Catenane-like molecules

Conductivity differences were seen in a bistable [3]catenane closed-loop molecule, with a naphthalene group as one station, and tetrathiafulvalene as the second station, decorated by a tetracationic catenane salt that could travel on the catenane, like a train between two stations, making a nano-switch [141]. 

A spectacular application of the acyloin ester condensation was the preparation of catenanes like ll7 These were prepared by a statistical synthesis which means that an acyloin reaction of the diester 10 has been carried out in the presence of an excess of a large ring compound such as 9, with the hope that some diester molecules would be threaded through a ring, and would then undergo ring closure to give the catena compound ... 

The picture on the right is an X-ray structure of a catenane—a molecule consisting of two interlocking rings joined like two links in a chain. The key to the synthesis depends on the selfstacking of the planar structures prior to ring closure. 

Catenanes, as the name of Latin origin suggests (catena = chain), are chain-like molecules composed of at least two cyclic compounds (i.e., a [2]catenane) that are not covalently linked to one another but nevertheless cannot be separated unless covalent bond breakage occurs. Catenanes thus belong, together with rotaxanes (Section 14.12.4.1), to the wider family of interlocked molecules. 

Some ten years ago only the few were aware of the existence of molecules without chemical bonds. Nowadays a new scientific discipline, chemical topology, appeared to study them. This science has succeeded in predicting and producing many remarkable chemical compounds (e.g. catenanes whose molecules are interconnected like links of a chain), in defining the electronic structure of molecules more precisely, and so on. 

Catenanes in Motion Electrochemically and Photochemically Driven Machine-Like Molecules... 

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. 

The discovery that DNA forms catenanes and knots, some of them extremely complex, initiated a new field of research which has been called Biochemical Topology [21]. In 1967, Vinograd and co-workers detected in HeLa cell mitochondria isolable DNA molecules that consist of independent, double-stranded, closed circles that are topologically interlocked or catenated like the links in a chain [22, 23]. A few years later, catenanes had been observed everywhere that circular DNA molecules were known [24] and the first knot was found by Liu and coworkers in single-stranded circular phage fd DNA treated with Escherichia coli co-protein [25]. In 1980, knots could also be generated in double-stranded circular DNA [26]. 

Numerous metallomacrocycles self-assemble from their components giving species possessing various structures, for instance triangular shape [9.30,9.31] containing a cavity [9.32], which may include a guest molecule [9.33] square [9.34,9.35] or star like [9.36-9.39] shapes wheel-shaped or toroidal hexameric [9.40], octameric [9.41] or decameric [9.42,9.43] structures square [9.44,9.45] rectangular [9.46,9.47] or bent [9.48] boxes into which substrate molecules may bind [9.49,9.50] adaman-tanoid shape [9.51,9.52] with cation inclusion [9.51c], formally related to that of the spheroidal macrotricycle 21 catenane type [9.53]. Coordination species of dendri-mer or arborol nature have been constructed [7.61, 8.27, 9.54]. 

We have recently reported that compound 7a shows a smectic-smectic phase transition associated with the change of the photoluminescent color (Fig. 7) [33]. Compound 7a has a 2,6-diethynylanthracene group as a photoluminescent core. This molecule has fork-like mesogens which consist of tetra(ethylene oxide) and p-(4-/ra .v-pentylcyclohexyl)phenyl moieties. A similar molecular design was previously applied to induce smectic liquid crystallinity for rotaxanes and catenanes [39—41 ]. 

Like other known (macro)molecules (e.g. molecular knots [41], catenanes, rotax-anes [42]), dendritic molecules can be named in accordance with the IUPAC Rules of Nomenclature. However, these rules are not always sufficiently comprehensive to adequately, unequivocally, and clearly name such complex structures as dendritic molecules [43]. 

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