Charge into Water Triggered by a Quick Light Pulse

May 21, 2023

 laser laboratory in the ZEMOS research building at Ruhr University Bochum

This study was made possible by the use of a new laser laboratory in the ZEMOS research building at Ruhr University Bochum (RUB), where all external interference signals are minimized. Image Credit: RUB, Marquard

New research from a consortium of Ruhr-Universität Bochum and UC Berkeley scientists has found that a proton can be released locally in some molecules, so-called photoacids, by being excited by light. A swift change occurs in the pH value of the solution, which is significant for many chemical and biological processes.

However, it had not yet been determined exactly what takes place when proton emission occurs. Researchers at the Ruhr University Bochum’s Cluster of Excellence: Ruhr Explores Solvation (RESOLV)  have now been able to record this event in an experiment with new technology. So far, the focus has been on dye or base pyranine, a fluorescent dye used, for instance, in yellow highlighters, which is one of the most researched photoacids.

The researchers noticed that a beating between the solute and solvent causes a brief quake that lasts for three to five picoseconds just before the proton begins to separate. The findings were published in the March 16th, 2023, issue of the journal Chemical Sciences. https://doi:10.1039/D2SC07126F

UC Berkeley Professor of Chemistry Martin Head-Gordon commented on the findings, “The interesting science in this project concerns understanding how photoacidity is initiated by light, leading to a transfer of excess energy from chromophore to solvent. To interpret the new optical pump, terahertz probe experiments were performed by our collaborators, led by Professor of Physical Chemistry Martina Havenith. Two outstanding postdocs in my group, Drs. Justin Talbot and James Shee, worked with me to perform dynamical simulations, quantum chemistry calculations, and develop a simple insightful model to aid the experiments.

Prof. Havenith Chemistry noted, "Despite a wealth of experimental studies, the process that is at the very beginning of proton detachment still remained the subject of controversial debate. Only 90 picoseconds, however, are needed for the complete detachment procedure to take place. (A picosecond is one-millionth of a millionth of a second.)”

The research team in Germany was able to witness the change of the solvent, in this instance water, during this process for the first time, whereas earlier studies had primarily concentrated on the change of the dye after light excitation. A recently created method called “Optical Pump THz Probe Spectroscopy” was used to accomplish this.

Ultra-Fast Energy Transmission

Prof. Havenith added, “We were able to follow how there is an oscillation at the beginning, which then subsides subsequently. It is exciting to see that the solvent response that promotes excited-state proton transfer could be caught in the act.”

The water molecules in the region are restructured because of the initial rapid energy transfer that occurs in less than a picosecond, which prepares the region for the subsequent proton migration.

Prof. Head-Gordon notes, “Amongst the most interesting conclusions of this study is that onset of protonation is triggered by rapid solvent reorganization driven by efficient sub-picosecond population of a solvent mode that makes and breaks the intermolecular hydrogen bond network.”

The new laser labs in the ZEMOS research building were integral in enabling the discovery. All external interference signals, such as those caused by electromagnetic radiation, temperature changes, and humidity variations, are minimized. Only then can the dye identify even the slightest vibrations in a water jet.

This research was supported by the German Research Foundation (DFG) through the Cluster of Excellence Ruhr Explores Solvation, grant number EXC 2033-390677874-RESOLV, the European Research Council through ERC Advanced Grant 695437 (THz Calorimetry), and the Mercator Research Center Ruhr through a professorship from the Universitätsallianz Ruhr.

The accompanying simulations were developed by Prof. Head-Gordon’s research group at the University of California, Berkeley. Their work was supported by CALSOLV, RESOLV’s sister organization

Journal Reference

Hoberg, C., et al. (2023) Caught in the act: real-time observation of the solvent response that promotes excited-state proton transfer in pyranine". Chemical Science.

ABOUT the RESOLV and CALSOLV partnership

This work is part of continuing partnership between the RESOLV Excellence Cluster led by Professor Havenith, and the CALSOLV group at UC Berkeley led by UC Berkeley Professor of Chemistry Teresa Head-Gordon.

CalSolv was established in 2014 as the intellectual home of Solvation Science at UC Berkeley. The goal of the research consortium is to study the fundamentals of solvation for a large variety of chemical systems using a combination of theory, spectroscopy, and surface science techniques. The CalSolv center involves the departments of Chemistry, Chemical and Biomolecular Engineering, Bioengineering, Materials Science and Engineering, and Environmental Sciences with collaboration through our sister center RESOLV (Ruhr Explores Solvation), a German Center of Excellence.

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