Catenane switching processes

Figure 13.36 Switching processes of catenane 41H5+ in solution. Starting from the deprotonated catenane 414+, the position of the ring switches under acid-base and redox inputs according to AND logic.

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]. 

MIMs, such as catenanes (see Self-Assembled Links Catenanes, Self-Processes) and rotaxanes (see Rotax-anes—Self-Assembled Links, Self-Processes), share aU the good attributes of supramolecular switches while dispensing with the concentration dependence. The cost involved in this functional simplicity is paid for by more lengthy syntheses. The benefits of the added conhol provided by the molecular format are greater than the sum... 

This process involves the covalent locking in of structures formed by reversible self-assembly. The irreversible, post-assembly step switches off the equilibrium process involved in the self-assembly. As we will see in the following sections, self assembly with covalent postmodification is involved in a range of biochemistry (e.g. insulin synthesis) and elegant abiotic supramolecular synthesis as in the formation of catenanes and knots. 

When one of the two macrocycles carries two different recognition sites, then the opportunity exists to control the dynamic processes in these switchable [2]catenanes in a manner reminiscent of the controllable molecular shuttles (Figure 29) [30-34, 41], In essence, the requirement for being able to switch between state 0 and state 1 in such a [2]catenane is that the symmetric macrocyclic component resides preferentially around one of the two different recognition sites incorporated within the non-... 

Niziol et al. review the linear and NLO properties of some catenanes and rotaxanes studied in solutions or tliln films. Techniques like UV-Vis spectrometry, second and tlilrd harmonic generation in thin films and electro-optic Kerr effects in solution have been employed. They review the synthesis and material processing of tlrese derivatives. Niziol et al. describe how the rotation rate of the macrocycle in catenane solutions is more than an order of magnitude larger than in rotaxanes. They comment on the factors on which the rate of rotation depends. This new class of molecules, with mobile subparts, is very likely to have useful applications including tire construction of synthetic molecular machines and all-optical switching elements. 

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. 

Like rotaxanes, catenanes are mechanically interlocked molecules. However, instead of interlocking one ring shaped macrocycle and a dumbbell shape, catenanes consist of interlocked macrocycles. The number of macrocycles contained in a catenane is indicated by the numeral that precedes it. Catenanes have bistable and multistable forms and a switchable, bistable [2]catenane is commonly exploited in nanotechnology and molecular electronics because its behavior can be controlled by electrochemical processes [89]. Collier et al. was the first to demonstrate the electroactivity of interlocked catenanes [90]. The authors affixed phospholipid counterions to a monolayer of [2]catenanes and then sandwiched this system between two electrodes. This work resulted in a molecular switching device that opened at a positive potential of 2 V and closed at a negative potential of 2 V. 

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