Microscope Designed To Study Molecular Oxygen in Fine Detail

Why does your favorite T-shirt fade over time in the sun? Why do you get sunburn and why does autumn herald with brown leaves? These questions all have one thing in common: the interplay between dye pigments and the oxygen in the air. Every child at school gets to know this chemical reaction of “oxidation” in the air we breathe. What else is there to research? For example, the fundamentals for a microscopic understanding of the oxidation reaction, which researchers at the University of Regensburg want to get to the bottom of.

Oxygen is an amazing molecule, it's magnetic. In liquid form, it can be lifted up with a magnet like iron filings. This property has something to do with the electrons in oxygen. All molecules consist of atomic nuclei and electrons, which in turn behave like tiny compass needles. Usually the compass needles of the electrons are opposite in pairs so that their magnetic forces cancel each other out. But in the oxygen molecule, which consists of two oxygen atoms, both compass needles point in the same direction and the oxygen acts magnetically.

Dye molecules, on the other hand, are not magnetic because the compass needles of the electrons point in opposite directions. If light falls on such a molecule, it is absorbed at a certain color, which gives the dye its characteristic appearance. The energy of the light is transferred to an electron in the dye molecule. This cancels the pairing of any two electrons so that the electron's compass needle can spontaneously change its orientation. It cannot go back to its original state because the needles are now pointing in the same direction. This effect is a magnetic state excited by light, the so-called “triplet”.

The international research team led by Prof. Jascha Repp has now investigated for the first time how this “triplet energy” is transferred from a single dye molecule to a single oxygen molecule. This process is of central importance in everyday life, as many oxidation reactions take place via the excited triplet state. As long as the molecule is in this state, the energy of the absorbed light remains in the molecule. This promotes chemical reactions. For a complete de-excitation of the molecule, another reversal of the orientation of the compass needle is necessary, which is unlikely and takes a long time. Alternatively, the energy can be transferred to another magnetic molecule, oxygen. Through this transfer, the dye molecule is de-energized, but the oxygen becomes reactive and can, for example, bleach the dye. This happens with the T-shirt in the sun, but also with the pigments of the skin when sunburned.

The team has now succeeded in tracking the energy transfer from dye to oxygen directly without destroying the dye. For this purpose, individual molecules were brought onto a surface at very low temperatures close to absolute zero and imaged with a so-called atomic force microscope, ie a fine needle with a single atom at its tip, which is moved over the molecule. The dye was prepared in the triplet state by means of a clever sequence of electrical pulses. The energy transfer to the oxygen can now be followed by measuring the force between the tip and the molecule over time. 

This spectacular breakthrough was published in the leading science magazine Science. The scientists hope to finally achieve the basis for a microscopic understanding of the oxidation reaction. In addition to the annoying fading of T-shirts, this interaction of triplet excitations in molecules also plays a central role in future-relevant technologies, for example in organic light-emitting diodes (OLEDs) and solar cells, in photocatalytic energy conversion and photosynthesis, and in photodynamic cancer therapy.

In terms of method, this is a new form of microscopy that resolves both temporally and spatially, in keeping with the research concept of the new Regensburg Center for Ultrafast Nanoscopy (RUN), whose research building is currently being built on the Regensburg campus.

Reference: Peng J, Sokolov S, Hernangómez-Pérez D, et al. Atomically resolved single-molecule triplet quenching. Science. 2021;373(6553):452-456. doi: 10.1126/science.abh1155

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