Rotaxanes ring shuttling

Figure 12.16 Schematic representation of (a) ring shuttling in rotaxanes and (b)...

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).

Among the various techniques that can be employed to investigate the ring shuttling between the two stations, the electrochemical ones are very useful, particularly the cyclic voltammetry when it is used for monitoring the behavior of the bipyridinium unit, which is one of the two stations involved in the ring shuttling. In protonated rotaxane 9H3 + (Fig. 13.10), the first and second one-electron reduction... 

Rotaxane 316+ was specifically designed36 to achieve photoinduced ring shuttling in solution,37 but it also behaves as an electrochemically driven molecular shuttle. This compound has a modular structure its ring component is the electron donor macrocycle 2, whereas its dumbbell component is made of several covalently linked units. They are a Ru(II) polypyridine complex (P2+), ap-terpheny 1-type rigid spacer... 

Figure 13.38 The electrochemically driven ring shuttling of rotaxane 434+ incorporated into an Au-SAM.

Figure 17.9 Schematic representation of the surface-bound photoswitchable rotaxane 10.104 Such a device is capable of transducing an optical signal into an electronic signal by means of the photocontrolled ring shuttling in the rotaxane molecules. (Adapted with permission from V. Balzani et al., ChemPhysChem 2008, 9, 202-220. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

Fig. 16 Structural formula of rotaxane 126+ (top) and intramolecular working mechanism for the photochemically induced ring shuttling (bottom). Right Potential energy profile for each molecular structure illustrated on the left [75, 76], Steps 1-4 are described in the text...

Figure 16. Optical electron transfer excitation of [2]rotaxane [46]. Shuttling does not take place because the direct and ferrocene-mediated back electron transfer reactions are much faster than the motion of the ring.

Interestingly, the dumbbell component of a molecular shuttle exerts on the ring motion the same type of directional restriction as imposed by the protein track for linear biomolecular motors (an actin filament for myosin and a microtubule for kinesin and dynein).4 It should also be noted that interlocked molecular architectures are largely present in natural systems—for instance, DNA catenanes and rotaxanes... 

Figure 13.8 Schematic operation of a two-station rotaxane as a controllable molecular shuttle, and idealized representation of the potential energy of the system as a function of the position of the ring relative to the axle upon switching off and on station A. The number of dots in each position reflects the relative population of the corresponding coconformation in a statistically significant ensemble. Structures (a) and (c) correspond to equilibrium states, whereas (b) and (d) are metastable states. An alternative approach would be to modify station through an external stimulus in order to make it a stronger recognition site compared to station A.

Sauvage has demonstrated both electrochemical and photochemical control over ring motions in a catenate, 18 [57,58]. The observed behavior of the catenate is essentially similar to the analogous rotaxane, the only difference being that the 4-coordinate to 5-coordinate (dpp -> terpy) shuttling process is slower in the catenate and the reverse step is faster. Again, the issue of directionality is not addressed in this system. 

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