Observation of the asphericity of 4f-electron density and its relation to the magnetic anisotropy axis in single-molecule magnets

The distribution of electrons in the 4f orbitals of lanthanide ions is often assigned a crucial role in the design of single-molecule magnets, which maintain magnetization in zero external field. Optimal spatial complementarity between the 4f-electron density and the ligand field is key to maximizing magnetic anisotropy, which is an important factor in the ability of lanthanide complexes to display single-molecule magnet behaviour. Here we have experimentally determined the electron density distribution in two dysprosium molecular complexes by interpreting high-resolution synchrotron X-ray diffraction with a multipole model. The ground-state 4f-electron density is found to be an oblate ellipsoid, as is often deduced from a simplified Sievers model that assumes a pure |+/- 15/2> ground-state doublet for the lanthanide ion. The large equatorial asymmetry-determined by a model wavefunction-was found to contain considerable M-J mixing of |+/- 11/2> and only 81% of |+/- 15/2>. The experimental molecular magnetic easy axes were recovered, and found to deviate by 13.1 degrees and 8.7 degrees from those obtained by ab initio calculations. Gaining a better understanding of the complex electronic structure of single-molecule magnets is essential for their design and development. The 4f-electron density distribution of a dysprosium single-molecule magnet has now been experimentally determined using synchrotron diffraction data interpreted with a multipole model. The magnetic easy axes were recovered by analysis of the 4f-electron density shape, which is clearly oblate.