Fullerenes are sphere-like bodies composed of nothing but tens of carbon atoms. Most famous member of this family is the football-shaped C60 fullerene. Since their discovery in the 80s, these unique molecules have fascinated researchers, and the Nobel Prize Committee, for their properties and applications. One property of fullerenes is to be electron acceptors, meaning they are easily reduced to a radical anion. Thanks to this, and other capabilities, fullerenes find applications in solar cells, OLEDs, molecular electronics and even medicine. However, fullerene radical anions usually live less than a second in solution. Besides, most fullerenes show bad solubility in many common liquids and require toxic aromatic solvents. Now, RESOLV scientists from TU University Dortmund have been able to stabilize the fullerene radical inside a protective nano-cage, dissolve it into acetonitrile solution and achieve the longest lifetime so far. The research has been published in the online edition of the Journal of the American Chemical Society.
To obtain the protective cages, the group of Guido Clever at TU University Dortmund mixed organic ligands with Palladium cations in acetonitrile solution. The components self-assemble into cages resembling Chinese lanterns that have the perfect shape and electronic configuration to host the C60 fullerene, once this is dispersed in solution as powder. The scientists were then able to photo-chemically reduce the fullerene into its radical anion. By using nuclear magnetic resonance, electron paramagnetic resonance (EPR, in collaboration with RESOLV scientist Müge Kasanmascheff), ultraviolet−visible−near-infrared spectroscopy and mass spectrometry, the scientists could confirm generation and encapsulation of the fullerene radical anion in the cage, as well as determine its lifetime.
3 Questions to the author Guido Clever:
What is the new discovery that you made - and why is it exciting?
In a previous publication, we already demonstrated that it is possible to encapsulate fullerenes in a self-assembled nano-cage and to dissolve it into a wider range of solvents. Here we use a very similar protective shell to go one step further. After encapsulating C60 in the cage in acetonitrile, we could selectively and quantitatively generate its anion. Our method not only allows to facilitate the reduction of the encapsulated fullerene, but also protects the formed radical anion from fast decay (e.g. by reaction with oxygen) so that its lifetime could now be increased to unprecedented lengths. In presence of oxygen, we could reach a lifetime of ca. 13 minutes in acetonitrile solution. But in de-aerated solution, using EPR spectroscopy, the group of Müge Kasanmascheff was able to detect remaining C60-radicals even 4 weeks after!
Why are your findings important?
Studying the one-electron-reduced form of a fullerene such as C60 is of high interest to better understand processes where this species is formed: For example, as consequence of light-powered charge separation in a solar cell or by accepting an electron from a neighboring functionality in an electronic circuit. Moreover, the ability to stabilize the fullerene radicals in solution opens perspectives for utilizing them as, to name a few, reagents in chemical reactions, novel EPR spin labels, or near IR emitters in a broad range of applications.
Is this related to Solvation Science? If yes, how?
The connection to Solvation Science is actually on two levels. First, being able to solubilize fullerenes in a selection of polar organic solvents opens up new possibilities to examine their chemistry, combine them with a wider range of reaction partners, and find alternative and better methods for their solution-processing, e.g. in surface modification and device fabrication. Second, by wrapping a protective shell around the fullerene, we have shielded the radical from the close encounter with the deactivating species, being it the solvent itself or dissolved oxidants (e.g. oxygen). At the same time, we keep the compound fully dissolved.
Original Publication: S. Hasegawa, S. L. Meichsner, J. J. Holstein, A. Baksi, M. Kasanmascheff, G. H. Clever: "Long-Lived C60 Radical Anion Stabilized inside an Electron-Deficient Coordination Cage", J. Am. Chem. Soc. 2021, 143, 9718, DOI: 10.1021/jacs.1c02860.