Rotaxanes constitution

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. 

The first example of electrochemically driven molecular shuttles is rotaxane 284+ (Fig. 13.25) constituted by the electron-deficient cyclophane 124+ and a dumbbellshaped component containing two different electron donors, namely, a benzidine and a biphenol moieties, that represent two possible stations for the cyclophane.10 Because benzidine is a better recognition site for 124+ than biphenol, the prevalent isomer is that having the former unit inside the cyclophane. The rotaxane... 

The chemical constitution of rotaxanes will depend, of course, on the methods used for their elaboration. Whereas the statistical synthesis (vide infra)... 

While the above studies are representative of the area, it should be noted that a considerable number of other [2]-pseudorotaxanes have been investigated. These include a system exhibiting enantioselective self-assembly during its formation as well as systems incorporating a new structural motif involving a l,2-bis(pyridini-um)ethane axle and a 24-crown-8 ether wheel .A number of poly-pseudoro-taxanes (and related poly-rotaxanes) as well as constitutionally asymmetric and chiral [2]-pseudorotaxanes have been reported. New, multiply-stranded and multiply-encircled systems have also been prepared. The solid state structure of... 

It is possible to synthesize [2]rotaxanes containing two identical recognition sites within their dumbbell component. The result is a degenerate equilibrium state in which the macrocyclic component can shuttle back and forth along the linear portion of the dumbbell. Such a system constitutes a molecular shuttle. Two examples [lla,b, 39] of [2]rotaxanes that behave as degenerate molecular shuttles are shown in Figure 12. 

The behavior of the systems constituted by the unstoppered compound Cu.29, or the semi-rotaxane Cu.30" [112], is related to the fully blocked rotaxane Cu.31, i.e., the same redox couple can be observed. Some variations of peak intensities with the scan rate also indicate an acceleration of the chemical processes as compared to Cu.31. However, additional signals corresponding to the species Cu (MeCN)4 (Ei/2 = +1.02 V) and Cu (6) (6-coordinate state, Ej/2 = —0.41 V) [95-97, 100] reveal that the dethreading process becomes significant and occurs primarily with the copper(II) species. The 6-coordinate complex evidenced by CV originates from the dethreading and coordination of the linear fragments via their tpy units to a copper center. 

It should be mentioned here that very interesting constitutional and translational isomerism is observed in the series of catenanes and rotaxanes which contain phenol derivatives such as macrocycUc phenylene-crown components as well as phenolic polyether chains (see also Lehn s recently published book ). 

The constitutional graphs of common compounds and those of Mobius compounds and knots (recently synthesized) are connected. The graphs of catenanes and rotaxanes, however consist of two or more components reflecting the dimeric (oligomeric) character of these compounds. 

Abstract The approach based on the copper(I)-templated synthesis of porphyrin catenanes and rotaxanes developed by the authors group is here reviewed. Zn(II) porphyrins and gold(III) porphyrins were chosen as electron donors and electron acceptors, respectively, to constitute the electro- and photoactive parts of the present systems. The processes—energy and electron transfer reactions—occiuring in the interlocked structures upon light absorption in the presence or absence of Cu(I) are presented, their rates and efficiencies critically compared and discussed with respect to properties of the components and of the ensemble. A detailed examination of differences and analogies in photoreactivity between the present and closely related systems reported by others is presented. 

The rotaxanes described in this paragraph constitute representative examples showing that complexing or decomplexing the appropriate metal in a coordination site can bring to close proximity, or spread a long distance... 

Several families of porphyrin rotaxanes and catenanes have been synthesized using the classic 3D-template effect of copper(I). The molecules contain up to 5 constitutive organic fragments ([5]rotaxane). By removing the metal template, totally different geometries are obtained. 

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