Lanthanide-Radical Magnetic Coupling in [LnPc2]0: Competing Exchange Mechanisms Captured via Ab Initio Multi-Reference Calculations

Hj Huang ,Willem Van Den Heuvel ,Alessandro Soncini

doi:10.20900/qmr20200003

Abstract

We present a computational investigation of the intramolecular exchange coupling in [LnPc$_2$]$^0$ (Ln = Tb, Dy, Ho, and Er) between the Ln$^{3+}$ 4f electrons and the spin-1/2 radical on the phthalocyanine ligands. A series of ab initio multi-configurational/multi-reference Complete/Restricted Active Space Self-Consistent-Field calculations (CASSCF/RASSCF), including non-perturbative spin--orbit coupling, were performed on [LnPc$_2$]$^0$ and on the smaller model compound [LnPz$_2$]$^0$. We find that the exchange coupling mechanisms are restricted by symmetry, but also dependent on the spin polarization effect triggered by the Pc$_2$ ligands $\pi$--$\pi^*$ excitations. The calculated exchange splittings are small, amounting to at most a few cm$^{-1}$, in disagreement with previous literature reports of strong antiferromagnetic coupling, but in good agreement with recent EPR experiments on [TbPc$_2$]$^0$. Furthermore, the coupling strength is found to decrease from [TbPc$_2$]$^0$ to [ErPc$_2$]$^0$, with decreasing number of unpaired electron spins in the lanthanide ground (Hund's rule) Russell--Saunders term.

Highlights

Single molecule magnets based on the lanthanide phthalocyanine double-decker ([LnPc2]±1/0, Ln = lanthanide, Pc = phthalocyanine) are of particular interest due to their large barrier to magnetic relaxation and high blocking temperatures, especially compared to traditional single molecule magnets based on transition metals [1,2,3,4,5,6,7]
The occurrence of ferromagnetic exchange interaction in the Complete Active Space Self-Consistent Field (CASSCF) calculations can be explained on the basis of a symmetry analysis in the approximate D4d point group of the molecule: The singly occupied molecular orbital (SOMO) of Pc2 transforms as a2, while the seven lanthanide 4f orbitals
We have presented results of a computational study of the intramolecular exchange coupling between Ln3+ 4f electrons and the Pc2 radical in [LnPc2]0 (Ln = Tb, Dy, Ho, and Er) molecules

Summary

Introduction

The occurrence of ferromagnetic exchange interaction in the CASSCF calculations can be explained on the basis of a symmetry analysis in the approximate D4d point group of the molecule: The SOMO of Pc2 transforms as a2 (see top right of Figure 1), while the seven lanthanide 4f orbitals The absence of the outer benzene rings will a ect the calculated spin density distribution and exchange coupling to some extent, but we expect that the physics of the exchange mechanisms will be correctly represented by the LnPz2 models.

Results

Conclusion