Strong X-ray magneto-chiral dichroism in paramagnetic molecular helices

30-03-2015

Chiral systems, objects that are not superimposable on their mirror images, show differential absorption of left and right circularly polarised light. When chiral systems are also magnetic, they can adsorb unpolarised light in a different way depending on the direction of the chirality, magnetisation, and propagation of light vectors. This effect, known as magneto-chiral dichroism, was observed for the first time only in 1997 with visible [1] light and even more recently with X-rays [2]. The magnetochiral effect has been speculated to be a possible origin of the homochirality of life on earth, as an alternative to the parity-violating electroweak nuclear interactions [3]. It is also of interest for photonic applications but very few examples of magneto-chiral dichroism are available in the literature, most of them reporting a very weak effect. Recently, very strong magneto-chiral dichroism in paramagnetic chiral molecular systems in the hard X-ray range has been detected at ESRF’s beamline ID12.

  • Share

The interplay between chirality and magnetism has previously attracted the interest of many eminent scientists including Pasteur [4], and it continues to be at the centre of intense research activity. Magnetism and chirality are indeed connected in the interaction of a chiral system with light through magneto-chiral dichroism, which is observable only in systems where both inversion and time-reversal symmetries are simultaneously broken. These symmetry conditions are satisfied in magneto-electric media and multiferroics and also in chiral media placed in a magnetic field. The striking features of magneto-chiral dichroism can be derived from symmetry considerations. It is independent of light polarisation but its sign depends on both the relative orientation of the light propagation direction, the applied magnetic field and the sample chirality. Magneto-chiral dichroism is in general very weak, commonly assumed proportional to the product of two other weak phenomena: natural and magnetic circular dichroism. Only a few examples have been reported in the literature and no systematic experimental studies on the origin of the phenomenon are available.

Two molecular helices crystallising in enantiopure trigonal crystals and comprising cobalt(II) or manganese(II) ions bridged by stable organic radicals (see Figure 1a) have been investigated at the K-edge of the transition metals using the unique capabilities of beamline ID12. All three dichroic spectra (X-ray magnetic circular dichroism (XMCD), X-ray natural circular dichroism (XNCD) and X-ray magnetochiral dichroism (XMχD)) have been measured on pairs of opposite enantiomers, using single crystals with a volume of the order of a mm3. The use of single crystals, or more generally of oriented samples, is mandatory given that the magneto-chiral dichroism in the X-ray range is dominated by the quadrupolar term of the expansion of the light-matter interaction, thus vanishing in isotropic media.

Simplified structure and X-ray absorption at the Co K-edge recorded on a single crystal.

Figure 1. a) Simplified structure of the [M(hfac)2(NITPhOMe)]∞ molecular helices (M=Co and Mn, hfac=hexafluoroacetylacetonate) winding clockwise or anticlockwise along the c crystallographic axis depending on the crystallisation in the P31 or P32 space groups. The bridging radical unit is highlighted by the red circle. b) The experimental X-ray absorption (XANES) at the Co K-edge recorded on a single crystal at 5 K and under a magnetic field of 3 Tesla is reported together with the three measured dichroic contributions: natural circular (XNCD), magnetic circular (XMCD) and magneto-chiral (XMχD) dichroism.

The results for the cobalt derivatives are reported in Figure 1b. The availability of  both enantiomers has allowed us to check the change of sign for XNCD and XMχD signals as well as the change in sign upon the reversal of the magnetic field for XMCD and XMχD. In addition, the magneto-chiral dichroism is comparable to the natural dichroism and larger than the magnetic dichroism, clearly indicating that the common assumption that the magneto-chiral dichroism is the product of two weak phenomena is not valid, at least in the hard X-ray regime.

Figure 1a clearly shows that the spectral shapes of the three dichroic contributions are very different, the magneto-chiral dichroism emerging as a narrow peak coincident with transitions from 1s to 3d orbitals of the cobalt atom induced by electric quadrupolar interactions. Interestingly, the isomorphous compound containing manganese(II), an isotropic magnetic ion given its d5 configuration, exhibits similar XNCD and XMCD signals but a vanishingly small magneto-chiral dichroism. The orbital magnetism of the cobalt ion is also responsible for the non collinear spin structure of this molecular helix, which seems therefore correlated to the observed strong magneto-chiral dichroism.

It is interesting to note that, while XMCD measured at the K-edge of transition metal ions provides information on the orbital magnetic moment of the ion, XMχD gives access to a more elusive quantity: the anapole orbital current or orbital toroidal moment. The two quantities are schematised in Figure 2. They are both time reversal odd, i.e. they change sign reversing the arrows of the current, but only the latter is non-symmetric with respect to the inversion symmetry, i.e. is parity odd.

Representation of the orbital dipolar currents (in blue) and orbital anapole currents (in red) of the Co atom.

Figure 2. Representation of the orbital dipolar currents (in blue) and orbital anapole currents (in red) of the Co atom detected at the K-edge through the measurement of the magnetic circular and magneto-chiral dichroism, respectively.

Orbital toroidal currents seem to be involved in different phenomena ranging from multiferroicity to superconductivity, and their accurate determination through hard X-ray spectroscopy has been demonstrated here.

 

Principal publication and authors
Strong magneto-chiral dichroism in a paramagnetic molecular helix observed by hard X-rays, R. Sessoli (a), M.–E. Boulon (a), A. Caneschi (a), M. Mannini (a), L. Poggini (a), F. Wilhelm (b) & A. Rogalev (b), Nature Physics 11, 69 (2015).
(a) Department of Chemistry ‘Ugo Schiff’ and INSTM Research Unit, University of Florence (Italy)
(b) ESRF

 

References
[1] G.L.J.A. Rikken and E. Raupach, Nature 390, 493 (1997).
[2] J. Goulon et al., Phys. Rev. Lett. 88, 237401 (2002).
[3] L.D. Barron, in Physical Origin of Homochirality in Life, ed. by D.B. Cline AIP, Woodbury, NY, 1996.
[4] L. Pasteur, Rev. Sci. 7, 2 (1884).

 

Top image: The orbital dipolar currents (in blue) and orbital anapole currents (in red) of the Co atom.