Rotaxanes cyclophane shuttling

Figure 12. Shuttling of a ring between two equivalent stations in a [2]rotaxane. (a) The bipyr-idinium-based cyclophane shuttles from one 1,4-dioxybenzene recognition site to the other at a rate of c. 2000 s in (CD3)2CO at ambient temperature [39a,bj. (b) The 1,4-dioxybenzene-based macrocyclic polyether shuttles from one bipyridinium recognition site to the other one at a rate of c. 3 X 10 s under the same conditions [39c. ...

The first example of electrochemically driven molecular shuttles is rotaxane 284+ (Fig. 13.25) constituted by the electron-deficient cyclophane 124+ and a dumbbellshaped component containing two different electron donors, namely, a benzidine and a biphenol moieties, that represent two possible stations for the cyclophane.10 Because benzidine is a better recognition site for 124+ than biphenol, the prevalent isomer is that having the former unit inside the cyclophane. The rotaxane... 

Figure 13.26 Structure formula of rotaxane 294+ and the electrochemically induced shuttling of the cyclophane along the dumbbell-shaped component (CH3CN, 298 K).

After this first report, a remarkable number of electrochemically controllable molecular shuttles have been designed, constructed, and studied. Rotaxane 294+ (Fig. 13.26), for instance, incorporates the electron-deficient cyclophane 124+ and a dumbbell containing two kinds of electron-rich units, namely, one 2,6-dioxyanthra-cene and two 1,4-dioxybenzene moieties.34 In solution, the rotaxane is present as the isomer with the 2,6-dioxyanthracene unit inside the cyclophane, owing to the fact that this unit is a better station in comparison to the 1,4-dioxybenzene recognition sites. 

Fig. 31 Mechanical actuation of a gold-coated microcantilever by molecular muscles [227]. (a) Structural formula of a palindromic, bistable [3]rotaxane with gold-binding dithiolane groups attached to the cyclophanes. (b) Reversible bending up and down of a cantilever by actuation of a monolayer ( 8 billion molecules) of the rotaxanes on its surface. The gold surface bends when the rotaxanes contract under the influence of an electrochemical oxidation that causes the cyclophanes to shuttle inward from the periphery of the molecule, (c) Electrochemical cell (Ag/AgCl, Pt, and the cantilever are the reference, counter, and working electrodes, respectively) and combined AFM device used to measure the bending by detecting a laser beam reflected off of the cantilever s surface...

Rotaxanes-Threading Molecular Rings Rotaxanes are obtained by threading linear polymers through molecular rings such as cyclodextrins, crown ethers and cyclophanes. Molecular shuttles based on the rotaxane structure have been proposed. 

Despite the fact that none of the three non-symmetrical [2]-rotaxanes just described fully meet the goal of having the cyclophane tetracation solely occupying the 7i-electron-donor site with the lowest oxidation potential, in two cases this isomer is the preferred one - provided that an appropriate choice of solvent is used. The results of these studies are still of considerable value in a general sense since they serve to delineate some of the subtleties inherent in attempting to use this approach to obtain a controllable shuttle. ... 

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]. 

Experimental and theoretical investigations on a related [2] rotaxane, which we call a molecular shuttle [29,30], have revealed that the tetracationic cyclophane can shuttle rapidly between two degenerate hydroquinone rings located in the dumbbell component. This discovery paved the way for the development [31] of a [2]rotaxane 104+ in which the recognition sites are different and in which their occupancy can be controlled (Fig. 15) by... 

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...

In 1991, Stoddart et al. reported the first molecular shuttle (Figure 14) based on a [2]rotaxane comprising a cyclophane... 

Willner et al. developed a redox-active rotaxane (Figure 50) as a monolayer assembly on an Au electrode. The rotaxane comprises a CBPQT + cyclophane threaded onto a molecular string, which includes a rr-donor diiminobenzene unit stoppered by an adamantane unit. The cyclophane localizes on the diiminobenzene unit initially, and shuttling can be induced by the reduction or oxidation of the cyclophane, which can be characterized... 

Display 15 reveals that the [2]rotaxane whose preparation is described in Display 14 behaves like a molecular shuttle. Dynamic NMR spectroscopy in CD3CXXTD3 solution indicates that the tetracationic cyclophane is darting back and forth 500 times per second between the two hydroquinol rings. See reference [12]. 

Display 18 describes the preparation of a [2]rotaxane which constitutes an asymmetric molecular shuttle. Preliminary examination of die tenqierature dependent NMR spectrum of the [2]rotaxane reveals that the tetracationic cyclophane prefers to encircle the hydroquinol ring rather than the p-xylyl residue, as predicted. The molecular assembly has still to be investigated electrochemically. The hoop represents the tetracationic cyclophane, [BBIPYBIXYCY] . 

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