Spin Crossover Systems Research Articles

Photoinduced Persistent Polarization Change in a Spin Transition Crystal.

Using light as a local heat source to induce a temporary pyroelectric current is widely recognized as an effective way to control the polarization of crystalline materials. In contrast, harnessing light directly to modulate the polarization of a crystal via excitation of the electronic bands remains less explored. In this study, we report an FeII spin crossover crystal that exhibits photoinduced macroscopic polarization change upon excitation by green light. When the excited crystal relaxes to the ground state, the corresponding pyroelectric current can be detected. An analysis of the structures, magnetic properties and the Mössbauer and infrared spectra of the complex, supported by calculations, revealed that the polarization change is dictated by the directional relative movement of ions during the spin transition process. The spin transition and polarization change occur simultaneously in response to light stimulus, which demonstrates the enormous potential of polar spin crossover systems in the field of optoelectronic materials.

Photoinduced Persistent Polarization Change in a Spin Transition Crystal

AbstractUsing light as a local heat source to induce a temporary pyroelectric current is widely recognized as an effective way to control the polarization of crystalline materials. In contrast, harnessing light directly to modulate the polarization of a crystal via excitation of the electronic bands remains less explored. In this study, we report an FeIIspin crossover crystal that exhibits photoinduced macroscopic polarization change upon excitation by green light. When the excited crystal relaxes to the ground state, the corresponding pyroelectric current can be detected. An analysis of the structures, magnetic properties and the Mössbauer and infrared spectra of the complex, supported by calculations, revealed that the polarization change is dictated by the directional relative movement of ions during the spin transition process. The spin transition and polarization change occur simultaneously in response to light stimulus, which demonstrates the enormous potential of polar spin crossover systems in the field of optoelectronic materials.

Static and dynamic properties of two-sublattice spin-crossover systems

We investigate the static and dynamic properties of spin-crossover systems in the presence of an external magnetic field using two-sublattice Ising-like Hamiltonian. At low temperatures both sublattices are in the LS state, and in the HS state at high temperatures. The new feature of spin-crossover is depicted for our model when we take the non-equivalent sublattices JA≠JB, the two-step spin-crossover phase transition is obtained with the presence of the hysteresis loop. In order to study the properties of two-step spin-crossover, we use the lowest approximation of the cluster variation method for static properties. The dynamic properties of the system with bilinear interactions are studied by the path probability method. We present high-spin state fraction nHS vs temperature and magnetic field variations for constant values of the degeneracy ratio between high-spin and low-spin states. In the present model, we obtained the relaxation curves of the order parameters SA and SB using different temperature values. Finally, the flow diagram of the order parameters is depicted for different values of rate constant k1 and temperature, which displays the stable, metastable, and unstable properties. Static and dynamic self-consistent equations of the system were performed by using Newton-Raphson and Runge-Kutta methods.

Evidence for long drift carrier lifetimes in [Fe(Htrz)2(trz)](BF4) plus polyaniline composites

An effort has been made to determine the majority carrier and drift carrier lifetimes in the promising spin crossover molecule [Fe(Htrz) 2 (trz)](BF 4 ) (where Htrz = 1H-1,2,4-triazole) plus polyaniline composite. The current voltage and capacitance voltage characteristics of the [Fe(Htrz) 2 (trz)](BF 4 ) plus polyaniline composite were studied at both the low spin state at T = 320 K and the high spin state at T = 380 K. In these composites, the drift carrier lifetimes are in excess of microseconds, and there are indications that the majority carriers are holes. While the drift carrier lifetime is much lower when the [Fe(Htrz) 2 (trz)](BF 4 ) plus polyaniline composite is in the low spin state, there are indications that for at least one polyaniline composite, the carrier mobility is much higher in the low spin state so that the mobility is consistent with the higher conductivity of the low spin state. • Very long carrier lifetime measured in spin crossover system. • Impressive mobility changes. • Transistors can be made of [Fe(Htrz) 2 (trz)](BF 4 ) plus polyaniline. • Comparison of conductance in low and high spin state.

Dynamics of Spin Crossover Molecular Complexes.

We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the “write” speed is an important criterion.

Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Co-crystallisation as a modular approach to the discovery of spin-crossover materials.

Herein we present co-crystallisation as a strategy for materials discovery in the field of switchable spin crossover (SCO) systems. Using [Fe(3-bpp)2]·2A (where 3-bpp = 2,6-bis(pyrazol-3-yl)pyridine, A = BF4−/PF6−) as a starting point, a total of 11 new cocrystals have been synthesised with five different dipyridyl coformers. Eight of these systems show spin crossover behaviour, and all show dramatically different switching properties from the parent complex. The cocrystals have been studied by variable temperature single-crystal X-ray diffraction and SQUID magnetometry to develop structure–property relationships. The supramolecular architecture of the cocrystals depends on the properties of the coformer. With linear, rigid coformer molecules leading to 1D supramolecular hydrogen-bonded chains, while flexible coformers form 2D sheets and bent coformers yield 3D network structures. The SCO behaviour of the cocrystals can be modified through changing the coformer and thus co-crystallisation presents a rapid, facile and highly modular tool for the discovery of new switchable materials. The wider applicability of this strategy to the design of hybrid multifunctional materials is also discussed.

Trend in light-induced excited-state spin trapping in Fe(II)-based spin crossover systems.

A computational study of the light-induced excited spin-state trapping (LIESST) in a number of Fe(II) spin crossover complexes, coordinated by monodentate, bidentate and multidentate ligands is carried out, with the goal to uncover the trend in the low temperature relaxation rate. A nine order of magnitude change in low temperature relaxation rate is observed among the complexes. The trend is rationalized in terms of the change in metal-ligand covalency, numerically estimated by the crystal orbital Hamiltonian population, thus influencing the back donation or delocalization of the electrons from the low-lying Fe(II)-centered molecular orbital to the empty low-lying ligand-centered π* antibonding molecular orbitals.

SPIN-CROSSOVER IRON(II) COORDINATION COM­POUNDS: FABRICATION OF FUNCTIONAL MATERIALS AND THEIR INTEGRATION INTO MICRO- AND NANOCONSTRUCTIONS

Development of micro- and nanosized spin-crossover (SCO) materials has become an important research direction within the past decade. Such an interest is associated with high perceptive of practical application of these materials in nanoelectronic devices. Therefore, researches working in the field of SCO put considerable efforts to obtain SCO complexes in various functional forms, such as nanoparticles, thin films, etc. Fabrication of these materials is realized through different chemical and/or lithographical approaches, which allow to adjust size, shape and even organization of nanoobjects.
 In this review theoretical background of SCO phenomenon is described, additionally different classes of coordination compounds exhibiting spin crossover are covered. It is demonstrated that electric field, temperature and light irradiation can be effectively used for switching and control of spin state in nanosized SCO systems. Cooperative SCO with transition close to room temperature, wide hysteresis loop and distinct thermochromic effect is most often observed for Fe(II) coordination complexes. Therefore, Fe(II) SCO compounds form one of the most perspective classes of compounds for obtaining functional materials. It is shown that integration of Fe(II) compounds into micro- and nanohybrid devi­ces allows to combine unique functional pro­perties in one material due to synergy between SCO and physical properties (luminescent, electrical, etc.) of the other component. As a result, SCO compounds are interesting not only from the fundamental point of view, but also from practical, thanks to the possibility of integration of SCO Fe(II) complexes as active materials in devices of different configurations.
 It is expected that obtaining of new Fe(II) coordination polymers with unique SCO cha­racteristics will favor the development of new functional materials and devices on their basis in the nearest future.

Correlation between Supramolecular Connectivity and Magnetic Behaviour of [FeIII(5-X-qsal)2]+-Based Salts Prone to Exhibit SCO Transition

We present an extensive study to determine the relationship between structural features of spin crossover (SCO) systems based on N-(8-quinolyl)salicylaldimine (qsal) ligand derivatives and their magnetic properties. Thirteen new compounds with general formula [FeIII(5-X-qsal)2]+ (X = H, F, Cl, Br and I) coupled to Cl−, ClO4−, SCN−, PF6−, BF4− and BPh4− anions were prepared and magnetically characterized. The structure/properties correlations observed in these compounds were compared to those of salts with the same [FeIII(qsal-X)2]+ cations previously reported in the literature. These cations favour the LS configuration in compounds with the weakest connectivity. As connectivity increases most of them present HS states at room temperature and structures may be described as arrangements of parallel layers of interacting cation dimers. All the compounds based on these cations undergoing complete SCO transitions within the 4–300 K temperature range have high intralayer connectivity. If, however, the interlayer connectivity becomes very strong they remain blocked in the HS or in the LS state. The SCO transition may be affected by the slightest change of solvent molecules content, disorder or even crystallinity of the sample and it remain difficult to predict which kind of ligand substituent should be selected to obtain compounds with the desired connectivity.

Cluster approach to ferromagnetic Ising-type model for spin-crossover systems

A cluster approach to ferromagnetic Ising-type model for spin-crossover systems is developed via the ghost bonds method, and exact relations between some thermal quantities and percolation variables are obtained. Simulations by the Swendsen-Wang cluster MC dynamics for 2D systems show an agreement between the clusters properties and the thermal quantities.

Desolvation–Solvation-Induced Reversible On–Off Switching of Two Memory Channels in a Cobalt(II) Coordination Polymer: Overlay of Spin Crossover and Structural Phase Transition

Desolvation–Solvation-Induced Reversible On–Off Switching of Two Memory Channels in a Cobalt(II) Coordination Polymer: Overlay of Spin Crossover and Structural Phase Transition

Cooperativity and Metal–Linker Dynamics in Spin Crossover Framework Fe(1,2,3-triazolate)2

Cooperative interactions are responsible for the useful properties of spin crossover (SCO) materials─large hysteresis windows, critical temperatures near room temperature, and abrupt transitions─with hybrid framework materials exhibiting the greatest cooperativity and hysteresis of all SCO systems. However, little is known about the chemical origin of cooperativity in frameworks. Here, we present a combined experimental–computational approach for identifying the origin of cooperativity in the metal–organic framework (MOF) Fe(1,2,3-triazolate)2 (Fe(TA)2), which exhibits the largest known hysteresis window of all SCO materials and unusually high transition temperatures, as a roadmap for understanding the manipulation of SCO behavior in general. Variable-temperature vibrational spectroscopy provides evidence that “soft modes” associated with dynamic metal–linker bonding trigger the cooperative SCO transition. Thermodynamic analysis also confirms a cooperativity magnitude much larger than those of other SCO systems, while electron density calculations of Fe(TA)2 support previous theoretical predictions that large cooperativity arises in materials where SCO produces considerable differences in metal–ligand bond polarities between different spin states. Taken together, this combined experimental–computational study provides a microscopic basis for understanding cooperative magnetism and highlights the important role of dynamic bonding in the functional behavior of framework materials.

Tutorial on elastic interaction models for multistep spin-crossover transitions

Spin-crossover (SC) compounds are fascinating materials that exhibit colorful phase transitions induced by temperature, pressure, photoirradiation, etc. From the microscopic point of view, the electronic (spin) state of a molecule changes between the low-spin (LS) and high-spin (HS) states by such stimuli, and a variation of the molecular size follows through the vibronic coupling between the electronic state and structure in the molecule. This causes an elastic distortion and then an elastic interaction. The elastic interaction is essential in cooperative properties of SC phenomena. In this paper, we present a tutorial study on elastic interaction models for SC phenomena, which are the recent trend of modeling of SC compounds. We focus on multistep transitions, which are a topic of SC phenomena. We analyze the phase diagrams including the metastable phases for several SC systems, in which antiferromagnetic-like and ferrimagnetic-like phases are associated in addition to the LS and HS phases. Making use of the phase diagrams, we show various patterns of thermally induced SC transition with steps. We also investigate SC transitions with steps in a core-shell SC nanocomposite composed of two different SC compounds. We focus on two cases: the core has a lower transition temperature than the shell, and the core has a higher transition temperature. We show characteristic features of difference in the two systems.

Elastic Modeling of Two-Step Transitions in Sterically Frustrated 1D Binuclear Spin-Crossover Chains

Among the large family of spin-crossover materials, binuclear systems play an important role due to their specific molecular configurations, allowing the presence of multi-step transitions and elastic frustration. Although this issue benefited from a significant number of spin-based theories, there is almost no elastic description of the spin transition phenomenon in binuclear systems. To overcome this deficiency, in this work we develop the first elastic modeling of thermal properties of binuclear spin-crossover solids. At this end, we investigated a finite spin-crossover open chain constituted of elastically coupled binuclear (A = B) blocks, ⋯A=B−A=B−A=B⋯, in which the considered equivalent A and B sites may occupy two configurations, namely low-spin (LS) and high-spin (HS) states. The sites of the binuclear unit interact via an intramolecular spring and couple to the neighboring binuclear units via other springs. The model also includes the change of length inside and between the binuclear units subsequent to the spin state changes. When injecting an elastic frustration inside the binuclear unit in the LS state, competing interactions between the intra- and the inter-binuclear couplings emerge. The latter shows that according to the intra- and inter-binuclear elastic constants and the strength of the frustration, multi-step transitions are derived, for which a specific self-organization of type (HS = HS)-(LS-LS)-(HS = HS)⋯ is revealed and discussed. Finally, we have also studied the relaxation of the metastable photoinduced HS states at low temperature, in which two relaxation regimes with transient self-organized states were identified when monitoring the elastic frustration rate or the ratio of intra- and intermolecular elastic interactions. These behaviors are reminiscent of the thermal dependence of the order parameters of the system. The present model opens several possibilities of extensions of elastic frustrations acting in polynuclear spin-crossover systems, which may lead to other types of spin-state self-organizations and relaxation dynamics.

Spin-state energetics of manganese spin crossover complexes: Comparison of single-reference and multi-reference ab initio approaches

Manganese spin crossover (SCO) complexes form a small but ever expanding family of compounds with thermally accessible states of different electronic configuration and total spin. Accurate prediction of spin-state energetics is essential for the theoretical description of these systems. However, this represents a challenging problem that necessitates recourse to correlated wave function methods rather than the more approximate density functional theory (DFT). Here we present a detailed study of spin-state energetics for eight Mn(III) and Mn(II) SCO complexes using the domain-based local pair natural orbital approach to coupled cluster theory with singles, doubles, and perturbative triples, DLPNO-CCSD(T). The effects of reference determinants, basis set, triples excitations, and pair natural orbitals (PNO) thresholds are evaluated and analysed in detail, enabling us to propose a robust and efficient computational protocol based on a combined and balanced mix of extrapolation to the complete basis set and infinite PNO space limits. The results are subsequently used to evaluate multireference wavefunction-based (CASSCF/NEVPT2) and DFT approaches, highlighting their inability to provide a balanced description of spin-state energetics for these complexes. The DLPNO-CCSD(T) protocol proposed in this study can serve as a generally applicable reference-quality quantum chemical method for studying spin crossover systems.

Correlation between anomalous thermal expansion coefficient and barocaloric effect: Application to spin crossover systems

We report the strong correlation between anomalous thermal expansion in solid-solid volumetric phase transition and the barocaloric effect. This investigation was carried out in {Fe[H2B(pz)2]2(bipy)} (pz = pyrazolyl; bipy = bipyridine) spin crossover system, in which the spin of Fe2+ changes from sLS = 0 to sHS = 2 coupled with high unit cell volume changes from VLS = 2356 to VHS = 2462 Å3, at T1/2–160 K. Giant barocaloric effect, through the isothermal entropy change (ΔST = 60 J kg−1 K−1) upon moderated hydrostatic pressure variation (ΔP = 1 kbar), was indirectly obtained from reported pressure dependence of magnetic susceptibility. Theoretical results, obtained from our microscopic model, updated with the reported metal-ligand asymmetrical stretching z-vibration modes, from DFT calculation, showed a remarkable agreement with the experimental data.

Tutorial on the elastic theory of spin crossover materials

In this paper, we present the ingredients that allow the building-up of the elastic model, one of the approaches that successfully describes the phenomena observed in complex spin-crossover systems at both the micro- and macroscopic level and we review its challenges and some of its main applications. After discussing the usefulness and the drawbacks of some of the previous models (such as mean-field and Ising-like ones), we introduce the premises that imposed the elastic approach in the study of spin-crossover compounds and present the steps to follow in order to build-up and implement the model. As illustrative applications, we first discuss the simulations of relaxation, thermal transition, and the nucleation phenomena and then introduce the effects of anisotropy in two-dimensional systems. Special sections are dedicated to particular structures like three-dimensional, spin-crossover micro- or nanoparticles as layers on substrates or embedded in polymer or surfactant matrices as well as to the study of ultra-fast phenomena.

Coupling and decoupling of spin crossover and ferroelastic distortion: Unsymmetric hysteresis loop, phase diagram, and sequence of phases

Spin-transition materials can exhibit thermal hysteresis due to cooperative elastic interactions in between active molecular sites. It results from the coupling of the nonsymmetry-breaking high spin fraction order parameter to the lattice volume strain. However, a symmetry-breaking order parameter responsible for crystallographic phase transition, like a ferroelastic distortion, can also couple to the volume strain. Here, we use the Landau theory to study the elastic coupling between a spin-crossover instability and a discontinuous ferroelastic distortion and the different phase transition lines. Below the triple point, the first-order line involves simultaneous ferroelastic distortion and spin transition. Above, the purely first-order ferroelastic distortion and the spin crossover occur sequentially. Our model, related to the coupling and decoupling of the ferroelastic phase transition and spin crossover, explains exotic behaviors reported experimentally in the literature for diverse spin-crossover systems, plastic crystals, or Prussian blue analogues. The unsymmetric hysteresis loops and the stepwise evolution of the spin conversion or volume strain under pressure result from different sequences of phase transitions. The model shows that the ferroelastic phase transition is the driving force of the cooperative spin transition hysteresis in this case.

Spin-state-dependent electrical conductivity in single-walled carbon nanotubes encapsulating spin-crossover molecules

Spin crossover (SCO) molecules are promising nanoscale magnetic switches due to their ability to modify their spin state under several stimuli. However, SCO systems face several bottlenecks when downscaling into nanoscale spintronic devices: their instability at the nanoscale, their insulating character and the lack of control when positioning nanocrystals in nanodevices. Here we show the encapsulation of robust Fe-based SCO molecules within the 1D cavities of single-walled carbon nanotubes (SWCNT). We find that the SCO mechanism endures encapsulation and positioning of individual heterostructures in nanoscale transistors. The SCO switch in the guest molecules triggers a large conductance bistability through the host SWCNT. Moreover, the SCO transition shifts to higher temperatures and displays hysteresis cycles, and thus memory effect, not present in crystalline samples. Our results demonstrate how encapsulation in SWCNTs provides the backbone for the readout and positioning of SCO molecules into nanodevices, and can also help to tune their magnetic properties at the nanoscale.