Benzidine station

The basic character of benzidine affords an alternate mechanism to influence the average position of the bead. Protonation of the amine functional groups by simple addition of trifluoroacetic acid (TFA) to the solution also generates positive charges on the benzidine unit and forces the bead to encircle the biphenol station. Neutralization with base (pyridine) returns the system to its initial state by removing the positive charges on the benzidine station (Scheme 6). These phenomena were verified in careful JH-NMR spectroscopic experiments [7], Thus, chemical (proton transfer) reactions are also useful to control the sliding motion of the macrocyclic bead in this rotaxane. 

Fig. 2. Potential energy profiles for bead sliding motions in the rotaxane (A) before and (B) after charge generation on the benzidine station...

The [2]rotaxane 12.4PF6, shown in Scheme tf, was self-assembled in 19% yield from the appropriate dumbbell-shaped compound, BBB, and [BBIPYXY][PF6]2-Analysis of the rotaxane showed that the anticipated control by external stimuli was possible. The position of the tetracationic cyclophane can be switched from the benzidine station (which corresponds to the major translational isomer with 84%... 

XH NMR was as fast as 300000 times a second. It is interesting if the shuttling speed can be controlled by light or an electrochemical method [96]. Bissell et al. [97] obtained the molecular shuttle shown in Fig. 29, in which benzidine and bisphenol units act as the stations. At 229 K the tetracation bead was found to stay on the benzidine side at a probability of 86%, but when the compound was treated with an acid or oxidized electrochemically it turned out that the bead can move to the bisphenol side at a higher probability. 

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... 

Using the bead 14, a rotaxane has been synthesized which contains two 7r-electron-rich moieties, namely biphenol and benzidine [76]. The thread portion 23 of this rotaxane therefore contains two stations upon which the tetracationic bead can potentially reside. For such a device, stability of the bead at its preferred station must be considered. This implies that a large energy difference [>kT) between stations must exist at the operating temperature to prevent dynamic shuttling of the bead between the stations. 

Electrochemical oxidation of the benzidine guest in 24+ creates a positive charge that forces the tetracationic bead to move over to the biphenol station (Scheme 5). This was clearly verified by the half-wave potential values obtained in cyclic voltammetric experiments with this rotaxane [7]. The reversible character of the one-electron oxidation of the benzidine unit in 24+ provides a useful electrochemical mechanism to control the sliding motion of this rotaxane s bead. Although we did not verify this point experimentally, it should be equally possible to oxidize the benzidine unit using homogeneous... 

From the standpoint of switchable properties it would be desirable to build a rotaxane capable of more sharply defined on-off conversions. Before switching, rotaxane 24+ presents two translational conformers, characterized by either benzidine or biphenol occupation by the bead. Although benzidine occupation is heavily favored (84% vs. 16%), the fraction of systems that show biphenol occupation are conceptually equivalent to a leak in the switching device. The selectivity of the bead for one of the two donor stations can be accomplished by one of two approaches ... 

In collaboration with Stoddart, the authors group reported in 1994 arotaxane in which the relative position of the wheel could be controlled by chemical and electrochemical methods.This rotaxane contains two n-donor stations in the axle benzidine and biphenol. The macro-cyclic wheel fcyclophane interacts preferentially... 

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