Researchers make molecular movies of photocatalysis about to happen
Newly visualized process could lead to better solar energy capture
A team led by researchers from the Technical University of Denmark (DTU) and including European XFEL scientists has shown that it is possible to record “molecular movies” of the interactions between light-activated molecules and their surroundings. These results bring scientists one step closer to exploiting the sun’s energy better than currently technologically possible. Sophisticated X-ray equipment and highly advanced computational chemistry methods were used in close collaboration with researchers from SLAC National Accelerator Laboratory in the USA, Lund University in Sweden, and KAIST University in South Korea.
The researchers were able to follow the detailed interaction between a photocatalyst, a substance that converts light to chemical energy, and a solvent with which it can react. The findings, which have been published in Nature Communications, provide a key step towards a general understanding of the dynamics, or motions within a molecule, involved in artificial photosynthesis through which molecules absorb and store solar energy in chemical bonds. Such results are expected to help pave the way for designing light-activated molecules that can produce hydrogen fuel from water, imitating natural photosynthesis through which nature transforms solar energy and carbon dioxide to plant sugars.
“Photocatalytic systems offer huge potential. If we can understand the underlying dynamics of chemical reactions in light-absorbing molecules, we can make rational decisions for how to improve their ability to produce fuels or other important molecules”, says Prof. Martin Meedom Nielsen from DTU Physics.
The photocatalyst investigated in these studies is an organometallic compound that represents a large class of molecules that perform a whole range of photochemical reactions. The combination of experiments and computational chemistry revealed that light excitation of the molecule initiates large-scale structural dynamics and completely changes the nature of the interaction with the surrounding solvent molecules. Before light absorption, the solvent molecules are attracted to the photocatalyst through diffuse and non-specific interaction. The light absorption changes these interactions to very specific interactions causing the solvent molecules to rotate 180 degrees and attach to the active site of the photocatalyst. This type of dynamics has never been visualized before.
Many important dynamics following light activation of photocatalytic molecules happen on ultrafast time scales—as fast as a few hundred femtoseconds. A hundred femtoseconds, or 0.000 000 000 000 1 seconds, is the time it takes light to travel 0.03 millimetres, the width of a human hair. This means that it takes highly specialized, sophisticated equipment to capture these dynamics. Nielsen and his group used the LCLS X-ray free-electron laser at SLAC National Accelerator Laboratory in California, USA.
“We are looking very much forward to taking the next step in the exploration of molecular dynamics”, says Nielsen, whose group is preparing to take full advantage of the European XFEL, which is due to start user operation in 2017.
“It was a very rewarding opportunity to be part of the team and exploit the potential of the ultrafast liquid X-ray scattering technique. We will also implement this technique at the European XFEL’s FXE instrument and make it available for users”, said European XFEL instrument scientist Wojciech Gawełda. “The results are extremely relevant for the FXE instrument at the European XFEL”, says FXE group leader Prof. Christian Bressler. “The unique capabilities of our instrument will allow us to employ X-ray scattering techniques simultaneously with X-ray spectroscopic tools. Combined with the unprecedented properties of the European XFEL and a dedicated Large Pixel Detector, the FXE instrument will enable even more sophisticated studies to be carried out in similar molecular systems”.