Rotaxanes supramolecular

In this review, we tried to cover all the supramolecular species that maybe classified as rotaxane dendrimers. We classified them by their structures - where in dendrimer rotaxane-hke features are introduced. Several different types of macrocycles have been employed as a ring component in the templated synthesis of rotaxane dendrimers. While the synthesis of Type I and II rotaxanes dendrimers is relatively straightforward, that of well-defined Type III rotaxane dendrimers, particularly those of second and higher generations, is still challenging. 

Other supramolecular structures such as catenanes and rotaxanes can be formed using zinc as a template ion for example, a benzylic imine catenate formed by Leigh et a/.288 The reversible five-component assembly of a [2]catenane from a chiral metallomacrocycle and a dinaphtho-crown ether has been achieved. Zinc is used as the metal component and drives assembly via the coordination to a bipyridyl unit 7r interactions between the aromatic components are also... 

In recent years, with increasing recognition of the roles played by specific noncovalent interactions in biological systems and chemical processes, the science of noncovalent assemblies- often called supramolecular science- has aroused considerable interest [76], The remaining part of this article reviews some important studies made on rotaxane and catenane, two classic types of supramolecular structure. 

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. 

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. 

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+. 

By using an incrementally staged strategy, the architectural features of switchable rotaxane 9H3+ (Fig. 13.9)15 were incorporated into those of a previously investigated16 triply threaded two-component supramolecular bundle. The result was the construction of two-component molecular devices like 10H39+ (Fig. 13.11) that behave like nanoscale elevators. 17 Compound 10H39+, which measures about... 

Tetrathiafulvalene and its derivatives are electroactive and can be easily and reversibly oxidized to TTF + and TTF2 +. The TTF skeleton now occupies a critical position as far as switchable properties are concerned, and behaves as a key unit for a number of supramolecular concepts. For instance, the recent years have seen an increasing contribution of TTF to the preparation of interlocked compounds such as rotaxanes and catenanes. These systems are of particular importance as candidates for molecular machines. 

Like the currently popular area, called nanoscience , the field of supramolecular chemistry has rather hazy boundaries. Indeed, both areas now share much common ground in terms of the types of systems that are considered. From the beginning, electrochemistry, which provides a powerful complement to spectroscopic techniques, has played an important role in characterizing such systems and this very useful book goes considerably beyond the volume on this same topic by Kaifer and Gomez-Kaifer that was published about 10 years ago. Some of the classic supramolecular chemistry topics such as rotaxanes, catenanes, host-guest interactions, dendrimers, and self-assembled monolayers remain, but now with important extensions into the realms of fullerenes, carbon nanotubes, and biomolecules, like DNA. 

Figure 32. Porphyrin blocking groups offer the possibility of further supramolecular deri-vatization. Amide-linked rotaxane with porphyrin blocking groups 85, its Zn complex 86, and an unsymmetrically capped rotaxane 87.

Catenanes and rotaxanes have made their way from being considered laboratory curiosities to become highlights in supramolecular chemistry. The synthesis of... 

The great variety of incorporable building blocks also offers the synthetic chemist many potential structural and functional design possibilities. The insertion of, e.g., photo-responsive elements, groups with further supramolecular derivatization potential, or sulfonamide units which enable subsequent inter- and intramolecular linkage of catenanes and rotaxanes might render good service in the development of molecular switches [64] and devices [65]. 

Figure 51. Anionic template synthesis of rotaxane 118 with a bis(phenyl ether) axle. The mechanical bond is formed by reaction of the phenolate-macrocycle complex [32-115] (supramolecular nucleophile) with the semiaxle intermediate 117. 

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