Spin Crossover Phenomenon Research Articles

Control of Spin-Crossover Phenomena in One-Dimensional Triazole-Coordinated Iron(II) Complexes by Means of Functional Counter Ions

The spin-crossover (SCO) phenomenon between a high-spin and a low-spin state has attracted much attention in the field of materials science. Among the various kinds of SCO complexes, the triazole-bridged iron(II) polymeric chain system, [Fe(II)(R-trz)3]X2·xH2O (where trz is triazole and X is the anion), exhibiting the SCO phenomenon with thermal hysteresis around room temperature, has been extensively studied from the viewpoint of molecular memory and molecular devices. In connection with this system, we have controlled the SCO phenomenon according to the characteristic properties of counter ions. In the case of X being CnH2n+1SO3−, the spin transition temperature (T1/2) increases with increasing the length (n) of the alkyl chain of the counter ion and saturates above n = 5, which is attributed to the increase in the intermolecular interaction of the alkyl chains of CnH2n+1SO3−, called the fastener effect. The hysteresis width of T1/2 decreases with increasing n, showing the even-odd, also known as parity, effect. In the cases where X is toluenesulfonate (tos: CH3C6H4SO3−) and aminobenzenesulfonate (abs: NH2C6H4SO3−), T1/2 and its hysteresis width vary drastically with the structural isomerism (ortho-, metha-, and para-substitution) of counter ions, which implies the possibility of photoinduced spin transition by means of the photoisomerization of counter ions. From this strategy, we have synthesized [Fe(II)(NH2-trz)3](SP150)2·2H2O (SP150 = N-alkylsulfonated spiropyran) and investigated the SCO phenomenon. Moreover, we have developed [Fe(II)(R-trz)3]@Nafion films exhibiting spin transition around room temperature, where the Nafion membrane behaves as a counter anion as well as a transparent substrate, and investigated the photogenerated high-spin state below 35 K. The lifetime of the photogenerated high-spin state strongly depends on the intensity of irradiated light.

Design and Control of Cooperativity in Spin-Crossover in Metal–Organic Complexes: A Theoretical Overview

Metal organic complexes consisting of transition metal centers linked by organic ligands, may show bistability which enables the system to be observed in two different electronic states depending on external condition. One of the spectacular examples of molecular bistability is the spin-crossover phenomena. Spin-Crossover (SCO) describes the phenomena in which the transition metal ion in the complex under the influence of external stimuli may show a crossover between a low-spin and high-spin state. For applications in memory devices, it is desirable to make the SCO phenomena cooperative, which may happen with associated hysteresis effect. In this respect, compounds with extended solid state structures containing metal ions connected by organic spacer linkers like linear polymers, coordination network solids are preferred candidates over isolated molecules or molecular assemblies. The microscopic understanding, design and control of mechanism driving cooperativity, however, are challenging. In this review we discuss the recent theoretical progress in this direction.

Correlating Properties in Iron(III) Complexes: A DFT Description of Structure, Redox Potential and Spin Crossover Phenomena

Calculations of structure, redox potential and spin transition energies were performed using DFT approximations for a series of [Fe(bztpen)OR]2+/+ type FeIII/FeII systems (R=Me, Et, nPr, nBu), which have a temperature dependent spin crossover behavior. These compounds exhibit changes in redox and magnetic properties, related to structural variations quite important for their possible applications among which signal generator materials stand out. Functionals B3LYP, ωB97X−D and TPSS along with PCM solvation model were evaluated for redox potential, whereas for spin crossover the TPSSh functional was also included. The multireference character of these compounds was tested as well. Calculations were compared to experimental measurements, and ωB97X−D proved able to accurately describe the geometries observed in solid state for the low spin (LS) and high spin (HS) states; moreover, it had the best correlation between calculated and experimental redox potential values. However, in the description of the spin transition energies the TPSS functional is needed to correctly describe the LS state as the observed ground state in the complexes at low temperature, which allows to calculate proper spin transition curves as a function of temperature. From these results, we obtained suitable approximations for an accurate description of redox potential and magnetic properties for the FeIII coordination compounds, which can be extended to model similar systems.

Reprint of “Surface-environment effects in spin crossover solids”

Abstract The impact of surface effects on thermal induced spin crossover phenomenon is a subject of a broad and current interest. Using the modified Ising-like model of spin crossover solids with the ligand field as function of the molecule’ positions and random component on surface by means of Metropolis Monte Carlo algorithm the thermal spin transition curves were calculated. The analysis of spin configuration during transition gives a general idea about contribution of molecules from the surface and inside the lattice into resulting magnetization of the systems. The behavior of hysteresis loop for various surface coupling and fluctuations strength has been described.

Control of the Phase Stability in Spin‐Crossover Core–Shell Nanoparticles through the Elastic Interface Energy

Size reduction leads to drastic changes in thermodynamic properties in spin‐crossover (SCO) nano‐objects in comparison with bulk materials. In particular, an important modification of the phase stability has been observed, reflected by a shift of the transition temperature. These changes are mostly attributed to the increasing role of surface properties at the nanoscale, especially the elastic properties, which play a key role in SCO phenomena. In this work, we propose a continuum mechanics approach to explore the possibility to tune the phase stability by controlling the interfacial elastic energy in core–shell nanoparticles with (semi‐)coherent interface. To this aim, the pressure at the particle surface/interface is analytically derived in the case of hollow and core–shell nano‐objects. Then, the extracted interfacial energy is injected in a thermodynamic model to mimic the spin‐transition curves in the framework of the effective medium approximation. A structural misfit between a SCO core and an inactive shell produces a negative or positive pressure on the SCO material, favoring either the high‐spin or the low‐spin states and thus can downshift or upshift the transition temperature. These theoretical results provide real perspectives for the control of the spin transition at the nanoscale via interfacial energies, as the design of core–shell coordination nanoparticles and other heterostructures has been considerably developed in recent years.

Computational design of magnetically active trinuclear heterometallic complexes on the basis of 1,3,5-triazapentadiene ligands

A series of trinuclear mixed-ligand complexes of iron(II) 2,6-di(pyrazol-1-yl)pyridine moieties with linkers on the basis of 1,3,5-triazapentadiene transition metal (M = Co, Ni, Cu, Zn) bischelates has computationally been designed using the density functional theory TPSSh/6-311++G(d,p) calculations. The systems possessing complete (M = Co, Ni) and partial (M = Cu, Zn) two-step spin-crossover phenomenon at ferrous ions have been revealed. Two spin-state switching mechanisms determining by spin-crossover at iron centers and configurational isomerism at cobalt ion may simultaneously occur in solution of corresponding heterometallic compound (M = Co). The nature of the exchange interactions between paramagnetic metal centers is controlled by variation of a metal in bischelate linker (M = Co, Ni, Cu, Zn). Energy and magnetic characteristics of electromeric forms of the complexes with nickel and copper central metal ions (M = Co, Ni) allow to consider them as promising candidates for the design of molecular switches.

Spatiotemporal Observation and Modeling of Remarkable Temperature Scan Rate Effects on the Thermal Hysteresis in a Spin-Crossover Single Crystal

The direct observation by optical microscopy of the thermally induced spin transition and the subsequent low-spin high-spin interface propagation inside the thermal hysteresis is a fascinating problem. The kinetic aspects of the spin crossover (SCO) transition investigated under various scan rates was mostly studied so far on powder sample which then average the kinetic effects of the thermal hysteresis. This problem is studied here on the robust spin-crossover single crystal [{Fe(NCSe)(py)2}2(m-bpypz)] for which we followed the dynamics of the thermal hysteresis at several controlled temperature scan rates. In addition to the expected shifts of the switching temperatures, sizable changes in the velocity of the high-spin (HS) low-spin (LS) interface dynamics are observed for the first time. Theoretical developments based on a spatiotemporal description of the SCO phenomenon using reaction diffusion equations including the heat balance between the crystal and its immediate environment are provided. We iden...

Chemical Information revealed by Mössbauer spectroscopy and DFT calculations

Mixed-valence state of binuclear metallocene derivatives and spin-crossover (SCO) phenomena of the assembled Fe(II) complexes have been studied by using Mossbauer spectroscopy. The understanding of the results obtained by Mossbauer spectra is well supported by means of X-ray structural analysis and density functional theory (DFT) calculation. Benchmark study of relativisitic DFT calculation by using Mossbauer isomer shifts of Eu, Np complexes reveals the validity of the calculation. Such study sheds light on the bonding character of 4f and 5f electron. These results are reviewed.

Surface-environment effects in spin crossover solids

The impact of surface effects on thermal induced spin crossover phenomenon is a subject of a broad and current interest. Using the modified Ising-like model of spin crossover solids with the ligand field as function of the molecule’ positions and random component on surface by means of Metropolis Monte Carlo algorithm the thermal spin transition curves were calculated. The analysis of spin configuration during transition gives a general idea about contribution of molecules from the surface and inside the lattice into resulting magnetization of the systems. The behavior of hysteresis loop for various surface coupling and fluctuations strength has been described.

Guest Programmable Multistep Spin Crossover in a Porous 2-D Hofmann-Type Material.

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e., competing ferro- and antiferro-elastic interactions) drive concerted lattice distortions. To this end, a new SCO framework scaffold, [FeII(bztrz)2(PdII(CN)4)]·n(guest) (bztrz = (E)-1-phenyl-N-(1,2,4-triazol-4-yl)methanimine, 1·n(guest)), has been prepared that supports a variety of antagonistic solid state interactions alongside a distinct dual guest pore system. In this 2-D Hofmann-type material we find that inbuilt competition between ferro- and antiferro-elastic interactions provides a SCO behavior that is intrinsically frustrated. This frustration is harnessed by guest exchange to yield a very broad array of spin transition characters in the one framework lattice (one- (1·(H2O,EtOH)), two- (1·3H2O) and three-stepped (1·∼2H2O) transitions and SCO-deactivation (1)). This variety of behaviors illustrates that the degree of elastic frustration can be manipulated by molecular guests, which suggests that the structural features that contribute to multistep switching may be more subtle than previously anticipated.

Development of hybrid diblock copolypeptide amphiphile/magnetic metal complexes and their spin crossover with lower-critical-solution-temperature(LCST)-type transition

Diblock copolypeptide amphiphiles have been attracting much interest as motifs for self-assembly in the production of nanostructures such as nanoparticles, nanosheets, and nanofibers. In this study, we first focus on the self-assembly of magnetic metal complexes in water using diblock copolypeptides. In addition, not only were nanocomposites formed, but also observed were the development of lower critical solution temperature (LCST) and LCST-induced spin crossover phenomenon, which are discussed. Three composites composed of the iron complex [FeII(ppi)2(NCS)2] (ppi = N-phenyl-2-pyridinalimine) and the diblock copolypeptide amphiphiles 1 and 2 or the polypeptide 3 (containing glutamic acid and leucine) were prepared. Supramolecular structures such as sheets and rectangular morphologies were obtained from composites. A perfectly reversible spin crossover with LCST was successfully generated in the case of composites made with 1. The technique of combining polypeptide molecules and discrete coordination compounds thus makes it possible to design flexible, reversible, and spin-controllable metastable systems.

Direct crystallographic evidence of the reversible photo-formation and thermo-rupture of a coordination bond inducing spin-crossover phenomenon.

A detailed crystallographic study of [FeII(LN5)(CN)2]·MeOH (LN5 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene) shows the light-induced formation and thermally-assisted rupture of a metal-ligand coordination bond concomitantly with a spin-crossover process occurring at the Fe(ii) center between its low spin (LS) and high spin (HS) states. These results illustrate the remarkable reversibility and non-destructive character of this photo-induced magneto-structural process in a single crystal.

Cooperative 1D Triazole-Based Spin Crossover FeII Material With Exceptional Mechanical Resilience

The magnetic spin change associated with the spin crossover (SCO) phenomenon represents a paradigm of bistability at the molecular level that is of current interest because of potential applications in the development of new generations of electronic devices such as nonvolatil memories, molecular sensors and displays.(1-5) The SCO phenomenon is especially observed in FeII complexes in which the paramagnetic high spin state (HS, S = 2) can be switched reversibly to the low spin state (LS, S = 0) by several external stimuli such as temperature, pressure or light irradiation.(6) A huge effort has been devoted to the understanding of the transition mechanisms using inter alia crystallographic tools...

Spin Crossover in Anionic Cobalt-Bridged Fullerene (Bu4N+){Co(Ph3P)}2(μ2-Cl-)(μ2-η2,η2-C60)2 Dimers.

A spin crossover phenomena is observed in an anionic (Bu4N+){Co(Ph3P)}2(μ2-Cl-)(μ2-η2,η2-C60)2·2C6H14 (1) complex in which two cobalt atoms bridge two fullerene molecules to form a dimer. The dimer has a triplet ground state with two weakly coupling Co0 atoms (S = 1/2). The spin transition realized above 150 K is accompanied by a cobalt-to-fullerene charge transfer that forms a quintet excited state with a high spin CoI (S = 1) and C60•- (S = 1/2).

An iron(II) complex tripodally chelated with 1,1,1-tris(pyridin-2-yl)ethane showing room-temperature spin-crossover behaviour.

The spin-crossover phenomenon is a reversible low- and high-spin transition caused by external stimuli such as heat. In the novel iron(II) complex salt tetraphenylphosphonium tris(thiocyanato-κN)[1,1,1-tris(pyridin-2-yl)ethane-κ3N,N',N'']ferrate(II), (C24H20P)[Fe(NCS)3(C17H15N3)], the Fe-N bond lengths are in the range 2.027 (2)-2.089 (2) Å, indicating that the specimen consists of comparable molar fractions of the low- and high-spin species at 296 K. A magnetic study confirmed that spin-crossover takes place at around 290 K.

Computational modeling of spin crossover phenomenon in adducts of iron bis-chelates with o-diiminobenzoquinones

A density functional B3LYP*/6-311++G(d,p) quantum chemical study of the interaction of FeII complexes with o-diiminobenzoquinones showed that adduct formation is accompanied by oxidation of the metal ion and conversion of the redox-active ligand to the semiquinonate form. Variation of substituents at nitrogen atoms of the bis-chelate and diimine made it possible to reveal the spin-crossover complexes. The nature and strength of the exchange interactions between the unpaired electrons of paramagnetic centers of the adducts studied depend on the spin state of their isomers and on the type of the iron complex.

Photoreversible Spin-crossover Ferrimagnetism in Fe2[Nb(CN)8](4-methylpyridine)8·2H2O

A cyanido-bridged Fe–Nb bimetal assembly, Fe2[Nb(CN)8](4-methylpyridine)8·2H2O, was synthesized. This compound exhibits a two-step spin-crossover phenomenon between FeII high-spin (FeIIHS, S = 2) a...

Spectral Signatures of Ultrafast Spin Crossover in Single Crystal [FeII(bpy)3](PF6)2

Solvated iron(II)-tris(bipyridine) ([Fe(II)(bpy)3](2+)) has been extensively studied with regard to the spin crossover (SCO) phenomenon. Herein, the ultrafast spin transition dynamics of single crystal [Fe(II)(bpy)3](PF6)2 was characterized for the first time using femtosecond transient absorption (TA) spectroscopy. The single crystal environment is of interest for experiments that probe the nuclear motions involved in the SCO transition, such as femtosecond X-ray and electron diffraction. We found that the TA at early times is very similar to what has been reported in solvated [Fe(II)(bpy)3](2+), whereas the later dynamics are perturbed in the crystal environment. The lifetime of the high-spin state is found to be much shorter (100 ps) than in solution due to chemical pressure exerted by the lattice. Oscillatory behavior was observed on both time scales. Our results show that single crystal [Fe(II)(bpy)3](PF6)2 serves as an excellent model system for localized molecular spin transitions.

Direct Optical Microscopy Observation of Photo-Induced Effects and Thermal Relaxation in a Spin Crossover Single Crystal

Background: Spin crossover (SC) compounds are among the most fascinating molecular solids exhibiting thermal and photo-switching. The spin crossover phenomenon manifests, generally, in iron (II) complexes in octahedral symmetry, surrounded by nitrogen atoms. The splitting of the energy of the d orbitals into the t2g (ground state) and eg (excited state) allows the existence of two possible electronic states, namely high-spin (HS, S=2) and low-spin (LS, S=0) for weak and strong ligand-field configurations, respectively. At the solid state, the switching between these two electronic states is possible with temperature, light, pressure, etc. and may lead to gradual or first-order transitions, accompanied with a thermal hysteresis loop. In addition, SC materials exhibit interesting non-equilibrium phenomena, like photo-induced metastable states at low-temperature and their non-linear thermally-induced relaxation, which will be investigated in the current work. Methods: Optical microscopy technique is used here to visualize the first-order thermal spin transition in the single crystal [{Fe(NCSe)(py)2}2 (m-bpypz)], where py = pyridine and bpypz= 3,5-bis(2-pyridyl)-pyrazolate, hereafter abbreviated Fe(NCSe). The experimental set-up was also adapted to include a photo-excitation through the objective of the microscope. We then succeeded in imaging, the spatiotemporal transformation of the SC single crystal under light at very low temperature, ~20K. Image processing allowed to furnish quantitative data on this transformation. Results: This report focusses on the first direct visualization by optical microscopy of the photo-transformation of the SC single crystal under light. The phenomenon behind this process, called LIESST effect (Light-induced Excited Spin State Trapping) is well known in the SC topic, and was studied in the past using magnetic and optical absorption techniques, but not yet visualized. Our results show that the transformation from LS to HS under light proceeds in a homogeneous way, without any presence of domain coarsening (at the resolution of the optical microscopy, ~500 nm), in good agreement with the general admitted idea that the LIESST effect is a non-cooperative process. We have also followed the spatiotemporal transformation (at higher temperature, T~60K) of the crystal during its thermal relaxation from the photo-induced metastable HS state to the stable LS state. We could derive the time dependence of the HS fraction which was found to follow the usual sigmoidal shape, in excellent agreement with previous magnetic data, thus confirming the cooperative character of this relaxation. As for the photo-excitation, our data demonstrated that the relaxation process from HS to LS proceeds without any domain formation: the crystal remains in the homogeneous state during the overall relaxation process. From the theoretical view point, the photoexcitation and the relaxation processes are described by the kinetic mean field model whose all parameters were derived from the experiment. Conclusion: We presented optical microscopy studies on the single crystal Fe(NCSe) which included the visualization of the first-order thermal transition and also the photo-excitation and the relaxation of one single crystal Fe(NCSe) under light. The thermal equilibrium transition, that is the first-order transition observed at ~112K, proceeds via the formation of a single domain which propagates overall the crystal. In contrast, the photoexcitation process and the subsequent thermal relaxation of the photo-induced HS state, did not show any domain formation. These two last processes take place through homogenous transformations of the crystal. The analysis of our optical microscopy data of photoexcitation/relaxation, in the frame of a kinetic mean-field description, allowed to derive all thermodynamic parameters of the model, such as the interaction parameter and the effective energy barriers. The current observations underline the role of the lattice dynamics and its crucial temperature dependence, in the conditions of occurrence of domains formation. In general, the existence of macroscopic domains requires the presence of an instability. Keywords: Mean-field kinetics, optical microscopy, photo-excitation, relaxation, spin-crossover, visualization.

Preparation of dihydroquinazoline carbohydrazone Fe(ii) complexes for spin crossover

Mononuclear spin-crossover complexes of dihydroquinazoline derivatives were synthesized to enable the spin crossover phenomenon via the variation of the solvent, anion and substituent.