The dynamics of light-harvesting chemistry

A new paper provides insights into the chemistry that underlies human-made, light-driven catalytic systems and paves the way to better understand how light-induced reactions can be controlled more effectively.

a person inside of a lab pouring liquid nitrogen into a container with a computer screen in the foreground
Graduate student Stephen Meloni at work in the physical chemistry lab of Jessica Anna.

Enzymes are highly selective and efficient protein catalysts that can speed up chemical reactions. The efficiency and selectivity of an enzyme is related to its chemical structure, so researchers study the minute details of these natural catalysts in order to create better catalysts in the lab. 

One example of an engineered system that has proven useful for human-made catalysts is known as a nanocage. These structures wrap around catalysts and hold them in place, which keeps the catalysts stable and makes them more effective and efficient during chemical reactions.

While researchers can create and use these engineered structures, they still don’t fully understand how nanocages influence the movement and shape of the catalysts trapped inside. This understanding is crucial in making the chemical reactions that they support more efficient. 

Now, a team of chemists in the lab of Jessica Anna has been able to catch a detailed glimpse inside these engineered structures using state-of-the-art laser spectrometry. The work, published in the Journal of Physical Chemistry, looks at metal carbonyl molecules inside self-assembled nanocages. Graduate student Stephen Meloni is a co-author on this publication, led by former Penn postdoc Rahul Gera. 

Since being inside a nanocage causes a change in the metal carbonyl’s chemical reactivity in response to light, Anna and her group had to figure out how to study the nanocage and the metal carbonyl complexes as a whole. To do this, the group uses ultra-fast, mid-infrared spectroscopy with pulses of laser light shot at incredibly short intervals. These time scales are within the femto-to-picosecond range, which is one quadrillionth to one trillionth of a second long.

One of the challenges faced by Anna’s group is the effort required to set up experiments. The lab has a custom laser spectrometer that’s made of an intricate set of mirrors and lenses spread across a long optics table. Each piece of the spectrometer has to be meticulously aligned. The first time that a new experiment is tried, it can take several days just to get the optics lined up.

Meloni, one of Anna’s first students, helped set up the lab, and the custom spectrometer, from scratch. While some students would have been hesitant to spend their time in a graduate program setting up a brand-new lab, Meloni saw the opportunity as a worthwhile challenge essential to his experience as a graduate student. “Our group is focused on developing new methods and being on the cutting-edge of those methods, so setting up the spectrometer is an integral part of our work. It’s very intertwined—both building the optics and doing a Ph.D. project in this lab,” Meloni says.

meloni in a lab standing behind a table covered with mirrors and lenses
Meloni was one of Anna’s first students and will be graduating in the next few months. He is currently exploring jobs at the interface between science and business and says that the critical thinking and problem solving skills he gained here at Penn will be instrumental for the next phase of his career.   

One of the technical challenges that Meloni faced on the nanocage project was how to collect data at “time zero” of the experiment, especially with the extremely short time scales they were trying to study. Solving this challenge required a large number of detailed measurements and alignments to ensure that the laser beams stayed within an area the width of a human hair while traveling across the meters-long optics table.

Thanks to the dynamic and detailed glimpse that the ultra-fast spectrometer provides, Anna and her group were able to see how the metal carbonyl complexes change shape and move within the nanocage. This study also led to some surprising findings on how very slight changes to the nanocage alters the shape and physical properties of the trapped metal carbonyl. This is also the first publication from the Anna lab that uses the infrared spectrometer setup that Meloni built. 

In the future, Anna’s group will look at the relationship between chemical structure and reactivity. This information will be essential to help chemists understand how chemical reactions can be controlled and how existing reactions could be done more efficiently. “We’re hoping that this study could lead to applications in areas such as materials engineering by enabling us to think about how to alter the cage or the metal carbonyl itself in order to control reactivity,” says Anna.

The research was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences under Award DE-SC-0016043; startup funds from the University of Pennsylvania; National Institutes of Health predoctoral training grant GM008275; and the University of Pennsylvania School of Arts and Sciences Mitchell Fellowship.

Jessica M. Anna is an Assistant Professor of Chemistry Elliman Faculty Fellow in the Department of Chemistry at the School of Arts and Sciences at the University of Pennsylvania.