Rotation Rotaxanes

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Figure 13.2 Schematic representation of the intercomponent motions that can be obtained with simple interlocked molecular architectures ring shuttling in rotaxanes (a), and ring rotation in rotaxanes (b) and catenanes (c).

In order to make the copper central core as easy as possible to rotate, we thought that the bulky stoppers should be located far away from the central complex. We thus prepared and studied a new bistable rotaxane, depicted in Fig. 14.10, whose stoppers are indeed very remote from the copper center.31 This new dynamic system can indeed be set in motion more rapidly than the previously described systems. 

Linnartz, P., Bitter, S., Schalley, C.A Deslipping of ester rotaxanes a kooperative interplay of hydrogen bonding with rotational barriers, Eur. J. Org. Chem. (2003), 4819-4829. 

Any artificial rotaxane-based mimic of the natural ATP synthase motor must bear a chiral element that helps to define the direction of rotation. Chirality can, of course, be implemented in such molecules by adding chiral groups as the stoppers or the wheel. Rotaxanes with cyclodextrins as the wheels have been described [11], and rotaxanes with glucose-containing stoppers are known [14]. Rotaxanes with elements of planar chirality have also been realized [15]. 

Because these approaches can often be equally well applied to the shuttling motion of rotaxanes, the circumrotation of catenanes, and the threading/dethreading of pseudorotaxanes, we do not restrict this discussion to rotation only, but present a small selection of different examples. 

Figure 2.35. Diagram of the molecular motions in copper(I)-complexed [2]-rotaxane 101 controlled by the redox state dependence of the stereoelectronic requirements of Cu. Cu(I) and Cu(II) are represented by black and white circles, respectively. Chelating sites are represented by thick lines. Initially, the string coordinates Cu(I) together with the bidentate site of the macrocycle in a tetrahedral geometry, affording the state 101(4). Electrochemical oxidation of Cu(I) to Cu(II) produces the state 101+(4), which slowly converts into the state 101+(5) after rotation of the Cu(II)-complexed macrocycle. The cycle is completed by reduction of Cu(II), which produces the state 101(5), converting to the initial state by back motion of the Cu(I)-complexed macrocycle.

One of the structural features seen in polyrotaxanes is the absence of any covalent binding between cyclic compounds and a linear polymeric chain capped with bulky end-groups at both terminals [1]. It looks like a necklace the cyclic compounds are mechanically locked by the linear polymeric chain. The cyclic compounds in a poly-rotaxane can slide and/or rotate along the axial polymeric chain if the polyrotaxane is soluble in a certain solvent. Furthermore, such mechanical locking between the cyclic compounds and the linear polymeric chain will be opened once one of the terminal bulky end-groups is cleaved by any external conditions. These characteristics are only observable in and specific to polyrotaxanes and are never seen in conventional polymeric architectures (Fig. 1). 

Rotation, around C—O bond 353, 354 Rotaxanes 1432 Rubazoic adds 349... 

When the cyclic component of the [3]rotaxane has low symmetry, planar chirality based on the direction of the rotation occurs (Scheme 35). Vuogtle... 

Figure 20. Pictorial representation of machine-like movements that can be obtained with pseudorotaxanes, rotaxanes, and catenanes (a) dethreading/rethreading of the molecular components in a [2]pseu-dorotaxane, (b) shuttling of the macrocyclic component along the axle in a [2]rotaxane, and (c) ring rotation in a [2]catenane.

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. 

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