Emergence of On-Surface Magnetochemistry

Abstract

The control of exchange coupling across the molecule−substrate interface is a key feature in molecular spintronics. This Perspective reviews the emerging field of on-surface magnetochemistry, where coordination chemistry is applied to surface-supported metal porphyrins and metal phthalocyanines to control their magnetic properties. The particularities of the surface as a multiatomic ligand or "surface ligand" are introduced. The asymmetry involved in the action of a chemical ligand and a surface ligand on the same planar complexes modifies the well-established "trans effect" to the notion of the "surface-trans effect". As ad-complexes on ferromagnetic substrates are usually exchange-coupled, the magnetochemical implications of the surface-trans effect are of particular interest. The combined action of the different ligands allows for the reproducible control of spin states in on-surface supramolecular architectures and opens up new ways toward building and operating spin systems at interfaces. Notably, spin-switching has been demonstrated to be controlled collectively via the interaction with a ligand (chemical selectivity) and individually via local addressing at the interface.A century ago, Alfred Werner laid the foundation of modern coordination chemistry, an achievement for which he was awarded the Nobel Prize in Chemistry in 1913. The innovative concept of primary and secondary valences in coordination complexes that he introduced has since then developed into oxidation states and coordination numbers and become textbook knowledge for introductory chemistry. 1 Briefly, the transition-metal ion in a given coordination complex with a fixed oxidation state can be either diamagnetic or paramagnetic, depending on the number of the ligands as well as their nature. It is the ligands that are responsible for the splitting of the d orbitals of the metal ion into t 2g and e g levels, which are at the heart of ligand field theory (LFT). 1 Thereby, ligand coordination directly alters the magnetic properties. Among the existing coordination complexes, particularly spin-bearing four-coordinated square-planar porphyrins and phthalocyanines have received much attention during the last six decades. Here, the coordination number of the metal ion and hence its magnetic properties can be easily modified by coordination with external axial ligands leading to five/six-fold coordination complexes. 2 Recently, on-surface coordination chemistry of metal porphyrins and metal phthalocyanines has raised increasing attention. 3−5 In this field of research, coordination chemistry of the planar molecules is studied in the presence of a surface ligand and an optional axial ligand binding to the free on-top site. A central issue is whether the known gas-phase coordination chemistry concepts remain valid for ligand coordination of spin-bearing planar metal−organic molecules assembled on (magnetic) surfaces. Emerging evidence shows that due to the surface−molecule interaction, novel magnetochemical effects arise beyond those known...

Controlling the Spin of Co Atoms on Pt(111) by Hydrogen Adsorption

“…DOI: 10.1103/PhysRevLett.114.106807 PACS numbers: 73.22.-f, 32.10.Dk, 75.30.Gw, 75.75.-c Individual surface-adsorbed magnetic atoms exhibit remarkably large orbital moments and anisotropies [1][2][3][4][5]. Like in adsorbed single ion molecules [6][7][8][9][10], their chemical environment can be tailored through exposure to reactive molecules, thus allowing the tuning of their magnetic properties [11]. Among the wealth of available molecules, H 2 is of particular interest.…”

Antiferromagnetic coupling of Cr-porphyrin to a bare Co substrate

“…The origin of this alignment has been much discussed [1,2,7,10,11] and assigned either to direct or indirect magnetic exchange coupling via the chemical bonds across the interface. Understanding and controlling molecule-substrate interactions are key ingredients for designing novel functional surfaces [15].To date, an antiferromagnetic (AFM) indirect exchange interaction has been reported for planar molecules assembled on oxygen reconstructed Ni and Co substrates [5,6] or when a graphene interlayer [12] was introduced between the molecule and substrate. An indirect AFM coupling was also observed for the rare-earth double-decker Tb-bis-phthalocyanine adsorbed on a metallic FM substrate, where one macrocycle acts as an interlayer [13].…”

“…The origin of this alignment has been much discussed [1,2,7,10,11] and assigned either to direct or indirect magnetic exchange coupling via the chemical bonds across the interface. Understanding and controlling molecule-substrate interactions are key ingredients for designing novel functional surfaces [15].…”

“…In "on-surface" configurations novel properties can arise when the magnetic and electronic characteristics of the metal-organic species are subtly modified by the surface-molecule interaction. Accordingly, it was discovered that metallophthalocyanines and metalloporphyrins with 3d and 4f block metal centers exhibit remarkable electronic, optoelectronic, and magnetic properties when adsorbed on surfaces [1,2, [5][6][7][8][9][10][11][12][13][14][15][16]. Strikingly, the magnetic exchange coupling of the spin of metalloporphyrins or metallophthalocyanines with that of ferromagnetic (FM) substrates (Co and Ni) was found to induce stable magnetic order in the paramagnetic molecules at and above room temperature [1,2,11].…”

Bulk spin-crossover in the complex [FeL(NCS)2] of a tris(pyridyl)ethane-derived N4-ligand—a temperature-dependent crystallographic study

“…This has led to a growing interest in the magnetochemistry of various types of complexes on surfaces and the possibility to preserve bulk behaviour in ensembles with reduced dimensions. 9,10 There are many examples where direct contact of a coordination compound with a surface efficiently quenches the SCO behaviour otherwise observed. For instance, electron-induced spin-state switching in ultra-high vacuum (UHV) deposited [Fe II (bpb) 2 ( phen)] (bpb: bis[1H-pyrazol-1-yl]borate; phen: 1,10-phenanthroline) on Au(111) only occurs from the second molecular layer onwards.…”