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( s 1 A , s 1 B ) ${(s_1^A, s_1^B )}$ and( s 2 A , s 2 B ) ${(s_2^A, s_2^B )}$ . The following 2-, 3-, and 4-bodyJ 1 ( s 1 A + s 1 B ) ( s 2 A + s 2 B ) ${J_1 (s_1^A + s_1^B )(s_2^A + s_2^B )}$ ,K 1 s 1 A s 1 B ( s 2 A + s 2 B ) ${K_1 s_1^A s_1^B (s_2^A + s_2^B )}$ , andK 2 ( s 1 A s 1 B ) ( s 2 A s 2 B ) ${K_2 (s_1^A s_1^B )(s_2^A s_2^B )}$ 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 4×4 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.

Spin crossover iron complexes with spin transition near room temperature based on nitrogen ligands containing aromatic rings: from molecular design to functional devices

During last decades, significant advances have been made in iron-based spin crossover (SCO) complexes, with a particular emphasis on achieving reversible and reproducible thermal hysteresis at room temperature (RT). This pursuit represents a pivotal goal within the field of molecular magnetism, aiming to create molecular devices capable of operating in ambient conditions. Here, we summarize the recent progress of iron complexes with spin transition near RT based on nitrogen ligands containing aromatic rings from molecular design to functional devices. Specifically, we discuss the various factors, including supramolecular interactions, crystal packing, guest molecules and pressure effects, that could influence its cooperativity and the spin transition temperature. Furthermore, the most recent advances in their implementation as mechanical actuators, switching/memories, sensors, and other devices, have been introduced as well. Finally, we give a perspective on current challenges and future directions in SCO community.

Bidirectional photomagnetism, exciplex fluorescence and dielectric anomalies in a spin crossover Hofmann-type coordination polymer

Stepped spin crossover (SCO) complexes with three or more spin states have promising applications in high-order data storage, multi-switches and multi-sensors. Further synergy with other functionalities, such as luminescence and dielectric properties, will provide a good chance to develop novel multifunctional SCO materials. Here, a bent pillar ligand and luminescent pyrene guest are integrated into a three-dimensional (3D) Hofmann-type metal-organic framework (MOF) [Fe(dpoda){Au(CN)2}2]·pyrene (dpoda = 2,5-di-(pyridyl)-1,3,4-oxadiazole). The magnetic data show an incomplete and two-step SCO behavior with the sequence of 1 ↔ 1/2 ↔ 1/4. The rare bi-directional light-induced excited spin-state trapping (LIESST) effect and light-induced stepped thermal relaxation after LIESST are observed. The pyrene guests interact with dpoda ligands via offset face-to-face π⋯π interactions to form intermolecular exciplex emissions. The competition between thermal quenching and stepped SCO properties results in a complicated and stepped exciplex fluorescence. Moreover, the stepped dielectric property with higher dielectric permittivity at lower temperature may be related to the more frustrated octahedral distortion parameters in the intermediate spin states. Hence, a 3D Hofmann-type MOF with bent pillar ligands and fluorescent guests illustrates an effective way for the development of multifunctional switching materials.

Highly Porous Cyanometallic Spin-Crossover Frameworks Employing Pyridazino[4,5-d]pyridazine Bridge

Single crystals of two spin-crossover (SCO) cyanometallic coordination polymers based on the pyridazino[4,5-d]pyridazine ligand (pp) of the composition [Fe(pp)M(CN)4]∙G (where M = Pd, Pt; G = guest molecules) were obtained by a slow diffusion technique. A single-crystal X-ray analysis showed that both compounds adopted the structure of porous 3D frameworks, consisting of heterometallic cyano-bridged layers and interlayer pillar pp ligands, with a total solvent accessible volume of ca. 160 Å3 per iron(II) ion (about 37% of the unit cell volume). These frameworks displayed hysteretic SCO behaviour with T1/2 of 150/190 K (heating/cooling) for Pd complex and 135/170 K (heating/cooling) for Pt complex, which was confirmed by variable-temperature SCXRD experiments. This research shows the perspective of using pp ligand for building porous MOFs with spin transitions.

Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)₂Pd(CN)₄]·nG (furpy: N-(pyridin-4-yl)furan-2-carboxamide, G = H₂O,EtOH (A·H₂O,Et), and H₂O (A·H₂O)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A·H₂O,Et and an abrupt single-step SCO for A·H₂O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A·H₂O,Et characterized by a unique Fe^II center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A·H₂O shows a triclinic symmetry with two inequivalent Fe^II centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.

Spin-State Modulation in FeII -Based Hofmann-Type Coordination Polymers: From Molecules to Materials

Spin crossover complexes that reversibly interconvert between two stable states imitate a binary state of 0 and 1, delivering a promising possibility to address the data processing concept in smart materials. Thus, a comprehensive understanding of the modulation of magnetic transition between high spin and low spin and the factors responsible for stabilizing the spin states is an essential theme in modern materials design. In this context, the present review attempts to provide a concise outline of the design strategy employed at the molecular level for fine-tuning the spin-state switching in Fe^II -based Hofmann-type coordination polymers and their effects on the optical and magnetic response. In addition, development towards the nanoscale architectures of HCPs, i. e., in terms of nanoparticles and thin films, are emphasized to bridge the gap between the laboratory and reality.

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.

Guest-Induced Multistep-to-One-Step Reversible Spin Transition with Enhanced Hysteresis in a 2D Hofmann Framework

The interplay of host-guest interactions and controlled modulation of spin-crossover (SCO) behavior is one of the most exploited topics regarding data storage, molecular sensing, and optical technologies. In this work, we demonstrate the experimental approach of tuning the SCO behavior via controlled modulation of the spin-state cooperativity in a 2D Hofmann coordination polymer, [Fe^IIPd(CN)₄(L)₂]·1.3MeOH (1·1.3MeOH; L = methyl isonicotinate). Removal of the solvent changes the four-step transition to a complete one-step spin transition with an enhanced hysteresis width (∼20 K). Structural analysis of solvated (1·1.3MeOH) and partially desolvated (1·0.3MeOH) compounds reveals that the crystal system changes from a monoclinic C2/c space group to an orthorhombic Imma space group, where the Fe^II sites are present in a more symmetrically equivalent environment. Consequently, the axial ligand-field (LF) strength and face-to-face interactions of the ligands were increased by removing the guest, which is reflected in the highly cooperative SCO in 1 (desolvated compound). Also, the shift of the CN bond stretching frequencies and decrease of their relative intensities from the variable-temperature Raman spectroscopic measurements further corroborate the SCO behavior. Besides, theoretical calculations reveal that the singlet (¹Γ) LF terms decrease by removing guest molecules, enhancing the molecular population in the low-spin state at low temperature, as experimentally observed for 1. Notably, fine tuning of the SCO behavior via host-guests interactions provides a great opportunity to design potential chemosensors.

Halobenzene Clathrates of the Porous Metal-Organic Spin-Crossover Framework [Fe(tvp)2(NCS)2]n. Stabilization of a Four-Step Transition

Here we show that the porous metal–organic spin crossover (SCO) framework [Fe(tvp)₂(NCS)₂]@4(CH₃CN·H₂O) [1@4(CH₃CN·H₂O)] is an excellent precursor material for the systematic synthesis, via single-crystal to single-crystal transformation, of a series of halobenzene clathrates. Immersion of samples constituted of single crystals of 1@4(CH₃CN·H₂O) in the liquid halobenzenes PhX_n, X = F (n = 1–6), X = Cl (n = 1, 2), and X = Br (n = 1) at room temperature induces complete replacement of the guest molecules by PhX_n to afford 1@2PhX_n. Single-crystal analyses of the new clathrates confirm the integrity of the porous framework with the PhX_n guests being organized by pairs via π-stacking filling the nanochannels. The magnetic and calorimetric data confirm the occurrence of practically complete SCO behavior in all of the clathrates. The characteristic SCO equilibrium temperatures, T_1/2, seem to be the result of a subtle balance in the host–guest interactions, which are temperature- and spin-state-dependent. The radically distinct supramolecular organization of the PhCl₂ guests in 1@2PhCl₂ affords a rare example of four-step SCO behavior following the sequence [HS₁:LS₀] ↔ [HS_2/3:LS_1/3] ↔ [HS_1/2:LS_1/2] ↔ [HS_1/4:LS_3/4] ↔ [HS₀:LS₁], which has been structurally characterized.

Guest exchange via single-crystal to single-crystal transformation has been revealed to be an excellent approach to systematically stow halobenzene molecules in the nanochannels of the porous spin-crossover (SCO) metal−organic framework [Fe^II(tvp)₂(NCS)₂]_n. This has enabled the unveiling of key structural factors leading to a four-step SCO behavior.

Two-Step Spin Crossover in Hofmann-Type Coordination Polymers [Fe(2-phenylpyrazine)2{M(CN)2}2] (M = Ag, Au)

Two 2D Hofmann-type complexes of the composition [Fe(Phpz)₂{M(CN)₂}₂] (where Phpz = 2-phenylpyrazine; M = Ag, Au) have been synthesized, and their spin-crossover (SCO) behavior has been thoroughly characterized. Single-crystal X-ray analysis reveals that these complexes contain a crystallographically unique Fe(II) center surrounded by two axial Phpz ligands and four equatorial cyanide [M(CN)₂]^- bridges. It is shown that, using of a ligand with two aromatic rings, an advanced system of weak supramolecular interactions (metal-metal, C-H···M, and π···π stacking contacts) is realized. This ensures additional stabilization of the structures and the absence of solvent-accessible voids due to dense packing. Both complexes are characterized by a highly reproducible two-step SCO behavior, as revealed by different techniques (superconducting quantum interference device magnetometry, optical microscopy, etc.). Research shows the exceptional role of the presence of various supramolecular interactions in the structure and the influence of the bulky substituent in the ligand on SCO behavior. Moreover, the perspective of substituted pyrazines for the design of new switchable materials is supported by this work.

Co-existence of five- and six-coordinate iron(ii) species captured in a geometrically strained spin-crossover Hofmann framework

The use of an angular ligand drives the formation of an irregular Hofmann framework whereby 6-coordinate and rare 5-coordinate FeII species co-exist – the 6-coordinate species show a spin-crossover transition.

2D/3D spin crossover porous coordination polymers based on isomeric tetrapyridyl benzene ligands

By employing cyanoborohydride and isomeric tetrapyridyl benzene ligands, a new three-dimensional (3D) [Fe(3-tpb)(NCBH3)2] (1·2H2O, 3-tpb = 1,2,4,5-tetra(pyridin-3-yl)benzene) and a two-dimensional (2D) [Fe(4-tpb)(NCBH3)2] (2·2-NapSMe and 2·H2O·3DMF, 4-tpb = 1,2,4,5-tetra(pyridin-4-yl)benzene, 2-NapSMe =...

An azido-bridged [FeII4] grid-like molecule showing spin crossover behaviour

The supramolecular self-assembly synthetic strategy provides a valid tool to obtain polynuclear Fe(II) complexes having effective communication between the metal centres and distinct spin crossover behaviour. Despite the great success in constructing various magnetic molecules, progress has not been made in SCO complexes based on azido bridges. In this article, the coordination-driven supramolecular assembly based on 3,6-substituted pyridazine and azide is presented to afford two Fe(II) grid-like complexes: [(L)₄FeII4(N₃)₄][BPh₄]₄·sol (1, L = 3,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridazine and 2, L = 3,6-di(pyridin-2-yl)pyridazine). The substitution of pyridinyl groups in 2 instead of pyrazolyl ones in 1 led to the only example exhibiting spin-crossover behaviour (T_1/2 = 230 K) among the azido-bridged complexes. In addition, a temperature-dependent photoluminescence study of 2 demonstrates a visible synergetic effect between the SCO event and the luminescence.

Neutral Nitrogen Donor Ligand-based MOFs for Sensing Applications

Neutral nitrogen donor (N-donor) ligand-based MOFs, with their enticing features inclusive of facile synthesis, labile metal-ligand bond, framework flexibility, atomic level tunability renders them appealing in molecular recognition-based studies. Intriguingly, the flexibility in such systems (owing to weaker metal-nitrogen bonds) promote maximization of host-analyte interactions, which is critical for the manifestation of a signaling response. Such host-analyte interactions can be tapped by discerning any change in the physical properties associated with the system, such as optical, fluorometric, chemiresistive, magnetic, dielectric constant, mass. This minireview presents a brief discussion on the various types of signal transduction pathways unveiled hitherto using neutral N-donor ligand-based MOFs and the fundamental insight into the signal's origin. Moreover, an elaborate compilation of the recent examples in this field has been presented. Also, the untapped prospects have been highlighted, which may serve as a beacon to drive future research.

Reverse Hofmann-Type Spin-Crossover Compound Showing a Multichannel Controllable Color Change in an Ambient Environment

Materials that demonstrate a multichannel controllable color change in response to external stimuli are fascinating for their potential applications in sensoring and displaying devices. Herein we report a Fe^II spin-crossover (SCO) compound that exhibits both solvatochromism and thermochromism under an ambient environment. This Hofmann-type compound possesses two different pores where the solvent guests can be removed in a two-step process. Because the loss of solvent guests modifies the spin state of magnetic centers, an unusual yellow-red-yellow two-step color change of crystals was detected. Moreover, because of the strong cooperativity of the spin centers, a dramatic red-to-yellow color change of crystals in response to a minute thermal perturbation around 303 K is triggered by an abrupt spin transition of the metal centers. The multichannel controllable dramatic color change demonstrated in the present compound highlights the sensoring and displaying roles of SCO materials.

A spin-crossover phenomenon in a 2D heterometallic coordination polymer with [Pd(SCN)4]2- building blocks

Two new two-dimensional (2D) coordination polymers, [Fe^II(L)₂{Pd^II(SCN)₄}] (L₁ = 3-(9-anthracenyl)-pyridine (1) and L₂ = 4-(9-anthracenyl)-pyridine (2)), were constructed by employing square-planar [Pd(SCN)₄]^2- building blocks. Compound 1 exhibits a complete spin-crossover (SCO) behaviour under normal atmospheric pressure, and represents the first SCO example in a 2D system containing [Pd(SCN)₄]^2- units. In contrast, compound 2 only shows paramagnetic behaviour at measured temperatures. It is clear that the fine-tuning of the monodentate ligand can modulate the ligand field and packing fashions, which sheds light on developing new SCO materials.

Reversible step spin crossover modulation via water absorption and dehydration in a 3D Hofmann-type framework

Reversible modulations of step spin crossover behaviors are achieved in a 3D Hofmann-type framework via water absorption and dehydration.

Thermal hysteresis induced by external pressure in a 3D Hofmann-type SCO-MOF

Two 3D Hofmann-type compounds [Fe^II(dbdpe)M^II(CN)₄]·4H₂O have been synthesized. The application of pressure on compound 1 shifted the transition temperature from 185 K to 298 K and led to a hysteresis loop of 13–25 K.

Solvent-Induced Reversible Spin-Crossover in a 3D Hofmann-Type Coordination Polymer and Unusual Enhancement of the Lattice Cooperativity at the Desolvated State

The new 3D Hofmann-type coordination polymer [Fe(dpyu){Pt(CN)₄}]·9H₂O [dpyu = 1,3-di(pyridin-4-yl)urea] exhibits reversible interchange between two- and one-step spin-crossover behavior, associated with desorption/resorption of lattice water molecules. Solvent water removal also induces an increase of the spin-transition temperature, indicating strong lattice cooperativity, observed for the first time in a 3D Hofmann-type coordination polymer.

A novel two-step Fe-Au type spin-crossover behavior in a Hofmann-type coordination complex {Fe(4-methylpyrimidine)2[Au(CN)2]2}

A Hofmann-type two-step spin-crossover (SCO) complex with a pyrimidine derivative ligand, Fe(4-methylpyrimidine)2[Au(CN)2]2, was synthesized, and this complex shows a two-step SCO phenomenon in the intermediate state. Symmetry breaking also occurs in the intermediate state. These results reveal three spin states within the complex for high-spin (HS), HS0.5LS0.5, and low-spin (LS).

Alternative Route Triggering Multistep Spin Crossover with Hysteresis in an Iron(II) Family Mediated by Flexible Anion Ordering

Multistep spin crossover (SCO) compounds have attracted much attention, since they can be great candidates for high-density multinary memory devices. The introduction of substituents, such as methyl (Me), chloro (Cl), bromo (Br), and methoxy (MeO) groups, at para positions to the phenyl-substituted tripodal N₆ ligand-coordinated SCO Fe^II material, [FeL^Ph](NTf₂)₂ [where L^Ph = tris(2-{[(1-phenyl-1H-1,2,3-triazol-4-yl)methylidene]amino}ethyl)amine and NTf₂ = bis(trifluoromethanesulfonyl)imide], affords a new family of solvent-free Fe^II complexes: [FeL^4-R-Ph](NTf₂)₂ {where L^4-R-Ph = tris[2-({[1-(4-R-phenyl)-1H-1,2,3-triazol-4-yl]methylidene}amino)ethyl]amine, where R = Me (1), Cl (2), Br (3), and MeO (4)}. 1 shows temperature invariant high-spin (HS) state, whereas the others show spin transitions with different characteristics, such as half-SCO (4), two-step SCO (3), and unusual three-step SCO with hysteresis (2). Mössbauer and X-ray absorption fine structure (XAFS) spectroscopic studies of them support the magnetic susceptibilities results. Density functional theory calculations indicate that the electronic effect of different substituents on magnetic properties is negligible in this Fe^II family. Single-crystal X-ray diffraction studies reveal that 1-4 has a similar packing arrangement with three-dimensional supramolecular network via intermolecular π-π and CH···π interactions between complex cations, and CH···X (X = O, N, and F) hydrogen bonding interactions between cations and inherently frustrated NTf₂ anions. Variable-temperature structural studies unveil a variety of stepped SCO behaviors of 2-4 and deactivation of SCO in 1 are governed by the regulation of ordering of NTf₂ counteranions through the subtle modification of terminal substituents of complex cations. Quantitative light-induced excited spin-state trapping (LIESST) effect was observed for 2-4 via green light irradiation (532 nm) at 10 K. This study opens up a new way for systematic control of magnetic response from no SCO to half-, two-step, and finally three-step SCO with hysteresis by precise tuning of the ordering of flexible NTf₂ anions included in the supramolecular network with potentially SCO-active complex cations.

Multi-step structural phase transitions with novel symmetry breaking and inverse symmetry breaking characteristics in a [Ag4I6]2- cluster hybrid crystal

In this study, a multi-step phase transition hybrid composed of (Pr-dabco)2Ag4I6 clusters (Pr-dabco+ = 1-propyl-1,4-diazabicyclo[2.2.2]octan-1-ium) has been prepared and characterized by microanalysis, IR and UV-vis spectroscopy, TG and DSC techniques, etc. This hybrid is thermally stable up to ∼486 K with five phases in the temperature region below 486 K. The phase transition shows symmetry breaking (SB) character between phases II (space group P21/c) and III (space group Pa3[combining macron]), while inverse symmetry breaking (ISB) between phases II and I (space group Pbca), and it is rather exceptional for matter to exhibit simultaneously SB and ISB nature in two successive phase transitions. Most importantly, each phase transition is associated with a dielectric anomaly, and phase V appears to be a plastic crystal with extra high ac conductivity (>10-2 S cm-1). Our work opens up new avenues to find a multi-phase transition material in silver halide hybrids.

Structure–property relationships and the mechanisms of multistep transitions in spin crossover materials and frameworks

Structure–property relationships are identified and applied to explain multi-step transitions and the different antiferroelastic patterns found in spin crossover frameworks.

Physical stimulus and chemical modulations of bistable molecular magnetic materials

In this Feature Article, we summarize the recent progress made in modulating the multifaceted magnetic behaviour of single-molecule magnets (SMMs) and spin-crossover (SCO) materials based on chemical modifications and external stimuli.

Asymmetric seven-/eight-step spin-crossover in a three-dimensional Hofmann-type metal–organic framework

An unprecedented hysteretic seven-/eight-step spin-crossover behavior is revealed. Most importantly, a molecular alloy based on a Hofmann-type framework is used as a strategy to explore multi-step spin-crossover materials for the first time.

Spin crossover in 2D iron(ii) phthalazine cyanometallic complexes

Two new spin-crossover analogues of Hofmann clathrates based on a bicyclic ligand phthalazine have been synthesized.

Homochiral versus racemic polymorphs of spin-crossover iron(ii) complexes with reversible LIESST effect

Homochiral and racemic polymorphs show different spin-crossover behaviours due to different intermolecular interactions, and reversible LIESST effects can be realized on homochiral complexes.

The substituent guest effect on four-step spin-crossover behavior

The fluoro substituent strategy on the guest in a three-dimensional Hofmann-type metal–organic framework is explored for four-step spin-crossover properties.