Thermal Spin Transition Research Articles

Measurement of Reverse-Light-Induced Excited Spin State Trapping in Spin Crossover Systems: A Study Case with Zn1−xFex(6-mepy)3tren(PF6)2·CH3CN; x = 0.5%

In an attempt to better understand the physics governing the apparition of reverse-light-induced excited spin state trapping (LIESST) phenomena in spin crossover (SCO) compounds, we have studied the LIESST effect and the possibility of a reverse-LIESST effect in the SCO complex Zn1−xFex(6-mepy)3tren(PF6)2·CH3CN, x = 0.5%. ((6-mepy)3tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-butenyl}amine)). This complex was chosen as a good candidate to show reverse-LIESST by comparison with its unsolvated analogue, since the introduction of acetonitrile in the structure leads to the stabilisation of the high-spin state and both exhibit a very abrupt thermal spin transition. Indeed, the steep thermal spin transitions of two differently polarised crystals of Zn1−xFex(6-mepy)3tren(PF6)2·CH3CN, x = 0.5% have been characterised in detail in a first step using absorption spectroscopy and no influence of the polarisation was found. These were then fitted within the mean field model to obtain the variation in the enthalpy and entropy and the critical temperatures associated with the process, which are significantly lower with respect to the unsolvated compound due to the incorporation of acetonitrile. In a second step, the light-induced low-spin-to-high-spin transition at low temperatures based on LIESST and its subsequent high-spin-to-low-spin relaxation at different temperatures were characterised by time-resolved absorption spectroscopy, with exponential behaviour in both cases. The stabilisation of the high-spin state due to the presence of acetonitrile was evidenced. Finally, light-induced high-spin-to-low-spin state transition at low temperature based on reverse-LIESST was attempted by time-resolved absorption spectroscopy but the Fe(II) concentration was too low to observe the effect.

Activating a high-spin iron(II) complex to thermal spin-crossover with an inert non-isomorphous molecular dopant.

[Fe(bpp)2][ClO4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C2/c) is high-spin between 5-300 K, and crystallises with a highly distorted molecular geometry that lies along the octahedral-trigonal prismatic distortion pathway. In contrast, [Ni(bpp)2][ClO4]2 (monoclinic, P21) adopts a more regular, near-octahedral coordination geometry. Gas phase DFT minimisations (ω-B97X-D/6-311G**) of [M(bpp)2]2+ complexes show the energy penalty associated with that coordination geometry distortion runs as M2+ = Fe2+ (HS) ≈ Mn2+ (HS) < Zn2+ ≈ Co2+ (HS) ≲ Cu2+ ≪ Ni2+ ≪ Ru2+ (LS; HS = high-spin, LS = low-spin). Slowly crystallised solid solutions [FexNi1-x(bpp)2][ClO4]2 with x = 0.53 (1a) and 0.74 (2a) adopt the P21 lattice, while x = 0.87 (3a) and 0.94 (4a) are mixed-phase materials with the high-spin C2/c phase as the major component. These materials exhibit thermal spin-transitions at T½ = 250 ± 1 K which occurs gradually in 1a, and abruptly and with narrow thermal hysteresis in 2a-4a. The transition proceeds to 100% completeness in 1a and 2a; that is, the 26% Ni doping in 2a is enough to convert high-spin [Fe(bpp)2][ClO4]2 into a cooperative, fully SCO-active material. These results were confirmed crystallographically for 1a and 2a, which revealed similarities and differences between these materials and the previously published [FexNi1-x(bpp)2][BF4]2 series. Rapidly precipitated powders with the same compositions (1b-4b) mostly resemble 1a-4a, except that 2b is a mixed-phase material; 2b-4b also contain a fraction of amorphous solid in addition to the two crystal phases. The largest iron fraction that can be accommodated by the P21 phase in this system is 0.7 ± 0.1.

Iron(II) Complexes of 2,6-Di[4-(ethylcarboxy)pyrazol-1-yl]pyridine with Reversible Guest-Modulated Spin-Crossover Behavior.

Three solvatomorphs of the iron(II) complex of 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (bpCOOEt2p) of formulas [Fe(bpCOOEt2p)2](ClO4)2·1.5MeNO2 (1), [Fe(bpCOOEt2p)2](ClO4)2·MeNO2 (2), and [Fe(bpCOOEt2p)2](ClO4)2·2MeNO2 (3) have been prepared and characterized. They show interesting spin-crossover (SCO) properties ranging from partial to complete thermal spin transitions and a light-induced excited spin-state trapping (LIESST) effect. In solvatomorph 2, a robust structure is formed with channels that enable the entrance or removal of solvent molecules by vapor diffusion without losing the crystallinity. Thus, solvent-exchanged samples [Fe(bpCOOEt2p)2](ClO4)2·MeNO2 (2·MeNO 2 ), [Fe(bpCOOEt2p)2](ClO4)2·MeCN (2·MeCN), [Fe(bpCOOEt2p)2](ClO4)2·0.5Me2CO (2·Me 2 CO), and [Fe(bpCOOEt2p)2](ClO4)2·MeCOOH (2·MeCOOH) were prepared by vapor diffusion of the solvents in a crystal of the compound previously heated to 400 K in a single-crystal to single-crystal (SCSC) fashion. Interestingly, this causes a change of spin state with a stabilization of the low-spin state in 2·Me 2 CO and the high-spin state in 2·MeCN. Therefore, the SCO properties of 2 can be tuned in a reversible way by exposure to different solvents.

Structural dynamics of a thermally silent triiron(II) spin crossover defect grid complex.

The structural evolution of spin crossover (SCO) complexes during their spin transition at equilibrium and out-of-equilibrium conditions needs to be understood to enable their successful utilisation in displays, actuators and memory components. In this study, diffraction techniques were employed to study the structural changes accompanying the temperature increase and the light irradiation of a defect [2 × 2] triiron(II) metallogrid of the form [FeII3LH2(HLH)2](BF4)4·4MeCN (FE3), LH = 3,5-bis{6-(2,2'-bipyridyl)}pyrazole. Although a multi-temperature crystallographic investigation on single crystals evidenced that the compound does not exhibit a thermal spin transition, the structural analysis of the defect grid suggests that the flexibility of the grid, provided by a metal-devoid vertex, leads to interesting characteristics that can be used for intermolecular cooperativity in related thermally responsive systems. Time-resolved photocrystallography results reveal that upon excitation with a ps laser pulse, the defect grid shows the first two steps of the out-of-equilibrium process, namely the photoinduced and elastic steps, occurring at the ps and ns time scales, respectively. Similar to a previously reported [2 × 2] tetrairon(II) metallogrid, FE3 exhibits a local distortion of the entire grid during the photoinduced step and a long-range distortion of the lattice during the elastic step. Although the lifetime of the pure photoinduced high spin (HS) state is longer in the tetranuclear grid than in the defect grid, suggesting that the global nuclearity plays a crucial role for the lifetime of the photoinduced species, the influence of the co-crystalising solvent on the lifetime of the photoinduced HS state remains unknown. This study sheds light on the out-of-equilibrium dynamics of a thermally silent defect triiron SCO metallogrid.

Electro-Elastic Modeling of Thermal Spin Transition in Diluted Spin-Crossover Single Crystals.

Spin-crossover solids have been studied for many years for their promising applications as optical switches and reversible high-density memories for information storage. This study reports the effect of random metal dilution on the thermal and structural properties of a spin-crossover single crystal. The analysis is performed on a 2D rectangular lattice using an electro-elastic model. The lattice is made of sites that can switch thermally between the low-spin and high-spin states, accompanied by local volume changes. The model is solved by Monte Carlo simulations, running on the spin states and atomic positions of this compressible 2D lattice. A detailed analysis of metal dilution on the magneto-structural properties allows us to address the following issues: (i) at low dilution rates, the transition is of the first order; (ii) increasing the concentration of dopant results in a decrease in cooperativity and leads to gradual transformations above a threshold concentration, while incomplete spin transitions are obtained for big dopant sizes. The effects of the metal dilution on the spatiotemporal aspects of the spin transition along the thermal transition and on the low-temperature relaxation of the photo-induced metastable high-spin states are also studied. Significant changes in the organization of the spin states are observed for the thermal transition, where the single-domain nucleation caused by the long-range elastic interactions is replaced by a multi-droplet nucleation. As to the issue of the relaxation curves: their shape transforms from a sigmoidal shape, characteristic of strong cooperative systems, into stretched exponentials for high dilution rates, which is the signature of a disordered system.

Clamping of spin-crossover solid inducing crystal bending and spatial spin organization

The present work investigates the effect of mechanical clamping on thermally induced spin transition in a deformable spin-crossover single crystal. We performed the analysis, which combined Monte Carlo and molecular dynamics simulations, on a 2D rectangular lattice using an electro-elastic model, taking into account for the change of spin states and volume along the transition process. The molecules (the sites) of the crystal can be either in the low-spin (LS) or in the high-spin (HS) states which are respectively characterized with small and bigger molecular volumes. We found that straining the system edge along the thermal spin transition not only influences the nature of the phase transition which changes from first-order to continuous but also causes the appearance of significant macroscopic bending effects. Furthermore, the spatiotemporal investigations of the strained spin transition networks revealed the formation of spatially modulated HS/LS patterns resulting from the system's bending which promotes spatially organized tensile and compressive regions. These results demonstrate the interplay/competition between the stretching effects favored by temperature and the macroscopic bending triggered by the external constraints imposed to the lattice, lead to the emergence of complex self-organized spin structures and shape deformation in the strained SCO solids.

Three-dimensional electroelastic modeling of the nucleation and propagation of the spin domains in spin-crossover materials

A three-dimensional version of the electroelastic model allowing the description of spin-crossover (SCO) materials taking into account for the volume change at the transition between the LS and the HS spin states is developed. The investigations are realized on a rectangular parallelepiped lattice with cubic symmetry. The SCO units are modeled by two-states fictitious spins coupled by springs whose equilibrium distances depend on the spin states. We implemented massive parallel simulations using CUDA (Compute Unified Device Architecture) programming where the spin states are updated using the Monte Carlo Metropolis algorithm, while the mechanical relaxation (lattice position) is performed by molecular dynamics. In this work, we investigated (i) the case of the thermal spin transition showing the macroscopic deformation of the parallelepiped accompanying the propagation of single domains and (ii) the isothermal relaxation of the photoinduced metastable HS fraction at low temperature. In both cases, the interplay between the electronic and the structural aspects of these transformations is analyzed and discussed in relation with the model parameters.

Magnetic phase transitions in an electroelastic model for magnetically ordered spin-crossover solids

Spin-crossover and Prussian blue analog materials are investigated in two dimensions with the three-state Blume-Emery-Griffiths model where each spin interacts with its nearest neighbors and next-nearest neighbors and may be either in high-spin or low-spin state. The interactions through the system are strongly dependent on the instantaneous distance between atoms and are magnetic and elastic in nature. Finite-size effects have been detected at finite temperature on the model. The thermal distortion of the lattice configuration due to lattice unit displacements strengthened the thermal spin transition that occurred. The generated numerical results are obtained by two-step Monte Carlo simulations where used thermodynamic parameters allowed to establish a rich phase diagram. Gradual and first-order transitions with thermally induced hysteresis phenomena have been observed. Near the thermal hysteresis loops, the model exhibits throughout relaxation curves lattice configurations evolving through two-dimensional nucleation and growth processes that are enhanced with suitable values of the model parameters.

Interplay between core and shell in a RbCoFe@RbNiCo Prussian blue analogue spin transition heterostructure

Core–shell particles with a Prussian blue analogue RbaCob[Fe(CN)6]c·mH2O core and different shell thicknesses are studied as the cores undergo both thermal and light-induced spin transitions, revealing different mechanisms for accommodating strain.

Hexakis-adducts of [60]fullerene as molecular scaffolds of polynuclear spin-crossover molecules.

A family of hexakis-substituted [60]fullerene adducts endowed with the well-known tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand for spin-crossover (SCO) systems has been designed and synthesized. It has been experimentally and theoretically demonstrated that these molecular scaffolds are able to form polynuclear SCO complexes in solution. UV-vis and fluorescence spectroscopy studies have allowed monitoring of the formation of up to six Fe(ii)–bpp SCO complexes. In addition, DFT calculations have been performed to model the different complexation environments and simulate their electronic properties. The complexes retain SCO properties in the solid state exhibiting both thermal- and photoinduced spin transitions, as confirmed by temperature-dependent magnetic susceptibility and Raman spectroscopy measurements. The synthesis of these complexes demonstrates that [60]fullerene hexakis-adducts are excellent and versatile platforms to develop polynuclear SCO systems in which a fullerene core is surrounded by a SCO molecular shell.

Three-Way Switchable Single-Crystal-to-Single-Crystal Solvatomorphic Spin Crossover in a Molecular Cocrystal

The Fe(III) spin-switching complexes [Fe(qsal-5-Y)2]X (where Y = F, Cl, Br, I or OMe; X = NCS–, Cl–, OTf–, NO3–, BF4–, PF6–, or BPh4–) display a variety of thermal spin transition profiles, includi...

Simultaneous Spin-Crossover Transition and Conductivity Switching in a Dinuclear Iron(II) Coordination Compound Based on 7,7',8,8'-Tetracyano-p-quinodimethane.

The reaction of Fe(OAc)2 and Hbpypz with neutral TCNQ results in the formation of [Fe2 (bpypz)2 (TCNQ)2 ](TCNQ)2 (1), in which Hbpypz=3,5-bis(2-pyridyl)pyrazole and TCNQ=7,7',8,8'-tetracyano-p-quinodimethane. Crystal packing of 1 with uncoordinated TCNQ and π-π stacking of bpypz- ligands produces an extended two-dimensional supramolecular coordination assembly. Temperature dependence of the dc magnetic susceptibility and heat capacity measurements indicate that 1 undergoes an abrupt spin crossover (SCO) with thermal spin transition temperatures of 339 and 337 K for the heating and cooling modes, respectively, resulting in a thermal hysteresis of 2 K. Remarkably, the temperature dependence of dc electrical transport exhibits a transition that coincides with thermal SCO, demonstrating the thermally induced magnetic and electrical bistability of 1, strongly correlating magnetism with electrical conductivity. This outstanding feature leads to thermally induced simultaneous switching of magnetism and electrical conductivity and a magnetoresistance effect.

Evidence of Photo-Thermal Effects on the First-Order Thermo-Induced Spin Transition of [{Fe(NCSe)(py)2}2(m-bpypz)] Spin-Crossover Material

We have investigated by means of optical microscopy and magnetic measurements the first-order thermal spin transition of the [{Fe(NCSe)(py)2}2(m-bpypz)] spin-crossover compound under various shining intensities, far from the light-induced spin-state trapping region. We found evidence of photo-heating effects on the thermally-induced hysteretic response of this spin-crossover material, thus causing the shift of the thermal hysteresis to lower temperature regions. The experimental results are discussed in terms of the apparent crystal temperature and are analyzed theoretically using two evolution equations of motion, written on the high-spin (HS) fraction and heat balance between the crystal and the thermal bath. A very good qualitative agreement was found between experiment and theory in the stationary regime, explaining the experimental observations well and identifying the key factors governing these photo-thermal effects.

High-spin → low-spin relaxation kinetics and metal dilution effects in 1D spin-crossover chain compounds

Light-Induced Excited Spin-State Trapping (LIESST) is a well-known phenomenon in spin-crossover (SCO) materials, studied since 1982 in solution and 1984 in the solid state, as it offers some interesting prospects in data storage. In this paper we discuss the influence of metal dilution on the lifetime of the photo-induced state in two 1D chain solid solution series: [FexZn1−x(btzp)3](BF4)2 and [FexZn1−x(endi)3](BF4)2 {btzp = 1,2-bis(tetrazol-1-yl)propane and endi = 1,2-bis(tetrazol-1-yl)ethane}. For each composition, we have determined the thermal spin transition temperature, T1/2, and the relaxation temperature of the photoinduced high-spin state, T(LIESST). A detailed analysis of the relaxation kinetics allowed a discussion of the evolution of this lifetime as a function of metal dilution.

Spin‐Crossover and Field‐Induced Single‐Molecule Magnet Behaviour in Metal(II)‐Dipyrazolylpyridine Complexes

The reaction between the 2,6‐di(pyrazol‐1‐yl)‐4‐(bromomethyl)pyridine (L) ligand and divalent transition metal salts led to the formation of three mononuclear complexes of formula [M(L)2](BF4)2·xMeNO2 [M = FeII (1·xMeNO2), x = 0 and 4, M = CoII, x = 2 (2·2MeNO2) and]. Static magnetic measurements highlighted thermal spin crossover at high temperature (320–340 K) for the two FeII solvatomorphs nevertheless both the solvation degree and crystal packing had drastic influence on the shape and width of the hysteresis loop. Thus the 1 displayed abrupt and straight (6 K) hysteresis loop while for 1·4MeNO2 the thermal spin transition could be described as a six‐step gradual transition with the widest magnetic bistability range (35 K) for this kind of complexes. Irradiation at 530 nm of 1·4MeNO2 provoked an efficient LIESST effect at low temperature (TLIESST = 55 K). Finally dynamic magnetic measurements of 2·2MeNO2 shown a field‐induced mononuclear Single‐Molecule Magnet behavior with Δ = 14.6(4) cm–1 and τ0 = 6.28(8) × 10–7 s.

Spatiotemporal Investigations on Light-Driven High-Spin–Low-Spin Interface Dynamics in the Thermal Hysteresis Region of a Spin-Crossover Single Crystal

We have investigated by optical microscopy the thermal spin transition of a single crystal of the spin-crossover compound [{Fe(NCSe)(py)2}2(m-bpypz)] under various shining intensities, far from the light-induced spin-state trapping region. We found evidence of photoheating on the thermally induced hysteretic response of the crystal, leading to the control of the transition temperature and the hysteresis width as a function of the light intensity. The inspections of the spatiotemporal behaviors of the spin-crossover transition, on heating and cooling, have also evidenced a significant dependence of the propagation speed of the high-spin–low-spin interface on the intensity of light. In particular, for strong shining intensities, a slowing of the interface speed at the transition is obtained, and an unprecedented dynamical two-step-like transition was observed in the thermal hysteresis. These results are analyzed theoretically using a spatiotemporal approach based on reaction–diffusion equations including th...

Fluorescence modulation via photoinduced spin crossover switched energy transfer from fluorophores to FeII ions.

Molecular materials possessing phototunable fluorescence properties have attracted great interest owing to their potential applications in optical switches and storage. However, most fluorescence modulation is realized through light-responsive structural isomerization in solution. It is a formidable challenge to achieve phototunable fluorescence emission with high fatigue resistance and a fast response rate in the solid state for the development of devices. Here, a mononuclear compound was constructed via the coordination of fluorophores with FeII ions, whose electronic configuration changed from low spin to high spin upon light irradiation. The photoinduced spin crossover of FeII ions was accompanied by a 20% increase in the fluorescence emission intensity. A temperature-dependent spectroscopic study together with time-dependent density functional theory calculations revealed that the effective spectral overlap between the emission of the fluorophores and the absorption band of the FeII ions differed between the low spin and high spin states. The photoinduced spin crossover switched the energy transfer from the fluorophore to the FeII ion, resulting in fluorescence modulation. The presented results provide a novel approach for developing optical memory and sensors via electron rearrangement of photoinduced spin crossover.

Thermal and light-induced spin transition in a nanometric film of a new high-vacuum processable spin crossover complex

Herein, we report the identification of a novel high-vacuum processable spin-crossover complex which can be efficiently used to prepare continuous ultrathin films with retention of switchable magnetic properties.

The influence of the sample dispersion on a solid surface in the thermal spin transition of [Fe(pz)Pt(CN)4] nanoparticles.

The thermal spin transition of [Fe(pz)Pt(CN)4], pz = pyrazine, nanoparticles is compared with the one of the microcrystalline powder by magnetic susceptibility measurements, absorption spectroscopy and X-ray powder diffraction (XRPD) using synchrotron radiation. The thermal transition shows less cooperativity when decreasing the size due to the reduction of cluster formation. Surprisingly, the dispersion of the nanoparticles on a surface entails important effects on the spin crossover properties of the system. These effects are simulated and explained within the framework of the mechanoelastic model.

Fe(abpt)2(NCSe)2] polymorph A: structural studies into the spin crossover behaviour

Crystallographic and UV-Vis analysis of the thermal spin-transition in [Fe(abpt)2(NCSe)2], polymorph A, is presented alongside LIESST and high pressure structures.