Mind the gap: Beckman researchers measure electron transport through space
The flow of electrical current through organic compounds is a central chemical process in science and technology, factoring into everything from photosynthesis to metabolism to energy storage in batteries.
Researchers at the Beckman Institute for Advanced Science and Technology designed a self-assembled molecular system to measure how electrons flow across the space between molecules. Their findings hold the potential to guide design and development of new organic electronic devices.
Hao Yu and Jialing (Caroline) Li, student researchers studying chemical and biomolecular engineering at the University of Illinois Urbana-Champaign, led the research under the direction of Charles Schroeder, a professor of chemical and biomolecular engineering.
“Our work holds the potential to enable new dynamic and functional devices for the emerging field of molecular electronics," said Schroeder, who is also the James Economy Professor of Materials Science and Engineering.
Prior work has sought to understand how electrical currents flow within single organic molecules — this is known as intramolecular transport. This study, published in the Journal of the American Chemical Society, focuses on through-space, or intermolecular, charge transport — how electrical currents flow in the space between stacked molecules inside a well-defined molecular complex.
“In organic electronics, it’s challenging to separate the effects of intramolecular and intermolecular charge transport. Our work stands out as providing a fundamental understanding of the intermolecular charge transport pathway," Li said.
The combination of intra- and intermolecular transport is key to developing efficient organic electronic devices such as new types of batteries. If electrons moving within a molecule are dogs roaming about behind a fence, electrons moving across the spaces between molecules are the dogs taking a leap over the fence and into another yard. How easy (or how unlikely) these leaps are depends on the gaps between the yards — or the spatial arrangement of the molecules.
To study through-space transport, the researchers assembled large molecular complexes — known as host-guest complexes — with parallel stacks of molecules enclosed as guests inside a synthetic host.
“The goal is to design well-aligned molecules to enhance conductivity and device performance. By using host-guest chemistry, we can fine-tune the electronic properties of these materials,” Li said.
This approach allowed the researchers to create well-defined molecular junctions to precisely characterize charge transport across adjacent stacked molecules. The design has potential applications to both basic science and applied research.
“To guide the design of materials for device fabrication, we need to understand why materials behave in certain way on the fundamental level before we can fine-tune the material on the device scale. It’s fascinating to merge two fields of science together, with one field motivating the other for an impactful goal,” Li said.
The study's success is built on the intersection of fundamental research and applied materials science; it's also built on collaborations between students and faculty members.
“This work is ideal for the Beckman Institute, which supports interdisciplinary research to achieve these bold collaborations," Yu said.
Beckman Director and chemistry professor Jeffrey Moore's lab group contributed expertise in molecular assembly.
“Credit for this project goes to the graduate students Hao and Caroline. From start to finish they worked together as a team to conceptualize this work and generate these intensely exciting findings. The breadth of their work is impressive, combining molecular design and synthesis, supramolecular chemistry, single-molecule charge transport measurements, and computational modeling," said Moore, who is also the Stanley O. Ikenberry Endowed Chair.
A group led by Nick Jackson, an assistant professor of chemistry, performed the simulation and computational modeling, and the Schroeder group performed the charge transport experiments.
“We are extremely fortunate to work with a highly collaborative team of scientists and graduate students in the Beckman Institute,” Schroeder said.
Contributing authors include Songsong Li, Department of Materials Science and Engineering; and Yun Liu, Department of Chemistry. For a full list of collaborators and their affiliations, please consult the publication.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, and by the Joint Center for Energy Storage Research.
The paper "Efficient intermolecular charge transport in π-stacked pyridinium dimers using cucurbituril supramolecular complexes can be accessed at https://doi.org/10.1021/jacs.1c12741.