Magnetochemistry: advances in theory and experimentation

The article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices. The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligoand polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

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Spin state switching in iron coordination compounds

Nanoscale spin crossover materials capable of undergoing reversible switching between two electronic configurations with markedly different physical properties are excellent candidates for various technological applications. In particular, they can serve as active materials for storing and processing information in photonic, mechanical, electronic, and spintronic devices as well as for transducing different forms of energy in sensors and actuators. In this progress report, a brief overview on the current state-of-the-art of experimental and theoretical studies of nanomaterials displaying spin transition is presented. Based on these results, a detailed analysis and discussions in terms of finite size effects and other phenomena inherent to the reduced size scale are provided. Finally, recent research devices using spin crossover complexes are highlighted, emphasizing both challenges and prospects.

Spin Crossover Nanomaterials: From Fundamental Concepts to Devices

In der Ubergangsmetallchemie gibt es eine Klasse von Komplexverbindungen, bei denen eine Temperaturerniedrigung einen Wechsel im Spinzustand des Zentralatoms vom High-Spin- in den Low-Spin-Zustand bewirkt. Dabei andern sich die magnetischen und optischen Eigenschaften, uber die der thermische Spinubergang (auch Spincrossover genannt) sehr gut verfolgt werden kann. Dieses Phanomen tritt sowohl in flussiger Phase als auch im Festkorper auf. Eine herausragende Stellung nehmen Eisen(II) - Spincrossover - Verbindungen ein, in denen der Spinubergang im Festkorper auf sehr unterschiedliche Weise - graduell, abrupt, mit Hysterese oder stufenweise - verlaufen kann und mit Mosbauer- und optischer Spektroskopie, mit magnetischen Suszeptibilitats- und Warmekapazitatsmessungen sowie durch Kristallstrukturanalyse intensiv untersucht worden ist. Die kooperative Wechselwirkung zwischen den einzelnen Komplexmolekulen kann befriedigend durch elastische Eigenschaften und durch die Anderung von Gestalt und Volumen der Komplexmolekule beim Spinubergang erklart werden. Bei Untersuchungen an Eisen(II)-Spincrossover-Verbindungen konnte man beobachten, das sich der Low-Spin-Zustand mit grunem Licht in den High-Spin-Zustand umschalten last, der bei tiefen Temperaturen eine nahezu unendlich lange Lebensdauer haben kann (LIESST = Light-Induced Excited Spin State Trapping). Mit rotem Licht last sich der metastabile High-Spin- wieder in den Low-Spin-Zustand zuruckschalten. Der Mechanismus des LIESST-Effekts ist aufgeklart, die Zerfallskinetik im Detail untersucht und im Rahmen der Theorie strahlungsloser Ubergange verstanden. Anwendungen des LIESST-Effekts in der optischen Informationstechnik sind denkbar.

Thermisch und optisch schaltbare Eisen(II)‐Komplexe

Spin-crossover coordination nanoparticles of the cyanide-bridged three-dimensional network Fe(pyrazine){Pt(CN)4} were prepared at three different sizes using a microemulsion. The 14 nm particles present a transition centered around 265 K with a hysteresis of 6 K.

Spin-crossover coordination nanoparticles.

Spin-crossover ferric complexes: unusual effects of grinding and doping solids

An explanation of unusual properties of spin-crossover ferric complexes

Etude par mesures de susceptibilite magnetique et par spectrometrie Mossbauer, RPE et IR de ce complexe ou Sal APA correspond a la base de Schiff derivee du salicylaldehyde et de l'(amino-3' propyl)-1 aziridine (coordinat tridente)

Dynamics of spin-state interconversion and cooperativity for ferric spin-crossover complexes in the solid state. 1. The unperturbed fast spin-flipping N4O2 complex [Fe(SalAPA)2]ClO4

Etude par spectrometrie RPE et Mossbauer principalement du meme complexe que celui etudie ibid., 3861-3870, mais prepare sous forme solvate, puis disolvate ou broye

Dynamics of spin-state interconversion and cooperativity for ferric spin-crossover complexes in the solid state. 2. Perturbations of the fast spin-flipping N4O2 complex [Fe(SalAPA)2]ClO4

Dynamics of spin-state interconversion and cooperativity for ferric spin-crossover complexes in the solid state. 1. the unperturbed fast spin-flipping n4o2 complex (fe(salapa)2)clo4

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