Ligand Influence on Local Magnetic Moments in Fe-Based Metal–Organic Networks

Planar metal–organic networks are highly promising materials due to their modular nature and wide-ranging possible applications from spintronics up to biosensing. Spin state transitions connect local magnetic properties with structural modifications. In this paper, we report on ab initio calculations for two metal–organic planar networks, the Fe-phthalocyanine (Pc) polymer and its precursor material Fe-tetracyano­benzene (TCNB). The spin-polarized generalized gradient approximation to density functional theory with an explicit treatment of the Hubbard-<i>U</i> correction (SGGA+<i>U</i>) indicates a spin state transition between the well confirmed <i>S</i> = 1 state for Fe-Pc and a local, high-spin <i>S</i> = 2 state at the Fe site for Fe-TCNB. The high-spin state at the Fe site is confirmed by X-ray absorption spectroscopy (XAS) measurements of the Fe-TCNB network on the Au(111) substrate in connection with a multiplet analysis. We propose a possible spin state transition between Fe-TCNB and Fe-Pc by the on-surface synthesis of the latter compound. The ab initio results prove also a high chemical stability of the Fe-TCNB network, metallic and ferromagnetic behavior, as well as a partial screening of the Fe spin <i>S</i> = 2 by two antiparallel electrons on the ligand sites to a state with total spin of <i>S</i> = 1. All of this makes the Fe-TCNB network an interesting material for spintronics applications

On-Surface Reaction between Tetracarbonitrile-Functionalized Molecules and Copper Atoms

Self-assembly at surfaces has proven to be very efficient in creating ordered, atomically controlled organic nanostructures. On-surface synthesis has recently emerged as a promising strategy to create stable structures bound by strong and irreversible covalent bonds. Here we present on-surface reaction between pyrazino phenanthroquinoxaline-tetracarbonitrile (PPCN) molecules and copper atoms on a Au(111) substrate. The reaction is monitored in ultrahigh vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). After a 475 K annealing, phthalocyanine cyclization occurs around a copper atom; the increase of annealing temperature to 540 and 675 K leads to the formation of 1D and 2D phthalocyanine polymers, respectively. This reaction is confirmed by the modification of the Cu 2p_3/2 and C 1s XPS spectra upon annealing

Self-Assembled Melamine Monolayer on Cu(111)

The self-assembled structure of melamine (1,3,5-triazine-2,4,6-triamine) deposited on Cu(111) was studied under ultra-high-vacuum conditions using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoemission spectroscopy (UPS). Molecular packing and conformation were investigated as a function of the annealing temperature. The molecules deposited at room temperature were found to be aligned with the molecular plane approximately perpendicular to the surface. At around 150 °C, the molecules became aligned in a row-type structure and were all coupled to the Cu(111) surface. At still higher annealing temperatures, a new two-dimensional (2D) network was formed via the polymerization of the adsorbed monolayer of melamine. A multistep model consistent with STM, XPS, and UPS results is proposed, starting with a self-organized vertically adsorbed melamine monolayer and ending with the formation of a 2D network lying on Cu(111) interpreted as a ring-opening polymerization of melamine. The reactive Cu­(111) surface is believed to be one of the keys in this multistep reaction

Microwave-Mediated Synthesis of Bulky Lanthanide Porphyrin–Phthalocyanine Triple-Deckers: Electrochemical and Magnetic Properties

Five heteroleptic lanthanide porphyrin–bis-phthalocyanine triple-decker complexes with bulky peripheral groups were prepared via microwave-assisted synthesis and characterized in terms of their spectroscopic, electrochemical, and magnetic properties. These compounds, which were easily obtained under our preparative conditions, would normally not be accessible in large quantities using conventional synthetic methods, as a result of the low yield resulting from steric congestion of bulky groups on the periphery of the phthalocyanine and porphyrin ligands. The electrochemically investigated triple-decker derivatives undergo four reversible one-electron oxidations and three reversible one-electron reductions. The sites of oxidation and reduction were assigned on the basis of redox potentials and UV–vis spectral changes during electron-transfer processes monitored by thin-layer spectroelectrochemistry, in conjunction with assignments of electronic absorption bands of the neutral compounds. Magnetic susceptibility measurements on two derivatives containing Tb^III and Dy^III metal ions reveal the presence of ferromagnetic interactions, probably resulting from magnetic dipolar interactions. The Tb^III derivative shows SMM behavior under an applied field of 0.1 T, where the direct and Orbach process can be determined, resulting in an energy barrier of <i>U</i>_eff = 132.0 K. However, Cole–Cole plots reveal the presence of two relaxation processes, the second of which takes place at higher frequencies, with the data conforming to a 1/<i>t</i> ∝ <i>T</i>⁷ relation, thus suggesting that it can be assigned to a Raman process. Attempts were made to form two-dimensional (2D) self-assembled networks on a highly oriented pyrolytic graphite (HOPG) surface but were unsuccessful due to bulky peripheral groups on the two Pc macrocycles