Rotaxanes redox-active units

When rotaxanes and catenanes contain redox-active units, electrochemical techniques are a very powerful means of characterization. They provide a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and the supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. 

Because of the presence of several redox-active units, the cyclic voltammogram of this rotaxane shows a complex redox pattern. However, the comparison to the electrochemical behavior of its molecular components and suitable model compounds (Fig. 13.29) enables to obtain useful information not only on its coconforma-tional features, but also, and most importantly, on its machine-like operation. 

Intriguing excited state electron transfer dynamics are displayed by Cu -complexed rotaxanes which combine several photo-redox active units [92, 331, 332], For example, a rotaxane [Cu(catphen)(phen-9,7-(C6o)2)] based on a Cu (phen)2... 

During the past 20 years, mechanically interlocked molecules, known as catenanes and rotaxanes, many of them redox-active, have become readily accessible using template-directed protocols that rely upon the precepts of molecular recognition and self-assembly and the tenets of supramolecular assistance to covalent synthesis. By incorporating different recognition units with dissimilar redox properties into appropriate components, these compounds can often be induced to switch hysteretically between ground and metastable co-con-... 

Willner et al. developed a redox-active rotaxane (Figure 50) as a monolayer assembly on an Au electrode. The rotaxane comprises a CBPQT + cyclophane threaded onto a molecular string, which includes a rr-donor diiminobenzene unit stoppered by an adamantane unit. The cyclophane localizes on the diiminobenzene unit initially, and shuttling can be induced by the reduction or oxidation of the cyclophane, which can be characterized... 

Bioelectronics is another apphcation area, in which rotaxanes, particularly redox-active rotaxanes, could make a significant impact Enzyme electrodes are altered in these apphcations by direct electron transfer between the electrode surface and the redox enzyme. Electronic communication between the surface and the redox enzyme centers is hindered, because a separation exists. This impediment can be circumvented by aligning the enzyme with the electrode and utilizing the redox relay units as go-betweens. The aforementioned concept has been exploited to associate an apoprotein, apo-gjucose oxidase (apo-GOx), onto relay-functionalized materials including flavin adenine dinucleotide (FAD) monolayers, nanoparticles, and carbon nanotubes [85-88]. Katz etal. used reversible redox-active rotaxane shuttles in the bioelectrocatalyzed oxidation of glucose [80]. 

The cellular unit that is active toward the contraction of skeletal muscles, known as the sarcomere, is comprised of alternatively stacked filaments of the proteins actin and myosin. During muscle contraction, the protein filaments slide past each other as a result of a rowing action of the surface myosin heads (Figure 6.92a). Hence, an effective biomimetic approach would entail the design of a linear architecture that features sliding components that will respond to a chemical stimulus. This approach has recently been demonstrated with the design of a rotaxane molecule that exhibits redox-controlled contraction and extension of the molecular architecture, in response to a chemical or electrochemical stimulus (Figure 6.92b). ... 

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