Switching Chiral Molecules With Light

An impression of a chiral molecule moving through various configurations as it transitions from one handedness to another. (Source: V. Valev & J. Collins)

For the first time scientists have created a way to model the inter­action between light and twisted molecules, as these molecules transition from left- to right-handed versions, or vice versa. The transi­tional forms offer a deeper insight into material sym­metries and their unexpected behaviour could lead to improved design of telecoms components.

Many molecules, including important pharma­ceuticals and valuable chemicals, exist in two chiral forms. They have the same chemical structure arranged in mirror images, termed left-handed and right-handed forms. This can alter their pro­perties and is therefore important to fully understand how the compound interacts with other molecules, or light. Typi­cally, it has only been possible to study either the left- or right-handed chiral form but nothing in between, however ideally scientists would like to gra­dually morph a shape from one handedness to the other and observe how the effects of this change translate into physical proper­ties.

Now a research team from the Depart­ment of Physics at the Univer­sity of Bath, working with colleagues at Univer­sity College London, Belgium and China, has created a way to do exactly that. Their unique method involves manu­facturing metallic nano-scale artificial molecules represen­tative of 35 inter­mediate stages along the way of a geometric trans­formation, from one handedness to the other. At this nano-scale, the shape of the arti­ficial molecule affects its optical pro­perties, so by using twisted laser light the team studied the pro­perties of the various stages, as the arti­ficial molecules morphed from left to right handed­ness.

PhD student Joel Collins said: “We were able to follow the proper­ties of a chiral arti­ficial molecule, as it was morphed from left- to right-handed form, through two different routes. No-one has done this before. Surpri­singly, we found that each route leads to a different behavior. We measured the dif­ference in absorption of left and right circu­larly polarized light, the circular-dichroism (CD). Along one route, the arti­ficial molecules behaves as might be expected, with progres­sively decreasing CD, and eventually a reversal of the CD, for the mirrored structure. However, along the second route, the CD reversed several times, even before the structure changed handed­ness.”

Ventsislav Valev who led the research said: “This is actually a very elegant idea but it has only become a possi­bility thanks to the recent advances in nano­fabrication. In chemistry, you can’t tune the twist of a chiral molecule, so every scientist who studies such molecules needs to tune the wavelength of light. We have demon­strated a new, comple­mentary physical effect, where we fix the wave­length and tune the twist of the chiral arti­ficial molecule. In many cases, our approach is more practical; for instance, when we’re designing telecoms compo­nents, where the optical wave­length is pre-deter­mined.” (Univ. Bath)

Reference: J. T. Collins et al.: Enantiomorphing Chiral Plasmonic Nanostructures: A Counterintuitive Sign Reversal of the Nonlinear Circular Dichroism, Adv. Opt. Mat. 1800153 (2018); DOI: 10.1002/adom.201800153

Link: Multiphoton Nanophysics (V. Valev), University of Bath, Bath, UK

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