First detailed look at how charge transfer distorts a molecule’s structure

Charge transfer is highly important in most areas of chemistry, including photosynthesis and other processes in living things. A SLAC X-ray laser study reveals how it works in a molecule whose lopsided response to light has puzzled scientists for nearly a decade. When light hits certain molecules, it dislodges electrons that then move from one location to another, creating areas of positive and negative charge. This “charge transfer” is highly important in many areas of chemistry, in biological processes like photosynthesis and in technologies like semiconductor devices and solar cells.

Even though theories have been developed to explain and predict how charge transfer works, they have been validated only indirectly because of the difficulty of observing how a molecule’s structure responds to charge movements with the required atomic resolution and on the required ultrafast time scales.

In a new study, a research team led by scientists from the University of Edinburgh, Brown University and the Department of Energy’s SLAC National Accelerator Laboratory used SLAC’s X-ray free-electron laser to make the first direct observations of molecular structures associated with charge transfer in gas molecules hit with light. Molecules of this gas, called N,N′-dimethylpiperazine or DMP, are normally symmetric, with a nitrogen atom at each end. Light can knock an electron out of a nitrogen atom, leaving a positively charged ion known as a “charge centre.” Intriguingly, this process is uneven; light absorption creates a charge centre in just one of the two nitrogen atoms, and this charge imbalance deforms the molecule’s atomic scaffolding, so atoms compensate by shifting position with respect to each other. But within three trillionths of a second, the charge redistributes itself between the two nitrogen atoms until it evens out and the molecules become symmetric again, the researchers report in a paper published in the Proceedings of the National Academy of Sciences recently. “We see the molecules breaking symmetry and reforming symmetry,” said Peter Weber, a chemistry professor at Brown University whose research group started studying DMP almost a decade ago. 

A lopsided response

Scientists in Weber’s group, including Xinxin Cheng – a PhD student who is now a SLAC associate staff scientist – discovered the molecule’s lopsided response to light eight years ago. It turned out that the molecule’s nitrogen atoms are just the right distance apart to make it an ideal model for studying charge transfer, a discovery that triggered a lot of discussion among theorists working to understand these processes as well as efforts to observe them in more detail.

Weber said the results demonstrate the value of the technique for extracting more detailed information than in previous experiments. The research team used that information to test theoretical models of how molecules respond, revealing flaws in the conventional approach known as density functional theory. Weber noted that the data seems to support detailed theoretical calculations of how these charge transfers take place by Hannes Jónsson of the University of Iceland, who was not involved in this study. 

Minitti, who has been working on DMP with the Brown lab from the start and participated in this study, said it has been difficult to get a theoretical understanding of how these asymmetric systems work because the experimental data on them has been so sparse and indirect.

“This work is a significant step forward,” he said, “giving us critical information about how the molecule responds during the charge transfer process. Research like this takes a village – we need experiments to inform the theory, and vice versa, to help us visualize this thing.”

Going forward, a big increase in the pulse repetition rate of the LCLS X-ray source is underway, with a leap from 120 pulses per second to 1 million pulses per second. This will allow researchers to study much more complex systems, informing the development of new approaches to solar energy generation and energy storage technologies, among many other applications. LCLS is a DOE Office of Science user facility, and the Office of Science funded this work.