ACS Nano http://doi.org/b4m5 (2017)
An open challenge in chemistry is the ability to follow a reaction at a single-molecule level. Techniques such as non-contact atomic force microscopy and aberration-corrected transmission electron microscopy (TEM) have been used, but neither have both the required temporal and spatial resolution for the task. Now, Chamberlain et al. use a particular TEM setup that allows them to track chemical reactions at a single-molecule level in real time and capture intermediate steps of the transformation.
The main advance of this setup is the fact that an electron beam is used both to image and trigger a chemical reaction. The intensity of the beam can also be tuned to adjust the reaction rate so that intermediate species can be captured.
In one example, the researchers trap perchlorocoronene molecules inside a single-walled carbon nanotube. The TEM image shows the molecules orderly stacked inside the nanotube. They then irradiate the sample with an 80 keV electron beam. The energy is transferred ballistically to the coronene molecule, causing the expulsion of a chlorine atom. The absence of a chlorine atom is seen in the TEM as a bright spot, instead of a distinctive dark spot typical of high-atomic-number atoms. The dechlorinated coronene then undergoes a Diels–Alder condensation reaction with a neighbouring molecule through the formation of an aryne intermediate. This step is seen in the TEM images as a loss of the stacking order of the coronene molecules. The adduct finally rearranges to yield an elongated molecule. As the irradiation continues, the researchers observe polycondensation and the formation of nanoribbons inside the nanotube.