LEMONT, Ill., Sept. 20, 2016 — To better understand how molecules undergo light-driven chemical transformations, a research team from the Argonne National Laboratory, Northwestern University, the University of Washington and the Technical University of Denmark used the Linac Coherent Light Source (LCLS) at the Department of Energy’s SLAC National Accelerator Laboratory to image molecules in their excited state. Results revealed an extremely short-lived state of excitation lasting only a few hundred femtoseconds (fs) before the molecule relaxed into a lower energy state. Learning more about this state is a step toward the further development of energy sources.
When the laser pulse hits the molecule, an electron from the outer ring moves into the center, creating a charge imbalance, which in turn creates an instability within the molecule. Another electron from the center migrates back to the outer ring, and the excited electron falls back into the lower open orbital to take its place.
To understand how molecules undergo light-driven chemical transformations, scientists need to be able to follow the atoms and electrons within the energized molecule as it gains and loses energy. In a recent study, a team of researchers at Argonne, Northwestern University and the Technical University of Denmark used the ultrafast high-intensity pulsed x-rays produced by the Linac Coherent Light Source to take molecular snapshots of these molecules. Courtesy of Scott Nychay.
"This first state appears and disappears so quickly, but it's imperative for the development of things like solar fuels," said Lin Chen, senior chemist at Argonne National Laboratory and professor of chemistry at Northwestern University. "Ideally, we want to find ways to make this state last longer to enable the subsequent chemical processes that may lead to catalysis, but just being able to see that it is there in the first place is important."
By using the LCLS, the researchers were able to capture atomic and electronic arrangements within the molecule that had lifetimes as short as 50 fs — about as long as it takes light to travel the width of a human hair.
"Although we had previously captured the molecular structure of a longer-lived state, the structure of this transient state eluded our detection because its lifetime was too short," Chen said.
The challenge, Chen added, is to prolong the lifetime of the excited state through the design of the molecule.
"From this study, we gained knowledge of which molecular structural element, such as bond length and planarity of the ring, can influence the excited state property," she continued. "With these results we might be able to design a system to allow us to harvest much of the energy in the excited state."
For the study, the research team used metalloporphyrin, a molecule similar to the building blocks needed for natural and artificial photosynthesis. Metalloporphyrins are of interest to scientists seeking to convert solar energy into fuel.
The research was published in the Journal of the American Chemical Society (doi: 10.1021/jacs.6b02176).
As the world's most powerful x-ray laser, the Linac Coherent Light Source creates unique light that can resolve detail the size of atoms and see processes that occur in less than one-tenth of a trillionth of a second. At these unprecedented speeds and scales, the LCLS has embarked on groundbreaking research in physics, structural biology, energy science, chemistry and a host of other fields.Courtesy of U.S. Department of Energy.