Catenane electroactive

As already seen for catenanes 134+ andl44+ (Fig. 13.15),ongoingffomseparated molecular components 16,124+, or 154+ to their catenanes substantial changes in the electrochemical behavior are expected because the electroactive units incorporated in the cyclophanes and macrocycle are engaged in donor-acceptor interactions and occupy spatially different sites. 

Several examples of catenanes and rotaxanes have been constructed and investigated on solid surfaces.1 la,d f 12 13 26 If the interlocked molecular components contain electroactive units and the surface is that of an electrode, electrochemical techniques represent a powerful tool to study the behavior of the surface-immobilized ensemble. Catenanes and rotaxanes are usually deposited on solid surfaces by employing the Langmuir-Blodgett technique27 or the self-assembled monolayer (SAM) approach.28 The molecular components can either be already interlocked prior to attachment to the surface or become so in consequence of surface immobilization in the latter setting, the solid surface plays the dual role of a stopper and an interface (electrode). In most instances, the investigated compounds are deposited on macroscopic surfaces, such as those of metal or semiconductor electrodes 26 less common is the case of systems anchored on nanocrystals.29... 

In this chapter, we will focus on transition metal-based catenanes and rotaxanes. We will restrict ourselves to compounds that are set in motion by an electrochemical signal. Indeed, the electrochemical techniques represent privileged methods for piloting these machines since they contain electroactive transition metal centers or complexes. In addition to triggering the motions, electrochemistry allows to investigate the dynamic properties of the compounds. 

Tetrathiafulvalene and its derivatives are electroactive and can be easily and reversibly oxidized to TTF + and TTF2 +. The TTF skeleton now occupies a critical position as far as switchable properties are concerned, and behaves as a key unit for a number of supramolecular concepts. For instance, the recent years have seen an increasing contribution of TTF to the preparation of interlocked compounds such as rotaxanes and catenanes. These systems are of particular importance as candidates for molecular machines. 

When conferred with a hydrophilic head (in this case a substituted trityl unit), and a hydrophobic (benzylic alcohol) tail, rotaxanes - branched [55] or otherwise - can be formed into Langmuir films in a manner similar to catenanes. Rotaxane 224+ - synthesized from its corresponding thread via the slipping approach - when incorporated into a device in a manner similar to the catenane 214+ also exhibited interesting electron-transport properties [56], Unlike the catenane-based device, there is no switching element built into the molecule. However, like the switchable catenane, the rotaxane 224+ has electroactive bipyridinium sites, whose presence can mediate the tunneling of... 

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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