Watching what happens in the first picosecond after a proton detaches from a dye after being exposed to light

A proton can be released locally by excitation with light in certain molecules, the so-called photoacids. The pH of the solution abruptly changes—a quick switch that is critical for numerous chemical and biological processes. What transpires at the time of proton release, however, has remained a mystery up until this point. In a recent experiment, researchers from the Cluster of Excellence Ruhr Explores Solvation RESOLV at Ruhr University Bochum in Germany were able to see precisely this.

In order to start the proton’s detachment, they saw a beating between the solute and solvent cause a miniscule earthquake that lasted three to five picoseconds. The journal Chemical Sciences publishes a report on this.

The emphasis has been on dye or base thus far.
The fluorescent dye pyranine, which is used, for example, in yellow highlighters, is one of the so-called photoacids that has received the most research attention. According to Professor Martina Havenith, a RESOLV spokesperson, “despite a wealth of experimental studies, the process that is at the very beginning of proton detachment still remains the subject of controversial debate.”. But the entire detachment procedure also takes place in just 90 picoseconds of time. A millionth of a millionth of a second is called a picosecond.

“Despite a variety of experimental research, the process that is at the very beginning of proton detachment has remained the subject of heated controversy,”

Professor Martina Havenith, spokesperson for RESOLV. 

The team was able to observe the change of the solvent, in this case water, during this process for the first time, whereas earlier studies had mainly focused on the change of the dye after light excitation. An innovative method known as “optical pump THz probe spectroscopy” was used to accomplish this.

Rapid energy transmission.
“We were able to follow how there is an oscillation at the beginning, which then subsides later,” says Martina Havenith. “It is fascinating to see that the excited-state proton transfer-promoting solvent response can be observed in action.” The initial ultrafast transfer of energy in less than a picosecond leads to a restructuring of the nearby water molecules and prepares the way for the ensuing proton migration.

The new laser labs in the ZEMOS research building allowed for the detection. All external interference signals, including those brought on by EM radiation, temperature changes, and humidity variations, are reduced there. The dye can then be used to detect even the slightest tremors in a water jet.

More information: Claudius Hoberg et al, Caught in the act: real-time observation of the solvent response that promotes excited-state proton transfer in pyranine, Chemical Science (2023). DOI: 10.1039/D2SC07126F

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