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.