Rotaxane redox active

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. 

In other work, the new redox-active [2]-rotaxane 39, incorporating a 1,4-phenylenediamine subunit in its linear component, has been synthesised in high yield (Figure 4.17). ° The redox behaviour of this product indicates considerable (increased) resistance to oxidation of the phenylenediamine subunit. This subunit appears even more promising with respect to its ability to repel the cationic cyclophane upon oxidation. 

Rotaxane 40 (Figure 4.18) was synthesised since it was reasoned that it might be possible to prolong the lifetime of any redox intermediate by the incorporation of terminal redox-active stoppers. The presence of the latter may provide a means of facilitating spatial separation of the primary redox changes. Preliminary cyclic voltammetry studies indicated that the 7t-radical dialkoxybenzene cation, formed by excitation of the charge-transfer complex, should favour oxidation of one of the... 

Figure 4.25 Formation of the isomeric redox-active rotaxanes 61a and 61b ...

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... 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

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... 

Katz E, Sheeney-Ichia L, Willner I (2004) Electrical contacting of glucose oxidase in a redox-active rotaxane configuration. Angew Chem Int Ed 43 3292-3300... 

For both rotaxane and catenane-based molecular machines, it is desirable to have recognition sites such that they can be easily controlled externally. Hence, it is preferable to build sites that are either redox-active or photo-active [144]. Catenanes can also be self-assembled [157]. An example of catenane assembled molecular motors is the electronically controllable bistable switch [158]. An intuitive way of looking at catenanes is to think of them as molecular equivalents of ball and socket and universal joints [153,159,160]. 

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]. 

Katz, E., Lioubashevsky, O. and Willner, I. (2004) Electromechanics of a redox-active rotaxane in a mono-layer assembly on an electrode. J.Am. Chem. Soc., 126, 15520-15532. 

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