Ring opening rotaxanes

As depicted in structures 1 and 2, rotaxanes are molecular composites consisting of cyclic and linear molecules in which the two components are connected together without any covalent bond, i.e., mechanically. Structure 1 is a pseudorotax-ane because the cyclic, represented by an open ring, can be disassociated from the linear species by external forces, e.g., dilution and heating in the solution state. Structure 2 is a true rotaxane, because the cyclic is permanently confined between two bulky blocking groups (BG), represented by the two balls, and can not slip off the linear molecule. 

Fig. 3 The macrocycle A incorporating a coordinating fragment thick line) interacts with a metal center black circle) and an asymmetrical open chain chelate B bearing one porphyrin and a precursor function X which is small enough to pass through the ring after the threaded intermediate C is formed, the additional porphyrin ring is constructed, affording the transition metal-complexed rotaxanes D and E. Demetalation leads to the free-ligand rotaxanes F and G from D and E, respectively...

Because of the reversibility of tiie RCM process the same type of rotaxane can be produced from the macrocycle containing a CH=CH bond that can be opened and closed reversibly. The macrocycle 34 with the salt 35 in the presence of catalyst 3 affords the catenane 36 in 95% yield, thus offering an example of a magic rin allowing tiie introduction of an ionic molecule 35 that can not mechanically enter the ring (Scheme 16, right-hand route) [47]. 

Like rotaxanes, catenanes are mechanically interlocked molecules. However, instead of interlocking one ring shaped macrocycle and a dumbbell shape, catenanes consist of interlocked macrocycles. The number of macrocycles contained in a catenane is indicated by the numeral that precedes it. Catenanes have bistable and multistable forms and a switchable, bistable [2]catenane is commonly exploited in nanotechnology and molecular electronics because its behavior can be controlled by electrochemical processes [89]. Collier et al. was the first to demonstrate the electroactivity of interlocked catenanes [90]. The authors affixed phospholipid counterions to a monolayer of [2]catenanes and then sandwiched this system between two electrodes. This work resulted in a molecular switching device that opened at a positive potential of 2 V and closed at a negative potential of 2 V. 

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