# Synopsis: Watching a Molecule Relax While it Reacts

Atomic force microscopy detects the subtle structural changes that take place when a molecule is ionized.

Redox reactions—where a molecule gains or loses an electrical charge—underpin important biological and chemical processes. Studies of these reactions with atomic-scale probes have been able to spot changes in a molecule’s charge, but changes in structure, which govern how fast redox reactions occur, have been harder to detect. Philipp Scheuerer, at the University of Regensburg, Germany, and colleagues coupled atomic force microscope (AFM) measurements with a computational model to identify the minute shifts in atom positions that occur when a molecule gains a single electron.

The team deposited copper pthalocyanine (CuPc)—a cross-shaped molecule with a single copper atom at its center—onto an insulating film. The film electrically isolates the CuPc molecule, allowing it to be stable in either its neutral or charged states. Scanning an AFM tip over a neutral molecule yielded an image whose pattern of bright and dark regions revealed the structure’s topography. This “contrast” image changed once the molecule gained an electron, indicating that different parts of the molecule responded differently to the extra charge. Computer simulations helped make sense of the change, revealing that a rearrangement of charge density within the CuPc pushed the copper atom up, while the rest of the atoms relaxed downward by about 10 pm.

The researchers say the significance of the work lies not in CuPc itself—a molecule with only limited application—but as a demonstration that charge-induced structural changes can now be detected with subangstrom resolution. They hope their proof-of-principle experiments will encourage studies of other molecules, such as those found in novel semiconductors and artificial light-harvesting systems.

This research is published in Physical Review Letters.

–Marric Stephens

Marric Stephens is a freelance science writer based in Bristol, UK.

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