Radical-lanthanide ferromagnetic interaction in a TbIII bis-phthalocyaninato complex

Eufemio Moreno-Pineda,Claudio Bonizzoni,Svetlana Klyatskaya,Mario Ruben,Alessandro Soncini,Stephen Hill,Dorsa Komijani,Alberto Ghirri,Marco Affronte

doi:10.1103/physrevmaterials.2.024405

Abstract

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.

Highlights

The study of electron delocalization involving spin-bearing ligands coordinated to metal ions is relevant to a wide range of research topics, including organic electronics, photovoltaics, and catalysis, as well as many important biological processes and biomedical applications [1,2,3,4]
CODE MM10060 charge transport through organic ligands provides a means of addressing electron and nuclear quantum states associated with lanthanide qubits integrated into single-molecule spin transistors [15,16,17]
Electron paramagnetic resonance (EPR) investigations have previously been employed to study Ln-radical systems [18,19], transitions are typically silent or forbidden at the low microwave frequencies of commercial electron paramagnetic resonance (EPR) spectrometers; this is due primarily to the large moment and strong crystalfield anisotropy of most lanthanides, resulting in an Ising-type coupling to the radical and appreciable zero-field gaps associated with allowed magnetic dipole transitions

Summary

Introduction

The study of electron delocalization involving spin-bearing (radical) ligands coordinated to metal ions is relevant to a wide range of research topics, including organic electronics, photovoltaics, and catalysis, as well as many important biological processes and biomedical applications [1,2,3,4] This subject became of interest within the molecular magnetism and nanomagnetism communities, given the demonstration that radical-bearing ligands can mediate strong exchange interactions between otherwise magnetically isolated lanthanide (Ln) ions, resulting in a leap forward in the development of spin-chain systems [5,6,7] and so-called single-molecule magnets (SMMs) [8,9,10]—molecules that can be magnetized below a characteristic blocking temperature TB.

Results

Discussion

Conclusion