Ising-like Model Hasn't Yet Said its Last Word: Exact and Mean-Field Investigations of 2, 3 and 4-Body Interactions in 1D Ising Chain of Binuclear Spin-Crossover Solids.

We investigate the static properties of a new class of 1D Ising-like Hamiltonian for binuclear spin-crossover materials accounting for two-, three-, and four-body short-range interactions between binuclear units of spins [[EQUATION]] and [[EQUATION]]. The following 2-, 3- , and 4-body [[EQUATION]], [[EQUATION]], and [[EQUATION]] terms are considered, in addition to intra-binuclear interactions, such as effective ligand-field energy and exchange-like coupling. An exact treatment is carried out within the frame of the transfer matrix method, leading to a [[EQUATION]] matrix from which, we obtained the thermal evolution of the thermodynamic quantities. Several situations of model parameter values were tested, among which that of competing intra- and inter-molecular interactions, leading to the occurrence of (i) one-step spin transition, (ii) two-, three-, and four-step transitions, obtained with a reasonable number of parameters. To reproduce first-order phase transitions, we accounted for inter-chains interactions, treated in the mean-field approach. Hysteretic multi-step transitions, recalling experimental observations, are then achieved. Overall, the present model not only suggests new landscapes of interaction configurations between SCO molecules but also opens new avenues to tackle the complex behaviors often observed in the properties of SCO materials.

BEG spin-1 model with random exchange magnetic interactions for spin-crossover solids.

We have investigated magnetic phase diagrams of spin-crossover (SCO) solids throughout the Blume-Emery-Griffiths spin-1 model where the spin states ±1 and 0 are associated to high-spin state and low-spin states respectively. In the present work, the quadrupolar interaction,K, parameter depends linearly on temperature and accounts for the role of the lattice phonons in the elastic interactions between the SCO units. Magnetic interactions are randomly distributed in the system and are controlled by a factorγ=Jij/Ksuch that forγ = 0 (Jij=0), magnetic ordering is not expected. The crystal-field that acts on SCO sites depends both on the ligand-field strength and the degeneracy ratio between HS and LS states as in some previous works. The system is also under the effect of a random local magnetic fieldhiacting on each sitei. The model is solved using a homogeneous mean field theory. Our investigations reveal the occurrence of thermally-induced gradual, and first-order spin-transitions by varying the model parameters. At vicinity of first-order transition, various types of isothermal magnetic hysteresis loops are obtained and their corresponding coercive field and loop patterns are discussed as function of temperature.

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.

Pressure-induced multi-step and self-organized spin states in an electro-elastic model for spin-crossover solids.

Spin transition materials are known to exhibit a rich variety of behaviors under several stimuli, among which pressure leads to major changes in their electronic and elastic properties. From an experimental point of view, thermal spin transitions under isotropic pressure showed transformations from (i) hysteretic to continuous transformations where the hysteresis width vanishes beyond some threshold pressure value; this is the conventional case. In several other cases very pathological and unexpected behaviours emerged, like (ii) persistent hysteresis under pressure; (iii) non-uniform behavior of the thermal hysteresis width which first increases with pressure and then decreases and vanishes at higher pressures; (iv) furthermore, double step transitions induced by pressure are also often obtained, where the pressure triggers the appearance of a plateau during the thermal transition, leading to two-step transitions, and finally (v) other non-conventional re-entrant transitions, where the thermal hysteresis vanishes at some pressure and then reappears at higher pressure values are also observed. In the present theoretical study, we investigate this problem with an electro-elastic description of the spin-crossover phenomenon by solving the Hamiltonian using a Monte Carlo technique. The pressure effect is here introduced directly in the lattice parameters, the elastic constants and ligand field energy. By considering spin state-dependent compressibility, we demonstrate that a large panel of experimental observations can be qualitatively described with this model. Among them, we quote (i) the conventional pressure effect decreasing the hysteresis width, (ii) the unconventional cases with pressure causing a non-monotonous behavior of the hysteresis width, (iii) re-entrant, as well as (iv) double step transitions accompanied with various types of spin state self-organization in the plateau regions.

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.

Downsizing of Nanocrystals While Retaining Bistable Spin Crossover Properties in Three-Dimensional Hofmann-Type {Fe(pz)[Pt(CN)4]}-Iodine Adducts.

Mastering nanostructuration of functional materials into electronic devices is presently an essential task in materials science. This is particularly relevant for spin crossover (SCO) compounds, whose properties are extremely sensitive to size reduction. Indeed, the search for materials displaying strong cooperative hysteretic SCO properties operative at the nanoscale close near room temperature is extremely challenging. In this context, we describe here the synthesis and characterization of 20-30 nm surfactant-free nanocrystals of the FeII Hofmann-type polymer {FeII(pz)[PtII,IVIx(CN)4]} (pz = pyrazine), which affords the first example of a robust three-dimensional coordination polymer, substantially keeping operational thermally induced SCO bistability at such a scale.

Pressure-Induced Conversion of a Paramagnetic FeCo Complex into a Molecular Magnetic Switch with Tuneable Hysteresis.

A key challenge in the design of magnetic molecular switches is to obtain bistability at room temperature. Here, we show that application of moderate pressure makes it possible to convert a paramagnetic FeIII 2 CoII 2 square complex into a molecular switch exhibiting a full dia- to paramagnetic transition: FeII CoIII ⇔ FeIII CoII . Moreover, the complex follows a rare behavior: the higher the pressure, the broader the magnetic hysteresis. Thus, the application of an adequate pressure allows inducing a magnetic bistability at room temperature with predictable hysteresis width. The structural studies at different pressures suggest that the pressure-enhanced bistability is due to the strengthening of intermolecular interactions upon pressure increase. An original microscopic Ising-like model including pressure effects is developed to simulate this unprecedented behavior. Overall, this study shows that FeCo complexes could be very sensitive piezo switches with potential use as sensors.

Interplay between spin-crossover and luminescence in a multifunctional single crystal iron(ii) complex: towards a new generation of molecular sensors.

We present a new example of a mononuclear iron(ii) complex exhibiting a correlated spin-crossover (SCO) transition and strong fluorescence, whose coordination sphere is saturated, for the first time, by six phosphorescent ligands. The interplay between SCO and light emission properties in the thermal region of the spin transition was investigated by means of magnetic, fluorescence, optical absorption and optical microscopy measurements on a single crystal. Overall, the results show an excellent correlation between fluorescence and magnetic data of the present gradual transition, indicating an extreme sensitivity of the optical activity of the ligand to the spin state of the active iron(ii) ions. These results open the way for conceiving new prototypes of pressure and temperature sensors based on this synergy between SCO and luminescence properties. In particular, the fact that cooperative SCO material is not a prerequisite for obtaining such synergetic effects, is useful for the design of thin films or nanoparticles, in which the cooperativity is reduced, for appropriate implementation in nanosized devices to enhance the sensing properties at the nanoscale.

An electro-elastic theory for the mechanically-assisted photo-induced spin transition in core-shell spin-crossover nanoparticles.

The development of heterostructure materials may lead to new features that cannot be obtained with natural materials. Here we simulate a model structurally hybrid core-shell nanoparticle with different lattice parameters between an electronically inert shell and an active spin crossover core. The nanoparticle consists of a 2D core with 20 × 20 size with square symmetry, surrounded by a shell made of 10 atomic layers. The low temperature photoexcitation of the core shows a significant environment-dependent behavior. In particular, we demonstrate that a shell with a large lattice parameter accelerates the low-spin to high-spin photoexcitation process of the core through the single domain nucleation mechanism while a moderate shell lattice parameter leads to spatially-homogeneous growth of the high-spin fraction. We found that the mechanical retro-action of the shell may cause elastic instability of the core leading to efficient control and manipulation of its photo-conversion.

Structural Insight into Order-Disorder Transition and Charge-Transfer Phase Transition in an Iron Mixed-Valence Complex (n-C3H7)4N[FeIIFeIII(dto)3] with a Two-Dimensional Honeycomb Network.

The structural properties of the iron mixed-valence complex ( n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato, C2O2S2) have been investigated by single-crystal X-ray diffraction (SCXRD) at low temperatures. ( n-C3H7)4N[FeIIFeIII(dto)3] has two-dimensional (2D) honeycomb layers consisting of alternating FeII and FeIII arrays bonded by bis-bidentate dithiooxalato ligands. Upon cooling, a superlattice structure with q = (1/3, 1/3, 0) was observed below 260 K, which corresponds to an order-disorder transition of the ( n-C3H7)4N+ ions between the honeycomb layers. The charge-transfer phase transition (CTPT) occurs at TC↑1/2 ∼ 120 K and TC↓1/2 ∼ 90 K upon heating and cooling, respectively, with an electron transfer between the FeII and FeIII ions, accompanied by a spin-state change, FeII ( S = 2; HS)-O2C2S2-FeIII ( S = 1/2; LS) ↔ FeIII ( S = 5/2; HS)-O2C2S2-FeII ( S = 0; LS). During the CTPT, the intersheet [FeIIFeIII(dto)3] distance decreased monotonously upon cooling, and an abrupt structural contraction was observed in the hexagonal 2D network. The volume contraction during the CTPT was quite small (∼0.7%), and differences in the structural distortions at the FeS6 and FeO6 sites were not found in the vicinity of the CTPT. We also calculated the orbital energies and the occupied spin states for the [Fe(O2C2S2)3] and [Fe(S2C2O2)3] octahedra in the vicinity of the CTPT by density functional theory (DFT). Because the local symmetry around the two coordinating iron ions is already lowered to trigonal symmetry, the CTPT did not cause any further deformation. This symmetry invariance resulted in an absence of orbital contributions to the total entropy change (Δ S) in the CTPT, which is in agreement with the previous heat capacity measurements. [Nakamoto, T; Miyazaki, Y; Itoi, M; Ono, Y; Kojima, N; Sorai, M. Heat Capacity of the Mixed-Valence Complex {[( n-C3H7)4N][FeIIFeIII(dto)3]}∞, Phase Transition because of Electron Transfer, and a Change in Spin-State of the Whole System. Angew. Chem., Int. Ed. 2001, 40, 4716-4719.].

Unprecedented Bistability in Spin-Crossover Solids Based on the Retroaction of the High Spin Low-Spin Interface with the Crystal Bending.

There has been in recent years a continuous increase in spatiotemporal investigations of the dynamics of the first-order transitions in spin-crossover (SCO) solids. In single crystals, this phenomenon proceeds via a single domain nucleation and propagation, characterized in some systems with the presence of two equivalent and symmetric interface orientations, between the high-spin (HS) and low-spin (LS) phases, due to the anisotropic structural change of the unit cell at the transition. The present investigations bring an experimental evidence of the reversible driving of the translational and rotational degrees of freedom of the HS-LS interface. In addition to its rectilinear displacement, the interface rotates between two stable angles, 60o and 120o. It is demonstrated that while the translation motion is accompanied by a crystal's length change, the interface rotation is controlled by the crystal's bending. These results are well-explained in the frame of an elastic theoretical description in which the effect of the crystal bending, on the stability of the interface's orientation, is simulated by applying a moment of forces on the crystal. It is found that the interface orientation becomes unstable beyond a threshold load value, announcing the emergence of a bistability in SCO solids, taking place at constant HS fraction and volume. This work underlines the sensitive character of the interface orientation to any macroscopic crystal bending, an idea that can be used to develop a new generation of robust stress sensors, working at constant volume, thus avoiding deterioration problems due to crystal fatigue.

Strain engineering of photo-induced phase transformations in Prussian blue analogue heterostructures.

Heterostructures based on Prussian blue analogues (PBA) combining photo- and magneto-striction have shown a large potential for the development of light-induced magnetization switching. However, studies of the microscopic parameters that control the transfer of the mechanical stresses across the interface and their propagation in the magnetic material are still too scarce to efficiently improve the elastic coupling. Here, this coupling strength is tentatively controlled by strain engineering in heteroepitaxial PBA core-shell heterostructures involving the same Rb0.5Co[Fe(CN)6]0.8·zH2O photostrictive core and isostructural shells of similar thickness and variable mismatch with the core lattice. The shell deformation and the optical electron transfer at the origin of photostriction are monitored by combined in situ and real time synchrotron X-ray powder diffraction and X-ray absorption spectroscopy under visible light irradiation. These experiments show that rather large strains, up to +0.9%, are developed within the shell in response to the tensile stresses associated with the expansion of the core lattice upon illumination. The shell behavior is, however, complex, with contributions in dilatation, in compression or unchanged. We show that a tailored photo-response in terms of strain amplitude and kinetics with potential applications for a magnetic manipulation using light requires a trade-off between the quality of the interface (which needs a small lattice mismatch i.e. a small a-cubic parameter for the shell) and the shell rigidity (decreased for a large a-parameter). A shell with a high compressibility that is further increased by the presence of misfit dislocations will show a decrease in its mechanical retroaction on the photo-switching properties of the core particles.

Control of the Speed of a Light-Induced Spin Transition through Mesoscale Core-Shell Architecture.

The rate of the light-induced spin transition in a coordination polymer network solid dramatically increases when included as the core in mesoscale core-shell particles. A series of photomagnetic coordination polymer core-shell heterostructures, based on the light-switchable Rb aCo b[Fe(CN)6] c· mH2O (RbCoFe-PBA) as core with the isostructural K jNi k[Cr(CN)6] l· nH2O (KNiCr-PBA) as shell, are studied using temperature-dependent powder X-ray diffraction and SQUID magnetometry. The core RbCoFe-PBA exhibits a charge transfer-induced spin transition (CTIST), which can be thermally and optically induced. When coupled to the shell, the rate of the optically induced transition from low spin to high spin increases. Isothermal relaxation from the optically induced high spin state of the core back to the low spin state and activation energies associated with the transition between these states were measured. The presence of a shell decreases the activation energy, which is associated with the elastic properties of the core. Numerical simulations using an electro-elastic model for the spin transition in core-shell particles supports the findings, demonstrating how coupling of the core to the shell changes the elastic properties of the system. The ability to tune the rate of optically induced magnetic and structural phase transitions through control of mesoscale architecture presents a new approach to the development of photoswitchable materials with tailored properties.

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

We investigated by means of optical microscopy (OM) the spatiotemporal features of the thermo-induced spin transition of [Fe(2-pytrz)2{Pd(CN)4}]·3H2O (1) (2-pytrz = 4-(2-pyridyl)-1,2,4,4H-triazole) single crystals having two different shapes (triangle and rectangle). While magnetic and calorimetric measurements, performed on a polycrystalline material, showed the respective average heating and cooling transition temperatures of (Tdown1/2 ∼ 152 K, Tup1/2 ∼ 154 K) and (Tdown1/2 ∼ 160.0 K, Tup1/2 ∼ 163.5 K), OM studies performed on a unique single crystal revealed significantly different switching temperatures (Tdown1/2 ∼ 152 K and Tup1/2 ∼ 162 K). OM investigations showed an interface spreading over all crystals during the spin transition. Thanks to the color contrast between the low-spin (LS) and the high-spin (HS) states, we have been able to follow the real time dynamics of the spin transition between these two spin states, as well as access the thermal hysteresis loop of each single crystal. After image processing, the HS-LS interface's velocity was carefully estimated in the ranges [4.4-8.5] µm s-1 and [2.5-5.5] µm s-1 on cooling and heating, respectively. In addition, we found that the velocity of the interface is shape-dependent, and accelerates nearby the crystal's borders. Interestingly, we observed that during the propagation process, the interface optimizes its shape so as to minimize the excess of elastic energy arising from the lattice parameter misfit between the LS and HS phases. All of these original experimental results are well reproduced using a spatiotemporal model based on the description of the spin-crossover problem as a reaction diffusion phenomenon.

Broadband Emission in Hybrid Organic-Inorganic Halides of Group 12 Metals.

We report syntheses, crystal and electronic structures, and characterization of three new hybrid organic-inorganic halides (R)ZnBr3(DMSO), (R)2CdBr4·DMSO, and (R)CdI3(DMSO) (where (R) = C6(CH3)5CH2N(CH3)3, and DMSO = dimethyl sulfoxide). The compounds can be conveniently prepared as single crystals and bulk polycrystalline powders using a DMSO-methanol solvent system. On the basis of the single-crystal X-ray diffraction results carried out at room temperature and 100 K, all compounds have zero-dimensional (0D) crystal structures featuring alternating layers of bulky organic cations and molecular inorganic anions based on a tetrahedral coordination around group 12 metal cations. The presence of discrete molecular building blocks in the 0D structures results in localized charges and tunable room-temperature light emission, including white light for (R)ZnBr3(DMSO), bluish-white light for (R)2CdBr4·DMSO, and green for (R)CdI3(DMSO). The highest photoluminescence quantum yield (PLQY) value of 3.07% was measured for (R)ZnBr3(DMSO), which emits cold white light based on the calculated correlated color temperature (CCT) of 11,044 K. All compounds exhibit fast photoluminescence lifetimes on the timescale of tens of nanoseconds, consistent with the fast luminescence decay observed in π-conjugated organic molecules. Temperature dependence photoluminescence study showed the appearance of additional peaks around 550 nm, resulting from the organic salt emission. Density functional theory calculations show that the incorporation of both the low-gap aromatic molecule R and the relatively electropositive Zn and Cd metals can lead to exciton localization at the aromatic molecular cations, which act as luminescence centers.

An unprecedented trinuclear FeII triazole-based complex exhibiting a concerted and complete sharp spin transition above room temperature.

We report a triazole-based trinuclear complex as the first example that displays a complete one-step first-order [HS-HS-HS] ↔ [LS-LS-LS] spin transition at 318 K. The strong ferro-elastic interactions, between the three metal centers, have been identified as the source of the concerted spin transition in this trinuclear complex.

Elastic Frustration Triggering Photoinduced Hidden Hysteresis and Multistability in a Two-Dimensional Photoswitchable Hofmann-Like Spin-Crossover Metal-Organic Framework.

We report a two-dimensional Hofmann-like spin-crossover (SCO) material, [Fe(trz-py)2{Pt(CN)4}]·3H2O, built from [FePt(CN)4] layers separated by interdigitated 4-(2-pyridyl)-1,2,4,4H-triazole (trz-py) ligands with two symmetrically inequivalent FeII sites. This compound exhibits an incomplete first-order spin transition at 153 K between fully high-spin (HS-HS) and intermediate high-spin low-spin (HS-LS) ordered states. At low temperature, it undergoes a bidirectional photoswitching to HS-HS and fully low-spin (LS-LS) states with green and near-IR light irradiation, respectively, with associated T(LIESST = Light-Induced Excited Spin-State Trapping) and T(reverse-LIESST) values of 52 and 85 K, respectively. Photomagnetic investigations show that the reverse-LIESST process, performed from either HS-HS or HS-LS states, enables access to a hidden stable LS-LS state, revealing the existence of a hidden thermal hysteresis. Crystallographic investigations allowed to identify that the strong metastability of the HS-LS state originates from the existence of a strong elastic frustration causing antiferroelastic interactions within the [FePt(CN)4] layers, through the rigid NC-Pt-CN bridges connecting the inequivalent FeII sites. The existence of the stable LS-LS state paves the way for a multidirectional photoswitching and allows potential applications for electronic devices based on ternary digits.

Elastic Frustration Causing Two-Step and Multistep Transitions in Spin-Crossover Solids: Emergence of Complex Antiferroelastic Structures.

Two-step and multistep spin transitions are frequently observed in switchable cooperative molecular solids. They present the advantage to open the way for three- or several-bit electronics. Despite extensive experimental studies, their theoretical description was to date only phenomenological, based on Ising models including competing ferro- and antiferro-magnetic interactions, even though it is recognized that the elastic interactions are at the heart of the spin transition phenomenon, due to the volume change between the low- and high-temperature phases. To remedy this shortcoming, we designed the first consistent elastic model, taking into account both volume change upon spin transition and elastic frustration. This ingredient was revealed to be powerful, since it was able to obtain all observed experimental configurations in a consistent way. Thus, according to the strength of the elastic frustration, the system may undergo first-order transition with hysteresis, gradual, hysteretic two-step or multistep transitions, and incomplete transitions. Furthermore, the analysis of the spatial organization of the HS and LS species in the plateau regions revealed the emergence of complex antiferro-elastic patterns going from simple antiferro-magnetic-like order to long-range spatial modulations of the high-spin fraction. These results enabled us to identify the elastic frustration as the fundamental mechanism at the origin of the very recent experimental observations showing the existence of organized spatial modulations of the high-spin fraction inside the plateau of two-step spin transitions.

Vibronic Theory of Ultrafast Intersystem Crossing Dynamics in a Single Spin-Crossover Molecule at Finite Temperature beyond the Born--Oppenheimer Approximation.

Quantum density matrix theory is carried out to study the ultrafast dynamics of the photoinduced state in a spin-crossover (SC) molecule interacting with a heat bath. The investigations are realized at finite temperature and beyond the usual Born-Oppenheimer (BO) approach. We found that the SC molecule experiences in the photoexcited state (PES) a huge internal pressure, estimated at several gigapascals, partly released in an "explosive" way within ∼100 fs, causing large bond length oscillations, which dampen in the picosecond time scale because of internal conversion processes. During this regime, the BO approximation is not valid. Depending on the tunneling strength, the ultrafast relaxation may proceed through the thermodynamic metastable high-spin state or prevent it. Interestingly, we demonstrate that final relaxation toward the low-spin state always follows a local equilibrium pathway, where the BO approach is valid. Our formulation reconciles the nonequilibrium and the equilibrium properties of this fascinating phenomenon and opens the way to quantum studies on cluster molecules.

Reversible Control by Light of the High-Spin Low-Spin Elastic Interface inside the Bistable Region of a Robust Spin-Transition Single Crystal.

By using a weak modulated laser intensity we have succeeded in reversibly controlling the dynamics of the spin-crossover (SC) single crystal [{Fe(NCSe)(py)2 }2 (m-bpypz)] inside the thermal hysteresis. The experiment could be repeated several times with a reproducible response of the high-spin low-spin interface and without crystal damage. In-depth investigations as a function of the amplitude and frequency of the excitation brought to light the existence of a cut-off frequency ca. 1.5 Hz. The results not only document the applicability of SC materials as actuators, memory devices, or switches, but also open a new avenue for the reversible photo-control of the spin transition inside the thermal hysteresis.