| Dendrimers involving mechanically interlocked components have been constructed. In one case, a kinetically driven template-directed synthetic approach was followed to prepare a core [3]rotaxane, which was subsequently functionalized with dendritic wedges. In another case, one type of dendritic wedge was functionalized with dibenzo[24]crown-8 (DB24C8) moieties, while the other one carried a complementary secondary dialkylammonium site, terminated with a cyclohexyl slippage stopper. Under thermodynamic control, mechanically interlocked dendrimers were assembled from these dendrons using a slippage protocol. These dendrimers were isolated by chromatographic methods and proved to be stable rotaxane-like entities under certain conditions. On changing the solvent polarity or temperature, the same species exhibited dissociation characteristic of pseudorotaxanes. An acid-base switchable [2]rotaxane, based on a DB24C8 ring component, which possesses amphiphilic character necessary for the fabrication of molecular electronic devices, has been prepared. The solution-phase switching behavior of this [2]rotaxane was investigated by 1H NMR spectroscopy. The experiments revealed an efficient all-or-nothing molecular switch. Another bistable [2]rotaxane was prepared involving a different pair of recognition sites complementary to DB24C8, where the difference between the affinities of the two sites for the macrocycle was minimal. As a result, this rotaxane exhibited switching properties under some conditions, and shuttling under others, revealing factors that maximize both sets of behavior. Finally, shuttling was investigated in a degenerate two-station [2]rotaxane involving recognition between bis-para-phenylene[34]crown-10 and a bipyridinium dication. Dynamic 1H NMR spectroscopy has shown that an increase in the steric bulk of the spacer group, separating the two degenerate sites, leads to an increase in the coalescence temperature, commensurate with an increase in the free energy barriers to shuttling. |
