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Molecular handedness controls spin

Chiral molecules help transmit quantum states

by Neil Savage, special to C&EN
October 19, 2023 | A version of this story appeared in Volume 101, Issue 35


Controlling the quantum state of electrons is crucial for quantum information processing, including new types of superpowerful computers and unbreakable cryptography. Scientists have known for a while that they could transmit quantum spin from one electron to another using molecules attached to metal or semiconductor substrates, but now they’ve shown that it’s the molecule, and not the substrate, that’s largely responsible for that effect (Science 2023, DOI: 10.1126/science.adj5328).

Striking a donor molecule (left) with a light source creates both spin-up and spin-down electrons, but only one type is efficiently transferred to an acceptor molecule depending on the chirality of a bridge molecule. Spin up is preferred in one version (top) and spin down is preferred in the other (middle).
Image shows three scenarios for a light source striking donor molecules in a liquid crystal. One scenario is on top, then one below it, and the other at the bottom. The image shows how the chirality of the donor and bridge molecules affect electron spin.
N. BURGESS/SCIENCE. From Joseph E. Subotnik, SCIENCE, 12 Oct 2023
Vol 382, Issue 6667, pp. 160-161, DOI: 10.1126/science.adk5634. Reprinted with permission from AAAS.

“A lot of people felt that it had to be intrinsically the molecule, [but] there was no direct evidence for it,” says Michael Wasielewski, a chemistry professor at Northwestern University who led the work, along with a team that included Matthew Krzyaniak of Northwestern and Stefano Carretta of the University of Parma.

Spin can be envisioned as a magnetic pole through the electron; spin up points to the north, down to the south. The chirality of a molecule, whether it’s essentially right-handed or left-handed, can affect how electrons with different spins move through a material, a phenomenon known as chirality-induced spin selectivity (CISS). A group of electrons can influence the spin of nearby electrons, so it was possible that the electronic structure of metal or semiconductor substrates was actually driving the phenomenon.

To test that, the researchers created nematic liquid crystals, made up of molecules that can change orientation. They aligned the molecules with a magnetic field, then froze them in place. They could alter the chirality of the crystal by rotating the frozen sample.

When the researchers excited one end of the molecule with light, it transmitted electrons to the other end. Whether there were more spin-up or spin-down electrons depended on the orientation of chiral molecules. The likelihood of getting a particular spin was 47% higher with chiral molecules than with symmetrical molecules. Wasielewski calls that “a surprisingly big effect” but not a pure effect. He plans to explore how to get 100% control of the spin.

Controlling the spin could produce paired quantum bits for quantum information systems. It could also increase the separation of charges in a solar cell, improving the conversion of light to electricity, and make photocatalysis more efficient. The new understanding of spin transfer could also help expand the understanding of the role of chirality in biological molecules.

Being able to measure the CISS effect in electron transfer represents a breakthrough, says Vladimiro Mujica, professor of molecular sciences at Arizona State University. “Spin-polarized electron transfer, as for example induced by the CISS effect, has very significant consequences in quantum biology, quantum sensors, and quantum information theory,” he says. David Limmer, a professor of chemistry at the University of California, Berkeley, calls the paper “a beautiful result.”



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