A Magneto‐optical Molecular Device: Interplay of Spin Crossover, Luminescence, Photomagnetism, and Photochromism

Di-Iron(II) [2+2] Helicates of Bis-(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties

Four bis[2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands have been synthesized, with butane-1,4-diyl (L¹ ), pyrid-2,6-diyl (L² ), benzene-1,2-dimethylenyl (L³ ) and propane-1,3-diyl (L⁴ ) linkers between the tridentate metal-binding domains. L¹ and L² form [Fe₂ (μ-L)₂ ]X₄ (X^- =BF₄ ^- or ClO₄ ^- ) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe₂ (μ-L¹ )₂ ][BF₄ ]₄ exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin-crossover, with examples of stepwise switching and partial spin-crossover to a low-temperature mixed-spin form. Salts of [Fe₂ (μ-L² )₂ ]⁴⁺ are high-spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L¹ -L³ vary with the ligand linker group, by mass spectrometry and ¹ H NMR spectroscopy. Gas-phase DFT calculations imply the butanediyl linker conformation in [Fe₂ (μ-L¹ )₂ ]⁴⁺ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.

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.

Manipulating Selective Metal-to-Metal Electron Transfer to Achieve Multi-Phase Transitions in an Asymmetric [Fe2Co]-Assembled Mixed-Valence Chain

Manipulation of multi-functions in molecular materials is promising for future switching and memory devices, although is currently difficult. Herein, we assembled the asymmetric {Fe2Co} unit into a cyanide-bridged mixed-valence chain {[(Tp)Fe(CN)3]2Co(BIT)}·2CH3OH (1) (Tp = hydrotris(pyrazolyl)borate and BIT = 3,4-bis-(1H-imidazol-1-yl)thiophen), which showed reversible multi-phase transitions accompanied by the photo-switchable single-chain magnet property and dielectric anomalies. Variable temperature X-ray structural studies revealed thermo-and photo-induced selective electron transfer (ET) between the Co and one of the Fe ions. Alternating-current magnetic susceptibility studies revealed that 1 displayed on and off of the single-chain magnet behavior by alternating 946-nm and 532-nm light irradiations. A substantial anomaly in dielectric constant was discovered during the electron transfer process, which is uncommon in similar ET complexes. These findings illustrate that 1 provided a new platform for multi-phase transitions and multi-switches adjusted by selective metal-to-metal ET.

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.

Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar-Ligand-Substituent Motion

Ferroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh-terpy)₂ ](BPh₄ )₂ ⋅3ac (1⋅3 ac; FPh-terpy=4'-((3-fluorophenyl)ethynyl)-2,2':6',2''-terpyridine) and is dependent on the degree of 180° flip-flop motion of the ligand's polar fluorophenyl ring. Single crystal X-ray structures at several temperatures confirmed the flip-flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid-state ¹⁹ F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip-flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin-state conversion being dependent on the molecular motion of a ligand-appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule-based magnetoelectric materials.

A Multifunctional Dysprosium-Carboxylato 2D Metall-Organic Framework

We report the microwave assisted synthesis of a bidimensional (2D) MOF of formula [Dy(MeCOO)(PhCOO)₂ ]_n (1) and its magnetically diluted analogue [La_0.9 Dy_0.1 (MeCOO)(PhCOO)₂ ] (1 d). 1 is a 2D material with single-ion-magnet (SIM) behaviour and 1 d is a multifunctional, magnetic and luminescent 2D material. 1 can be exfoliated into stable nanosheets by sonication.

Multiscale Approach of Spin Crossover Materials: A Concept Mixing Russian Dolls and Domino Effects

The spin crossover (SCO) phenomenon corresponds to a modification that originates at the atomic scale. However, the simple consideration of the transformations that occur following the SCO at this scale or in its close vicinity does not allow anyone to truly understand, anticipate and thus take advantage of what happens at the scale of the material, and even less at the device one. As the fruit of years of work and experience on this phenomenon, we formalize here the concept of the multiscale understanding of SCO. Clearly, the deflagration generated by the initial impressive atomic modification on all the physical scales of the solid must be understood in terms of structure-properties relationships that fit together, like Russian dolls, and propagate according to a kind of domino effect. Each scale can both give different and independent consequences from those of the other scales but at the same time can influence those of a larger or smaller scale, the whole being imperatively to take into account. The concept appears well illustrated by the volume modification, always the same at the atomic level but drastically different and adaptable, in amplitude and sense, at any other physical scale. This approach results in a much wider range of potential applications than the atomic level alone initially suggests, including one serious path to shape memory materials.

Encapsulation of a CrIII Single-Ion Magnet within an FeII Spin-Crossover Supramolecular Host

Single functional molecules are regarded as future components of nanoscale spintronic devices. Supramolecular coordination chemistry provides unlimited resources to implement multiple functions to individual molecules. A novel coordination [Fe₂ ] helicate exhibiting spin-crossover is demonstrated to be ideally suited to encapsulate a [Cr(ox)₃ ]^3- complex anion (ox=oxalate), unveiling for the first-time single ion slow relaxation of the magnetization for this metal. A possibility of tuning the dynamics of this relaxation as well as the performance of the Cr^III center as qubit arises from the observation that metastable high spin Fe^II centers from the host can be generated by irradiation with green light at low temperature.