Chemists reveal method for differentiating PCET mechanisms – a key step for steering fundamental energy conversion and redox catalysis processes.
Redox reactions form the basis of many fundamental processes of life. Without them, neither cellular respiration nor photosynthesis could take place. Redox reactions also play a crucial role in applications in the domains of chemistry, biochemistry, and the use of light for energy generation. Understanding the fundamental principles of these reactions is therefore important for driving forward new technologies. Using an innovative method based on high pressures, a team led by LMU chemist Professor Ivana Ivanović-Burmazović, member of the "e-conversion" Cluster of Excellence, and Professor Dirk Guldi from FAU Erlangen-Nürnberg has managed for the first time to differentiate two related reaction mechanisms.
Balance between electrons and protons
In redox reactions, electrons are transferred between molecules. Because electrons have a negative charge, this can cause the charge of the reactants to change, which is energetically demanding. Nature has found an elegant solution to prevent this: Often, the transfer of electrons is coupled with the transfer of positively charged protons. This proton-coupled electron transfer (PCET), as it is known, does not produce any change in charge – the most efficient way for a redox reaction to occur.
There are two possible mechanisms here: Either electrons and protons are transferred simultaneously ("concerted"), or the transfer occurs in stepwise fashion – that is to say, with electrons and protons transferred separately. "To be able to optimize these processes, we need to know the exact mechanisms," says Ivanović-Burmazović. "Before now, however, there has been no direct method for differentiating the two alternatives with certainty. Our work set out to remedy this."
Pressure yields the answer
For their study, the researchers investigated the influence of pressure on the very rapid (within nanoseconds) light-induced reaction of a photosensitive molecule in solution. It was already known that this molecule transfers both protons and electrons to corresponding acceptor molecules, but the exact course of these processes – the mechanism – was unknown. "Our results show that measuring the effect of pressure on the reaction rate allows direct inferences to be drawn about the mechanisms," explains Ivanović-Burmazović.
If high pressure – in the experiment, up to 1,200 atmospheres – is applied and the reaction rate remains unchanged, it is a concerted reaction. "When electrons and protons are transferred simultaneously, charge of reacting species does not change and neither does the associated solvation sphere – that is, the cluster of solvent molecules surrounding the molecules. Therefore, pressure has no influence on reaction rate – a clear sign of a concerted mechanism," explains Ivanović-Burmazović. If the rate changes, however, this points to changes in the charge and to a change in the volume of the solvation sphere – indicating a stepwise process.
To their surprise, the researchers were able not only to determine the type of mechanism, but also influence the process: "By increasing the pressure, we managed to steer the reaction from a stepwise mechanism toward a concerted mechanism," says Ivanović-Burmazović.
The new findings are highly significant for numerous research areas that deal with the motion of electrons and protons, emphasize the authors. They not only offer new insights into fundamental chemical processes, but could also help advance new technologies concerned with the conversion and storage of chemical energy – such as redox catalysis for the generation of solar fuels or for hydrogen production.
