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Study Shows How Monomer Sequence Affects Charge Transport in Chain Molecules

In the University of Illinois at Urbana-Champaign, scientists in the Moore and Schroeder groups have reported a new study demonstrating how variations in the polymer sequence influence the properties of charge transport. This research needed the ability to develop and analyze chain molecules with high levels of accuracy.

Researchers in the Schroeder and Moore groups, from left, Hao Yu, a graduate student in chemical and biomolecular engineering; Jeff Moore, professor of chemistry; Charles Schroeder, professor of chemical and biomolecular engineering; and Songsong Li, a graduate student in materials science and engineering. Image Credit: Beckman Institute for Advanced Science and Technology.

The study titled “Charge Transport in Sequence-Defined Conjugated Oligomers” was published in the Journal of the American Chemical Society.

In modern society, polymers or chain molecules can be found everywhere, and there has been increasing use of organic electronic materials in sensors, flat panel displays, and solar cells. But more often, traditional materials are prepared by statistical polymerization, where the order of the monomers or subunits—that is, the monomer sequence—is random.

Traditional polymerization methods do not give us a perfect level of control of sequence. As a result, it has been challenging to ask how the monomer sequence affects its properties.

Charles Schroeder, Associate Head and Ray and Beverly Mentzer Professor in Chemical and Biomolecular Engineering, Beckman Institute for Advanced Science and Technology, University of Illinois

Schroeder is also a full-time faculty member at the Beckman Institute for Advanced Science and Technology. The scientists created a technique known as iterative synthesis to address the issue.

Protein synthesis in our cells occurs by adding the amino acids one by one. We use the same method for making synthetic polymers where we add distinct monomers in a one-by-one fashion. This allows us to precisely control the sequence in a linear arrangement.

Hao Yu, Graduate Student, Beckman Institute for Advanced Science and Technology, University of Illinois

Yu belongs to the Schroeder Group, as well as the Moore Group headed by Jeff Moore, the Stanley O. Ikenberry Endowed Chair and professor of chemistry.

The researchers synthesized the materials and then analyzed the charge transport properties with the help of single-molecule methods. Thus, they were able to quantify the conductance via single chains, similar to a “molecular wire.”

Molecular wires are generally good at transporting charge. We wanted to know how the charge transport properties change if the overall sequence changes,” stated Schroeder.

To facilitate the characterization, Yu added molecular anchors at both ends of the chain molecule.

We used a technique called the scanning tunneling microscope-break junction method, where the anchors link to two gold electrodes and form a molecular junction. Then we impose an applied bias or voltage across the molecule, and this allows us to measure the charge transport properties of these polymers.

Songsong Li, Graduate Student, Beckman Institute for Advanced Science and Technology, University of Illinois

Li belongs to the Schroeder group.

Currently the synthesis method is labor intensive,” added Schroeder. “Moving forward, we are developing automated synthesis methods in the Beckman Institute to generate large libraries of sequence-defined molecules.”

The implications of this work are significant,” stated Dawanne Poree, program manager at the Army Research Office that supports the study. “It’s often been wondered if the sequence-dependent properties observed in biological polymers could translate to synthetic polymeric materials. This work represents a step toward answering this question.

Poree continued, “Additionally, this work provides key insights into how molecular structure can be rationally designed and manipulated to render materials with designer properties of interest to the Army such as nanoelectronics, energy transport, molecular encoding, and data storage, self-healing, and more.”

The study was funded by the U.S. Department of Defense by a multidisciplinary university research initiative of the Army Research Office, an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory.


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