Mobility, rotaxanes/catenanes

Intramolecular mobility of metal complexes of rotaxanes and catenanes with macroheterocyclic fragments 98ACR611. 

A special issue devoted to molecular machines appeared in Accounts of Chemical Research in 2001. It reflects the current interest for this field in which ruthenium complexes act as important tools. Molecular machines are characterized by a mobile part and a stationary part. Photochemical and electrochemical inputs can make a machine work, offering the advantage of being switched on and off easily and rapidly. Mechanically interlocked molecules, such as rotaxanes and catenanes, are suitable candidates. Crown ethers, cyclophanes, and calixarenes are representative families of the cyclic... 

The nonlinear optical properties of rotaxanes and catenanes were studied mainly by three techniques the optical second and third harmonic generation and the electro-optic Kerr effect. As already mentioned, the harmonic generation techniques give the fast, electronic in origin, molecular and bulk hyperpolarizabili-ties, whereas the electro-optic methods are sensitive to all effects which induce optical birefringence, such as e.g. the rotation of molecules. Therefore the last technique is very useful to study the rotational mobility of molecules and/or their parts. 

Niziol et al. review the linear and NLO properties of some catenanes and rotaxanes studied in solutions or tliln films. Techniques like UV-Vis spectrometry, second and tlilrd harmonic generation in thin films and electro-optic Kerr effects in solution have been employed. They review the synthesis and material processing of tlrese derivatives. Niziol et al. describe how the rotation rate of the macrocycle in catenane solutions is more than an order of magnitude larger than in rotaxanes. They comment on the factors on which the rate of rotation depends. This new class of molecules, with mobile subparts, is very likely to have useful applications including tire construction of synthetic molecular machines and all-optical switching elements. 

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