Velocity of the high-spin low-spin interface inside the thermal hysteresis loop of a spin-crossover crystal, via photothermal control of the interface motion

Exploring the equilibrium and non-equilibrium properties of a cooperative trinuclear spin-crossover chain: The role of elastic frustration

Among the large family of spin-crossover (SCO) solids, recent investigations focused on polynuclear SCO materials, whose specific molecular configurations allow the presence of multi-step transitions and elastic frustration. In this contribution, we develop the first elastic modeling of thermal and dynamical properties of trinuclear SCO solids. For that, we study a finite SCO open chain constituted of successive elastically coupled trinuclear (A=B=C) blocks, in which each site (A, B, and C) may occupy two electronic configurations, namely, low-spin (LS) and high-spin (HS) states, accompanied with structural changes. Intra- and inter-molecular springs couple the sites inside and between trimers. The model also includes the change of length inside and between the trinuclear units subsequent to the spin states changes. First, we studied the mechanical relaxation of a LS chain initially prepared with HS distances, from which we dissected the dynamics of the atomic displacements for various strengths of intra- and inter-molecular elastic constants. Second, we investigated the thermal properties of the chain at equilibrium, which revealed the existence of a rich variety of behaviors, going from: gradual LS to HS transition to multiple spin transitions with the presence of self-organized spin state structures in the plateaus. The latter were identified as emerging from antagonist short- and long-range elastic interactions between intra- and inter-block size changes. The present model opens several possible extensions, among which are the cases of coupled non-linear trimer molecules as well as that of inter-chain interactions with block-block interactions, leading to unexpected hysteretic spin transitions.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Abrupt Spin Transition in a Heteroleptic Fe(II) Complex with Pendant Naphthalene Functionality

A heteroleptic spin-crossover (SCO) complex, [Fe(tpma)(xnap-bim)](ClO₄)₂ (1; tpma = tris(2-pyridylmethyl)amine, xnap-bim = 8,15-dihydrodiimidazo[1,2-a:2',1'-c]naphtho[2,3-f][1,4]diazocine), has been obtained by reacting a Fe(II) precursor salt with tetradentate tpma and bidentate xnap-bim ligands. Depending on crystallization conditions, two different solvates have been obtained, 1·2.25py·0.5H₂O and 1·py. The former readily loses the interstitial solvent to produce either a powder sample of 1 upon filtration or crystals of 1 if the solvent loss is slowed by placing the crystals of 1·2.25py·0.5H₂O in diethyl ether. In contrast, 1·py exhibits higher stability toward solvent loss. The crystal packing of both solvates and of the solvent-free structure features double columns of [Fe(tpma)(xnap-bim)]²⁺ cations formed by efficient π-π interactions between the pyridyl groups of tpma ligands, as well as by stacks supported by π-π interactions between interdigitated naphthalene fragments of xnap-bim ligands. While both solvates show a gradual SCO between the high-spin (HS) and low-spin (LS) states of the Fe(II) ion, solvent-free complex 1 exhibits an abrupt spin transition centered at 127 K, with a narrow 2 K thermal hysteresis. Complex 1 also shows a light-induced excited spin state trapping effect, manifested as LS → HS conversion upon irradiation with white light at 5 K. The metastable HS state relaxes to the ground LS state when heated above 65 K.

Optical microscopy imaging of the thermally-induced spin transition and isothermal multi-stepped relaxation in a low-spin stabilized spin-crossover material

The thermal spin transition and the photo-induced high-spin → low-spin relaxation of the prototypical [Fe(ptz)₆](BF₄)₂ spin-crossover compound (ptz = 1-propyltetrazole) diluted in the isostructural ruthenium host lattice [Ru(ptz)₆](BF₄)₂, which stabilizes the Fe(II) low-spin state, have been investigated. We demonstrate the presence of a crystallographic phase transition around 145 K (i.e. from the high-temperature ordered high-spin phase to a low-temperature disordered low-spin phase) upon slow cooling from room temperature. This crystallographic phase transition is decoupled from the thermal spin transition. A supercooled ordered low-spin phase is observed as in the pure Fe(II) analogue upon fast cooling. A similar order-disorder phase transition is also observed for pure [Ru(ptz)₆](BF₄)₂ but at relatively higher temperature (i.e. at around 150 K) without involving any spin transition. For Ru-diluted [Fe(ptz)₆]²⁺, the crystallographic phase transition as well as strong cooperative effects involving various degrees of elastic frustration are at the origin of stepped sigmoidal high-spin → low-spin relaxation curves, which are modelled in the framework of a classical mean field model, considering both the tunnelling and thermally activated regimes. Optical microscopy studies performed on two different single crystals showed the existence of hysteretic thermal transitions with slight domain formation, hardly visible in the static crystal images. This behavior is attributed to the double effect upon Ru dilution, which decreases the cooperative character of the transition and simultaneously reduces the optical contrast between the LS and HS states. Moreover, the transition temperature revealed to be slightly crystal dependent, highlighting the crucial role of the spatial distribution of Ru from one crystal to another, in addition to the well-known effects of crystal shape and size.

Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling

Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn³⁺ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [Mn^III(3,5-diCl-sal₂(323))]BPh₄ reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P1 → P1_(1/2). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn–Teller-active Mn³⁺ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P1 and P1 → P1_(1/2) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1→ P1_(1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

Asymmetric Design of Spin-Crossover Complexes to Increase the Volatility for Surface Deposition

A mononuclear complex [Fe(tBu₂qsal)₂] has been obtained by a reaction between an Fe(II) precursor salt and a tridentate ligand 2,4-di(tert-butyl)-6-((quinoline-8-ylimino)methyl)phenol (tBu₂qsalH) in the presence of triethylamine. The complex exhibits a hysteretic spin transition at 117 K upon cooling and 129 K upon warming, as well as light-induced excited spin-state trapping at lower temperatures. Although the strongly cooperative spin transition suggests substantial intermolecular interactions, the complex is readily sublimable, as evidenced by the growth of its single crystals by sublimation at 573 → 373 K and ∼10^-3 mbar. This seemingly antagonistic behavior is explained by the asymmetric coordination environment, in which the tBu substituents and quinoline moieties appear on opposite sides of the complex. As a result, the structure is partitioned in well-defined layers separated by van der Waals interactions between the tBu groups, while the efficient cooperative interactions within the layer are provided by the quinoline-based moieties. The abrupt spin transition is preserved in a 20 nm thin film prepared by sublimation, as evidenced by abrupt and hysteretic changes in the dielectric properties in the temperature range comparable to the one around which the spin transition is observed for the bulk material. The changes in the dielectric response are in excellent agreement with differences in the dielectric tensor of the low-spin and high-spin crystal structures evaluated by density functional theory calculations. The substantially higher volatility of [Fe(tBu₂qsal)₂], as compared to a similar complex without tBu substituents, suggests that asymmetric molecular shapes offer an efficient design strategy to achieve sublimable complexes with strongly cooperative spin transitions.

Elastic Origin of the Unsymmetrical Thermal Hysteresis in Spin Crossover Materials: Evidence of Symmetry Breaking

The jungle of experimental behaviors of spin-crossover materials contains a tremendous number of unexpected behaviors, among which, the unsymmetrical hysteresis loops having different shapes on heating and cooling, that we often encounter in literature. Excluding an extra effect of crystallographic phase transitions, we study here these phenomena from the point of view of elastic modeling and we demonstrate that a simple model accounting for the bond lengths misfits between the high-spin and low-spin states is sufficient to describe the situation of unsymmetrical hysteresis showing plateaus at the transition only on cooling or on heating branches. The idea behind this effect relates to the existence of a discriminant elastic frustration in the lattice, which expresses only along the high-spin to low-spin transition or in the opposite side. The obtained two-step transitions showed characteristics of self-organization of the spin states under the form of stripes, which we explain as an emergence process of antagonist directional elastic interactions inside the lattice. The analysis of the spin state transformation inside the plateau on cooling in terms of two sublattices demonstrated that the elastic-driven self-organization of the spin states is accompanied with a symmetry breaking.

Spin Crossover and High-Spin State in Fe(II) Anionic Polymorphs Based on Tripodal Ligands

Two new mononuclear Fe(II) polymorphs, [(C₂H₅)₄N]₂[Fe(py₃C-OEt)(NCS)₃]₂ (1) and [(C₂H₅)₄N][Fe(py₃C-OEt)(NCS)₃] (2) (py₃C-OEt = tris(pyridin-2-yl)ethoxymethane), have been synthesized and characterized by single-crystal X-ray diffraction, by magnetic and photomagnetic measurements, and by detailed variable-temperature infrared spectroscopy. The molecular structure, in both complexes, is composed of the same anionic [Fe(py₃C-OEt)(NCS)₃]^- complex (two units for 1 and one unit for 2) generated by coordination to the Fe(II) metal center of one tridentate py₃C-OEt tripodal ligand and three terminal κN-SCN coligands. Magnetic studies revealed that polymorph 2 displays a high-spin (HS) state over the entire studied temperature range (300-10 K), while complex 1 exhibits an abrupt and complete spin crossover (SCO) transition at ca. 132.3 K, the structural characterizations of which, performed at 295 and 100 K, show a strong modification, resulting from the thermal evolutions of the Fe-N bond lengths and of the distortion parameters (∑ and Θ) of the FeN₆ coordination sphere, in agreement with the presence of HS and low-spin (LS) states at 295 and 100 K, respectively. This thermal transition has been also confirmed by the thermal evolution of the maximum absorbance for ν(NCS) vibrational bands recorded in the temperature range 200-10 K. In 1 the signature of a metastable photoinduced HS state has been observed using photomagnetic and photoinfrared spectroscopy, leading to a similar T(LIESST) relaxation temperature (LIESST = light-induced excited spin-state trapping) of 70 K.

Giant Magnetoelectric Coupling and Magnetic-Field-Induced Permanent Switching in a Spin Crossover Mn(III) Complex

We investigate giant magnetoelectric coupling at a Mn³⁺ spin crossover in [Mn^IIIL]BPh₄ (L = (3,5-diBr-sal)₂323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη² coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

Fully Controllable Structural Phase Transition in Thermomechanical Molecular Crystals with a Very Small Thermal Hysteresis

The construction of a practical crystalline molecular machine faces two challenges: to realize a collective molecular movement, and to amplify this movement into a precisely controlled mechanical response in real time and space. Thermosalient single crystals display cooperative molecular movements that are converted to strong macroscopic mechanical responses or shape deformations during temperature-induced structural phase transitions. However, these collective molecular movements are hard to control once initiated, and often feature thermal hystereses that are larger than 10 °C, which greatly hamper their practical applications. Here, it is demonstrated that the phase boundaries of the thermomechanical molecular crystal based on a fluorenone derivative 4-DBpFO can be used to finely control its structural phase transition. When this phase transition is triggered at two opposite crystal faces, it is accompanied by two parallel phase boundaries that can be temperature controlled to move forward, backward, or to halt, benefitting from the stored elastic energy between the parallel boundaries. Moreover, the thermal hysteresis is greatly decreased to 2-3 °C, which allows for circular heating/cooling cycles that can produce a continuous work output.

Stress-Induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry-breaking phase transitions in the mononuclear Mn3+ compound [Mn(3,5-diBr-sal2 (323))]BPh4 , 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress-induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc-P1 transition.

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.

Spatio-temporal Investigations of the Incomplete Spin Transition in a Single Crystal of [Fe(2-pytrz)2{Pt(CN)4}]·3H2O: Experiment and Theory

Optical microscopy technique is used to investigate the thermal and the spatio-temporal properties of the spin-crossover single crystal [Fe(2-pytrz) 2 {Pt(CN) 4 }]·3H 2 O, which exhibits a first-order spin transition from a full high-spin (HS) state at high temperature to an intermediate, high-spin low-spin (HS-LS) state, below 153 K, where only one of the two crystallographic Fe(II) centers switches from the HS to HS-LS state. In comparison with crystals undergoing a complete spin transition, the present transformation involves smaller volume changes at the transition, which helps to preserving the crystal’s integrity. By analyzing the spatio-temporal properties of this spin transition, we evidenced a direct correlation between the orientation and shape of HS/HS-LS domain wall with the crystal’s shape. Thanks to the small volume change accompanying this spin transition, the analysis of the experimental data by an anisotropic reaction-diffusion model becomes very relevant and leads to an excellent agreement with the experimental observations.

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

The dispersion on a Sapphire surface of [Fe(pz)Pt(CN)₄], pz = pyrazine nanoparticles influences the thermal spin transition, as shown using magnetic, spectroscopic and diffraction data. This is explained within the framework of the mechanoelastic model.

Interplay between a crystal's shape and spatiotemporal dynamics in a spin transition material

Experimental (top) and theoretical (bottom) snapshots of the interface propagation along the spin transition in the spin-crossover single crystal [Fe(2-pytrz)₂{Pd(CN)₄}]·3H₂O, showing its interplay with the crystal shape.

Rapid and robust spatiotemporal dynamics of the first-order phase transition in crystals of the organic-inorganic perovskite (C12H25NH3)2PbI4

The dynamics of the thermally induced first-order structural phase transition in a high-quality single crystal of the organic-inorganic perovskite (C₁₂H₂₅NH₃)₂PbI₄ was investigated by optical microscopy. The propagation of the straight phase front (habit plane) during the phase transition along the cooling and heating pathways of the thermal hysteresis was observed. The thermochromic character of the transition allowed monitoring of the thermal dependence of average optical density and aided the visualization of the interface propagation. The thermal hysteresis loop is 10 K wide, and the interface velocity is constant at V ≈ 1.6 mm s^–1. The transition is accompanied with sizeable change in crystal size, with elongation of ~6% along the b axis and compression of ~ –2% along the a axis, in excellent agreement with previously reported X-ray diffraction data. The progression of the habit plane is at least 160 times faster than in spin-crossover materials, and opens new prospects for organic-inorganic perovskites as solid switching materials. Moreover, the crystals of (C₁₂H₂₅NH₃)₂PbI₄ are unusually mechanically robust and present excellent resilience to thermal cycling. These hitherto unrecognized properties turn this and possibly similar hybrid perovskites into perspective candidates as active medium for microscopic actuation.

A two-in-one pincer ligand and its diiron(II) complex showing spin state switching in solution through reversible ligand exchange

A novel pyrazolate-bridged ligand providing two {PNN} pincer-type compartments has been synthesized. Its diiron(II) complex LFe2(OTf)3(CH3CN) (1; Tf = triflate) features, in solid state, two bridging triflate ligands, with a terminal triflate and a MeCN ligand completing the octahedral coordination spheres of the two high-spin metal ions. In MeCN solution, 1 is shown to undergo a sequential, reversible, and complete spin transition to the low-spin state upon cooling. Detailed UV/Vis and (19)F NMR spectroscopic studies as well as magnetic measurements have unraveled that spin state switching correlates with a rapid multistep triflate/MeCN ligand exchange equilibrium. The spin transition temperature can be continuously tuned by varying the triflate concentration in solution.

Spin crossover with thermal hysteresis: practicalities and lessons learnt

Memory applications of spin crossover require bistability: magnetic data must be appropriately collected and reported, and consideration given to lifetimes.

High-temperature photo-induced switching and pressure-induced transition in a cooperative molecular spin-crossover material

The thermal and photo-induced switching properties of the recently published molecular spin crossover complex [Fe(H4L)2](ClO4)2·H2O·2(CH3)2CO () have been investigated in detail through Raman spectroscopy. Magnetometric and single crystal X-ray diffraction kinetic studies within the hysteresis loop have proven its metastable character, which allowed establishing the shape of the true, quasi-static hysteresis loop. This is related to the proximity of the temperatures of the thermal high to low spin SCO (T1/2↓) to those of the relaxation of the thermally or photo-generated metastable states (T(TIESST) and T(LIESST), respectively). Green light irradiation within the hysteresis loop results in a complete low to high spin photo-switch. In addition, the SCO of at room temperature can be induced by applying pressure, as followed by Raman spectroscopy.

AFM imaging of molecular spin-state changes through quantitative thermomechanical measurements

Quantitative atomic force microscopy is used in conjunction with microwire heaters for high-resolution imaging of the Young's modulus changes across the spin-state transition. When going from the high spin to the low spin state, a significant stiffening is observed.

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.