Rotaxanes shuttling process

In a recent report [141] Stoddart et al. reported a new class of rotaxanes with dendritic stoppers by using a so-called threading approach (Fig. 25). Alkylation of bipyridinium based units with Frechet s third tier branched aryl ethereal dendron, in the presence of BPP34C10 afforded 58 as one of the products. Variable temperature H-NMR spectroscopy in different NMR solvents helped determine the novel shuttling process of BPP34C10 from one bipyridinium unit to the other in 58. The dendritic framework of 58 assists in its solubility in a wide range of solvents. 

Figure 20. The shuttling process associated with the [2]rotaxane 56-4PFg in solution.

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. 

Figure 30 The pH-dependent photoisomerization and shuttling processes of the [2]-rotaxanes 118-120.

Vetter and Abraham [97] designed and synthesized rotaxanes where the axle has a 9-aryl-9-methoxy-acridane at each end, and where, consequently, the macrocycle CBPQT shuttles between one end and another. Where the axle has one aromatic ring in the central position, addition of acid produces acridinium cations at the ends of the axle, and thus forces the macrocycle to occupy the central position due to interactions with the uncharged central aromatic ring. Where the axle has two aromatic rings in the central position, addition of acid forces the macrocycle to shuttle between these two. The process can be reversed by addition of base. 

Stoddart and co-workers have developed molecular switch tunnel junctions [172] based on a [2]rotaxane, sandwiched between silicon and metallic electrodes. The rotaxane bears a cyclophane that shuttles along the molecular string toward the electrode and back again driven by an electrochemical translation. They used electrochemical measurements at various temperatures [173] to quantify the switching process of molecules not only in solution, but also in self-assembled monolayers and in a polymer electrolyte gel. Independent of the environment (solution, self-assembled monolayer or solid-state polymer gel), but also of the molecular structure - rotaxane or catenane - a single and generic switching mechanism is observed for all bistable molecules [173]. 

If we take the approximate distance between the two donor stations in rotaxane 24+ as 16 A and approximate the bead diffusion coefficient by a reasonable value (2 x 10-6 cm2/s), the time required for shuttling between the two stations would be approximately 6 ns. This calculated shuttling time is about six orders of magnitude shorter than the millisecond regime that we detected experimentally. This difference reveals that the interactions between the bead and the two donor stations, as well as those between the bead and the oligoethyleneoxy tethers, play a crucial role at controlling the bead sliding motions. In the absence of more detailed information on these bead-thread interactions, it is reasonable to postulate that the kinetics of the bead-station dissociation process probably determines the overall rate at which the bead can transfer from one station to the other. 

Fig. 14a-d. Intramolecular mechanism for the light-driven switching of the ring R between the two stations Aj and A2 in the [2]rotaxane represented in Fig. 13 [7f]. The dashed lines indicate processes that are in competition with those needed to perform shuttling...

Fig. 16. A photo- and electrochemically controllable molecular shuttle. The unperturbed rotaxane 116+ exists preferentially in the translational isomer in which the BPP34C10 crown ether resides around the bipyridinium unit, a Photochemical excitation of the Ru(bipy)3 unit results in PET to the bipyridinium site, and consequent translation of the crown ether to the 3,3dimethylbipyridinium unit, which is a less efficient recognition site for the cyclophane CBPQT4+ than a bipyridinium system. This process occurs only in the presence of a sacrificial reductant which reduces the Ru(III) center back to its Ru(II) state in order to prevent charge recombination, b Conversely, upon electrochemical reduction of the bipyridinium unit, the crown ether takes up residency around the 3,3 -dimethylbipyridi-nium site. This process is reversed through electrochemical oxidation of the bipyridinium radical cation back to the dication...

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