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Chemical Bonding

Electrochemistry of single molecules under the microscope

AFM explores relationships between charge, bonding, and structure

by Mark Peplow, special to C&EN
July 12, 2019 | APPEARED IN VOLUME 97, ISSUE 28

 

09728-scicon7-afmcxd.jpg
Credit: IBM Research
Atomic force microscopy shows how structure and bonding change in four molecules—azobenzene, pentacene, tetracyanoquinodimethane, and porphine (top to bottom)—as they transition between the charge states +1 (cyan), neutral (green), –1 (orange), and –2 (red).
09728-scicon7-porphinevid.jpg
Credit: IBM Research
The pattern of bonding conjugation in porphine changes as the neutral molecule acquires two electrons and shifts it from aromatic to antiaromatic.

An atomic force microscope (AFM) has revealed how the structure and bonding patterns of organic molecules change as they take on electrical charges (Science 2019, DOI: 10.1126/science.aax5895). The technique could be used to study photosynthesis, organic photovoltaic devices, and molecular electronics. “How structure changes with charge is at the very heart of these processes; it’s fundamental,” says Leo Gross of IBM Research–Zurich. The researchers used an AFM to study four molecules adsorbed on an insulating sodium chloride film. By changing the voltage between probe and sample, the AFM could remove or add electrons from single molecules and then map the positions of the atoms. Adding an electron to azobenzene, for example, disrupted its conjugated system and caused its phenyl rings to twist out of their usual coplanar arrangement. In the diradical dianion pentacene, AFM showed that carbon atoms in the second and fourth rings hosted the unpaired electrons. And for porphine, the parent compound of porphyrins such as chlorophyll and heme, the technique tracked how bonding conjugation changed between the neutral, aromatic system and its antiaromatic dianion. The team hopes to use this method to perform single-molecule synthesis, moving atoms and charging them to make or break individual chemical bonds.

Credit: IBM Research
The pattern of bonding conjugation in porphine changes as the neutral molecule acquires two electrons and shifts from aromatic to anti-aromatic.
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