Spin Transition Compounds Research Articles

Facile Approach for the Fabrication of Vapor Sensitive Spin Transition Composite Nanofibers

In this work polymer nanofibers were functionalized by incorporation of the spin transition (ST) compound [Fe(H2btm)2(H2O)2]Cl2 (FeH2btm) (H2btm = di(1H‐tetrazol‐5‐yl)methane). FeH2btm is an interesting compound due to its ability to reversibly and sensitively switch between high spin (HS) and low spin (LS) state when exposed to common volatile compounds (VOC) like ammonia and methanol. By using polyvinylidene fluoride (PVDF) as the main compound, inhibiting interactions between the complex and polymer were minimized. By using UV‐Vis spectroscopy, the visible and reversible switching between HS and LS state when exposed to an ammonia or hydrochloric acid atmosphere was confirmed. Powder X‐Ray diffraction (PXRD), scanning electron microscopy (SEM) and energy dispersive X‐Ray spectroscopy (EDX) show a homogenous distribution of FeH2btm with no major crystalline accumulations and a mean fiber diameter of 106 ± 20 nm. The composite fiber has a similarly high thermal stability as the pure FeH2btm, as shown by thermogravimetric analysis (TGA). Mössbauer spectroscopy indicates an incomplete spin transition after exposition to ammonia. This could be due to low permeability of the VOC into the composite fiber.

Anomalous Pressure Response of Temperature-Induced Spin Transition and a Pressure-Induced Spin Transition in Two-Dimensional Hofmann Coordination Polymers.

Spin transition (ST) compounds have been extensively studied because of the changes in rich physicochemical properties accompanying the ST process. The study of ST mainly focuses on the temperature-induced spin transition (TIST). To further understand the ST, we explore the pressure response behavior of TIST and pressure-induced spin transition (PIST) of the 2D Hofmann-type ST compounds [Fe(Isoq)2M(CN)4] (Isoq-M) (M = Pt, Pd, Isoq = isoquinoline). The TISTs of both Isoq-Pt and Isoq-Pd compounds exhibit anomalous pressure response, where the transition temperature (T1/2) exhibits a nonlinear pressure dependence and the hysteresis width (ΔT1/2) exhibits a nonmonotonic behavior with pressure, by the synergistic influence of the intermolecular interaction and the distortion of the octahedral coordination environment. And the distortion of the octahedra under critical pressures may be the common behavior of 2D Hofmann-type ST compounds. Moreover, ΔT1/2 is increased compared with that before compression because of the partial irreversibility of structural distortion after decompression. At room temperature, both compounds exhibit completely reversible PIST. Because of the greater change in mechanical properties before and after ST, Isoq-Pt exhibits a more abrupt ST than Isoq-Pd. In addition, it is found that the hydrostatic properties of the pressure transfer medium (PTM) significantly affect the PIST due to their influence on spin-domain formation.

105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism

Little is known about the mechanisms behind the bistability (memory) of molecular spin transition compounds over broad temperature ranges (>100 K). To address this point, we report on a new discrete FeII neutral complex [FeIIL2]0 (1) based on a novel asymmetric tridentate ligand 2-(5-(3-methoxy-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl))pyridine (L). Due to the asymmetric cone-shaped form, in the lattice, the formed complex molecules stack into a one-dimensional (1D) supramolecular chain. In the case of the rectangular supramolecular arrangement of chains in methanolates 1-A and 1-B (both orthorhombic, Pbcn) differing, respectively, by bent and extended spatial conformations of the 3-methoxy groups (3MeO), a moderate cooperativity is observed. In contrast, the hexagonal-like arrangement of supramolecular chains in polymorph 1-C (monoclinic, P21/c) results in steric coupling of the transforming complex species with the peripheral flipping 3MeO group. The group acts as a supramolecular latch, locking the huge geometric distortion of complex 1 and in turn the trigonal distortion of the central FeII ion in the high-spin state, thereby keeping it from the transition to the low-spin state over a large thermal range. Analysis of the crystal packing of 1-C reveals significantly changing patterns of close intermolecular interactions on going between the phases substantiated by the energy framework analysis. The detected supramolecular mechanism leads to a record-setting robust 105 K wide hysteresis spanning the room temperature region and an atypically large TLIESST relaxation value of 104 K of the photoexcited high-spin state. This work highlights a viable pathway toward a new generation of cleverly designed molecular memory materials.

Phase Trapping in Multistep Spin Crossover Compound.

The dimeric motif is the smallest unit for two interacting spin centers allowing for systematic investigations of cooperative interactions. As spin transition compounds, dinuclear complexes are of particular interest, since they potentially reveal a two-step spin crossover (SCO), switching between the high spin-high spin [HS-HS], the high spin-low spin [HS-LS], and the low spin-low spin [LS-LS] states. Herein, we report the synthesis and characterization of six dinuclear iron(II) complexes [FeII2(μ2-L1)2](BF4)4 (C1), [FeII2(μ2-L1)2](ClO4)4 (C2), [FeII2(μ2-L1)2](F3CSO3)4 (C3), [FeII2(μ2-L2)2](BF4)4 (C4), [FeII2(μ2-L2)2](BF4)4 (C5), and [FeII2(μ2-L2)2](BF4)4 (C6), based on the 1,3,4-thiadiazole bridging motif. The two novel bis-tridentate ligands (L1 = 2,5-bis{[(1H-imidazol-2-ylmethyl)-amino]-methyl}-1,3,4-thiadiazole and L2 = 2,5-bis{[(thiazol-2-ylmethyl)-amino]-methyl}-1,3,4-thiadiazole) were employed in the presence of iron(II) salts with the different counterions. Upon varying ligands and counterions, we were able to change the magnetic properties of the complexes from a temperature-independent [HS-HS] spin state over a one-step spin transition toward a two-step SCO. When cooled slowly from room temperature, the two-step SCO goes along with two distinct phase transitions, and in the intermediate mixed [HS-LS] state distinct HS/LS pairs can be identified unambiguously. In contrast, rapid cooling precludes a crystallographically observable phase transition. For the mixed [HS-LS] state Mössbauer spectroscopy confirms a statistical (random) orientation of adjacent [HS-LS]·[HS-LS]·[HS-LS] chains.

Solvatomorphism-Induced 45 K Hysteresis Width in a Spin-Crossover Mononuclear Compound.

Spin-transition compounds are coordination complexes that can present two stable or metastable high-spin and low-spin states at a given temperature (thermal hysteresis). The width of the thermal hysteresis (difference between the maximum and minimum temperature between which the compound exhibits bi-stability) depends on the interactions between the coordination complexes within the compound, and which may be modulated by the absence or presence of solvent within the structure. The new compound [Fe(3-bpp)2 ][Au(CN)2 ]2 (1, 3-bpp=2,6-di-(1H-pyrazol-3-yl)pyridine) was synthesized and its properties were compared with those of the solvated compound [Fe(3-bpp)2 ][Au(CN)2 ]2 ⋅2 H2 O (1.H2 O) already described. 1 has a two-steps thermal hysteresis of 45 K, in contrast to the compound 1.H2 O which exhibits a gradual conversion without hysteresis. This hysteretic transition is accompanied by a reversible reconstructive structural transition and twinning. This stepped behaviour is also observed in the photomagnetic properties despite the low efficiency of photoswitching. Single-crystal photocrystallographic investigations confirm this low conversion, which we attributed to the high energy cost to form the high-spin structure, whose symmetry differs from that of the low-spin phase.

Nanoparticles of Spin Transition Compounds Made Electrochemically

Materials exhibiting geometrically frustrated magnetism are at the forefront of the search for novel magnetic ground states (i.e., quantum magnets), best achieved in synthetic herbertsmithite [1]. A method for synthesizing crystalline Zn4-xCux(OH)6Cl2 (in which x=1 for herbertsmithite, x=0 for clinoatacamite and 0.33<x<1 for paratacamite) was first discovered in 2012 (while being first found in nature in 2004) [2]. These compounds are typically synthesized by hydrothermal or solvothermal methods (e.g., 185 - 200°C). To date, these methods have been limited to very low production rates and yields and focus on the production of macroscopic crystals, due to the need to obtain single crystals in order to reveal their novel magnetic properties. Here we introduce the electrochemically-driven synthesis of mondisperse nano-particles of herbertsmithite at room temperature (18 °C). The synthesis was carried out using a mixture of Cu2+ and Zn2+ ions as the metal precursors and O2 (in air) as the oxidant gas through a gas-diffusion cathode. XRD patterns obtained for different [Cu2+]0/[Zn2+]0 ratios are presented (Figure 1a). Zero-field-cooled (ZFC) and field-cooled (FC) mass magnetization (M) in a field of 7.98 kA m-1, over the temperature range of 2 to 300 K, showed a ferromagnetic response below Tc ~ 5 K that is accompanied by bifurcation of FC data (Figure 1b). Upon reaching Tc, the sample showed a spin glass (SG)-like state, in which the magnetic moments were frozen in random directions [3]. We believe that the extracted ferromagnetic hysteresis at T = 2 K was caused by an impurity phase (Figure 1b inset). As the purity of the herbertsmithite nanoparticles is increased, a clear distinction of the quantum spin liquid state is expected. Our new synthesis method, namely gas-diffusion electrocrystallization (GDEx), also produces other magnetic nanoparticles with controllable properties on demand, such as a precise degree of magnetization for e.g., pure phase magnetite. Given the electrochemical nature of the synthesis and the nano-scale dimensions of the magnetic particles formed, GDEx can be easily coupled to other electrochemical methods to produce magnetic thin films either in aqueous or non-aqueous media. Yet, we can achieve solid powders or stable colloidal dispersions for processing by other methods.

Room-Temperature Spin-Transition Iron Compounds

Spin transition complexes are an interesting class of materials exhibiting molecular bistability with potential applications in nanotechnological devices as memory storage units, sensors or displays. Since the discovery of the spin transition phenomenon in tris(N,Ndialkyldithiocarbamato) iron(III) complexes, numerous investigations have been devoted to this field of molecular magnetism. The spin transition phenomenon is probably the most spectacular example of bistability in molecular chemistry. However, it is a challenge to obtain spin transition materials when working under ambient conditions (eg. room temperature and pressure), which would be highly advantageous for potential applications. So far, only some Fe(II) and Fe(III) molecular systems have shown temperature-induced spin transitions around and even above room temperature. Within this review, we discuss the characteristics of this class of bistable compounds in detail and we try to draw more general conclusions regarding the integration, implementation and application of spin transition compounds as switching elements in hybrid molecular devices.

Study of switching in spin transition compounds within the mechanoelastic model with realistic parameters.

Here we reproduce the static and dynamical properties of spin-crossover complexes in the framework of the mechanoelastic model applied to triangular lattices. The switching processes between the high-spin and low-spin states are studied by combining the Monte Carlo method with the elastic lattice relaxation. The transition probabilities between the two states take into account intrinsic parameters, the values of which are approximated from experimental quantities (e.g., the energy gap, and the degeneracy ratio from the thermodynamic enthalpy and the entropy difference between the states), and the elastic force or elastic energy stored in the springs connecting the spin-changing centres. The value of the corresponding spring constant is estimated from the experimentally determined variation of the ligand-field strengths in the two spin states due to the cooperativity and the bulk modulus. Both simulated hysteresis loops and relaxation curves are in agreement with experimental data. Cooperativity related phenomena such as like-spin domain formation and the evolution of the interaction distribution with the HS fraction are also analysed.

Liquid‐Filled Valence Tautomeric Microcapsules: A Solid Material with Solution‐Like Behavior

The integration of stimuli‐responsive valence tautomeric (VT) molecular systems into solid materials without compromising their functionality is a major bottleneck for the use of these compounds in high‐added value applications. In this work, an innovative, simple, and universal approach is described to tackle this challenge based on the confinement of the active species into liquid‐filled polymeric capsules. A microstructured solid with optimized solution‐like behavior is obtained in this way, whose VT properties can be rationally tuned upon variation of the encapsulated solvent. Incorporation of the resulting capsules into thin films or other matrices of interest allows successful transfer of valence tautomerism from the liquid phase to solid materials, thus paving the way to the fabrication of functional devices based on spin transition compounds.

Polymorphism dependent light induced spin transition.

A photomagnetic study of orthorhombic and tetragonal low spin polymorphs of compound [Fe(L)2](BF4)2 revealed different properties and thermal stability of their metastable high spin states. Furthermore, the reversible thermally induced phase/spin transition between low spin and high spin orthorhombic phases was studied by variable temperature UV/VIS spectroscopy.

Synergy between polymorphism, pressure, spin-crossover and temperature in [Fe(PM-BiA)2(NCS)2]: a neutron powder diffraction investigation

The pressure dependencies of the lattice parameters of the spin transition compound [Fe(PM-BiA)2(NCS)2] have been derived from neutron powder diffraction measurements at low temperature. The study of the compound [Fe(PM-BiA)2(NCS)2]-pI has first confirmed the atypical spin crossover behaviour under pressure of this compound that shows a pressure induced structural transition inducing the transformation into a different polymorph, [Fe(PM-BiA)2(NCS)2]-pII. This phenomenon avoids a first-order spin transition in favour of continuous transition around 0.75 GPa at ambient temperature. Low temperature measurements under pressure up to 1.07 GPa allowed us not only to describe the spin-crossover for both polymorphs but also to reach phase-diagram regions where both polymorphs co-exist in different spin-states. Finally, the reversibility of the structural variations has been demonstrated.

Micro- and nanocrystals of the iron(iii) spin-transition material [FeIII(3-MeO-SalEen)2]PF6

The elaboration of micro- and nanometric particles of [FeIII(3-MeO-SalEen)2]PF6 (H-3-MeO-SalEen being the condensation product of 3-methoxy-substituted salicylaldehyde and N-ethylethylenediamine), a spin-transition compound of molecular nature, is reported. A sudden precipitation approach, using butan-1-ol as an anti-solvent, has been shown to produce objects, whose size and shape can be modulated by varying the selected experimental parameters (temperature, solvent, and polymeric confining agent). They are in the form of either needle-shaped (7500 × 640 × 215 (a); 3500 × 350 × 125 (b); 950 × 270 × 35 nm3 (c)) or spherical particles (18 ± 3 nm (d)). The chemical nature of these molecular materials has been confirmed by Energy-dispersive X-ray and Raman spectroscopies. The room temperature powder X-ray diffractograms indicate the formation of size-reduced particles (a–d) of crystalline character, which correspond to the unperturbed phase previously reported by Hendrickson and co-workers. Magnetic, EPR and Raman investigations demonstrate the occurrence of a low-spin ↔ high-spin transition. In agreement with the literature, disappearance of the small hysteresis and decrease of the cooperative character indicate weaker interactions within size-reduced particles. Two striking features: (i) the small shift in the transition temperatures from 162 K (bulk) to 153 K (18 nm) and (ii) the very quantitative spin transformation (fraction of high-spin residues ≤5%) contrast to those usually reported for coordination networks and point towards specificity of molecular nanomaterials.

Study of the relaxation in diluted spin crossover molecular magnets in the framework of the mechano-elastic model

We model here the behavior of spin transition compounds, considering molecules arranged in a 2D hexagonal lattice and interacting via springs. The role of impurities in the clustering and nucleation phenomena is analyzed, as well as the manner in which the impurities affect the relaxation curves. The switching of the individual molecules is checked using a Monte Carlo procedure. When a molecule changes its state, it also modifies its volume, and the new equilibrium positions of all the molecules are calculated. As in previously reported experiments, it is found here that bigger impurities slow down the relaxation from the metastable high-spin state to the low-spin state, while smaller impurities act in an opposite way. It is shown that if the concentration of the impurities is higher than a certain threshold, then they act as a barrier, trammeling the fast evolution of domains developing from the edges.

The 1-D polymeric structure of the [Fe(NH2trz)3](NO3)2·nH2O (with n = 2) spin crossover compound proven by single crystal investigations

The first single crystal-based description of a spin transition compound from the patented [Fe(Rtrz)(3)]X(2)·nH(2)O family is reported, properly the compound with Rtrz = 4-amino-1,2,4-triazole, X = NO(3) and n = 2. This study definitively proves the 1-D polymeric nature of the crystal structure and allows a description of the [Fe(Rtrz)(3)] chains, as well as of the anions and water implementation within the crystal lattice opening the possibility of subsequent investigations of the structure-property correlation within this peculiar family of spin transition compounds.

Monte Carlo simulations of phase transitions and lattice dynamics in an atom–phonon model for spin transition compounds

We apply here the Monte Carlo Metropolis method to a known atom–phonon coupling model for 1D spin transition compounds (STC). These inorganic molecular systems can switch under thermal or optical excitation, between two states in thermodynamical competition, i.e. high spin (HS) and low spin (LS). In the model, the ST units (molecules) are linked by springs, whose elastic constants depend on the spin states of the neighboring atoms, and can only have three possible values. Several previous analytical papers considered a unique average value for the elastic constants (mean-field approximation) and obtained phase diagrams and thermal hysteresis loops. Recently, Monte Carlo simulation papers, taking into account all three values of the elastic constants, obtained thermal hysteresis loops, but no phase diagrams. Employing Monte Carlo simulation, in this work we obtain the phase diagram at T=0 K, which is fully consistent with earlier analytical work; however it is more complex. The main difference is the existence of two supplementary critical curves that mark a hysteresis zone in the phase diagram. This explains the pressure hysteresis curves at low temperature observed experimentally and predicts a “chemical” hysteresis in STC at very low temperatures. The formation and the dynamics of the domains are also discussed.

Room temperature hysteretic spin transition in 1D iron(II) coordination polymers

The 1D spin transition compound [Fe(L1)3](ClO4)2 (1) with L1 = ethyl-4H-1,2,4-triazol-4-yl-acetate, a novel neutral bidentate ligand, has been synthesised. The temperature dependence of the high-spin molar fraction derived from 57Fe Mossbauer spectroscopy reveals an exceptionally abrupt single step transition between low-spin and high-spin states with a hysteresis loop of width 5 K (Tc↑ = 298 K and Tc↓ = 293 K). This spin transition operating around room temperature presents striking reversible thermochromism from white at 295 K to pink at ice temperature, behaving as an optical alert towards temperature variations. This first order phase transition was additionally followed by differential scanning calorimetry in good agreement with Mössbauer spectroscopy data. The spin transition properties of a freshly prepared sample of 1, dried under a N2(g) stream which was formulated as [Fe(L1)3](ClO4)2·MeOH (2), are shifted below room temperature (T1/2↑ = 273 K and T1/2↓ = 263 K), thus showing a remarkable influence of solvent inclusion on the spin state of these chain compounds.

Supramolecular lattice-solvent control of iron(ii) spin transition parameters

We report on the synthesis of spin transition compounds 1, 2 of formula [Fe(L)2](A)2 (where L = 2′,6′-bis(pyrazol-1-yl)-3,4′-bipyridine, A = ClO4−—compound 1; A = BF4−—compound 2) and compound 3 of formula [Fe(L)(LH)](BF4)3·H2O·CH3CN (where LH = 3-(2,6-bis(pyrazol-1-yl) pyridine-4-yl)-pyridinium(+)). Compounds 1, 2 and 3 were characterized by single-crystal X-ray diffraction, ESI-ToF mass spectrometry, 1H NMR and elemental analysis. The single-crystal X-ray diffraction study of the counter anion analogues 1 and 2 reveals almost identical molecular structures without any significant presence of intermolecular interactions. However, in the case of compound 3, the crystal structure reveals supramolecular interactions involving molecular cations, BF4− anions and, most importantly, lattice solvent molecules. The presence of solvent water molecules induces the presence of two different types of hydrogen bonding: (i) water molecules interacting with the fluorine atoms of BF4− anions and (ii) water molecules interconnecting protonated and nonprotonated nitrogens of pyridine-3-yl substituents of neighboring complex cations. These overall hydrogen bonding pattern between the neighboring iron(II) complex cation moieties is responsible for the formation of a one dimensional (1D) hydrogen bonded zig-zag chain. The magnetic investigations elucidate high temperature spin transition behavior for both anion analogues 1 and 2, while compound 3 exhibits a lattice-solvent dependency of the temperature-driven spin transition accompanied with stepwise solvent liberation above room temperature. After complete solvent removal the solvent-free compound 3d, [Fe(L)(LH)](BF4)3, shows an abrupt spin transition accompanied with thermal hysteresis loop; T1/2(↑) = 240 K and T1/2(↓) = 231 K, ΔT1/2 = 9 K. The Ising-like model that includes two vibrational modes has been applied in a direct fitting of magnetic data. The model recovers the temperature evolution of the χT product functions for all compounds under study, involving also compound 3d with the thermal hysteresis.

Model for Elastic Relaxation Phenomena in Finite 2D Hexagonal Molecular Lattices

The relaxation in a spin transition compound is modeled on the basis of molecules interacting by the way of connecting springs and situated in a bidimensional open boundary hexagonal lattice. The switch of individual molecules is randomly checked using a standard Monte Carlo procedure. The switching probability depends on the energy gap between the two states in the absence of interactions and on the elongations of the nearest springs. The main characteristics of the experimental relaxation curves are reproduced and clustering and nucleation phenomena are detected.

Room-temperature spin-transition iron compounds

Since the discovery of the spin transition phenomenon in tris(N,N-dialkyldithiocarbamato) iron(III) complexes [1], numerous investigations have been devoted to this field of molecular magnetism. The spin transition phenomenon is probably the most spectacular example of bistability in molecular chemistry. However, it is a challenge to obtain spin transition materials when working under ambient conditions (e.g. room temperature and pressure), which would be highly advantageous for potential applications. So far, only some Fe(II) and Fe(III) molecular systems have shown temperature-induced spin transitions around and even above room temperature. Within this review we discuss the characteristics of this class of bistable compounds in detail and we try to draw more general conclusions regarding the integration, implementation and application of spin transition compounds as switching elements in hybrid molecular devices.

Light-Induced Excited Spin State Trapping: Ab Initio Study of the Physics at the Molecular Level

This paper provides a qualitative analysis of the physical content of the low-energy states of a spin-transition compound presenting a light-induced excited spin state trapping (LIESST) phenomenon, namely, [Fe(dipyrazolpyridine)2](BF4)2, which has been studied using the wave function-based CASPT2 method. Both the nature of the low-energy states and the relative position of their potential energy wells as a function of the geometry are rationalized from the analysis of the different wave functions. It is shown that the light-induced spin transition occurring in such systems could follow several pathways involving different excited spin states. In an ideal octahedral geometry, the interconversion from the excited singlet state to the triplet of lower energy, which is usually seen as an intermediate state in the LIESST mechanism, is quite unlikely since there is no crossing between the potential energy curves of these two states. On the contrary, in lower-symmetry complexes, the geometrical distortion of the coordination sphere due to ligand constraints is responsible for the occurrence of a crossing between these two states in the Franck-Condon region, leading to a possible participation of this triplet state in the LIESST mechanism. In the reverse LIESST process, a crossing between the potential energy curves of another triplet state and the excited quintet state occurs in the Franck-Condon region as well.