The First Observation of Hidden Hysteresis in an Iron(III) Spin‐Crossover Complex

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.

Observation of TLIESST above Liquid Nitrogen Temperature and Disclosure of Hidden Hysteresis in Multiresponsive Hofmann-type Coordination Polymers

Photoresponsive spin-crossover (SCO) molecules are an important class of bistable magnetic molecules with intriguing potential in device applications. The light-induced excited spin state trapping (LIESST) and the combined application of light and temperature can provide access to the metastable region of the SCO profile. The primary obstacle in utilizing light stimuli is the manifestation of light-induced trappings at extremely low temperatures. Herein, we report two novel multiresponsive 2D Hofmann-type coordination polymers exhibiting light-induced excited spin state trapping above liquid nitrogen temperature (TLIESST = 82 and 81 K). Stimulating the samples in conjugation with light and temperature successfully unveils hysteresis, which is otherwise concealed. Apart from light and temperature, we found that the SCO phenomenon is also responsive to external hydrostatic pressure and exhibits modulation of the hysteresis width and transition temperature shifts with changes in pressure.

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.

Optical Phenomena in Molecule-Based Magnetic Materials

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

Impact of flexible hexyl chain ordering in a mononuclear spin crossover iron(III) complex

A novel FeIII complex [Fe(Hex-tnal)2]BPh4 (1) with a tridentate N2O ligand having an n-hexyl chain, Hex-Htnal (=1-((((1-hexyl-1H-1,2,3-triazol-4-yl)methyl)imino)methyl)naphthalen-2-ol), is reported. Temperature-dependent magnetic susceptibility measurements revealed that 1 exhibits a two-step spin crossover (SCO) transition in the 400-10 K temperature range, including an unusual gradual χMT change above RT (300-345 K) and a hysteretic χMT jump in a narrow temperature range of 345-357 K. These behaviors were also characterized by differential scanning calorimetry. Variable-temperature single-crystal X-ray diffraction studies revealed that the order-disorder transition and conformational change of the hexyl chains and the symmetry change associated with the re-entrant structural phase transition, namely triclinic P1̄ (100-275 K) ↔ monoclinic C2/c (296-340 K) ↔ triclinic P1̄ (360 K), are coupled to variations in intermolecular interactions and the N4O2 coordination environment, resulting in the occurrence of the unusual two-step SCO transition of 1. This study demonstrates that the flexible motion of alkyl substituents in the supramolecular lattice influences the occurrence of anomalous magnetic switching properties.

Extremely Slow Thermally-Induced Spin Crossover in the Two-Dimensional Network [Fe(bbtr)3 ](BF4 )2

Cooling [Fe(bbtr)3 ](BF4 )2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) triggers very slow spin crossover below 80 K (T1/2 ↓ =76 K). The spin crossover (SCO) is accompanied by a hysteresis loop (T1/2 ↑ =89 K). In contrast to isostructural perchlorate analogue [Fe(bbtr)3 ](ClO4 )2 in which spin crossover during cooling is preceded by phase transition at TPT =126 K in tetrafluoroborate phase transition does not occur to the beginning of spin crossover (80 K). Studies of mixed crystals [Fe(bbtr)3 ](BF4 )2(1-x) (ClO4 )2x (0.5≤x≤0.9) showed that a phase transition precedes spin crossover, however, for x≅0.46 intersection of T1/2 (x) and TPT (x) dependencies takes place. The application of pressure of 1 GPa shifts the spin crossover in [Fe(bbtr)3 ](BF4 )2 to a temperature above 270 K. High-pressure studies of neat tetrafluoroborate and perchlorate, as well as mixed crystals [Fe(bbtr)3 ](BF4 )2(1-x) (ClO4 )2x (0.1≤x≤0.9), revealed that at 295 K P1/2 value changes linearly with x indicating similar mechanism of spin crossover under elevated pressure in all systems under investigation. Variable pressure single crystal X-ray diffraction studies confirmed that in contrast to thermally induced spin crossover undergoing differently in tetrafluoroborate and perchlorate an application of high pressure removes this differentiation leading to a similar mechanism depending at first on start spin crossover and then P-3→P-1 phase transition occurs. In this report we have shown that 2D coordination polymer [Fe(bbtr)3 ](BF4 )2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) treated to date as spin crossover silent shows thermally induced spin crossover phenomenon. Spin crossover in tetrafluoroborate is extremely slow. Determination of the spin crossover curve required carrying measurement in the settle mode-cooling from 85 to 70 K took about 600 h (average velocity of change of temperature ca. 0.0004 K/min).

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO complexes with access to [LS-LS], [LS-HS], and [HS-HS] states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of [{FeIII(tpa)}2spiro](PF6)2 (1), [{FeIII(tpa)}2Br4spiro](PF6)2 (2), and [{FeIII(tpa)}2thea](PF6)2 (3) (tpa = tris(2-pyridylmethyl)amine, spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol, Br4spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol, theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the [HS-HS] state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed [LS-HS] state. In solution, all complexes undergo SCO from [HS-HS] at room temperature, via [LS-HS] to mixtures including [LS-LS] at 77 K, with the extent of SCO increasing in the order 1 < 2 < 3. Gas phase density functional theory calculations suggest a [LS-LS] ground state for all complexes, with the [LS-HS] and [HS-HS] states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro4- < Br4spiro4- < thea4-, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the [LS-HS] state in relation to the midpoint energy between [LS-LS] and [HS-HS]. The relative stability of the [LS-HS] state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 < 2 < 3. The bromo substituents of Br4spiro4- increase the ligand field strength relative to spiro4-, while the stronger ligand field provided by thea4- arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states.

Bidirectional photoswitchability in an iron(iii) spin crossover complex: symmetry-breaking and solvent effects

The impact of solvent on spin crossover (SCO) behaviour is reported in two solvates [Fe(qsal-I)₂]NO₃·2ROH (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate; R = Me 1 or Et 2) which undergo abrupt and gradual SCO, respectively. A symmetry-breaking phase transition due to spin-state ordering from a [HS] to [HS-LS] state occurs at 210 K in 1, while T_1/2 = 250 K for the EtOH solvate, where complete SCO occurs. The MeOH solvate exhibits LIESST and reverse-LIESST from the [HS-LS] state, revealing a hidden [LS] state. Moreover, photocrystallographic studies on 1 at 10 K reveal re-entrant photoinduced phase transitions to a high symmetry [HS] phase when irradiated at 980 nm or a high symmetry [LS] phase after irradiation at 660 nm. This study represents the first example of bidirectional photoswitchability and subsequent symmetry-breaking from a [HS-LS] state in an iron(iii) SCO material.

Nitrogen-Vacancy Magnetometry of Individual Fe-Triazole Spin Crossover Nanorods

[Fe(Htrz)₂(trz)](BF₄) (Fe-triazole) spin crossover molecules show thermal, electrical, and optical switching between high spin (HS) and low spin (LS) states, making them promising candidates for molecular spintronics. The LS and HS transitions originate from the electronic configurations of Fe(II) and are considered to be diamagnetic and paramagnetic, respectively. The Fe(II) LS state has six paired electrons in the ground states with no interaction with the magnetic field and a diamagnetic behavior is usually observed. While the bulk magnetic properties of Fe-triazole compounds are widely studied by standard magnetometry techniques, their magnetic properties at the individual level are missing. Here we use nitrogen vacancy (NV) based magnetometry to study the magnetic properties of the Fe-triazole LS state of nanoparticle clusters and individual nanorods of size varying from 20 to 1000 nm. Scanning electron microscopy (SEM) and Raman spectroscopy are performed to determine the size of the nanoparticles/nanorods and to confirm their respective spin states. The magnetic field patterns produced by the nanoparticles/nanorods are imaged by NV magnetic microscopy as a function of applied magnetic field (up to 350 mT) and correlated with SEM and Raman. We found that in most of the nanorods the LS state is slightly paramagnetic, possibly originating from the surface oxidation and/or the greater Fe(III) presence along the nanorods' edges. NV measurements on the Fe-triazole LS state nanoparticle clusters revealed both diamagnetic and paramagnetic behavior. Our results highlight the potential of NV quantum sensors to study the magnetic properties of spin crossover molecules and molecular magnets.

Chemical Manipulation of the Spin-Crossover Dynamics through Judicious Metal-Ion Dilution

In 2019, our groups described a unique Fe^II complex, [Fe(^2MeL)(NCBH₃)₂] (^2MeL = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine) possessing a low-spin ground state that is not easily accessible due to the extremely slow dynamics of the high-spin to low-spin phase transition. Herein, we report the successful chemical manipulation of this spin-crossover (SCO) process through controlled metal-ion dilutions. The emergence or suppression of the thermally induced SCO behavior was observed depending on the radius of the metal ion used for the dilution (Ni^II or Zn^II). Reversible photo-switching has been confirmed in all mixed-metal complexes whether the low-spin state is thermally accessible. Remarkably, the dilution with Zn^II metal ions stabilizes HS Fe^II complexes with complete suppression of the thermally induced SCO process without destroying the reversible photoswitchability of the material.

Interface versus Bulk Light-Induced Switching in Spin-Crossover Molecular Ultrathin Films Adsorbed on a Metallic Surface

Spin-crossover molecules present the unique property of having two spin states that can be controlled by light excitation at low temperature. Here, we report on the photoexcitation of [Fe^II((3, 5-(CH₃)₂Pz)₃BH)₂] (Pz = pyrazolyl) ultrathin films, with thicknesses ranging from 0.9 to 5.3 monolayers, adsorbed on Cu(111) substrate. Using X-ray absorption spectroscopy measurements, we confirm the anomalous light-induced spin-state switching observed for sub-monolayer coverage and demonstrate that it is confined to the first molecular layer in contact with the metallic substrate. For higher coverages, the well-known light-induced excited spin-state trapping effect is recovered. Combining continuous light excitation with thermal cycling, we demonstrate that at low temperature light-induced thermal hysteresis is measured for the thicker films, while for sub-monolayer coverage, the light enables extension of the thermal conversion over a large temperature range. Mechanoelastic simulations underline that, due to the intermolecular interactions, opposite behaviors are observed in the different layers composing the films.

Low-Frequency Sub-Terahertz Absorption in HgII -XCN-FeII (X=S, Se) Coordination Polymers

Self-assembly Fe^II complexes of phenazine (Phen), quinoxaline (Qxn), and 4,4'-trimethylenedipyridine (Tmp) with tetrahedral building blocks of [Hg^II (XCN)₄ ]^2- (X=S or Se) formed six new high-dimensional frameworks with the general formula of [Fe(L)_m ][Hg(XCN)₄ ]⋅solvents (L=Phen, m/X=2/S, 1; L=Qxn, m/X=2/S, 2; L=Qxn, m/X=1/S, 3; L=Qxn, m/X=1/Se, 3-Se; L=Tmp, m/X=1/S, 4; and L=Tmp, m/X=1/Se, 4-Se). 1, 3, and 3-Se show an intense sub-terahertz (sub-THz) absorbance of around 0.60 THz due to vibrations of the solvent molecules coordinated to the Fe^II ions and crystallization organic molecules. In addition, crystals of 1, 4, and 4-Se display low-frequency Raman scattering with exceptionally low values of 0.44, 0.51, and 0.53 THz, respectively. These results indicate that heavy metal Fe^II -Hg^II systems are promising platforms to construct sub-THz absorbers.

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.

Toward High-Temperature Light-Induced Spin-State Trapping in Spin-Crossover Materials: The Interplay of Collective and Molecular Effects

Spin-crossover (SCO) materials display many fascinating behaviors including collective phase transitions and spin-state switching controlled by external stimuli, e.g., light and electrical currents. As single-molecule switches, they have been fêted for numerous practical applications, but these remain largely unrealized-partly because of the difficulty of switching these materials at high temperatures. We introduce a semiempirical microscopic model of SCO materials combining crystal field theory with elastic intermolecular interactions. For realistic parameters, this model reproduces the key experimental results including thermally induced phase transitions, light-induced spin-state trapping (LIESST), and reverse-LIESST. Notably, we reproduce and explain the experimentally observed relationship between the critical temperature of the thermal transition, T_1/2, and the highest temperature for which the trapped state is stable, T_LIESST, and explain why increasing the stiffness of the coordination sphere increases T_LIESST. We propose strategies to design SCO materials with higher T_LIESST: optimizing the spin-orbit coupling via heavier atoms (particularly in the inner coordination sphere) and minimizing the enthalpy difference between the high-spin (HS) and low-spin (LS) states. However, the most dramatic increases arise from increasing the cooperativity of the spin-state transition by increasing the rigidity of the crystal. Increased crystal rigidity can also stabilize the HS state to low temperatures on thermal cycling yet leave the LS state stable at high temperatures following, for example, reverse-LIESST. We show that such highly cooperative systems offer a realistic route to robust room-temperature switching, demonstrate this in silico, and discuss material design rationale to realize this.

Homochiral Mn3+ Spin-Crossover Complexes: A Structural and Spectroscopic Study

Structural, magnetic, and spectroscopic data on a Mn³⁺ spin-crossover complex with Schiff base ligand 4-OMe-Sal₂323, isolated in crystal lattices with five different counteranions, are reported. Complexes of [Mn(4-OMe-Sal₂323)]X where X = ClO₄^– (1), BF₄^– (2), NO₃^– (3), Br^– (4), and I^– (5) crystallize isotypically in the chiral orthorhombic space group P2₁2₁2 with a range of spin state preferences for the [Mn(4-OMe-Sal₂323)]⁺ complex cation over the temperature range 5–300 K. Complexes 1 and 2 are high-spin, complex 4 undergoes a gradual and complete thermal spin crossover, while complexes 3 and 5 show stepped crossovers with different ratios of spin triplet and quintet forms in the intermediate temperature range. High-field electron paramagnetic resonance was used to measure the zero-field splitting parameters associated with the spin triplet and quintet states at temperatures below 10 K for complexes 4 and 2 with respective values: D_S=1 = +23.38(1) cm^–1, E_S=1 = +2.79(1) cm^–1, and D_S=2 = +6.9(3) cm^–1, with a distribution of E parameters for the S = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex 1 at room temperature reveal a 2:1 ratio of enantiomers in the chiral conglomerate, and solution CD measurements on the same sample in methanol show that it is stable toward racemization. Solid-state UV–vis absorption spectra on high-spin complex 1 and mixed S = 1/S = 2 sample 5 reveal different intensities at higher energies, in line with the different electronic composition. The statistical prevalence of homochiral crystallization of [Mn(4-OMe-Sal₂323)]⁺ in five lattices with different achiral counterions suggests that the chirality may be directed by the 4-OMe-Sal₂323 ligand.

Zero-field splitting parameters of the spin triplet and quintet forms of a spin-crossover Mn³⁺ complex stabilized in lattices with different counterions are measured by high-field electron paramagnetic resonance at different frequencies. The homochiral crystallization of the enantiopure Δ or Λ forms of the chelate complex, despite the use of achiral anions, is attributed to the steric influence of the ligand substituent.

The strong correlations between thermal conductivities and electronic spin states in crystals of Fe(III) spin crossover complexes

Solids that change their thermal conductivity during a phase transition can be useful in the development of a thermal switch to allow control of heat flow and reduce energy consumption....

Light-induced hidden multistability in a spin crossover metal-organic framework

The pursuit of spin crossover (SCO) materials with photo-switchable multistability is driven by the fascinating perspectives toward light-response switches and opto-magnetic memory devices. Herein, we report a 3D Hofmann-type metal...

Guest-Driven Light-Induced Spin Change in an Azobenzene Loaded Metal-Organic Framework

Stimuli-responsive materials that can be reversibly switched by light are of immense interest. Among them, photo-responsive spin crossover (SCO) complexes have great promises to combine the photoactive inputs with multifaceted outputs into switchable materials and devices. However, the reversible control the spin-state change by photochromic guests is still challenging. Herein, we report an unprecedented guest-driven light-induced spin change (GD-LISC) in a Hofmann-type metal-organic framework (MOF), [Fe(bpn){Ag(CN)₂ }₂ ]⋅azobenzene. (1, bpn=1,4-bis(4-pyridyl)naphthalene). The reversible trans-cis photoisomerization of azobenzene guest upon UV/Vis irradiation in the solid-state results in the remarkable magnetic changes in a wide temperature range of 10-180 K. This finding not only establishes a new switching mechanism for SCO complexes, but also paves the way toward the development of new generation of photo-responsive magnetic materials.

Laser-Induced Cooperative Transition in Molecular Electronic Crystal

The competing and non-equilibrium phase transitions, involving dynamic tunability of cooperative electronic and magnetic states in strongly correlated materials, show great promise in quantum sensing and information technology. To date, the stabilization of transient states is still in the preliminary stage, particularly with respect to molecular electronic solids. Here, a dynamic and cooperative phase in potassium-7,7,8,8-tetracyanoquinodimethane (K-TCNQ) with the control of pulsed electromagnetic excitation is demonstrated. Simultaneous dynamic and coherent lattice perturbation with 8 ns pulsed laser (532 nm, 15 MW cm^-2 , 10 Hz) in such a molecular electronic crystal initiates a stable long-lived (over 400 days) conducting paramagnetic state (≈42 Ωcm), showing the charge-spin bistability over a broad temperature range from 2 to 360 K. Comprehensive noise spectroscopy, in situ high-pressure measurements, electron spin resonance (ESR), theoretical model, and scanning tunneling microscopy/spectroscopy (STM/STS) studies provide further evidence that such a transition is cooperative, requiring a dedicated charge-spin-lattice decoupling to activate and subsequently stabilize nonequilibrium phase. The cooperativity triggered by ultrahigh-strain-rate (above 10⁶ s^{- 1} ) pulsed excitation offers a collective control toward the generation and stabilization of strongly correlated electronic and magnetic orders in molecular electronic solids and offers unique electro-magnetic phases with technological promises.

Spin-crossover iron(ii) long-chain complex with slow spin equilibrium at low temperatures

A mononuclear complex with long alkyl chains, [Fe^II(H₂Bpz₂)₂(C₉bpy)] (1; H₂Bpz₂ = dihydrobis(1-pyrazolyl)borate, C₉bpy = 4,4'-dinonyl-2,2'-bipyridine), was synthesized. Single-crystal X-ray crystallographic studies revealed that the Δ- and Λ-forms of the complex co-crystallized in the lattice asymmetric unit, while magnetic measurements unveiled that this complex underwent incomplete one-step spin crossover (SCO) with the transition completeness and temperature depending on the measurement velocity because of slow spin equilibrium. Multivariable approaches such as varying scan rate, annealing the sample, light irradiation and pressure have been adopted to effectively overcome the slow spin equilibrium and thus improve the SCO completeness.

Halogen-Substituted Spin-Crossover Fe(III) Compounds with Photoresponsive Properties

Halogen-substituted Fe(III) compounds, [Fe(Hphsalpm^X)₂]PTFB (Hphsalpm^X = 5-X-(R,S)-((phenyl(2-pyridyl)methylimino)methyl)phenol, PTFB = phenyltrifluoroborate; X = F for 1, Cl for 2, Br for 3) and [Fe(Hphsalpm^X)₂]PTFB·MeOH (X = I for 4·MeOH), were synthesized. Compounds 1, 4·MeOH, and its desolvated form 4 exhibited an invariant high-spin state in the whole temperature range, while 2 and 3 underwent gradual, nonhysteretic, and incomplete spin crossover (SCO) with transition temperatures (T_C) of 153 and 220 K, respectively. Interestingly, the SCO-active compounds 2 and 3 showed light-induced excited spin-state trapping (LIESST) effects at 10 K, and light-induced reversible ON/OFF switching behaviors were realized by alternately using 880 and 1064 nm light, while the thermally inert compound 4·MeOH unexpectedly showed a reverse-LIESST effect. These results may help to design and synthesize new photoresponsive SCO Fe(III) compounds for the development of switchable materials.

Second harmonic generation on chiral cyanido-bridged FeII-NbIV spin-crossover complexes

Incorporating chiral organic ligands into cyanido-bridged FeII-NbIV assemblies synthesized chiral spin-crossover complexes, FeII2[NbIV(CN)8](L)8·6H2O (L = R-, S-, or rac-1-(3-pyridyl)ethanol: R-FeNb, S-FeNb, or rac-FeNb). Rietveld analyses based on a racemic complex of rac-FeNb indicate that the chiral complexes have a cubic crystal structure in the I213 space group with a three-dimensional cyanido-bridged FeII-NbIV coordination network. All the complexes exhibit spin crossover between the high-spin (HS) and the low-spin (LS) FeII states without thermal hysteresis. Chiral complexes of R-FeNb and S-FeNb show second harmonic generation (SHG) due to their non-centrosymmetric structure. The I213 space group provides second-order susceptibility tensor elements of χxyz, χyzx, and χzxy, which contribute to SHG. The temperature-dependent second harmonic light intensity change is due to spin crossover between FeIIHS and FeIILS.

Tailoring the properties of 3d transition metal complexes with different N-cycloalkyl-substituted tetrazoles

N-Cycloalkyl-substituted tetrazoles were coordinated with different late 3d transition metal(ii) salts to give moderately sensitive energetic coordination compounds.

Light-induced excited spin state trapping in iron(iii) complexes

This review discusses the correlation of the local and whole molecular structure of iron(iii) complexes with the magnetic properties including the light-induced excited spin-state trapping (LIESST) effect.

Bivariate Metal-Organic Frameworks with Tunable Spin-Crossover Properties

In this work, pyrazine (A), aminopyrazine (B), quinoxaline (C), and 5,6,7,8-tetrahydroquinoxaline (D) have been screened out among a large number of pyrazine derivatives to construct Hofmann-type metal-organic frameworks (MOFs) Fe(L)[M(CN)₄ ] (M=Pt, Pd) with similar 3D pillared-layer structures. X-ray single-crystal diffraction reveals that the alternate linkage between M and Fe^II ions through cyano bridges forms the 2D extended metal cyanide sheets, and ligands A-D acted as vertical columns to connect the 2D sheets to give 3D pillared-layer structures. Subsequently, a series of bivariate MOFs were constructed by pairwise combination of the four ligands A-D, which were confirmed by ¹ H NMR, PXRD, FTIR, and Raman spectroscopy. The results demonstrated that ligand size and crystallization rate play a dominant role in constructing bivariate Hofmann-type MOFs. More importantly, the spin-crossover (SCO) properties of the bivariate MOFs can be finely tuned by adjusting the proportion of the two pillared ligands in the 3D Hofmann-type structures. Remarkably, the spin transition temperatures, T_c ↑ and T_c ↓ of Fe(A)_x (B)_1-x [Pt(CN)₄ ] (x=0 to 1) can be adjusted from 239 to 254 K and from 248 to 284 K, respectively. Meanwhile, the width of the hysteresis loops can be widened from 9 to 30 K. Changing Pt to Pd, the hysteresis loops of Fe(A)_x (B)_1-x [Pd(CN)₄ ] can be tuned from 9 (T_c ↑=215 K, T_c ↓=206 K) to 24 K (T_c ↑=300 K, T_c ↓=276 K). This research provides wider implications in the development of advanced bistable materials, especially in precisely regulating SCO properties.

A rare octacoordinated mononuclear iron(III) spin-crossover compound: synthesis, crystal structure and magnetic properties

The chemistry of transition-metal complexes with unusually high coordination numbers has been of interest because of their application in catalytic and biological systems. Deprotonation of the ionogenic tetradentate ligand 6,6′-bis(1H-tetrazol-5-yl)-2,2′-bipyridine [H2bipy(ttr)2] in the presence of iron(III) and tetra-n-butylammonium bromide, [n-Bu4N]Br, in solution resulted in the synthesis of a rare octacoordinated anionic mononuclear complex, tetra-n-butylammonium bis[6,6′-bis(tetrazol-1-id-5-yl)-2,2′-bipyridine]iron(III) methanol hemisolvate dihydrate, (C16H36N)[Fe(C12H6N10)2]·0.5CH3OH·2H2O or [n-Bu4N][Fe{bipy(ttr)2}2]·0.5CH3OH·2H2O (1), which has been structurally characterized by elemental analysis, powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. In 1, the coordination sphere of the iron(III) ion is a distorted bis-disphenoid dodecahedron, in which the eight coordination positions are occupied by eight N atoms from two independent tetradentate [bipy(ttr)2]2− anionic ligands, therefore forming the anionic [Fe{bipy(ttr)2}2]− unit, with the negative charge balanced by a free [n-Bu4N]+ cation. An investigation of the magnetic properties of 1 revealed a gradual incomplete spin-crossover behaviour below 150 K.

Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex

A proton-electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.

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

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

Build 3D Nanoparticles by Using Ultrathin 2D MOF Nanosheets for NIR Light-Triggered Molecular Switching

The coordination interactions between transition-metal ions (Cu²⁺, Ag⁺) and sulfur atoms on ultrathin two-dimensional (2D) nanosheets of spin-crossover (SCO) metal-organic frameworks {[Fe(1,3-bpp)₂(NCS)₂]₂}_n (1,3-bpp = 1,3-di(4-pyridyl)propane), which constructed the ultrathin 2D nanosheets into three-dimensional (3D) nanoparticles, have made a profound effect on the SCO performance. Compared with 2D nanosheets, both the intraligand π-π* transition band and the metal-to-ligand charge transition band from the d(Fe) + π(NCS) to π*(1,3-bpp), for the 3D nanoparticles, have shown dramatic blue-shifts; meanwhile, the d-d transition band for the high-spin (HS) state Fe(II) ions has been generated, suggesting significantly the influence of 3D assemble-caused dimensional changes on the solid-state SCO performance of ultrathin 2D nanosheets. More importantly, by loading on the ytterbium ion (Yb³⁺)-sensitized hexagonal phase upconverting nanoparticles in the aqueous colloidal suspension, the near infrared (NIR) light (980 nm) triggered HS (high spin) to LS (low spin) state transitions have been observed, demonstrating the achievement of challenging target of NIR light-triggered molecular conversion under environment conditions.

Light- and temperature-assisted spin state annealing: accessing the hidden multistability

Among responsive multistable materials, spin crossover (SCO) systems are of particular interest for stabilizing multiple spin states with various stimulus inputs and physical outputs. Here, in a 2D Hofmann-type coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂] (isoq = isoquinoline), a medium-temperature annealing process is introduced after light/temperature stimulation, which accesses the hidden multistability of the spin state. With the combined effort of magnetic, crystallographic and Mössbauer spectral investigation, these distinct spin states are identified and the light- and temperature-assisted transition pathways are clarified. Such excitation-relaxation and trapping-relaxation joint mechanisms, as ingenious interplays between the kinetic and thermodynamic effects, uncover hidden possibilities for the discovery of multistable materials and the development of multistate intelligent devices.

Two new two-stage manipulation protocols, namely light- and temperature-assisted spin state annealing (LASSA/TASSA), are applied to a spin crossover coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂], revealing the hidden multistability of spin states.

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.

Bistable molecular materials with dynamic structures

In this Feature Article, we introduce how to manipulate the motion of electrons or molecules by external stimuli, to achieve switchable properties in molecule-based single crystals.

Slow Dynamics of the Spin-Crossover Process in an Apparent High-Spin Mononuclear FeII Complex

A mononuclear Fe^II complex that shows a high-spin (S=2) paramagnetic behavior at all temperatures (with standard temperature-scan rates, ≈1 K min^-1 ) has, in fact, a low-spin (S=0) ground state below 100 K. This low-spin state is not easily accessible due to the extremely slow dynamics of the spin-crossover process-a full relaxation from the metastable high-spin state to the low-spin ground state takes more than 5 h below 80 K. Bidirectional photo-switching of the Fe^II state is achieved reproducibly by two selective irradiations (at 530-590 and 830-850 nm). The slow dynamics of the spin-crossover and the strong structural cooperativity result in a remarkably wide 95-K hysteresis loop induced by both temperature and selected light stimuli.