Credit: © 2007 ACS

Since the discovery of buckminsterfullerene (C60), chemists have been designing and making large organic molecules that possess hollow cavities capable of binding this small carbon compound. This enables C60 to be dissolved in solvents in which it is not usually possible, offering a greater range of conditions under which its properties can be studied.

Now, a team of researchers in Japan at the University of Tokyo and the Rigaku Corporation led by Takuzo Aida and Kentaro Tashiro have developed a molecular box that not only encircles C60, but also forms transient chemical bonds to it1. The box has flat porphyrin molecules — each with an iridium metal atom at its centre — at both the top and the bottom that are linked in a face-to-face arrangement by two hydrocarbon chains. When a C60 molecule is sandwiched inside the box, its carbon–carbon double bonds interact strongly with the iridium atoms, causing the fullerene to lengthen along the Ir–C60–Ir axis. It was found, however, that the C60 could still rotate inside the box, but much more slowly than in the case of a control compound with no iridium present.

This indicates that the bonds formed between the iridium atoms and C60 are reversible, but an overall braking effect is observed in the rotation of the fullerene. It is suggested that this work may serve as a model to study the concept of 'chemical friction' on graphitic surfaces, with obvious implications for other carbon nanostructures, such as nanotubes and graphene.