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In cages, bowls, pills: How to dissolve football-shaped fullerenes

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Portrait photo of Guido Clever. © Oliver Schaper​/​TU Dortmund
Prof. Guido Clever: “We see chances to further develop the concept into tools for selective purification of larger fullerenes and their derivatives.”
Fullerenes are useful molecular structures, but they only dissolve in a few toxic solvents. Prof. Guido Clever and colleagues from Nagasaki University, Japan, have found new options to solve this problem.

Fullerenes are spherical molecular structures made of a single carbon layer. The most famous fullerene is the C60, a football-shaped object of 60 carbon atoms. Despite these molecules being of technological relevance, for example in organic solar cells and medical research, chemists find it difficult to tinker with fullerenes, because they dissolve only in a few toxic solvents.

Now, solvation scientist Guido Clever from TU Dortmund University and colleagues from Nagasaki University, Japan, describe supramolecular cage- and bowl-like structures that can host fullerenes and dissolve them in many more solvents. Their results have been published on 8 May 2019 in the online edition of the Journal of the American Chemical Society.

“In this paper we show how to solubilize notoriously insoluble fullerenes in a number of polar organic solvents which then allows chemical modification of the fullerenes”, says Clever. To demonstrate the new approach towards the solvation of fullerenes, the Clever Lab synthesized a short organic building block (“ligand”) and a longer one, and mixed them in solution with Palladium cations. The short ligand, based on pyridine arms, interacts with Palladium to form a symmetric cage structure, which selectively takes one C60 from its solid form into the solution phase. The long ligand, based on quinolines, forms instead a bowl-like compound with a large opening that binds C60 but also the larger C70. Two bowls can also be combined into a pill-shaped dimer to host a pair of fullerenes in close contact.

The findings were confirmed by NMR and UV-Visible spectroscopy, mass spectrometry and X-ray single crystal experiments. Interestingly, Clever was able to make the bowl-confined C60 to react in acetonitrile with one molecule of anthracene in a Diels-Alder reaction. The reaction took place only on the solvent-exposed area of the football-like fullerene and was highly selective, generating only a limited amount of undesired side products.

1. What is the most exciting result about this research?

The modular concept! We could turn new organic ligands, with small footprint, into either a cage or an open bowl with differential fullerene binding abilities.

2. Why are your findings  important?

We see chances to further develop the concept into tools for selective purification of larger fullerenes and their derivatives. We could apply it in selective chemical transformations and the development of fullerene-based devices, e.g. in the field of organic electronics and photovoltaics.

3. Which solvents can you use with your system? 

So far our system allows solubilization of fullerenes in acetonitrile, acetone, nitromethane and DMF. While some of these solvents are still rather unpleasant in terms of toxicity and environmental aspects, we see good chances to extend solvent scope to alcohols and even water.

Original Publication: B. Chen, J. J. Holstein, S. Horiuchi, W.G. Hiller and G.H. Clever: Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls and Heteroleptic Pills Binding One or Two Fullerenes, in J. Am. Chem. Soc., 2019, DOI: 10.1021/jacs.9b02207 

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