Rotaxane structure, donor-acceptor

Fig. 12 Examples of the use of color in depictions of various types of MIMs. Note how the colors and positions of constituent parts in the three-dimensional structures reflect those in the structural drawings to enhance clarity between representations of (a) a donor-acceptor [2]catenane [75] and (b) an ammonium-binding [2]rotaxane [76] from our group, (c) a transition metal-templated Solomon Knot from the Sauvage and Fujita groups [77], and (d) a benzylic amide [2]catenane from the Leigh group [78]. Reproduced with permission from [75] (copyright 1991 Royal Society of Chemistry), [76] (copyright 2000 Wiley-VCH), [77] (copyright 1999 Royal Society of Chemistry), [78] (copyright 1995 Wiley-VCH)...

Stoddart and co-workers also made use of aromatic electron donor-acceptor (EDA) interactions for highly effective synthesis of catenanes and rotaxanes (Figure 1-3) [62]. In this case, however, one of the ligsons acts as template, which is subsequently incorporated in the covalent structure of the ligand product (catenane or rotaxane). 

Conversely, the template responsible for molecular association can be discrete, separate from the interlocked structure, and thus removable (Figure 10.2b). The tern-plating charge-transfer donor-acceptor in rotaxane 3 (Scheme 10.3) is an example of integrated templation. The copper(I) metal in catenane 2 (Scheme 10.2) is strictly... 

Dumbbell 34+ exhibits two bielectronic and reversible processes that can be attributed to the simultaneous first and second reduction of the two bipyridinium units contained in its axle-like section. The bielectronic nature of the processes indicates, as expected, that the bipyridinium units are equivalent and behave independently. Also, model rotaxane 44+ shows two bielectronic and reversible processes that are straightforwardly assigned to the bipyridinium units contained in its dumbbell component they are, however, shifted to more negative potentials compared to dumbbell 34+. These shifts can be attributed to the CT interactions with the electron donor ring that make the electron acceptor bipyridinium units more difficult to reduce, whereas the bielectronic nature of the processes indicates the such units are noninteracting and equivalent—both of them are surrounded by a ring—in full agreement with the supramolecular structure of 44+. 

Structurally related to these species are the triply branched compound 56+ and its rotaxanes 66+, 76+, and 86+ (Fig. 13.6)9, in which one, two, or three acceptor units are encircled by the electron donor macrocyclic compound 2. Although these rotaxanes cannot behave as degenerate molecular shuttles because of their branched topology, they are nevertheless interesting from the electrochemical viewpoint. 

In interlocked compounds such as rotaxanes and catenanes, electron-donor and -acceptor units not only cause the presence of CT interactions, but are also responsible for the occurrence of intercomponent and intermolecular oxidation and reduction processes. Such processes weaken or even destroy the CT interactions that stabilize the structure of the compound, with a consequent change in its coconformation. External inputs, like electrons or photons, can be used to cause the redox processes and the structural rearrangements that follow. Suitably designed rotaxanes and catenanes can therefore exhibit machine-like movements that correspond to a binary logic behavior. 

The studies described in this section were started shortly after the X-ray crystal structure of the RC of Rh. viridis was disclosed [73]. During these years, the role of the so-called accessory bacteriochlorophyll BCh was under debate [79]. In particular, the possibility was considered that it could play the role of a superexchange relay between SP and BPh (see Figure 22). In this respect, the copper(I)-complexed [2]rotaxane Cu.20+ represented a functional artificial model of the SP/BPh/BCh triad, the central Cu complex fragment between the Zn porphyrin donor and the Au porphyrin acceptor mimicking the function of BCh between SP and BCh. However, the kinetic scheme shown in Figure 22a has been revised, being now quite firmly established that (at least at room temperature) BCh is directly involved in the electron transfer reaction the transfer from the electronically excited special pair SP to BCh takes about 3 ps, and the next transfer step to the BPh, 0.65 ps [80]. In the earlier experiments, detection of the intermediate state SP+BCh was prevented by its relatively slow population and fast decay. 

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. 

Electron-transfer processes have also been investigated in porphyrin-containing [2]-rotaxanes [106,120]. hi these systems, two Zn(ll)-porphyrin (ZnP) electron donors were attached as stoppers on the rod, while a macrocycle attached to a Au(III)-porphyrin (AuP ) acceptor was threaded on the rod. By selective excitation of either porphyrin unit, electron transfer could be induced from ZnP to the AuP unit that generated the same charge-transfer state irrespective of which porphyrin was excited. Additional metal ions like Ag+ and Cu were introduced into the system by coordination of phenan-throlines as shown in structure 45. When Zn(ll)-porphyrin was excited, no effect of Ag" or Cu" on the electron-transfer process was observed. However, the excitation of Au(III)-porphyrin enhanced the electron-transfer rate in the presence of Ag" as well as Cu". These results show that it is possible to tune the rate of electron transfer between noncovalently linked reactants by appropriate modification of the link. 

In this kind of pseudorotaxanes, rotaxanes, and catenanes, the stability of a specific supramolecular structure is largely due to the CT interaction. In order to cause mechanical movements, such a CT interaction has to be destroyed. This can be done by reduction of the electron-acceptor units or by oxidation of the electron-donor units through chemical, electrochemical, or photochemical redox processes. In most cases, the CT interaction can be restored by an opposite redox process, which thus promotes a reverse mechanical movement leading back to the original structure. 

Figure 8 Donor-acceplor mulliporphyrin conjugalcs. The donor porphyrin (empty diamond) and the acceptor porphyrin (hatched diamond) are mechanically linked in cither catcnanc (a) or rotaxane (b) structure. Tlie arrows show the direction of photoinduced electron transfer.

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