Molekular Magnetismus

Der molekulare Magnetismus ist ein multidisziplinäres Feld, das die Synthese von Molekülen mit gewünschten magnetischen Eigenschaften kombiniert und theoretische Untersuchungen zum mechanistischen Verständnis der Beziehung dieser Eigenschaften zur elektronischen Struktur enthält. Spektakuläre Fortschritte bei der Entwicklung und Implementierung von spin-abhängigen relativistisch korrelierten Elektronenstrukturverfahren für größere Systeme durch Prof. Dr. Frank Neese und sein Entwicklerteam im Programm ORCA lieferten die Infrastruktur und dienen als feste theoretische Grundlage für dieses Projekt. Die Kombination von Experiment und Theorie auf einer Eins-zu-Eins-Basis ermöglicht es, magnetische Eigenschaften von magnetischen Zielmaterialien sowohl zu interpretieren als auch vorherzusagen (rational zu gestalten). Molekularer Magnetismus hat große Auswirkungen auf eng verwandte Disziplinen wie molekulare Elektronik und chemische Reaktivität (homogene und heterogene Katalyse). Letzteres wurde von der Wirkung von Übergangsmetall-Spinzentren als aktiven Zentren in Enzymen wie die Spaltung von Wasser auf einem vierkernigen gemischtvalenten Mangankomplex (Photosystem II) und dem Sauerstoff- und Wasserstoffatomtransfer in Häm und Nicht-Häm hochvalente Eisenenzymen, die zu Energiespeichermolekülen führen. All dies motivierte 2013 die Gründung der Gruppe molekularer Magnetismus. Sie kombiniert Experiment: Spektroskopien –Elektron Paramagnetischer Resonanz(EPR), Absorption- und Emission Spektroskopie(UV-VIS), Magnetischer Circular Dichroismus (MCD) und grundlegende theoretische  Handwerkzeuge: i) Computerprotokolle, die es erlauben, magnetische und spin-Hamilton (SH) Parameter aus ab-initio korrelierten Wellenfunktionen zu extrahieren, und ii) eine ab-initio basierte Ligandenfeldtheorie (AILFT), die es erlaubt die magnetischen Parameter mit der Art der Metall-Ligand-Bindungen und den komplexen Geometrien zu knüpfen.

Open d- or f-shell transition metal or lanthanides and actinides in complexes  give rise to an unique response to an external magnetic field governed by the preferred alignments of their magnetic moments. This property is termed magnetic anisotropy. In axial symmetry this anisotropy is described by one parameter D which  quantifies  the zero-field splitting of the 2S+1 sublevels M ( -S<M<S )  of the spin S (E=DM2) of the non-relativistic ground state.  Magnetic anisotropy arises from coupling between orbital and spin angular momenta.  Below a given temperature (called the blocking temperature, TB) the resulting total angular momentum and its magnetization induced by an external magnetic  field  may persists after switching off  the field. The specific  life-time of  this magnetic state  -the  relaxation time depends on the magnitude of the orbital angular momentum and its coupling to its dissipative surrounding (thermal bath). Orbital momenta are maximal in atoms and ions with orbitally degenerate ground states, such as FeI 4F(d7) or FeII 2D(d6). However, because of their spherical symmetry, such orbital moments are isotropic.  Anisotropic magnetic moments are intrinsic for  molecules possessing axial (four of threefold) symmetries, which in addition, are in orbitally degenerate or nearly-denerate ground states. Spin-orbit coupling  introduces atomic like orbital moments which couple with the spin and this leads to an entirely anisotropic magnetization (Ising anisotropy).  This magnetization  is maximal along the axis of quantization  (the easy axis, D<0)  and (in weak magnetic fields) very small in directions perpendicular to it.

The project focuses on magnetic properties of transition metal complexes with open d-shells. Open d- or f-shell transition metal or a lanthanide complex can give rise to an unique response to an external magnetic field governed by the preferred alignments of their magnetic moments. This property is termed magnetic anisotropy. In axial symmetry this anisotropy is described by one parameter D which quantifies the zero-field splitting of the 2S+1 sublevels M ( -S≤M≤S ) of the spin S (E=DM²) within a given electronic state (normally the non-relativistic ground state). Magnetic anisotropy arises from coupling between orbital and spin angular momenta. Below a given temperature (called the blocking temperature, T_B) the resulting total angular momentum and its magnetization induced by an external magnetic field may persist after switching off the field. The specific life-time of this magnetic state -the relaxation time depends on the magnitude of the orbital angular momentum and its coupling to its dissipative surrounding (thermal bath). Orbital momenta are maximal in atoms and ions with orbitally degenerate ground states, such as Co(II) ⁴F or Fe(II) ²D. However, because of the spherical symmetry, such orbital moments are isotropic and do not create magnetic anisotropies. Anisotropic magnetic moments are intrinsic for molecules possessing axial four of threefold symmetries, which in addition, are in orbitally degenerate ground states. First order spin-orbit coupling in such systems introduces atomic like orbital moments which couple with the spin and this leads to an entirely anisotropic magnetization. This magnetization is maximal along the axis of quantization (the easy axis, D<0) and (in weak magnetic fields) very small in directions perpendicular to it (Ising type anisotropy). A series of pseudo-tetrahedral Fe(II) complexes of this type has been theoretically predicted [1] by first principles calculations and synthesized and magnetically characterized [2,3] at about the same time (see Figure 1). These were shown to display slow relaxation of the magnetization and were termed single ion magnets (SIM). These discoveries opened a new field in magneto chemistry. The ultimate goal of the studies carried out within this project is to increase the blocking temperature and the relaxation time making such systems potential candidates for magnetic memory devices. Systems with magnetically bi stable ground states in molecules well isolated from their dissipative surroundings can also display quantum coherence and are therefore of potential interest in future quantum computers.

Based on the infrastructure given by the development and implementation of multi reference electronic structure methods in the ORCA program by Prof. Frank Neese and his group, we developed and tested computational protocols for first principles computations of the magnetic and spectroscopic properties of SIM which allow to both interpret and predict such phenomena [4]. We applied these tools to the interpretation of existing SIM and to assist rational design of novel ones with improved magnetic properties. We studied trigonal four coordinate (Figure 1, [4]) and pseudo linear two coordinate Fe(II) complexes (Figure 2, [5,6]) the latter displaying magnetic anisotropies on unprecedented size.

A fruitful scientific exchange with our collaborators (Prof. J. Long, Berkeley, USA) culminated in understanding of the magnetic properties of these class of compounds and the first SIM displaying a magnetic hysteresis and magnetic blocking temperature T_B=4K (Figure 3) [7].