Yann Garcia

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.

Pressure and temperature spin crossover sensors with optical detection

Iron(II) spin crossover molecular materials are made of coordination centres switchable between two states by temperature, pressure or a visible light irradiation. The relevant macroscopic parameter which monitors the magnetic state of a given solid is the high-spin (HS) fraction denoted nHS, i.e., the relative population of HS molecules. Each spin crossover material is distinguished by a transition temperature T1/2 where 50% of active molecules have switched to the low-spin (LS) state. In strongly interacting systems, the thermal spin switching occurs abruptly at T1/2. Applying pressure induces a shift from HS to LS states, which is the direct consequence of the lower volume for the LS molecule. Each material has thus a well defined pressure value P1/2. In both cases the spin state change is easily detectable by optical means thanks to a thermo/piezochromic effect that is often encountered in these materials. In this contribution, we discuss potential use of spin crossover molecular materials as temperature and pressure sensors with optical detection. The ones presenting smooth transitions behaviour, which have not been seriously considered for any application, are spotlighted as potential sensors which should stimulate a large interest on this well investigated class of materials.

Insights into the Origin of Cooperative Effects in the Spin Transition of [Fe(NH2trz)3](NO3)2: the Role of Supramolecular Interactions Evidenced in the Crystal Structure of [Cu(NH2trz)3](NO3)2·H2O

The thermally induced hysteretic spin transition (ST) that occurs in the polymeric chain compound [Fe(NH(2)trz)(3)](NO(3))(2) (1) above room temperature (T(c)(upward arrow) = 347 K, T(c)(downward arrow) = 314 K) has been tracked by (57)Fe Mössbauer spectroscopy, SQUID magnetometry, differential scanning calorimetry (DSC), and X-ray powder diffraction (XPRD) at variable temperatures. From the XRPD pattern indexation, an orthorhombic primitive cell was observed with the following cell parameters: a = 11.83(2) A, b = 9.72(1) A, c = 6.361(9) A at 298 K (low-spin state) and a = 14.37(2) A, b = 9.61(4) A, c = 6.76(4) A at 380 K (high-spin state). The enthalpy and entropy variation associated to the ST of 1, have been evaluated by DSC as DeltaH = 23(1) kJ mol(-1) and DeltaS = 69.6(1) J mol(-1) K(-1). These thermodynamic data were used within a two-level Ising like model for the statistical analysis of First Order Reversal Curve (FORC) diagram that was recorded for 1, in the cooling mode. Strong intramolecular cooperative effects are witnessed by the derived interaction parameter of J = 496 K. The crystal structure of [Cu(NH(2)trz)(3)](NO(3))(2).H(2)O (2) was obtained thanks to high quality single crystals prepared by slow evaporation after hydrothermal pretreatment. The catena poly[mu-tris(4-amino-1,2,4-triazole-N1,N2) copper(II)] dinitrate monohydrate (2) crystallizes in the monoclinic space group C2/c, with a = 16.635(6) A, b = 13.223(4) A, c = 7.805(3) A, beta = 102.56(3) degrees, Z = 4. Complex 2 is a 1D infinite chain containing triple N1,N2-1,2,4-triazole bridges with an intra-chain distance of Cu...Cu = 3.903(1) A. A dense H-bonding network with the nitrate counteranion involved in intra-chain and inter-chain interactions is observed. Such a supramolecular network could be at the origin of the unusually large hysteresis loop displayed by 1 (DeltaT approximately 33 K), as a result of an efficient propagation of elastic interactions through the network. This hypothesis is strengthened by the crystal structure of 2 and by the absence of crystallographic phase transition for 1 over the whole temperature range of investigation as shown by XRPD.

Spin Transition Charted in a Fluorophore-Tagged Thermochromic Dinuclear Iron(II) Complex

The first crystal structures of a dinuclear iron(II) complex with three N1,N2-1,2,4-triazole bridges in the high-spin and low-spin states are reported. Its sharp spin transition, which was probed using X-ray, calorimetric, magnetic, and (57)Fe Mossbauer analyses, is also delineated in the crystalline state by variable-temperature fluorimetry for the first time.

Prediction of the Spin Transition Temperature in Fe(II) One-Dimensional Coordination Polymers: an Anion Based Database.

One-dimensional (1D) coordination polymers of formula [Fe(NH(2)trz)(3)]A.nH(2)O, {A = TiF(6)(2-), n = 0.5 (1) and n = 1 (2); A = ZrF(6)(2-), n = 0.5 (3) and n = 0 (4); A = SnF(6)(2-), n = 0.5 (5) and n = 1 (6); A = TaF(7)(2-), n = 3 (7) and n = 2.5 (8); A = GeF(6)(2-), n = 1 (9) and n = 0.5 (10), NH(2)trz = 4-amino-1,2,4-triazole} have been synthesized, fully characterized, and their spin crossover behavior carefully studied by SQUID magnetometry, Mossbauer spectroscopy, and differential scanning calorimetry. These materials display an abrupt and hysteretic spin transition around 200 K on cooling, as well as a reversible thermochromic effect. Accurate spin transition curves were derived by (57)Fe Mossbauer spectroscopy considering the corrected f factors for the high-spin and low-spin states determined employing the Debye model. The unusual hysteresis width of 3 (28 K), was attributed to a dense hydrogen bonding network involving the ZrF(6)(2-) counteranion and the 1D chains, an organization which is also revealed in [Cu(NH(2)trz)(3)]ZrF(6).H(2)O (11). Trinuclear spin crossover compounds of formula [Fe(3)(NH(2)trz)(10)(H(2)O)(2)](SbF(6))(6).S {S = 1.5CH(3)OH (12), 0.5C(2)H(5)OH (13)} were also obtained. A structural property relationship was derived between the volume of the inserted counteranion and the transition temperature T(1/2) of the 1D chains. Two linear size regimes were identified for monovalent anions (0.04 <or= V (nm(3)) <or= 0.09) and for divalent anions (above V >or= 0.11 nm(3)) with saturation around T(1/2) = 200 K. These characteristics allowed us to derive an anion based database that is of interest for the prediction of the transition temperature of such functional switchable materials. Diffuse reflectivity measurements under hydrostatic pressure for 3,4 combined with calorimetric data allow an estimation of the electrostatic pressure between cationic chains and counteranions in the crystal lattice of these materials. The chain length distribution that ranges between 1 and 4 nm was also derived.

Insights into the Origin of Solid-State Photochromism and Thermochromism of N-Salicylideneanils: The Intriguing Case of Aminopyridines.

The relationships between the crystal structure and optical properties of switchable N-salicylideneanils have been revised and discussed on the basis of new experimental results and a computational approach. N-salicylidene-3-aminopyridine (L(3)) is a versatile thermo- and photochromic molecule. It also exhibits an infinitely slow thermal back relaxation (k = 9.9x10(-8) s(-1)) after photoswitching that is suitable for optical memories. Contrary to reports in the literature, N-salicylidene-4-aminopyridine (L(4)) is exclusively thermochromic. To explain these unexpected optical properties in the solid state, crystallography combined with UV-visible spectroscopic data was exploited. L(3) was also used as a ligand in new thermochromic coordination complexes [M(CH(3)OH)(2)(L(3))(2)(NCX)(2)], in which M(II) = Fe, Co, Ni, Cu or Mn and X = S or Se (1-6), which allowed the fine-tuning of the electron density in the photochromic moiety. The influence of the coordination through the nitrogen of the pyridine ring is also fully discussed.