CHIMMM · Discovery of novel chiral magnetic molecular materials for the study of magnetochiral effects

Discovery of novel chiral magnetic molecular materials for the study of magnetochiral effects

Chirality is a property found in everyday objects (e.g. screws and springs), in the human body (e.g. hands and feet), or in molecules (e.g. the DNA helix) that originates from the lack of an inversion center. Chiral objects are distinct from their mirror images, which, on the molecular level, are called enantiomers (Fig. 1). An enantiomeric pair has identical physical properties in non-chiral media, and can only be distinguished by their interaction with other chiral media. Molecular chirality was discovered by Louis Pasteur in the 19th century. During his experiments, he observed that a solution of natural tartaric acid obtained from wine yeast rotates polarized light, while a solution of the same product synthesized in the lab has no such effect. This is because the natural tartaric acid is a homochiral product, consisting of only one enantiomer that rotates the polarization plane of light, while synthetic tartaric acid contains a 50/50 mixture of enantiomers, or a racemic mixture, with each component rotating polarized light in opposite sense and thus cancelling the effect. Because two enantiomers are chemically identical, a chemical reaction always produces a mixture of enantiomers, unless a chiral agent is present. However, the building blocks of life, such as amino acids, DNA, and carbohydrates are not found as racemic mixtures, but are instead homochiral. This raises the question of what was the external chiral influence that generated the preference for one handedness at the origin life. One explanation is based on the fact that enantiomers react differently to polarized electromagnetic radiation. This phenomenon is called natural optical activity and some authors have hypothesized that it could be responsible for biological homochirality based on studies of enantioselective reactions using circularly polarized light. Note that this type of polarized light is very rare on earth and thus homochirality according to this hypothesis would probably have an extraterrestrial origin. Another source could be the magnetochiral effect, which is the different interaction of enantiomers with nonpolarized light in the presence of a magnetic field. Although the magnetochiral effect was predicted in the 1980s, to date, there have only been a few investigations of this phenomenon. Because the magnetochiral effect is very small, we wanted to try to maximize it by using strongly chiral compounds with a high spin, (e.g. many unpaired electrons). Such compounds are quite rare, as although molecular magnetism has been an active topic of research for the last 30 years, homochiral compounds have been scarcely explored mainly because of the absence of rational preparation procedures. This is a undoubtedly a subject that deserves more attention as the combination of chirality and magnetism can provide important information about the origin of homochirality in life and give rise to advanced materials including piezoelectrics, pyroelectrics, non-linear optical devices or molecular multiferroics.

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