Strong magneto-chiral dichroism in a paramagnetic molecular helix observed by hard X-rays

Magneto-chiral dichroism is a non-reciprocal—that is, directional—effect observed in magnetized chiral systems, featuring an unbalanced absorption of unpolarized light depending on the direction of the magnetization. Despite the fundamental interest in a phenomenon breaking both parity and time-reversal symmetries, magneto-chiral dichroism is one of the least investigated aspects of light–matter interaction most likely because of the weakness of the effect in most reported experiments. Here we have exploited the element selectivity of hard X-ray radiation to investigate the magneto-chiral properties of enantiopure crystals of two isostructural molecular helicoidal chains comprising either cobalt(II) or manganese(II) ions. A strong magneto-chiral dichroism, with Kuhn asymmetry of the order of a few per cent, has been observed in the cobalt chains system, whereas it is practically absent for the manganese derivative. The spectral features of the X-ray magneto-chiral dichroism signal differ significantly from the natural and magnetic dichroic contributions and have been rationalized here using the multipolar expansion of matter–radiation interaction.