Spin Crossover Systems Research Articles

Evolution of self-organized spin domains under light in single-crystalline[Fe(ptz)6](BF4)2

We study domain growth upon the light-induced phase separation (LIPS) in the spin-crossover $[\text{Fe}{(\text{ptz})}_{6}]{({\text{BF}}_{4})}_{2}$ single crystal under clarified conditions of intensity of light and temperature. Our primary motivation is to model the relaxation behavior of a spin-crossover system under light in the spinodal regime. At this end, we built a discrete spatiotemporal hierarchy of coupled equations, which can be regarded as an efficient scheme for simulating the phase separation under light as well as for obtaining the equilibrium mean-field solutions of lattice models having complex structures. We found that in the spinodal regime under light, initial homogeneous states self-organize into spin domains after some incubation time. The evolution patterns of the high- and low-spin domains revealed that the self-organization proceeds roughly in two regimes, viz. diffusion of interfaces in the early stage followed by a growth regime. The analysis of the correlation function shows that the characteristic domain size behaves as $R(t)\ensuremath{\sim}{t}^{1/2}$, following the Allen-Cahn law.

Accurate Modeling of Spin-State Energetics in Spin-Crossover Systems with Modern Density Functional Theory

The energies of different spin multiplicities of a range of iron complexes are computed using modern density functional theory (DFT) methods of the generalized gradient approximation (GGA; BP86 and OPBE), meta-GGA (TPSS), hybrid meta-GGA (TPSSh), hybrid (B3LYP), and double-hybrid (B2PLYP) types. It is shown that so far only the double-hybrid density functional B2PLYP, in conjunction with large and flexible basis sets (def2-QZVPP), is able to provide qualitatively correct results of spin-state energetics for the investigated non-spin-crossover complexes. An energy difference of -6 to 0 kcal/mol is proposed to be indicative of spin-crossover behavior.

Light‐Induced Stored Information in Nanoparticles

AbstractWe investigate here the consequence on light‐induced and thermally induced spin‐crossover (SCO) properties with particle size reduction from the macroscopic to microscale to nanoscale domains. Three samples with distinct particle sizes of the SCO coordination polymer [Fe(NCS)2(bpe)2] [bpe = 1,2‐bis(4′‐pyridyl)ethane] have been prepared by water‐in‐oil reverse micelle methods. Comparison of the magnetic properties with particle size reduction of these and the original macroscale slow‐grown crystals revealed that the spin transition becomes more gradual, more incomplete and concomitantly the transition temperature (T1/2) decreases – much like what is observed in metal dilution studies. Importantly, here, in the first photoinduced magnetic studies on a nanoparticle SCO system, we see that even on the nanoscale photoconversion of the low spin species to a metastable high‐spin state is possible. Furthermore, particle size reduction appears to have little effect on the temperature at which the stored photomagnetic information is erased. These results highlight that light‐induced SCO properties are governed by direct metal coordination environment (i.e., on the molecular scale), whereas, thermally induced magnetic properties rely more on crystal packing and ligand field effects.

Anomalous thermodynamic properties in ferropericlase throughout its spin crossover

The thermodynamics properties of ferropericlase Mg1�xFexO where x = 0.1875Fp throughout its spin crossover were investigated by first principles. Fp was treated as an ideal solid solution of pure high-spin and low-spin states. The Gibbs free energies of the pure states were addressed using the LDA+U method. A vibrational virtual-crystal model was developed to address the vibrational properties of the pure spin cases and used in conjunction with quasiharmonic theory to compute their vibrational free energies. The thermodynamics properties of Fp display significant anomalies that should be typical of spin crossover systems in general. In Fp, in particular, they are fundamental for understanding the state of earth’s interior, where the pressure and temperature conditions of the crossover are realized.

Thermal- and pressure-induced spin transition coupled to crystallographic phase transition in some Fe(II) molecular crystals

The thermal spin transition in spin-crossover compounds is accompanied by a change in most of their physical properties, like optical, magnetic and structural properties. Iron (II) spin-crossover crystals provide insight into some most interesting phenomena of solid-state physics [P. Gütlich, A. Hauser, and H. Spiering, Angew. Chemie 106 (1994), pp. 2109–2141; Topics in Current Chemistry: “Spin Crossover in Transition Metal Compounds I, II and III”, P. Gütlich and H.A. Goodwin eds., Vols. 233, 234 and 235, 2004.]. To fine-tune the zero-point energy difference between the high-spin (HS) and the low-spin (LS) states, external pressure can be used as a tool [H.G. Drickamer and C.W. Frank, Electronic Transitions and the High Pressure Chemistry and Physics of Solids, Chapman and Hall, London, 1973; P. Adler, A. Hauser, A. Vef, H. Spiering, and P. Gütlich, Hyperfine Interactions 47 (1989), pp. 343–356. P. Adler, H. Spiering, P. Gütlich, J. Phys. Chem. Solids 50 (1989), pp. 587–597.]. The large difference in metal–ligand distances between HS and LS complexes, typically of 0.2 Å [B. Gallois, J.A. Real, C. Hauw, and J. Zarembowitch, Inorg. Chem. 29 (1990), pp. 1152–1158.] and the concomitant large volume difference, typically of 30 Å3 [L. Wiehl, H. Spiering, P. Gütlich, and K. Knorr, J. Appl. Cryst. 23 (1990), pp. 151–160.] is a general feature of iron (II) spin-crossover systems. In fact, the macroscopic change in volume of the crystal that accompanies the spin transition is evidenced by X-ray crystallography [L. Wiehl, H. Spiering, P. Gütlich, and K. Knorr, J. Appl. Cryst. 23 (1990), pp. 151–160.]. In this paper, we discuss the thermodynamic behaviour, which is the energy restructuration between the units in an iron (II) molecular crystal that undergoes a spin-crossover transition due to an external perturbation such as temperature or pressure. The results are given in comparison with Slichters’ and Drickamers’ expression for a regular solution extended to three components [H.G. Drickamer, and C.W. Frank, Electronic Transitions and the High Pressure Chemistry and Physics of Solids, Chapman and Hall, London, 1973; P. Adler, A. Hauser, A. Vef, H. Spiering, and P. Gütlich, Hyperfine Interactions 47 (1989), pp. 343–356. P. Adler, H. Spiering, P. Gütlich, J. Phys. Chem. Solids 50 (1989), pp. 587–597; J. Jeftić, H. Romstedt, and A. Hauser, J. Phys. Chem. Solids 57 (1996), pp. 1743–1750.]. The expression of the Gibbs’ free energy of the system is discussed in relation to its structure.

Spectrum-sensitive phonon wipeout due to a fluctuating spin state in aFe2+coordination polymer

Raman scattering in the spin-crossover system [Fe(pmd)(H2O){Au(CN)2}2]*H2O reveals a complex three-phase spin-state transition in contrast to earlier observations in magnetization measurements. We observe different spin state phases as function of temperature and electromagnetic radiation in the visible spectral range. There exists a fluctuating spin state phase with an unexpected wipeout of the low frequency phonon scattering intensity. Furthermore we observe one phase with reduced symmetry that is attributed to a cooperative Jahn-Teller effect. Pronounced electron-phonon interaction manifests itself as a strong Fano-resonance of phonons related to {FeN6} and {FeN4O2} coordination octahedra. Density functional theory supports this interpretation.

Fe(ii) complex with the octadentate btpa ligand: a DFT study on a spin-crossover system that reveals two distinct high-spin states

Density functional theory calculations (DFT) were performed for the spin-crossover system [Fe(btpa)]2+ (btpa = N,N,N',N'-tetrakis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine), and for the predominantly low-spin [Fe(b(bdpa))]2+ complex (in the solid state) (b(bdpa) = N,N'-bis(benzyl)-N,N'-bis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine). The calculations confirmed that the former complex exhibits two high-spin isomers of the complexes, i.e. with C1 quasi hepta-coordinated (long-lived isomer) and C2 hexa-coordinated (short-lived isomer) structures that have been suggested previously based on time-resolved Raman and flash photolysis experiments. Application of B3LYP and B3LYP* functionals together with the CEP-31G basis yielded reasonable estimates of electronic energies (E(el) = E(el)(HS) - E(el)(LS)) for both isomers (calculated E(el) of ca. 24 and 31 kJ mol(-1) for long- and short-lived HS isomers, respectively, vs. the experimentally determined value of 27.5 kJ mol(-1)). Further calculations yielded the electronic structure of the low-spin isomer together with lowest lying singlet and triplet excited states of the [Fe(btpa)]2+ as well as the energy profile of the C2 <--> C1 isomerisation pathway for the high-spin [Fe(btpa)]2+ within the framework of the QST (quadratic synchronous transit) approach. The data obtained are discussed in relation to the observed ultrafast intersystem crossing in Fe(II) polypyridine complexes. The importance of ligand strain in relation to the destabilisation of the low-spin isomers is also discussed. In that context, calculations for a further 15 Fe(II) spin-crossover complexes of hexa-coordinating nitrogen-donor ligands have shown that the LS-HS conversion is associated with a release of ligand stress of 95 +/-16 kJ mol(-1), on average. On the basis of the calculations presented in this paper we propose that octahedral high-spin d-6 isomers are far more elastic regarding the angular distortions (equatorial and meridional strain, i.e. the declination of cis- and trans-L-M-L angle from the regular values of 90 and 180 degrees, respectively) than their low-spin counterparts.

Monte Carlo simulations for 1- and 2D spin crossover compounds using the atom–phonon coupling model

In this contribution we have simulated, using the Monte Carlo–Metropolis technique, the thermal behavior of an one dimensional and of a square lattice spin crossover system. In the one dimensional case, a long-range interaction parameter must be included in order to obtain a thermal hysteresis. For the square lattice we have tacked into account the nearest as well the next-nearest neighbors. We show the role of the elastic constant ratio on the hysteresis width.

Relaxation process from photoinduced states of double-step spin-crossover systems using a kinetic two-sublattice Ising-like model including intra-site coupling

A two-sublattice kinetic mean-field Ising-like model is proposed to describe the relaxation process from photo-induced metastable states in double-step spin-crossover complexes. Its originality lies in the rigorous treatment of intra-site coupling that yields dynamical equations by using additionally the Glauber stochastic treatment of master equations. The numerical resolution of these equations permits one to extract not only the high-spin fraction but also the intra-site correlation and the symmetry-breaking parameter which are hidden order parameters in these systems. Our simulation can demonstrate interesting relaxation behavior. As the result of synergy of inter- and intra-site interactions, double-step relaxation curves can be obtained in most cases. When decreasing the magnitude of interactions, one-step sigmoidal relaxation or stretched exponential relaxations appear. Asymmetry between $A$ and $B$ sublattices may even induce curious dynamics such as triple steplike behavior that is also observed for weak fluctuations in the initial configuration. For stronger fluctuations the system relaxes to an ordered metastable state instead of reaching the ground state. This model is finally applied to genuine systems, namely, the binuclear spin-crossover complex ${[\text{Fe}(\text{bt}){(\text{NCS})}_{2}]}_{2}(\text{bpym})$ and the polymeric spin-crossover compound ${\text{Fe}(\text{pmd})[\text{Ag}{(\text{CN})}_{2}]{\text{Ag}}_{2}{(\text{CN})}_{3}}$ that exhibit both a two steps thermal spin transition. The relaxation curves are then conveniently reproduced considering weak-interaction parameters whose possible origins will be highlighted.

Magnetostructural Relationship in the Spin-Crossover Complex t-{Fe(abpt)2[N(CN)2]2}: Polymorphism and Disorder Phenomenon

t-{Fe(abpt)(2)[N(CN)(2)](2)} [abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] is an intriguing spin-crossover system that crystallizes in two polymorphs. Polymorph A is paramagnetic; its crystal structure consists of a single molecule located at the center of inversion symmetry. Polymorph B, on the other hand, exhibits a rather complicated two-step-like spin transition; its crystal structure consists of two symmetry-independent molecules. The crystal structure of polymorph B has been derived in the different spin states: above the high-temperature step (300 K), between the two steps (90 K), below the incomplete low-temperature step (50 K), in the light-induced metastable state (15 K), in the thermally quenched metastable state (15 K), and after relaxation from the quenched state (15 K). The correlation between the structure and magnetic properties is precisely established, allowing the complicated magnetic behavior of polymorph B to be well understood. A unique order-disorder phase transition, resulting in a modulation of the metastable state structures, is detected for the first time on such spin-transition compounds. The modulation of the structure originates from a particular ordering of the dicyanamide ligand at one of the two Fe sites.

Femtosecond Time-Resolved Optical and Raman Spectroscopy of Photoinduced Spin Crossover: Temporal Resolution of Low-to-High Spin Optical Switching

A combination of femtosecond electronic absorption and stimulated Raman spectroscopies has been employed to determine the kinetics associated with low-spin to high-spin conversion following charge-transfer excitation of a FeII spin-crossover system in solution. A time constant of tau = 190 +/- 50 fs for the formation of the 5T2 ligand-field state was assigned based on the establishment of two isosbestic points in the ultraviolet in conjunction with changes in ligand stretching frequencies and Raman scattering amplitudes; additional dynamics observed in both the electronic and vibrational spectra further indicate that vibrational relaxation in the high-spin state occurs with a time constant of ca. 10 ps. The results set an important precedent for extremely rapid, formally forbidden (DeltaS = 2) nonradiative relaxation as well as defining the time scale for intramolecular optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.

Light-Induced Excited-State Spin Trapping in Tetrazole-Based Spin Crossover Systems

Ab initio calculations have been performed on Fe (II) (tz) 6 (tz = 1- H-tetrazole) to establish the variation of the energy of the electronic states relevant to (reverse) light-induced excited-state spin trapping (LIESST) as function of the Fe-ligand distance. Equilibrium distances and absorption energies are correctly reproduced. The deactivation of the excited singlet is proposed to occur in the Franck-Condon region through overlap of vibrational states with an intermediate triplet state or an intersystem crossing along an asymmetric vibrational mode. This is followed by an intersystem crossing with the quintet state. Reverse LIESST involves a quintet-triplet and a triplet-singlet intersystem crossing around the equilibrium distance of the high-spin state. The influence of the transition metal is studied by changing Fe (II) for Co (II), Co (III), and Fe (III). The calculated curves for Fe (III) show remarkable similarity with Fe (II), indicating that the LIESST mechanism is based on the same electronic conversions in both systems.

Computational Chemistry of Modified [MFe3S4] and [M2Fe2S4] Clusters: Assessment of Trends in Electronic Structure and Properties

The aim of this work is to understand the molecular evolution of iron-sulfur clusters in terms of electronic structure and function. Metal-substituted models of biological [Fe(4)S(4)] clusters in oxidation states [M(x)Fe(4-x)S(4)](3+/2+/1+) have been studied by density functional theory (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pd, with x = 1 or 2). Most of these clusters have not been characterized before. For those that have been characterized experimentally, very good agreement is obtained, implying that also the predicted structures and properties of new clusters are accurate. Mean absolute errors are 0.024 A for bond lengths ([Fe(4)S(4)], [NiFe(3)S(4)], [CoFe(3)S(4)]) and 0.09 V for shifts in reduction potentials relative to the [Fe(4)S(4)] cluster. All structures form cuboidal geometries similar to the all-iron clusters, except the Pd-substituted clusters, which instead form highly distorted trigonal and tetragonal local sites in compromised, pseudocuboidal geometries. In contrast to other electron-transfer sites, cytochromes, blue copper proteins, and smaller iron-sulfur clusters, we find that the [Fe(4)S(4)] clusters are very insensitive to metal substitution, displaying quite small changes in reorganization energies and reduction potentials upon substitution. Thus, the [Fe(4)S(4)] clusters have an evolutionary advantage in being robust to pollution from other metals, still retaining function. We analyze in detail the electronic structure of individual clusters and rationalize spin couplings and redox activity. Often, several configurations are very close in energy, implying possible use as spin-crossover systems, and spin states are predicted accurately in all but one case ([CuFe(3)S(4)]). The results are anticipated to be helpful in defining new molecular systems with catalytic and magnetic properties.

Reentrant magnetic ordering and percolation in a spin-crossover system

Spin-crossover compounds, which are characterized by magnetic ions showing low-spin and high-spin states at thermally accessible energies, are ubiquitous in nature. We here focus on the effect of an exchange interaction on the collective properties for the case of nonmagnetic low-spin ions, which applies to ${\text{Fe}}^{2+}$ compounds such as FeO under high pressure. Monte Carlo simulations are used to study a three-dimensional spin-crossover model for the full parameter range from essentially pure high spin to essentially pure low spin. We find that as the low-spin state becomes more favorable, the Curie temperature drops, the universality class deviates from the three-dimensional Heisenberg class, and the transition eventually changes to first order. A heat-bath algorithm that grows or shrinks low-spin and high-spin domains is developed to handle the first-order transition. When the ground state has low spin, a reentrant magnetic transition is found in a broad parameter range. We also observe a percolation transition of the high spins, which branches off the first-order magnetic transition.

Synthesis, crystal structure and magnetic properties of the spin crossover system [Fe(pq) 3] 2+

Three new compounds formulated (ClO 4) 2[Fe(pq) 3] ( 1), (BF 4) 2[Fe(pq) 3] · EtOH ( 2) and {(ClO 4)[MnCr(C 2O 4) 3][Fe(pq) 2(H 2O) 2]} ( 3), where pq is 2,2′-pyridylquinoline, have been synthesised and characterised. Despite the different crystal packing exhibited by 1 and 2, the cationic species [Fe(pq) 3] 2+ are structurally quite similar. At 293 K, the Fe–N bond lengths are characteristic of the iron(II) in the high-spin state. In contrast to 1, 2 undergoes a continuous spin transition. Indeed, at 95 K its structure experiences a noticeable change in the Fe–N bonds and angles, i.e. the Fe–N bonds shorten by 0.194 Å on the average. The magnetic behaviour confirms that 1 is fully high-spin in the 4–300 K temperature range while 2 shows a spin transition centred at T 1/2 = 150 K. The corresponding enthalpy, entropy and interaction parameter are Δ H = 7.49 kJ mol −1, Δ S = 50 J K −1 mol −1and Γ = 1.35 kJ mol −1. Compound 3 has been obtained as a microcrystalline powder. The magnetic properties of 3 point at the occurrence of ferromagnetic coupling below 100 K and the onset of a ferromagnetic ordering below 10 K (Weiss constant equal to 6.8 K). The Mössbauer spectra of 3 show the occurrence of a magnetic order at T ⩽ 4.2 K.

Molecular magnetism, status and perspectives

A short review is made of molecular magnetism, trying to discuss what is alive and well, with perspectives for the future. All the main fields of activity are mentioned, ranging from the so-called spin cross-over systems to the quest for organic (molecular) ferromagnets. Particular attention is devoted to some of the recent advances in these fields, highlighting also the opportunities for the development of applications. Low dimensional magnets are perhaps the best opportunity to use molecules, and the status of single-molecule and single chain magnets is discussed. The last part is devoted to the organization of magnetic molecules and to the development of techniques which allow to measure the magnetic properties of thin layers and, in perspective, of single molecules.

Spin transition and relaxation dynamics coupled to a crystallographic phase transition in a polymeric iron(II) spin-crossover system

The spin-crossover compound [Fe(bbtr) 3](ClO 4) 2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane) forms a polymeric hexagonal sheet structure. It shows an abrupt thermal spin transition with 13 K wide hysteresis around 105 K, as evidenced by single crystal optical spectroscopy. The transition temperature for the thermal high-spin→low-spin transition on cooling as well as the relaxation kinetics just below T c ↓ depend upon the history of the sample. This is typical for a nucleation and growth mechanism and domain formation. In contrast, the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is governed by the intersystem crossing process.

Size and pressure effects in the atom-phonon coupling model for spin crossover compounds

The smallest size of a spin crossover particle which still exhibits hysteric behavior is an open question. It is clear that spin crossover is a cooperative phenomenon, which implies the dependence of the high spin fraction variation on the number of molecules within the system. We use the “atom-phonon coupling” model to explain the different results that have been recently published with regard to the critical dimension of a spin crossover system. In the second part of the paper, the applicability of the model to the pressure induced transition has been developed, and the results are in good agreement with what has been observed experimentally.

Ab initioMolecular and Solid State Studies of the FeII Spin Cross-over System [Fe(btz)2(NCS)2] (btz = 2.2’-bis-4.5-dihydrothiazine)

Ab initio computations within the density functional theory are reported for the spin cross-over complex [Fe(btz)2(NCS)2] (btz = 2.2'-bis-4.5-dihydrothiazine), where 3d6 FeII is characterized by high-spin (HS t2g 4, eg 2) and low-spin (LS t2g 6, eg 0) states. Results of infrared and Raman spectra for the isolated molecule are complemented for the crystalline solid with a full account of the electronic band structure properties: the density of states assessing the crystal field effects and the chemical bonding, assigning a specific role to the Fe-N interactions within the coordination sphere of FeII

Sub-picosecond time resolved infrared spectroscopy of high-spin state formation in Fe(ii) spin crossover complexes

The photoinduced low-spin (S = 0) to high-spin (S = 2) transition of the iron(ii) spin-crossover systems [Fe(btpa)](PF(6))(2) and [Fe(b(bdpa))](PF(6))(2) in solution have been studied for the first time by means of ultrafast transient infrared spectroscopy at room temperature. Negative and positive infrared difference bands between 1000 and 1065 cm(-1) that appear within the instrumental system response time of 350 fs after excitation at 387 nm display the formation of the vibrationally unrelaxed and hot high-spin (5)T(2) state. Vibrational relaxation is observed and characterized by the time constants 9.4 +/- 0.7 ps for [Fe(btpa)](PF(6))(2)/acetone and 12.7 +/- 0.7 ps for both [Fe(btpa)](PF(6))(2)/acetonitrile and [Fe(b(bdpa)](PF(6))(2)/acetonitrile. Vibrational analysis has been performed via DFT calculations of the low-spin and high-spin state normal modes of both compounds as well as their respective infrared absorption cross sections. The simulated infrared difference spectra are dominated by an increase of the absorption cross section upon high-spin state formation in accordance with the experimental infrared spectra.