Pump-Probe Spectroscopy Could Help to Control Chiral Molecules

An international research team at Freie Universität Berlin, in collaboration with colleagues at the DESY research center, Kiel University, and Kansas State University, has proposed a quantum-chemical calculation-based approach to induce and probe chiral vibrational motion using pump-probe spectroscopy. Such an approach aims provide a solution to achieve absolute asymmetric synthesis, or the control of product chirality using only light fields.

It would also demonstrate a method to use light to change a planar molecule into a chiral molecule that exhibits only one type of handedness — which would be particularly useful in the chemical synthesis of compounds.

In a chiral molecule, the atomic spatial arrangement is either left-handed or right-handed. Molecules that consist of four or more atoms are often chiral. “Each chiral molecule has a twin molecule from a mirror world with the opposite handedness,” said Christiane Koch, a professor from the Department of Physics at Freie Universität Berlin. The left-handed and the right-handed versions of the molecule — called enantiomers — are equivalent to each other in every way, except that they interact with their surroundings and with each other differently, depending on the handedness.

Biologically relevant molecules are mainly homochiral, Koch said. This means that in the living cells, only one enantiomer is present.

“Despite being made up of exactly the same atoms, the two mirror images of a molecule differ in many properties,” she said.

For example, one enantiomer might smell like oranges and the other might smell like lemons.

In medicine, this potential property poses a potential problem, according to Koch. One enantiomer could be a cure for a disease, while the other is a dangerous poison.

“Controlling the formation of one specific handedness of chiral molecules is therefore an important goal in chemical synthesis,” Koch said.

Absolute asymmetric synthesis is an alternative to the control of chirality with chiral chemicals — the approach that scientists usually use. Previously, however, it was unclear how absolute asymmetric synthesis could be achieved.

Starting from a gaseous ensemble of the planar molecule carbonyl chlorofluoride (COFCl), as the nonchiral reagent, the carbon molecules are made chiral by exciting vibrational motion out of the molecular plane. This preparation step is referred to as pump. As the molecule vibrates back and forth from above to below the plane, the molecular handedness changes. This change can be measured by ionizing the molecules and varying the delay in time between the pump and probe steps.

The required combination of electric fields for the pump step can be realized by a left and a right circularly polarized pulse, which together induce a Raman transition as well as a static electric field. The probe step involves ionizing the vibrating molecules by an extreme ultraviolet pulse that probes the time-dependent net handedness via the photoelectron circular dichroism, the researchers said.

The researchers inferred the molecules’ handedness by measuring the directions into which electrons are emitted.

According to the researchers, their proposal for pump-probe spectroscopy of molecular chirality is feasible with current experimental technology. In their paper, the researchers said, “Such studies of time-dependent chirality, following the change of handedness in the course of vibrational motion, would provide a stepping stone to lastly realize long-standing proposals for chiral molecules, including the measurement of parity violation and enantiomeric purification with light.”

The research was published in Science Advances (https://www.science.org/doi/10.1126/sciadv.ade0311).