Catenane electrochemical behavior

Electrochemical techniques can also give interesting information in the case of catenanes of higher complexity, as shown by the results obtained by investigating a series of catenanes made of up to seven interlocked rings.20 The three basic components of these catenanes are the tetracationic cyclophanes 124+ and 154+, and macrocycle 16 containing three electron donor DMN units (Fig. 13.16). For space reasons, only the electrochemical behavior of catenanes 174 1, 188+, 194+, and 204+ (Fig. 13.17), compared to those of their molecular components, is reported. 

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. 

Figure 11. Correlation diagram for the electrochemical behavior of [2]catenane 11 +, [3]catenane 12 +, and their l and 10 components (MeCN solution, 25 C, potentials versus SCE) bielectronic processes are indicated [26c],... 

CT interactions and electron transfer processes play a fundamental role in the chemistry of rotaxanes and catenanes. CT interactions are often responsible for the driving forces that lead to the syntheses of these compounds such interactions live on when the components have been interlocked, and therefore contribute to determine the actual structure of the resulting compound. Because of the presence of CT interactions, the electronic absorption and emission spectra, as well as the electrochemical behavior, of many rotaxanes and catenanes exhibit characteristic features, quite different from those exhibited by the separated components. 

Catenane 5 was here again mainly used for this study, and comparison of the electrochemical behavior of the catenate complexes and the analogous complexes of the acyclic ligand 1 will be particularly emphasized in this section. The cations include Li+, H+, Co +, Ni +, Zn +, Ag+, Cd +, and Pd +. The electrochemical data are collected in Table 3. 

From the previously described electrochemical behavior of the catenane-type complexes [95-97], and assuming some analogous values for the redox couples Cu (4)/ Cu (4) and Cu (5)/Cu (5j, the same type of electron transfer-induced reaction is expected [111]. 

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. 

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... 

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... 

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. 

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. 

On the oxidation side, the behavior of the folly desymmetrized catenanes 6 and 9 is particularly interesting from the viewpoint of molecular machines and it is the only one here discussed. Their electrochemical patterns are very similar and consist of three oxidative processes (for 6 " see Fig. 9.5a) the first two (Fig. 9.5c) are assigned to the two consecutive monoelectronic TTF oxidations [12], while the third one is ascribed to the oxidation of the DON unit. The first and second TTF oxidations exhibit the same features observed for a previously studied catenane [13, 14] and can be interpreted as follows after the TTF " oxidation, the electron donor ring circumrotates with respect to the electron accepting ring, delivering the DON unit into its cavity. 

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