Self-Assembly of [<i>n</i>]Rotaxanes Bearing Dendritic Stoppers

We have developed a synthetic approach to the self-assembly of [2]-, [3]-, and [4]rotaxanes, incorporating bis-p-phenylene-34-crown-10 as the ring component(s) surrounding bipyridinium-based dumbbell-shaped components bearing dendritic stoppers at both ends. As a result of the hydrophobic dendritic framework, these [n]rotaxanes are soluble in a wide range of organic solvents, despite the polycationic natures of their bipyridinium-based backbones. In all cases, they could be purified by means of column chromatography employing relatively low polar eluants. The molecular shuttling action of the [2]rotaxane containing two bipyridinium units on the rod portion of the dumbbell shaped component has been investigated by variable-temperature 1H-NMR spectroscopy in a range of solvents [CDCl3, CD2Cl2, THF-d8, and (CD3)2CO] for the first time. This investigation reveals a marked dependence of the rate of the shuttling process upon the polarity of the media. On going from CDCl3 to (CD3)2CO, the rate constant increases from ca. 200 to 33000 times per second. Molecular dynamics simulations, performed in CHCl3 and Me2CO on the [2]rotaxane, suggest that significant conformational changes occur upon changing the polarity of the medium resulting in both steric and electronic hindrance of the shuttling process in CHCl3. Three-dimensional representations, as well as the approximate sizes - i.e. overall lengths and molecular volumes which range from 3 to 6 nm and from 4 to 6 nm3, respectively - of these molecular compounds, were obtained by means of molecular modeling studies. Thus, these nanometer-scale dendritic rotaxanes resemble naturally-occurring chemical systems incorporating an active component, in so far as the rotaxane-like core with its distinctive recognition features is surrounded by a molecular shell in the form of the dendritic framework.