Enantiospecific interactions between chiral molecules and magnetic surfaces

The Chiral-Induced Spin Selectivity (CISS) effect was reported for the first time in 1999, and later it was found that photo-electrons moving through a self-assembled monolayer of DNA showed a 60% spin-polarization. This effect shows that when electrons move through chiral molecules, their transport is spin-dependent, with the preferred spin-orientation determined by the handedness of the molecule and the direction of motion. More recent studies demonstrated that upon adsorption of a chiral self-assembled monolayer (SAM), a soft ferromagnetic layer could be permanently magnetized in one direction, depending on the handedness of the adsorbed molecule. This effect originates from the well-known phenomenon in which the formation of a SAM with a large dipole moment involves charge transfer that equalizes the electrochemical potential of the adsorbed layer and the sample surface: as a result of the CISS effect, this charge transfer is spin-polarized and thus can magnetize the ferromagnetic layer. Based on these studies, we investigated the complimentary phenomenon, the possible use of a magnetized ferromagnetic layer to induce enantioselective adsorption of chiral molecules. The results indicate that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations. These results suggest that the same approach can be used to achieve chiral resolution in the crystallization processes of conglomerates. The spin-polarized surface promotes crystallization of enantiomorphous crystals depending on the direction of the magnetic moment.