Mechanisms behind chemical reactions unraveled using high pressure

Chemists use pressure to reveal the mechanisms behind electron transfer – an important step towards controlling energy conversion and redox catalysis.

They form the basis of many fundamental processes in life. Without them, cell respiration and photosynthesis would be impossible. Redox reactions also have a decisive role to play in applications in chemistry, biochemistry or in gaining energy from light. Gaining an understanding of their basic principles is therefore important to advance new technologies. Researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and the University of Munich have now succeeded for the first time in differentiating between two connected reaction mechanisms using an innovative method. Their findings have been published in the journal Nature Chemistry 2025.*

Balance between electrons and protons

During redox reactions, electrons are transferred between molecules. As electrons have a negative charge, this can lead to a change in the charge of their reaction partner, which is undesirable from an energy point of view. Nature has found an elegant solution to sidestep this: the transfer of electrons is often coupled with the transfer of positively charged protons. During this proton-coupled electron transfer (PECT) there is no change in charge, thereby triggering a redox reaction in the most effective way.

There are two different possible mechanisms: either electrons and protons are transferred at once, in a concerted way, or the transfer is made in stages, split according to electrons and protons. In order to optimize these processes, it is important to know which of the mechanisms is currently involved. “Until now, there has been no direct method of differentiating without doubt between the two methods. That is where our work comes in,” explains Prof. Dr. Dirk M. Guldi from the Chair of Physical Chemistry I, who also conducts research into energy transition at Profile Center FAU Solar.

Pressure as the key

For their study, the researchers investigated the influence of pressure on the light-induced reaction of a photosensitive molecule in a solution, lasting just nanoseconds. It was already known that this molecule transfers protons and electrons to the relevant acceptor molecules, but the exact mechanism behind these processes was unclear. The results indicate that measuring the effect of pressure on reaction speed allows direct conclusions to be drawn considering the mechanisms taking place.

If high pressure – in the experiment up to 1,200 times the pressure of Earth’s atmosphere – is exerted and the reaction speed remains unaltered, then it is a concerted reaction. If electrons and protons are transferred at the same time, neither the charge nor the connected sequence of the molecules in the solvents surrounding the molecules change. “Accordingly, pressure has no influence on the speed of the reaction – a clear indication of a concerted mechanism,” explains Prof. Dr. Ivana Ivanović-Burmazović from LMU. If the speed of the reaction changes, this indicates a change in charge and a change in the sequences of the solvent’s molecules around the molecules – an indication that the process is taking place step by step. To their surprise, researchers were not only able to determine the type of the mechanism, they were also able to influence the process. By increasing pressure, the researchers were able to control the reaction of a step by step mechanism and steer it towards a concerted mechanism.

The new findings are of major significance for numerous research areas concerning the movement of electrons and protons, underlines Daniel Langford from the Chair of Physical Chemistry I. In his opinion, they not only offer new insights into fundamental chemical processes, but may also make a contribution towards advancing new technologies connected to the transformation and storing of chemical energy, for example creating solar fuels or producing hydrogen.

DOI: 10.1038/s41557-025-01772-5

Further information:

Prof. Dr. Dirk M. Guldi
Chair of Physical Chemistry I
dirk.guldi@fau.de