Study provides detailed insight into intriguing properties of chiral materials
Research focuses on light-matter interactions in chiral materials under a magnetic field
BUFFALO, NY – In nature, many molecules have a property called chirality, which means they cannot be superimposed on their mirror images (like a left hand and a right hand).
Chirality can influence function, affecting the effectiveness of a drug or enzyme, for example, or the perceived aroma of a compound.
Now, a new study is advancing scientists’ understanding of another property related to chirality: how light interacts with chiral materials under a magnetic field.
Previous research has shown that in such a system, the left and right shapes of a material absorb light differently, so as to reflect each other when light flowing parallel to an external magnetic field changes direction, adopting anti-parallel flow. This phenomenon is called magneto-chiral dichroism (MChD).
However, past experiments lacked confirmation that the experimental observations match the predictions made by the MChD theory – a necessary step to verify the theory and understand the effects observed by scientists.
The new article, which will be published April 21 in Science Advances, changes that. The study was led by Geert LJA Rikken, PhD, director of the National Laboratory of Intense Magnetic Fields in France, and Jochen Autschbach, PhD, Larkin Professor of Chemistry at the University of Buffalo in the US The first authors were Matteo Atzori, PhD, researcher at the National Laboratory of Intense Magnetic Fields, and doctoral student UB chemistry Herbert Ludowieg.
“The first theoretical predictions of MChD for light appeared in the 1980s. Since then, an increasing number of observations of the effect have been reported, but no quantitative analysis has been possible to confirm whether the theory under -jacent of the MChD is correct, ”says Rikken. “The new study offers detailed measurements on two well-defined model systems and advanced quantum chemistry calculations on one of them.”
“Dr. Rikken’s team made the first experimental observation of MChD in 1997 and has since reported further experimental studies of the effect in different systems, ”says Autschbach. “However, it is only now that a direct comparison between an experiment and ab-initio theoretical quantum calculations becomes possible, for a verification of the MChD theory.”
The research focused on crystals made up of mirrored forms of two compounds: tris (1,2-diaminoethane) nickel (II) nitrate and tris (1,2-diaminoethane) cobalt (II) nitrate. As Autschbach explains, “The molecular shape of the tris (1,2-diaminoethane) metal (II) ion in the crystal is shaped like a helix. The propellers are also available in mirror image pairs, which cannot be overlaid. “
Rikken’s lab performed detailed experimental measurements for the two systems studied, while Autschbach’s group took advantage of UB’s supercomputing facility, the Center for Computational Research, to perform quantum chemistry calculations. Difficulties relating to the absorption of light by the nickel (II) compound.
The results, as explained in the Science Advances article: “We report the experimental low temperature MChD spectra of two archetypal chiral paramagnetic crystals taken as model systems, nickel (II) and cobalt (II) nitrate, for the light propagating parallel or perpendicular to the c-axis of the crystals, and the calculation of MChD spectra for the Ni (II) derivative by advanced quantum chemistry calculations.
“By incorporating vibronic coupling, we find a good agreement between experience and theory, which paves the way for MChD to develop into a powerful chiral spectroscopic tool and provide fundamental information for the chemical design of new magnetochiral materials for technological applications. “
While the study is a matter of basic science, Rikken notes the following regarding the future potential of MChD: “We find experimentally that (for the materials we studied), at low temperature, the difference in light transmission parallel and anti-parallel to a modest magnetic field of 1 Tesla, little more than what a refrigerator magnet produces, can reach 10%. Our calculations allow us to understand this in detail. The size of the effect and its detailed understanding now opens the door to future MChD applications, which could range from optical diodes to new methods of optical data storage. “
The research was funded by the French National Research Agency and the US National Science Foundation, with additional support from the National Center for Scientific Research, the University of Bordeaux, the Nouvelle Aquitaine Regional Council, the ‘European Union (EU) Horizon 2020 and research innovation program and the EU Ministry of Higher Education and Research.