Radical-lanthanide ferromagnetic interaction in a T bIII bis-phthalocyaninato complex

Recent studies have highlighted the importance of organic ligands in the field of molecular spintronics, via which delocalized electron-spin density can mediate magnetic coupling to otherwise localized 4f moments of lanthanide ions, which show tremendous potential for single-molecule device applications. To this end, high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy is employed to study a neutral terbium bis-phthalocyaninato metalorganic complex, [TbPc2]0, with the aim of understanding the magnetic interaction between the Ising-like moment of the lanthanide ion and the unpaired spin density on the coordinating organic radical ligand. The measurements were performed on a previously unknown [TbPc2]0 structural phase crystallizing in the Pnma space group. EPR measurements on powder samples of [TbPc2]0 reveal an anisotropic spectrum, which is attributed to the spin-12 radical coupled weakly to the EPR-silent TbIII ion. Extensive double-axis rotation studies on a single crystal reveal two independent spin-12 signals with differently oriented (albeit identical) uniaxial g-tensors, in complete agreement with x-ray structural studies that indicate two molecular orientations within the unit cell. The easy-axis nature of the radical EPR spectra thus reflects the coupling to the Ising-like TbIII moment. This is corroborated by studies of the isostructural [YPc2]0 analog (where Y is nonmagnetic yttrium), which gives a completely isotropic radical EPR signal. The experimental results for the terbium complex are well explained on the basis of an effective model that introduces a weak ferromagnetic Heisenberg coupling between an isotropic spin-12 and an anisotropic spin-orbital moment, J=6, that mimics the known, strong easy-axis TbPc2 crystal-field interaction.