Self-Assembly of [n]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 1 H-NMR spectroscopy in a range of solvents [CDCl 3, CD 2 Cl 2, THF- d 8, and (CD 3 ) 2 CO] 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 CDCl 3 to (CD 3 ) 2 CO, the rate constant increases from ca. 200 to 33000 times per second. Molecular dynamics simulations, performed in CHCl 3 and Me 2 CO 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 CHCl 3 . 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 nm 3, 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.