The birth of a subnanometric soccer ball

video: TEM observation of irradiated truxene under an electron beam.
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Credit: Institute of Basic Sciences, University of Tokyo

Ever since the existence of molecules was proven and molecular reactions predicted, humans have wanted to visually observe how such events unfold. Such observations of single-molecule reactions are very important for the fundamental understanding of chemical sciences, which would aid in the development of new catalysts, materials or drugs, and help us decipher complex biochemical processes. However, this has not been possible for very long in modern chemistry, and until now information about dynamic processes at the nanoscale has only been obtained by indirect methods because the molecules were too small. to be viewed.

Recent findings from researchers at the Center for Nanomedicine at the South Korea Institute of Basic Sciences, as well as researchers from Japan and Germany, may have been a game-changer. The group successfully observed the upward synthesis of fullerene C60, which is a football-like allotrope of carbon, and produced a video image detailing the process using single-molecule atomic-resolution real-time electron microscopy. (SMART-EM). This was made possible with the advent of aberration-corrected transmission electron microscopy (TEM) and the establishment of the resolution conditions for sub-nanometric sized objects such as single molecules and even atoms.

In their experiment, the researchers worked with a tailor-made truxene derivative (C60H30), which is shaped like a flat soccer ball as the starting material. For TEM observation, truxene was attached to a monolayer of graphene, which prevents the molecule from undergoing rapid translation across the surface or even detachment in vacuum. By isolating a single molecule on the surface, they were able to study dynamic processes without interference from other molecules. This two-dimensional flat material was then irradiated with a highly energetic electron beam of up to 80,000 V, which is hundreds of times higher than the voltage found in household electrical outlets.

What happens to the molecule if it is exposed to such a powerful electron beam? If the molecule follows the rules of classic organic chemistry textbooks, the extreme condition would force the truxene to lose its hydrogens through a process known as cyclodehydrogenation, which causes the remaining carbon atoms in the molecule to fold back into a spherical shape ( Figure 1) . But if high-energy pathways dominate, it would result in unpredictable decomposition, until the molecule is completely atomized.

By largely correlating the actual TEM images with those of the simulated models (Figure 2), the researchers discovered that the truxene molecule initially undergoes a kinetically and thermodynamically controlled cyclodehydrogenation reaction. TEM observations revealed that the reaction pathway occurs via key intermediates thermodynamically promoted through apparently classical organic reaction mechanisms, which have been identified and captured on video. Thus, they showed that the electron beam transfers kinetic energy to nuclei and excites the vibrational states of the molecule, which gives the molecule enough energy to undergo chemical reactions. It is important to note that the cross section (probability) of the conventional chemical pathway is greater than that of the destructive cleavage of the CH bond.

These results describe for the first time the real-space, real-time analysis of a discrete molecule-to-molecule transformation, captured on video. This real-space observation of a discrete chemical reaction is an important step in the chemical sciences and will lead to a better understanding of fundamental chemical processes at the nanoscale. The identification of key intermediates also revealed new information about electron beam-driven reactions. The researchers plan to explore the full scope of the SMART-EM technique by applying it to larger systems, such as analysis of liquid media. This will help advance research in fields ranging from the chemistry of nanomaterials to biomedical sciences, where understanding beam-matter interactions is of the utmost importance. The knowledge gained in these studies will also help design new strategies to synthesize nanomaterials using electron beam lithography.

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