P. Gütlich

Mössbauer spectroscopy and transition metal chemistry

1. Introduction.- 2. Basic Physical Concepts.- 3. Hyperfine Interactions.- 4. Experimental.- 5. Mathematical Evaluation of Mo ssbauer Spectra.- 6. Interpretation of Mo ssbauer Parameters of Iron Compounds.- 7. Mo ssbauer-Active Transition Metals Other than Iron.- 8. Some Special Applications.

Light-induced excited spin state trapping in a transition-metal complex: The hexa-1-propyltetrazole-iron (II) tetrafluoroborate spin-crossover system

We report the first example of a light-induced conversion of the low-spin (LS) state (1A1g) to the high-spin (HS) state (5T2g) in the spin-crossover iron (II) complex [Fe(ptz)6] (BF4)2, where ptz = 1-propyltetrazole. The spin state conversion is quantitative, and the trapped HS state does not decay within many hours at temperatures < 50 K. At α 50 K the excited HS state relaxes back to the normal LS state. Around 135 K, one sees the well-known thermally induced spin transition LS (1A1) α HS (5T2). Mossbauer studies demonstrate unambiguously this hitherto unknown photophysical phenomenon which we call “light-induced excited spin state trapping” (LIESST).

Jarosite and Hematite at Meridiani Planum from Opportunity's Mossbauer Spectrometer

Mössbauer spectra measured by the Opportunity rover revealed four mineralogical components in Meridiani Planum at Eagle crater: jarosite- and hematite-rich outcrop, hematite-rich soil, olivine-bearing basaltic soil, and a pyroxene-bearing basaltic rock (Bounce rock). Spherules, interpreted to be concretions, are hematite-rich and dispersed throughout the outcrop. Hematitic soils both within and outside Eagle crater are dominated by spherules and their fragments. Olivine-bearing basaltic soil is present throughout the region. Bounce rock is probably an impact erratic. Because jarosite is a hydroxide sulfate mineral, its presence at Meridiani Planum is mineralogical evidence for aqueous processes on Mars, probably under acid-sulfate conditions.

Spin crossover phenomena in Fe(II) complexes

The behaviour of spin crossover compounds is among the most striking and fascinating shown by relatively simple molecular species. This review aims to draw attention to the various ways in which spin crossover phenomena are manifested in iron(II) complexes, to offer some rationalisation for these, and to highlight their possible applications. Typical examples have been selected along with more recent ones in order to give an overall view of the scope and development of the area. The article is structured to provide the basic material for those who wish to enter the field of spin crossover.

Spin crossover in transition metal compounds

C.N.R. Rao, M.M. Seikh, C. Narayana: Spin-State Transition in LaCoO3 and Related Materials .- H.A. Goodwin: Spin Crossover in Cobalt(II) Systems .- Y. Garcia, P.Gutlich: Thermal Spin Crossover in Mn(II), Mn(III) Cr(II) and Co(III) Coordination Compounds .- D.N. Hendrickson, C.G. Pierpont: Valence Tautomeric Transition Metal Complexes .- P. Guionneau, M. Marchivie, G.Bravic, J.-F. Letard, D. Chasseau: Structural Aspects of Spin Crossover. Example of the [Fe(II)Ln(NCS)2] Complexes .- J. Kusz, P. Gutlich, H. Spiering: Structural Investigations of Tetrazole Complexes of Iron(II) .- A. Hauser: Light-Induced Spin Crossover and the High-Spin Low-Spin Relaxation .- F. Varret, K. Boukheddaden, E. Codjovi, C. Enachescu, J. Linares: On the Competition Between Relaxation and Photoexcitations in Spin Crossover Solids under Continuous Irradiation .- P. Gutlich: Nuclear Decay Induced Excited Spin State Trapping (NIESST) .- M.-L. Boillot, J. Zarembowitch, A. Sour: Ligand-Driven Light-Induced Spin Change (LD-LISC): A Promising Photomagnetic Effect

Photoswitchable coordination compounds

Photoswitchable compounds represent an attractive class of materials in coordination chemistry. Recent progress dealing with transition metal compounds involving photo-induced changes of the magnetic and/or optical properties to long-lived metastable states are covered in the present review article. The basic photophysical phenomena together with representative examples such as nitroprusside derivatives, relevant spin crossover complexes, stilbenoid complexes and finally Prussian blue analogues are discussed. Some possible applications regarding energy and information storage are suggested at the end.

Spin state switching in iron coordination compounds

Summary 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-, oligo- and 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.

Spin Crossover in a Supramolecular Fe4II [2×2] Grid Triggered by Temperature, Pressure, and Light

A multiplex electronic switch on the molecular level has been realized by using a tetranuclear FeII complex of the [2×2] grid type. The four metal ions can be switched stepwise between their high-spin and low-spin states by temperature, pressure, and light, thus representing a triple level, triple switch system as illustrated in the picture.

Mössbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity's journey across sulfate-rich outcrop, basaltic sand and dust, and hematite lag deposits

Additonal co-authors: P Gutlich, E Kankeleit, T McCoy, DW Mittlefehldt, F Renz, ME Schmidt, B Zubkov, SW Squyres, RE Arvidson

A polymeric two-dimensional mixed-metal network. Crystal structure and magnetic properties of {[P(Ph)4][MnCr(ox)3]}

Abstract The mixed-metal ferromagnet {[P(Ph) 4 ][MnCr(ox) 3 ]} n , where Ph is phenyl and ox is oxalate, has been prepared and a two-dimensional network structure, extended by Mn(II)-ox-Cr(III) bridges, has been determined from single crystal X-ray data. Crystal data: space group R 3 c , a = b =18.783(3), c =57.283(24) A, α=β=90, γ=120°, Z =24 (C 30 H 20 O 12 PCrMn). The magnetic susceptibility data obey the Curie-Weiss law in the temperature range 260–20 K with a positive Weiss constant of 10.5 K. The temperature dependence of the molar magnetization exhibits a magnetic phase transition at T c =5.9 K. The structure is discussed in relation to the strategy for preparing molecular based ferromagnets and, in addition, it is a solution to the question of the dimensionality of the [MM′(ox) 3 ] n network, which in principle can extend two- or three-dimensionally to the crystal lattice. The optical absorption spectra of the single crystals are assigned to the ‘CrO 6 ’ chromophores. Their polarization patterns reflect the electric dipole selection rules for D 3 symmetry. A strong site selective luminescence from the chromium(III) 2 E states is observed at low temperature and the system may be suitable for studying energy transfer mechanisms.