with the electrochemical switching of polypyrrole. Changes in free volume are mainly due to two effects electrostatic repulsions between fixed positive charges and exchange of cations, anions, and solvent molecules between the polymer and the solution. (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, J. Phys. Chem. 101, 3688, 1997, Figs. 1, 3,6, 7, 13. Copyright 1997. Reprinted with the permission of the American Chemical Society.)...

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

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. 

These types of switchable electrode surfaces have been used to selectively pattern two different cell populations onto a surface [151] and additionally these surfaces can selectively release different cells at different applied potentials [152]. However, it is important to recognize that electrochemically switching a surface from inactive to conjugation and active to conjugation has been well explored with nitro-terminated aryl diazonium salts. In such studies, the application where very anodic potential resulted in a six-electron reduction to an amine [139], to which proteins could be attached [153-155]. The key difference is that the interaction of the biological medium with the surface is controlled by the presence of the antifouling layer. In many ways these electrode surfaces developed by Mrksich and coworkers [150-152, 156] are very similar to the antifouling surfaces with molecular wires discussed in Section 1.4.2 [131, 132, 138, 142]. In both cases the electrode is... 

Electrochemical switching of the emission of a metal complex occurs in a system in which a luminescent (tris-bipyridine)ruthenium(ll) centre is linked to a quinone unit. Interconversion of the redox couple 124-125 allows the reversible switching... 

The structures of the systems under investigation and the various processes involved in their photochemical and electrochemical switching are illustrated in Scheme 9. 

GSCC and the red CV trace displays the presence of an MSCC at a low temperature, (c) Electrochemical switching of the SAMs. 

More interestingly, a reversible photo-induced transformation of the cyclic voltam-mograms between the open-ring form and closed-ring form of the TPF was observed. This property could be used as the detecting signal for application in electrochemical switches. 

Kapui et al. prepared a novel type of polypyrrole films [168]. The film was impregnated by spherical styrene-methacrylic acid block copolymer micelles with a hydrophobic core of 18 nm and a hydrophilic corona of 100 nm. The properties of the micelle-doped polypyrrole films were investigated by cyclic voltammetry and SECM. It was found that the self-assembled block copolymer micelles in polypyrrole behave as polyanions and the charge compensation by cations has been identified during electrochemical switching of the polymer films. 

Cyclic voltammetry shows that the [2]rotaxane 22 1 h can also undergo electrochemical switching (Figure 16) by a monoelectronic oxidation process to give the radical pentacationic species 225+. On oxidation, the benzidine unit is converted to its monocationic radical state, generating an electrostatic repulsion, which causes the tetracationic cyclophane to move to the biphenol unit in [2]rotaxane 224+. This redox procedure is completely reversible. 

Next we consider the case where a low overpotential exists at some point in the electrochemical switching process, as is necessarily the case in cyclic voltammetric or chronopotentiometric experiments. Now, four additional mechanisms become possible on the time scale of the electron/ion transfer process. This is so because the electron/ion transfer step, E, is no longer constrained to be the first. Consequently, either one or both of the chemical steps may precede the coupled electron/ion transfer. During the oxidation of R to Of, if solvation of Ra is more rapid than its reconfiguration, the two mechanisms, CEC and CC E may occur. If reconfiguration of R is more rapid than its solvation, two additional mechanisms, C EC and C CE, become possible. 

KoxY = exP [-F ( Ox/Red) (OxY/RedY) ) / ]. Here, E oxY/RedY) the formal potential of the redox couple OxY/RedY. Electrochemical switching has received much attention with respect to anion and cation recognition, analytical applications, biomimetics of ion transport through membranes, and generally molecular recognition. 

Ion transport, especially cation transport, was one of the early focal points in macrocyclic chemistry, revolving primarily around the crown ethers and cryptands. Later efforts have been to provide switches to control the rates of cation transport. Two examples of the types of switches that have been developed include photo switches using cryptands,and electrochemical switches using anthraquinone-derived lariat ethers. 

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