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Unlocking the Mysteries of Chirality-Induced Spin Selectivity (CISS) in Isolated Molecules

New observation of CISS has implications for quantum information, energy, and biology

Arleigh Truesdale | November 6, 2023
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Deciphering the movement and behavior of electrons has been key to unlocking advancements in energy technologies such as solar cells and semiconductor devices. For decades, scientists have been captivated by a curious phenomenon known as chirality-induced spin selectivity (CISS), which influences the activity of electrons in molecules that have a specific “handedness”, or so-called chirality.

Since its discovery in the late nineties, the CISS effect has been observed in molecules attached to metal substrates or semiconductor surfaces. Now, new research has demonstrated the CISS effect to be an intrinsic property of electron motion within isolated chiral molecules.

The pioneering study elucidates the underlying mechanisms of CISS and may have ranging applications for quantum computing, energy generation, and lighting efficiency as well as biological processes. The study, led by Michael R. Wasielewski, Clare Hamilton Hall Professor of Chemistry, was published in Science in mid-October. The full team included researchers from Northwestern University and the University of Parma.

"If electrons move through a chiral material…only one of the spin directions is transmitted, while the other is inhibited. If the material has the opposite handedness, the effect is reversed," explained Wasielewski, who is also Co-Executive Director of the Paula M. Trienens Institute for Sustainability and Energy and Director of the Center for Molecular Quantum Transduction (CMQT).

To generate and measure the CISS effect, the international research team carried out an experiment that involved synthesizing molecules made up of three components: a photo-responsive electron donor, a chiral bridge, and an electron acceptor. Using short laser pulses, an electron was transferred from donor to acceptor along the chiral bridge. The team used a liquid crystal to align the molecules so that the electron transfer could be detected using electron paramagnetic resonance (EPR) spectroscopy.

“We have a demonstrated method to understand the molecular design factors that shape the CISS effect. Our team worked with molecular design theories from our collaborators at the University of Parma and brought them to life using the equipment and cutting-edge facilities at Northwestern,” said Wasielewski.

Now that the CISS effect has been observed in standalone molecules, it can be more widely and efficiently exploited. In magnetic molecules, electron spins can act as qubits. Qubits are the fundamental units of information in quantum computing, just as bits are the units of information in classic computing. The CISS effect could be used to manipulate the state of individual qubits in quantum devices, such as sensors and computers. Temperatures approaching absolute zero are typically required for the operation of quantum systems, but the CISS effect is efficient at room temperature which may create new functional flexibility.

The CISS effect could also be used to improve the efficiency of semiconductor devices like light-emitting diodes (LEDs). In the natural world, the new discovery may shed light on the role that chirality plays in various biological processes. “There is still much to discover and comprehend about this fascinating phenomenon that has the potential to revolutionize research and innovation across numerous disciplines,” remarked Wasielewski.

The full research team included Jillian M. Bradley, Stefano Carretta, Alessandro Chiesa, Hannah J. Eckvahl, Matthew D. Krzyaniak, Nikolai A. Tcyrulnikov, Michael R. Wasielewski, and Ryan M. Young. This work was supported by the National Science Foundation; the Center for Molecular Quantum Transduction, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences; and co-funded by the European Union (ERC-SyG CASTLE).