Thermal Spin Transition Research Articles

Light-induced excited spin state trapping effect on [Fe(mepy)3tren](PF6)2solvated crystals

A spin-crossover solvated compound [Fe(mepy)3tren](PF6)2·C7H8·C2H3N has been prepared and its switching properties have been compared to those reported for the non-solvated solid. The thermal spin transition occurs at 88 K with the opening of a 3.5 K wide hysteresis loop, while a fairly steep transition at 215 K without hysteresis has been previously reported for the non-solvated analogue. This feature has been rationalized by the analysis of the high-spin (HS) and low-spin (LS) structures, evidencing a relative stabilization of the high-spin state, as well as strong intermolecular interactions in the solvated compound. The photoswitching of the solvated solid, based on the light-induced excited spin state trapping effect, leads to a quantitative transformation from the low-spin to the high-spin state at 10 K. The long lifetime of the metastable HS state at 10 K allows the measurement of the photo-induced HS structure, where the cooperative interactions are enhanced, compared to those of the thermally populated HS structure. Then,the HS-to-LS relaxations have been studied between 45 and 60 K. They are sigmoidal in shape and can be well fitted in the frame of the mean-field approximation. The relative stability of the photo-induced HS state in this family of spin crossover compounds is not directly related to their thermal spin transition temperature. This unexpected observation is rationalized by a careful analysis of their structural characteristics.

A facile route for preparation of monodisperse nanoparticles of one-dimensional Fe(ii)-4-amino-1,2,4-triazole coordination polymers with hysteretic spin-crossover near room temperature

Monodisperse nanoparticles of one-dimensional Fe(II)-4-amino-1,2,4-triazole coordination polymers show hysteretic spin-crossover near room temperature.

The role of iron(ii) dilution in the magnetic and photomagnetic properties of the series [FexZn1−x(bpp)2](NCSe)2

The effects of metal dilution on the spin-crossover behavior of iron(II) in the mixed crystal series [Fe(x)Zn(1-x)(bpp)2](NCSe)2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) have been studied using magnetic susceptibility, photomagnetism and diffuse reflectivity measurements. For each mixed-crystal system, the thermal spin transition temperature, T(1/2), and the relaxation temperature of the photo-induced high-spin state, T(LIESST), have been systematically determined. It appears that T(1/2) decreases with the metal dilution while T(LIESST) remains unchanged. Dilution also tends to decrease the hysteresis width and smooth the transition curves. These effects were discussed first qualitatively and then quantitatively on the basis of a kinetic study governing the photo-induced back conversion taking into account the relative sizes of Zn(II) and Fe(II) ions. Interestingly, single crystals were obtained for [Fe(0.6)Zn(0.4)(bpp)2](NCSe)2 allowing the X-ray diffraction crystal-structure determination.

Influence of particle size on the phase behavior associated with the thermal spin transition of the Prussian blue analogue K0.4Co1.3[Fe(CN)6]·4.4 H2O

The Prussian blue analogue K0.4Co1.3[Fe(CN)6]·nH2O has previously been shown to exhibit three distinct phases at low temperature. In addition to the well-studied high-temperature ‘high spin’ and low-temperature ‘low spin’ phases, with unit cells of ∼10.3 and ∼9.95Å, respectively, an intermediate phase with unit cell a=10.09Å was recently reported, although it existed as a minority constituent in a mixed-phase material. Herein we report the synthesis of 130nm K0.4Co1.3[Fe(CN)6]·4.4H2O particles that form solely the intermediate phase at temperatures below the normal charge transfer induced spin transition. Compared with those formed from a bulk synthesis, the nanosized particles are shown to have the same chemical composition at high temperature, but they undergo different structural transitions with thermal cycling.

Size and Shape Dependence of Thermal Spin Transitions in Nanoislands

Theoretical calculations of thermal spin transitions in nanoscale clusters on a surface are presented. The mechanisms for magnetization reversal are identified and the activation energy and pre-exponential factor for the rate are evaluated using a recently developed harmonic transition state theory and a Heisenberg-type Hamiltonian. A maximum is found in the pre-exponential factor as a function of cluster size corresponding to a crossover from a uniform rotation mechanism to temporary domain wall formation. As the islands grow, the energy barrier increases up to a limit where the domain wall is fully established. For larger islands, the minimum energy path becomes flat resulting in a significant recrossing correction to the transition state theory estimate of the rate. The parameters of the Hamiltonian are chosen to mimic Fe clusters on a W(110) surface, a system that has previously been studied extensively in the laboratory and the calculated results are found to be in close agreement with the reported measurements.

Atomic force microscopy and near-field optical imaging of a spin transition

We report on atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) investigations of single crystals of the spin crossover complex {Fe(pyrazine)[Pt(CN)4]} across the first-order thermal spin transition. We demonstrate for the first time that the change in spin state can be probed with sub-micrometer spatial resolution through various topographic features extracted from AFM data. This original approach based on surface topography analysis should be easy to implement to any phase change material exhibiting sizeable electron-lattice coupling. In addition, AFM images revealed specific topographic features in the crystals, which were correlated with the spatiotemporal evolution of the transition observed by far-field and near-field optical microscopies.

Synthesis, structures, and magnetic properties of iron(II) complexes, [FeII(HLMe)2](ClO4)2 and its ethanol adduct [FeII(HLMe)2](ClO4)2·EtOH (HLMe = 2-methylimidazol-4-yl-methylideneamino-2-ethylpyridine): Their structural distortion and spin states

Two new iron(II) complexes, [FeII(HLMe)2](ClO4)2 (1) and its ethanol adduct [FeII(HLMe)2](ClO4)2·EtOH (2) were synthesized by selective crystallization in methanol and ethanol, respectively, where HLMe is a tridentate N3 ligand, 2-methylimidazol-4-yl-methylideneamino-2-ethylpyridine, and their structures and magnetic properties were investigated. Each compound contains the same octahedrally N6 coordinated complex-cation [FeII(HLMe)2]2+ and two ClO4− ions. The non-solvated compound 1 is the HS (S=2) complex without spin transition over the 5–300K temperature range, and has the homochiral 1D helical-chain structure constructed of the intermolecular NH···O− hydrogen bonds between ClO4− ions and neighboring imidazole NH groups of the [FeII(HLMe)2]2+ cations. The ethanol adduct 2 shows a gradual one-step spin transition between the HS (S=2) and LS (S=0) states, and has two crystallographically independent iron(II) sites with two NH(imidazole)···OH(EtOH)···O(ClO4−) hydrogen bonds of Fe(1) site and two NH(imidazole)···O(ClO4−) hydrogen bonds of Fe(2) site. There are no further intermolecular interactions to produce an extended structure in a crystal lattice of 2. The presence or absence of thermal spin transition is related to the structural distortion of each HS molecule of these perchlorate salts associated with their surroundings of intermolecular hydrogen bonds.

Thermal and Light-Induced Spin Switching Dynamics in the 2D Coordination Network of {[Zn1-xFex(bbtr)3](ClO4)2}∞: The Role of Cooperative Effects

The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn(1-x)Fe(x)(bbtr)(3)](ClO(4))(2)}(∞) (x = 0.02-1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane) are discussed. In the neat compound, the thermal spin transition with a hysteresis of 13 K is accompanied by a crystallographic phase transition (Kusz, J.; Bronisz, R.; Zubko, M.; Bednarek, H. Chem. Eur. J.2011, 17, 6807). In contrast, the diluted crystals with x ≤ 0.1 stay essentially in the high-spin state down to low temperatures and show typical first order relaxation kinetics upon photoexcitation, and the structural phase transition is well separated from the spin transition. With increasing Fe(II) concentration, steeper thermal transitions and sigmoidal relaxation curves indicate increasingly important cooperative effects. Already at x = 0.38, the spin relaxation is governed by cooperative interactions between Fe(II) centers, and the crystallographic phase transition begins to influence the spin transition. The kinetic behavior of the thermal spin transition is reproduced within the framework of a dynamic mean-field model.

Photomagnetism of a sym‐cis‐Dithiocyanato Iron(II) Complex with a Tetradentate N,N′‐Bis(2‐pyridylmethyl)1,2‐ethanediamine Ligand

A comprehensive study of the magnetic and photomagnetic behaviors of cis-[Fe(picen)(NCS)(2) ] (picen = N,N'-bis(2-pyridylmethyl)1,2-ethanediamine) was carried out. The spin-equilibration was extremely slow in the vicinity of the thermal spin-transition. When the cooling speed was slower than 0.1 K min(-1), this complex was characterized by an abrupt thermal spin-transition at about 70 K. Measurement of the kinetics in the range 60-70 K was performed to approach the quasi-static hysteresis loop. At low temperatures, the metastable HS state was quenched by a rapid freezing process and the critical T(TIESST) temperature, which was associated with the thermally induced excited spin-state-trapping (TIESST) effect, was measured. At 10 K, this complex also exhibited the well-known light-induced excited spin-state-trapping (LIESST) effect and the T(LIESST) temperature was determined. The kinetics of the metastable HS states, which were generated from the freezing effect and from the light-induced excitation, was studied. Single-crystal X-ray diffraction as a function of speed-cooling and light conditions at 30 K revealed the mechanism of the spin-crossover in this complex as well as some direct relationships between its structural properties and its spin state. This spin-crossover (SCO) material represents a fascinating example in which the metastability of the HS state is in close vicinity to the thermal spin-transition region. Moreover, it is a beautiful example of a complex in which the metastable HS states can be generated, and then compared, either by the freezing effect or by the LIESST effect.

シアン化物イオン架橋環状四核錯体における分子内電子移動に基づく多段階相転移

Controlled intramolecular electron transfers were demonstrated in a series of cyanide-bridged Fe-Co molecular squares, [Co2Fe2(CN)6(L1)2(L2)4](PF6)2. The electronic structures of the molecular squares can be varied between [FeIIILS2CoIIHS2] and [FeIILS2CoIIILS2] state by intramolecular electron transfers, which were significantly affected by the redox potential differences between metal ions. Chemical modifications of the capping ligands allowed the systematic control of the electronic states, and the occurrence of a novel two-step thermal electron transfer-coupled spin transition (ETCST). The two-step ETCST was originating from three thermodynamically stable phases, LT, IM, and HT phases. The electronic structure of each phase was investigated by X-ray structural analyses, and the IM phase showed long-range order of [FeIIILS2CoIIHS2] and [FeIILS2CoIIILS2] species. In addition, novel thermal and protonation-induced ETCST in the solution state were found using variable temperature UV-vis absorption spectroscopy.

Self-Decelerating Relaxation of the Light-Induced Spin States in Molecular Magnets Cu(hfac)2LR Studied by Electron Paramagnetic Resonance

Molecular magnets Cu(hfac)(2)L(R) (hfac = hexafluoroacetylacetonate) called "breathing crystals" exhibit thermally and light-induced magnetic anomalies very similar to iron(II) spin-crossover compounds. They are physically different systems, because the spin-state switching occurs in exchange-coupled nitroxide-copper(II)-nitroxide clusters, in contrast to classical spin crossover in d(4)-d(7) transition ions. Despite this difference, numerous similarities in physical behavior of these two types of compounds have been observed, including light-induced excited spin-state trapping (LIESST) phenomenon recently found in the Cu(hfac)(2)L(R) family. Similar to iron(II) spin-crossover compounds, the excited spin state in breathing crystals relaxes to the ground state on the time scale of hours at cryogenic temperatures. In this work, we investigate this slow relaxation in a series of breathing crystals using electron paramagnetic resonance (EPR). Three selected compounds represent the cases of relatively strong or weak cooperativity and different temperature of thermal spin transition. They all were studied in a neat magnetically concentrated form; however, sigmoidal self-accelerating relaxation was not observed. On the contrary, the relaxation shows pronounced self-decelerating character for all studied compounds. Relaxation curves and their temperature dependence could be fitted assuming a tunneling process and broad distribution of effective activation energies in these 1D materials. A number of additional experimental and theoretical arguments support the distribution-based model. Because self-decelerating relaxation behavior was also found in 1D polymeric iron(II) spin-crossover compounds previously, we compared general relaxation trends and mechanisms in these two types of systems. Both similarities and differences of copper-nitroxide-based breathing crystals as compared to iron(II) spin-crossover compounds make future research of light-induced phenomena in these new types of spin-crossover-like systems topical in the field of molecule-based magnetic switches.

ChemInform Abstract: Structure:Function Relationships in Molecular Spin‐Crossover Complexes

AbstractReview: 198 refs.

Visualization and quantitative analysis of spatiotemporal behavior in a first-order thermal spin transition: A stress-driven multiscale process

We investigated by optical microscopy the propagation of the thermal spin transition in [Fe(btr)${}_{2}$(NCS)${}_{2}$]\ifmmode\cdot\else\textperiodcentered\fi{}H${}_{2}$O\ifmmode\cdot\else\textperiodcentered\fi{}(btr = bis-triazole) single crystals. Using high-quality fresh crystals embedded in oil we could follow the transformation front of the on-cooling transition. We observed in detail a linear front propagating through the entire crystal at extremely slow velocity, \ensuremath{\sim}2.5 \ensuremath{\mu}m/s on average. The analysis of the temporal dependences of the optical densities (OD) at given positions within a Kolmogorov-Johnson-Mehl-Avrami (KJMA) law led to an exponent \ensuremath{\sim}0.6, typical for a process governed by planar diffusion. It is inferred that the spin transition is a multiscale process. The agreement with a previous structural investigation, analyzed in terms of an average KJMA law, is shown. We conclude that the stresses generated by the volume change associated with the spin transformation are the driving force for the propagation of the nucleation and growth mechanism. The role of defects is discussed and optical effects specific of the experiment are evidenced.

Pressure-driven thermal spin transition behaviors in mono-nuclear iron(II) compounds: Synthesis, crystal structure and magnetic properties

Abstract Three isostructural mononuclear iron(II) complexes with the similar tripodal ligands had been synthesized and structurally characterized, which exhibit three-dimensional (3D) supramolecular network constructed by S···H―C hydrogen bonding interactions between adjacent iron centers. The variable-temperature magnetic properties of these three complexes under different pressure have been investigated to probe the possibilities of spin transition behavior under external pressure. For 1, the pressure-driven SCO behavior was firstly observed as the application of external pressure, and then the pressure-driven thermal SCO behavior occurs subsequently as the external pressure reached up to a specific value. However, for 2 and 3, no any SCO behaviors could be observed even under high external pressure. Comparison of the crystal structures of the three complexes show that the differences are mostly due to steric effects and weak ligand field originated from the similar tripodal ligands.

The kinetics features of a thermal spin transition characterized by optical microscopy on the example of [Fe(bbtr)3](ClO4)2 single crystals: Size effect and mechanical instability

We investigated by optical microscopy the thermal spin transition in single crystals of [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl) butane). The growth of the low-spin phase was observed for different crystal orientations and sizes. The process always started from a corner of the crystal but its further development depended on the size, shape and thermal history of the crystal. In crystals of smaller size, under isothermal conditions, the low-spin phase developed in a continuous way, through the propagation of a rather well defined transformation front, with a higher propagation velocity inside the planes perpendicular to the c axis. In larger crystals the spontaneous occurrence of inhomogeneous stresses led to a stepwise propagation process.

The propagation of the thermal spin transition of [Fe(btr)2(NCS)2]·H2O single crystals, observed by optical microscopy

We observed by optical microscopy the propagation of the thermal spin transition in single crystals of the archetype compound [Fe(btr)2(NCS)2]·H2O (btr = 4,4′-bis-1,2,4-triazole). In fresh crystals we could follow well-defined transformation fronts of various shapes. The propagation conditions revealed to depend very sensitively on the structural quality of the crystals. The best observations were obtained on cooling selected fresh crystals, rather thin, protected by an oil layer, under isothermal conditions. They evidenced the progression of a well-defined transformation front through the entire crystal. This is typical for a nucleation and growth process governed by inhomogeneous nucleation. The front shape was simple (circular or linear) and progression was regular in the best cases. Crystals of lesser quality (or already cycled through the transition) exhibited a more complex behaviour, assigned to the impact of internal stresses, either genuine or generated by the previous transformation of the crystals. These observations provide new insights into the physics of like-spin domains, which are briefly discussed.

Structure:function relationships in molecular spin-crossover complexes

Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).

Thermally Two-stepped Spin Transitions Induced by Intramolecular Electron Transfers in a Cyanide-bridged Molecular Square

Abstract A cyanide-bridged molecular square, [Co2Fe2(CN)6(tp*)2(dtbbpy)4](PF6)2·2MeOH (1) (tp* = hydrotris(3,5-dimethylpyrazol-1-yl) borate and dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) was prepared by the reaction of (Bu4N)[Fe(CN)3(tp*)] with Co(CF3SO3)2·6H2O and dtbbpy in methanol. Variable temperature X-ray structural analyses, magnetic and spectroscopic data revealed that 1 exhibited novel thermal two-stepped spin transitions induced by intramolecular electron transfers.

Two‐Step Thermal Spin Transition and LIESST Relaxation of the Polymeric Spin‐Crossover Compounds Fe(X‐py)2[Ag(CN)2]2 (X=H, 3‐methyl, 4‐methyl, 3,4‐dimethyl, 3‐Cl)

In the series of polymeric spin-crossover compounds Fe(X-py)(2)[Ag(CN)(2))](2) (py=pyridine, X=H, 3-Cl, 3-methyl, 4-methyl, 3,4-dimethyl), magnetic and calorimetric measurements have revealed that the conversion from the high-spin (HS) to the low-spin (LS) state occurs by two-step transitions for three out of five members of the family (X=H, 4-methyl, and X=3,4-dimethyl). The two other compounds (X=3-Cl and 3-methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin-crossover behaviour of the compound undergoing two-step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X=3-methyl undergo light-induced excited spin state trapping (LIESST) at low temperatures. Isothermal HS-to-LS relaxation curves at different temperatures support the low-cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX=H and X=3,4-dimethyl the behaviour is well described by a double exponential function in accordance with the two-step thermal spin transition. The thermodynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the experimental data.

Theoretical Study of Thermal Spin Transition between the Singlet State and the Quintet State in the [Fe(2-picolylamine)3]2+Spin Crossover System

The spin transition between the low-spin singlet state and the high-spin quintet state in the [Fe(2-pic)(3)](2+) (2-pic: 2-picolylamine) complex is studied by using density functional theory (DFT) calculations. After careful comparison of density functionals BLYP, B3LYP, and B3LYP* (which has 15% Hartree-Fock exchange compared with 20% for B3LYP), we concluded that the spin-state splitting can be accurately reproduced by using the B3LYP* functional. The potential energy surfaces along minimum energy pathways of the three spin states were calculated at the B3LYP*/6-311+G** level of theory to find minimum energy crossing points (MECPs). The MECPs between the singlet and quintet states (SQ(M)) were found (E(SQ) = 6.8 kcal/mol), as well as the MECPs between the triplet and singlet states (ST(M), E(ST) = 12.9 kcal/mol) and the triplet and quintet states (TQ(M), E(TQ) = 12.8 kcal/mol). Although the distortion leading to SQ(M) from the singlet equilibrium geometry is mainly a symmetric expansion of the Fe-N bonds, the distortions leading to ST(M) and SQ(M) are asymmetric. Normal mode analysis demonstrates that these geometrical distortions contain a combination of several low-frequency normal modes, and therefore, these modes play a significant role in the intersystem crossing via the crossing seam.