Researchers have captured the rapid motions of solvent molecules that impact light-driven electron transfer in a molecular complex for the first time.Greg Stewart/SLAC National Accelerator Laboratory
Light-absorbing molecules can transform photons into electricity or fuels by shuttling electrons from one atom to another. In many cases the molecules are surrounded by a solvent - such as water, in the case of the electron shuttling that underlies photosynthesis - and studies have shown that the solvent plays an important role in these electron transfers. But measuring the motions of solvent molecules to find out how they influence the process has been difficult.
In a study published Feb. 15 in Nature Chemistry, a research team led by Munira Khalil, professor and chair of chemistry at the University of Washington, has captured the rapid motions of solvent molecules that impact light-driven electron transfer in a molecular complex for the first time. This information could help researchers learn how to control energy flow in molecules, potentially leading to more efficient clean energy sources.
"This is the first time that we've been able to experimentally capture a specific motion of a solvent that's in this sort of lockstep with what's happening inside the molecular complex," said Khalil, a co-senior author on the paper.
The team, which includes scientists at the SLAC National Accelerator Laboratory and the Pacific Northwest National Laboratory, which are both U.S. Department of Energy facilities, overcame this obstacle using a combination of X-ray techniques and simulations.
"It's a long-standing challenge in chemistry to understand, at a microscopic level, the crucial role solvents play in chemical reactions," said lead author Elisa Biasin, a research associate at the SLAC-based Stanford PULSE Institute. "Until recently we didn't have tools that were directly sensitive to atomic motion at very fast time scales to investigate this."
The team focused on a molecular complex containing two metal atoms that can exchange an electron between them. This system serves as a platform to study electron transfer reactions. First, they dissolved the complex in water, where it formed strong hydrogen bonds with surrounding water molecules. They kicked off the electron transfer process between the metal atoms using an optical laser pulse. Then they scattered X-ray pulses from SLAC's Linac Coherent Light Source off the sample to monitor the motion of the atoms in the complex and the surrounding solvent molecules during the electron transfer.
The ultrashort X-ray pulses, just millionths of a billionth of a second long, captured the synchronized motions of the water molecules that were bonded to the complex. As an electron transferred from one metal atom to the other, the hydrogen bonds weakened and the solvent molecules moved away from the complex. When the electron returned to the first metal atom, the solvent molecules oscillated back to their original position.
The team was able to analyze and interpret the experimental results using molecular simulations. Co-senior author Niri Govind, a physicist at PNNL, and co-author Amity Andersen, a PNNL computational chemist, contributed to these simulations with NWChem, an open-source, PNNL-developed computational chemistry software package.
"The combination of experiment and molecular simulation was crucial to understanding the coupled dance that occurs during ultrafast electron transfer between the metal atoms and the surrounding water molecules," said Govind.
