Modifying the Magnetic Anisotropy of an Iron Porphyrin Molecule by an on-Surface Ring-Closure Reaction

Abstract

The magnetic properties of adsorbed metalloporphyrin molecules can be altered or tuned by the substrate, additional axial ligands, or changes to the molecules' macrocycle. These modifications influence the electronic configuration of the fourfold-coordinated central metal ion that is responsible for the metalloporphyrins' magnetic properties. We report a substantial increase in the effective spin moment obtained from sum-rule analysis of X-ray magnetic circular dichroism (XMCD) for an iron metalloporphyrin molecule on Au(111) through its conversion from iron(II)-octaethylporphyrin (FeOEP) to iron(II)-tetrabenzoporphyrin (FeTBP) in a surface-assisted ring-closure ligand reaction.Density functional theory calculations with additional strong Coulomb correlation (DFT+U ) show that the on-surface reaction alters the conformation of the molecule, increasing its planarity and the ion-surface distance. A spin-Hamiltonian fit of the magnetization as a function of field reveals a substantial increase in the intra-atomic magnetic dipole term (T z ) and a decrease in the magnitude of the easy-plane anisotropy upon ring closure. This consequence of the ring closure demonstrates how new magnetic properties can be obtained from on-surface reactions, resulting here in significant modifications to the magnetic anisotropy of the Fe ion, and sheds light onto the molecule-substrate interaction in these systems.

Room temperature magnetism of ordered porphyrin layers on Fe

“…Tuning these properties in view of obtaining a regular assembly of magnetic units is one of the main goals of magnetochemistry. 11,12 Magnetic ordering at room temperature (RT) and without the need of external magnetic fields is obtained by choosing a ferromagnetic substrate 13,14 and stems either from the direct exchange between the unpaired electrons of the molecule and the substrate electronic states or from indirect superexchange mediated by the N ligands. 9,15 However, the choice of using traditional magnetic substrates, such as transition metals (TMs), is usually precluded by the poor morphological quality and structural integrity of the molecular layers, due to the strong chemical interaction with the surface.…”

Insight into intramolecular chemical structure modifications by on-surface reaction using photoemission tomography

“…For instance, a substantial increase in the effective spin moment has been reported in the case of iron(II)-TBP formed upon on-surface reaction on Au(111) from iron(II)-OEP. 5 In contrast to this system, CoOEP on Cu(100) has shown a sizable magnetic moment, while CoTBP displayed a quenched moment on this substrate. 4 Notably, the manipulation of the magnetic anisotropy in metal porphyrin molecules induced by ringclosure reactions is a key issue for the development of molecular-based spintronic devices.…”

“…Together with the surface-driven conformational changes, the porphyrin molecules may undergo on-surface temperatureinduced chemical changes of their molecular structure. [2][3][4][5][6] A well-known surface-assisted ring-closure reaction in the case of metal-OEP involves the peripheral ethyl groups and results in the transformation of the OEP in tetrabenzoporphyrin (TBP), which occurs via the dehydrogenation of two neighbouring ethyl groups, followed by the formation of six-membered carbon rings (see Fig. 1).…”

“…1). 5,7,8 This on-surface reaction can deeply alter the intriguing properties of the porphyrin array. For instance, a substantial increase in the effective spin moment has been reported in the case of iron(II)-TBP formed upon on-surface reaction on Au(111) from iron(II)-OEP.…”

“…16 In the present work, we will show that the PT approach can also be extended to the study of surfaceinduced intramolecular modifications, i.e., ring-closure reaction, in single molecules self-assembled in molecular arrays. More frequently, intramolecular reactions are monitored by STM, temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS) and NEXAFS techniques, [2][3][4][5][6]17,18 which do not always allow the authoritative identification of an adsorbed species. Only indirect conclusions can be made from, e.g., the presence or absence of specific vibrational modes in infrared spectroscopy or chemical shifts in core-level spectroscopy.…”