Catenanes electrochemical properties

The above section describes the pioneering work which opened the route to the synthesis of interlocked rings, i.e., catenanes. The displayed X-ray stuctures of the prototypical free [2]catenane and of some metallo catenanes allow one to visualize the interlocking of the coordinating macrocycles, the intramolecular interactions responsible for the overall structures, and to understand their particular behavior (such as electrochemical properties, enhancement of basicity, etc.) due to their topography and to their topology. 

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

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. 

If molecules such as these are to be employed in molecular devices, one of the challenges is to learn how to use external stimuli to interact with the system. Such stimuli could be chemical, electrochemical, or photochemical in nature. In view of this, the two [2]-catenanes 14 and 15 were synthesised (Figure 5.8). Related to their characteristic redox properties, the precursor vinylogous viologen building... 

Stoddart et al. also incorporated a bistable [2]catenane (Figure 39) into macromolecules to form a side-chain poly[2]catenane (Figure 82). This poly[2]catenane could also behave as a molecular switch that can be addressed in an on or off state electrochemically by the virtue of the redox properties of the TTF moiety. Furthermore, the switching properties remain even in spherical aggregates. This stndy provides a good mechanically interlocked switchable polymeric scaffold for the construction of solid-state molecular electronic devices. 

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