Flat fullerene fragments attract electrons

June 01, 2023

(Nanowerk News) Researchers at Kyoto University in Japan have gained new insights into the unique chemical properties of spherical molecules composed entirely of carbon atoms, called fullerenes. They did this by creating flat fractions of the molecule, which surprisingly preserved and even enhanced some of the key chemical properties.

The team published their findings in a journal Nature Communications (“Flattened 1D fragment of C fullerene60 which exhibits resistance to multi-electron reduction”).

“Our work can lead to new opportunities in a wide range of applications, such as semiconductors, photoelectric conversion devices, batteries and catalysts,” said group leader Aiko Fukazawa at the Institute for Integrated Cell-Material Sciences (iCeMS). Even without the symmetry and curvature of the fullerene, the designed flat fullerene fragment retaining the pentagonal substructure displays the same electron-accepting properties as the fullerene. (Image: Kyoto University)

Buckminsterfullerene (or simply ‘buckyball’) is a molecule in which 60 carbon atoms are bonded to form a spherical shape. Named for its structural similarity to the geodesic dome designed by renowned architect Buckminster Fuller, and its unique structure continues to interest scientists. Buckminsterfullerene and related spherical carbon clusters with different numbers of carbon atoms are colloquially known as fullerenes, after the Fuller family name.

One of their most attractive characteristics is the ability to accept electrons, a process known as reduction. Due to their electron-accepting character, fullerenes and their derivatives have been extensively investigated as electron-carrying materials in organic thin-film transistors and organic photovoltaics. Nevertheless, fullerenes are an anomalous class of materials compared to other conventional organic electron acceptors, due to their robustness in accepting many electrons.

Theoretical chemists have proposed three possible factors that may be behind fullerene’s electron-accepting abilities: the high symmetry of the entire molecule, its carbon atoms with bonds arranged in a pyramidal manner, and the presence of a pentagonal substructure distributed between the six-membered rings.

The Kyoto team focused on the influence of the pentagonal ring. They designed and synthesized flattened fullerene fragments, and experimentally confirmed that these molecules can accept up to the number of electrons equal to the number of five-membered rings in their structure without decomposition.

“This surprising discovery highlights the importance of pentagonal substructures for producing stable multi-electron receiving systems,” said Fukazawa.

Experiments also revealed that the fragments display increased absorbance of ultraviolet, visible, and near-infrared light compared to the more limited absorbance of the fullerene itself. This might open up new possibilities in photochemistry, such as using light to start chemical reactions or developing light sensors or solar powered systems.

The team will now explore the possibilities that their flat fullerene fragments have in a wide variety of applications related to electron transfer processes. It is unusual to obtain such high electron-accepting abilities in a molecule consisting only of carbon, circumventing the common requirement to incorporate other electron-withdrawing atoms or functional groups into a carbon-based framework. Continuing to explore the effects of combining atoms or other chemical groups, however, may yield additional control and versatility in chemical properties.

“We hope to pioneer the science and technology of what we call super electron-accepting hydrocarbons, by exploiting their high degree of freedom to explore the effects of structural modifications,” said Fukazawa.

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