Rotaxanes - Threading Molecular Rings

Dendrimer chemistry shows that we can prepare a confined volume by designing and synthesizing an appropriate arrangement of segments. The rotaxanes described in this section and the catenanes described in the next section have 

Researchers attempted to design and syntheze artificial rotaxane structures long before biological rotaxanes were discovered. A stepwise process involving the fixation of cyclic molecules, insertion of a linear molecule and then stopper fixation at both ends of this linear molecule, produced rotaxane structures in low yields, was reported in the 1960s. When supramolecular concepfs were 

If the cyclodextrins in the rotaxane structure are linked covalently, then the cyclodextrin array is maintained even after the stoppers and the polymer chain have been removed (Fig. 3.19). Reacting the cyclodextrins with epichlorohydrin under weak alkali conditions induces crosslinking between facing hydroxyl groups. The stoppers are then removed under stronger alkali 

Catenanes and Molecular Capsules - Complex Molecular Associations 

While rotaxanes are composed of wires and rings, catenanes consist of two or more interlocked rings. The word catenan comes from the Latin word catena , which means linked chains. Although the interlocked rings in catenanes are not bonded together by covalent bonds, they cannot be separated from each other. The molecule is stabilized simply by spatial interlocking. This characteristic is different to other supermolecules, where specific interactions play crucial roles when fixing the structures of complexes. 

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Rotaxanes-Threading Molecular Rings Rotaxanes are obtained by threading linear polymers through molecular rings such as cyclodextrins, crown ethers and cyclophanes. Molecular shuttles based on the rotaxane structure have been proposed. 

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. 

Chambron, Jean-Claude, Rotaxanes From Random to Transition Metal-templated Threading of Rings at the Molecular Level, 5, 225. 

The techniques for including the polymers are the crystallization from the melt with suitable hosts, and the crystallization from a solvent common to the polymer and the host by self-assembly. In some cases simple milling of the host and the guest leads to the inclusion. A particular case is that of large molecular rings, that can accept linear alkanes and polymer chains like a thread in the eye of a needle, forming rotaxanes and pseudorotaxanes the better known examples are formed by cyclodextrins [43-45]. 

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