Catenanes properties

Most of the reactions that can be used to prepare distannanes can be extended to the preparation of oligo- and polystannanes, which have attracted interest for potential use in electronic and optical devices.455 The structures of the products are not necessarily those of completely linear catenanes, (R2Sn) , and different degrees of branching confer different properties. The formation of polymers is also frequently accompanied by the formation of cyclic pentamers or hexamers, and samples prepared by different methods may show substantially different properties. 

The molecules with distinct topological properties are not a mere curiosity, since they can be found in Nature. Circular DNA schematically presented as 42 are sometimes found in living organisms in the form of catenanes and knots [38], and special enzymes topoisomerases take part in their formation and transformations [39]. Circular DNA molecules can even form nets of catenated structures like that schematically presented in Figure 2.7 [40]. A discussion of biological topological structures falls outside the scope of this monograph it should be stressed, however, that their role in Nature is not understood and warrants an explanation. 

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. 

In this chapter, we will focus on transition metal-based catenanes and rotaxanes. We will restrict ourselves to compounds that are set in motion by an electrochemical signal. Indeed, the electrochemical techniques represent privileged methods for piloting these machines since they contain electroactive transition metal centers or complexes. In addition to triggering the motions, electrochemistry allows to investigate the dynamic properties of the compounds. 

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. 

This conclusion stresses the importance of comparison of poly [2]catenanes containing catenane units of various chemical structures for the understanding of their properties. 

The plectonemic nature of the DNA double-helix makes it a tractable molecule for experiments in molecular topology. This is a very rich vein for the exploration of the topological properties of matter. In this chapter, we have tried to illuminate some of the techniques by which the single-stranded topology of DNA can be directed in synthetic molecules. Catenanes and knots, periodic braids, and Borro-mean rings are available from simple protocols, and it is to be hoped that the physical properties associated with complex topologies (Moffatt 1990) will become available through the medium of DNA constructions. 

Chen, J., Seeman, N.C. (1991b) The electrophoretic properties of a DNA cube and its sub-structure catenanes. Electrophoresis 12, 607-611 (1991). 

Catenane 10.76 displays some very interesting dynamic properties, which may be followed by 41 NMR spectroscopy. In particular, at 81°C the hydroquinone ring protons appear as a singlet signal... 

The electrochemistry of catenane 10.76 can be studied by cyclic voltammetry (Box 4.1). The compound possesses very different redox properties in comparison with the two isolated components. 

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