This story by Kristina Shidlauski is shared with the University of Illinois Urbana-Champaign newsroom. John R. Reynolds is a professor in the School of Chemistry and Biochemistry and School of Materials Science and Engineering at Georgia Tech. He served as founder of the Georgia Tech Polymer Network (GTPN) and is a member of the Center for Organic Photonics and Electronics (COPE).

Chirality, a property where structures have a distinct left- or right- “handedness,” allows natural semiconductors to move charge and convert energy with high efficiency by controlling electron spin and the angular momentum of light. A new study has revealed that many conjugated polymers, long considered structurally neutral, can spontaneously twist into chiral shapes. This surprising behavior, overlooked for decades, could pave the way for development of a new class of energy-efficient electronics inspired by nature.

The research, a collaborative project that included researchers from the University of Illinois Urbana-Champaign, Georgia Institute of Technology, University of North Carolina, and Purdue University was recently published in the Journal of the American Chemical Society.

“Many molecules essential to life are chiral,” said Ying Diao, professor of chemical and biomolecular engineering at Illinois, who led the project. “The question that has remained a really a big fascination across the field is how chiral symmetry breaking happens in the first place: that is how life selects one handedness over the other. Our work mainly focuses on the origin of chirality: why chirality spontaneously emerges in absence of any chiral sources.” 

To answer this question, the team tested 34 different conjugated polymers. Each polymer was dissolved in a solvent, then the researchers gradually increased the polymer concentration to observe whether liquid–liquid phase separation (LLPS) occurred. When LLPS was detected, they used circular dichroism spectroscopy to analyze the samples, revealing a strong correlation between phase separation and the emergence of chirality. The researchers refer to this phenomenon as spontaneous chiral symmetry breaking.

They found that approximately two-thirds of the polymers spontaneously formed chiral structures when their concentration in the solution increased.

“That took our community by surprise, because conjugated polymers have been studied for half a century,” Diao said. “These new chiral helical states of matter have basically been hiding in plain sight.”

To understand why some of the polymers developed chirality while others did not, Illinois chemistry professor and senior co-author Nicholas E. Jackson applied machine learning to analyze molecular features across the polymer library. The analysis, later backed up by additional testing, revealed that polymers with longer molecular chains were more likely to form chiral assemblies. Unexpectedly, the researchers also found that the presence of oxygen atoms in the side chains was a strong predictor of chiral behavior.

“Machine learning uncovered hidden patterns across dozens of conjugated polymers, relating subtle chemical details to chiral phase formation,” Jackson said. “Such insights would have been very difficult to derive by human intuition alone.”

Diao noted that the discovery not only deepens our fundamental understanding of chiral emergence but also holds significant technological promise. In nature, chiral systems – such as those involved in photosynthesis – enable highly efficient electron transport. Looking ahead, Diao said that mimicking this behavior could lead to major performance gains in electronic devices and innovation of new device types.

“We are thinking about using chirality to control conductivity – for example, in transparent conductors for phones or in solar cells that could be more stable and efficient,” she said. “In our computers, electrons bounce around and heat is a big problem. But if we make chiral versions, we think charge transfer could be extremely efficient, just like in nature.”

“What’s nice about this is, this is not the end of the story,” said Georgia Institute of Technology chemistry professor John Reynolds, a senior co-author on the study. “This work provides guidance to polymer scientists in the field for studying the many, many conjugated polymers that have been synthesized over the years, and for designing new polymers with enhanced properties.”

This study was supported by the U.S. Office of Naval Research, the Air Force Office of Scientific Research, the Molecule Maker Lab Institute, and the National Science Foundation. Polymers for the study were provided by Reynolds, University of North Carolina chemistry professor Wei You, University of Illinois chemistry professor Jeff Moore, and Purdue University chemistry professor Jianguo Mei.

In addition to her appointment in Chemical & Biomolecular Engineering, Diao is a full-time faculty member at the Beckman Institute for Advanced Science and Technology, holds a faculty appointment with Chemistry in the College of Liberal Arts & Sciences, and is affiliated with Materials Science & Engineering in The Grainger College of Engineering. In addition to his appointment in Chemistry, Jackson is a group leader at the Beckman Institute and affiliate faculty member in the departments of Chemical & Biomolecular Engineering and Materials Science & Engineering.

The paper, "Ubiquitous Chiral Symmetry Breaking of Conjugated Polymers via Liquid Liquid Phase Separation," is available online at https://pubs.acs.org/doi/abs/10.1021/jacs.5c07995