Dumbbell-shaped Component Research Articles

Template-Directed Syntheses of Rotaxanes

Rotaxanes are molecules comprised of a dumbbell-shaped component encircled by one or more macrocyclic components. The early syntheses of rotaxanes were mainly based upon statistical threading or upon directed methodologies involving chemical conversion. However, with the advent of supramolecular chemistry, a series of host-guest and template-directed approaches to rotaxanes have been developed and employed successfully. We have devised a template-directed approach to rotaxanes incorporating π-electron deficient bipyridinium-based components and π-electron rich hydroquinone-based polyether components. The noncovalent bonding interactions responsible for the self-assembly of these molecular compounds are (i) π-π stacking interactions between the complementary aromatic units, (ii) hydrogen bonding interactions between the acidic hydrogen atoms in the α-positions with respect to the nitrogen atoms on the bipyridinium units and some of the polyether oxygen atoms, as well as, (iii) edge-to-face T-type interactions between some of the hydrogen atoms on the hydroquinone rings and the π-clouds of the aromatic spacers separating the bipyridinium units. By employing these methodologies, we have synthesised a range of [2]rotaxanes, [3]rotaxanes, and [4]rotaxanes. Furthermore, the dynamic processes involving the shuttling of the cyclic components along the dumbbell-shaped components associated with some of these rotaxanes have been investigated in some detail. The reversible control of the process via external stimuli - such as chemical and electrochemical - has been achieved in the case of a [2]rotaxane incorporating benzidine and biphenol recognition sites. These results suggest the possibility of generating, on the nanoscopic level, molecular devices in the shape of rotaxanes able to store and process informations, thus, affording molecular machines.

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 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 sizesi.e. overall lengths and molecular volumes which range from 3 to 6 nm and from 4 to 6 nm3, respectivelyof 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.

A new design strategy for the self-assembly of molecular shuttles

A strategy for the construction of a [2]rotaxane—comprised of a dumbbell-shaped component containing two π-electron-deficient 4,4′-bipyridinium units encircled by one π-electron-rich bisparaphenylene-34-crown-10 macrocycle—by a constitutionally determined self-assembly process is described.