Experimental 2 rotaxane

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

The principle of the second synthetic approach to polycatenanes, i.e. stepwise polycondensation, has been proposed by Shaffer and Tsay, but not experimentally demonstrated [42, 43], This approach has the advantage over multifunctional polycondensation that a linear polymer is formed before cyclization (Scheme 7). However, the second step, which consists of the cyclization of n macrocycles along the polymer chain 19, is likely, again, to give rise to an undefined network, containing some rotaxane and catenane units 21, similar to the multifunctional polycondensation approach. 

Also known are other types of topological isomers which draw the researchers attention, but they exist only on paper so far. Catenanes and rotaxanes are the only topological structure classes which have been investigated experimentally. 

Owing to the size of supramolecules semi-empirical methods are still in use. For instance, semi-empirical PM3 calculations were carried out for metalacryp-tands and metalacryptates and with lead as metal the theoretically predicted favorable cryptand formation was subsequently verified experimentally [123]. In another example, the rate of shuttling motions in rotaxanes was examined with semiempirical AMI calculation by Ghosh et al. [124] who generated different structures with a Monte Carlo procedure and subsequently optimized low-energy conformers. 

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... 

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. 

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... 

I" ", were interpreted as confirmation of the superexchange mechanism for reaction (1) [54]. This was further supported by the results obtained with 12", the free rotaxane, where photophysical properties very similar to I were evidenced, Scheme 1. The experimental kinetic data collected for these structures (Table 1) allowed us to derive a good correlation between the rates of electron transfer and the reciprocal of the energy gap between the relevant orbitals on the porphyrin excited state and on the phenanthroline ligand, confirming a superexchange mechanism for electron transfer, for reactions (1),... 

In the same year as reporting the rotaxane, the redox-driven circumrotations of a bistable catenate Cat-1+ (Figure 15) were reported. The philosophy behind the experimental designs that were required to identify that motion had occurred (see square scheme in Figure 15a) were largely the same as for Rot-l +. However, the motions were so slow that different experiments were ultimately conducted. 

Typical data from one of our molecular switches is shown in Figure 4 (we have incorporated additional data from various control molecules in this figure). For the structures shown in Figure 2, the controls include [2]catenanes with identical recognition sites (i.e., two DN groups), the dumbbell component of the [2]rotaxane structure, the TCP ring, and others. One always does control experiments, of course, but controls are critical here, because these devices are difficult to characterize fully, and one of the few experimental variables is mo-... 

Pseudorotaxanes complexes can be rendered kinetically inert, i.e. transformed into rotaxanes, by attaching bulky groups at the extremities of the axle to prevent dethreading [1]. Therefore, the pseudorotaxane or rotaxane behavior of a given axle-macrocycle pair depends on the threading-dethreading rate constants, which in turn are determined by the intrinsic kinetic parameters of such processes— particularly, the energy barriers—and the experimental conditions [2, 3],... 

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