Catenanes electrochemically driven

As discussed in Section 13.2.4, when one of the two rings of a catenane carries two different recognition sites, the dynamic processes of one ring with respect to the other can be controlled. In particular, if redox units are incorporated into the catenane structure, there is the possibility of controlling these processes upon electrochemical stimulation. Catenanes that exhibit such a behavior can be seen as electrochemically driven molecular rotors. An example is offered by catenane 384+ (Fig. 13.33a), which incorporates macrocycle 2 and a tetracationic cyclophane comprising one bipyridi-nium and one trans-l,2-bis(4-pyridinium)ethylene unit.19,40... 

Electrochemically Driven Molecular Machines Based on Transition-metal Complexed Catenanes and Rotaxanes... 

ELECTROCHEMICALLY DRIVEN MOTIONS IN COPPER-COMPLEXED CATENANES... 

Electrochemically Driven Ring Gliding Motion in Catenanes I 271... 

Scheme 9 Chemically and electrochemically driven translational isomer switching of [2] catenane 17 [56]...

The other examples of electrochemically driven ring motions in [2]catenanes are from the class of metal complexed catenanes (i.e., catenates) that have been synthesized and studied in our groups. These compounds, the synthesis of which relies on the ability of copper( I) to gather the bidentate phenanthroline ligand around its tetrahedral coordination sphere, are produced in remarkable yield [9, 28, 57f]. The principle of operation is essentially based on the different stereoelectronic requirements of copper(I) and copper(II). Whereas a coordination number of 4, with a tetrahedral or distorted tetrahedral arrangement is preferred by copper(I),... 

A schematic representation of a catenane complex with donors from each of two interlinked rings binding to a central metal ion, and (at right) the process by which a system with differing ring components may operate as an electrochemically-driven molecular switch, involving ring rotation. 

Figure 37 An electrochemically driven molecular shuttle based on a heteronuclear [2]catenane...

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary TT-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controUable molecular shuttles (6,236). The TT-donors in the shuttles are located in the dumb-beU shaped component of the rotaxane and the 7T-acceptors in the macrocycHc component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

Catenanes in Motion Electrochemically and Photochemically Driven Machine-Like Molecules... 

Stoddart and co-workers have developed molecular switch tunnel junctions [172] based on a [2]rotaxane, sandwiched between silicon and metallic electrodes. The rotaxane bears a cyclophane that shuttles along the molecular string toward the electrode and back again driven by an electrochemical translation. They used electrochemical measurements at various temperatures [173] to quantify the switching process of molecules not only in solution, but also in self-assembled monolayers and in a polymer electrolyte gel. Independent of the environment (solution, self-assembled monolayer or solid-state polymer gel), but also of the molecular structure - rotaxane or catenane - a single and generic switching mechanism is observed for all bistable molecules [173]. 

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