Proton-Induced Reversible Spin-State Switching in Octanuclear FeIII Spin-Crossover Metal-Organic Cages.

Responsive spin-crossover (SCO) metal-organic cages (MOCs) are emerging dynamic platforms with potential for advanced applications in magnetic sensing and molecular switching. Among these, FeIII-based MOCs are particularly noteworthy for their air stability, yet they remain largely unexplored. Herein, we report the synthesis of two novel FeIII MOCs using a bis-bidentate ligand approach, which exhibit SCO activity above room temperature. These represent the first SCO-active FeIII cages and feature an atypical {FeN6}-type coordination sphere, uncommon for FeIII SCO compounds. Our study reveals that these MOCs are sensitive to acid/base variations, enabling reversible magnetic switching in solution. The presence of multiple active proton sites within these SCO-MOCs facilitates multisite, multilevel proton-induced spin-state modulation. This behavior is observed at room temperature through 1H NMR spectroscopy, capturing the subtle proton-induced spin-state transitions triggered by pH changes. Further insights from extended X-ray absorption fine structure (EXAFS) and theoretical analyses indicate that these magnetic alterations primarily result from the protonation and deprotonation processes at the NH active sites on the ligands. These processes induce changes in the secondary coordination sphere, thereby modulating the magnetic properties of the cages. The capability of these FeIII MOCs to integrate magnetic responses with environmental stimuli underscores their potential as finely tunable magnetic sensors and highlights their versatility as molecular switches. This work paves the way for the development of SCO-active materials with tailored properties for applications in sensing and molecular switching.

An elastic single crystal composed of one-dimensional chiral coordination polymers.

A single crystal composed of one-dimensional coordinated polymers, [CdCl2(1-methyl-2-pyridone)]n, has been synthesized and characterized. This compound exhibits outstanding elastic bending due to the molecular spring nature of the CdCl2 coordination framework and weak intermolecular interactions between the coordination chains. Owing to the helical arrangement of organic ligands surrounding the coordination structure, the compound crystallizes in a chiral space group. As a result, it displays compelling circular dichroism spectra and second harmonic generation properties.

High-performance Pyroelectric Property Accompanied by Spin Crossover in a Single Crystal of Fe(II) Complex.

Pyroelectric materials hold significant potential for energy harvesting, sensing, and imaging applications. However, achieving high-performance pyroelectricity across a wide temperature range near room temperature remains a significant challenge. Herein, we demonstrate a single crystal of Fe(II) spin-crossover compound shows remarkable pyroelectric properties accompanied by a thermally controlled spin transition. In this material, the uniaxial alignment of polar molecules results in a polarization of the lattice. As the molecular geometry is modulated during a gradual spin transition, the polar axis experiences a colossal thermal expansion with a coefficient of 796×10-6 K-1. Consequently, the material's polarization undergoes significant modulation as a secondary pyroelectric effect. The considerable shift in polarization (pyroelectric coefficient, p=3.7-22 nC K-1cm-2), coupled with a low dielectric constant (ϵ'=4.4-5.4) over a remarkably wide temperature range of 298 to 400 K, suggests this material is a high-performance pyroelectric. The demonstration of pyroelectricity combined with magnetic switching in this study will inspire further investigations in the field of molecular electronics and magnetism.

Guest water-induced structural transformation and spin-crossover variation of a two-dimensional Hofmann-type compound.

In this paper, we report a two-dimensional (2D) Hofmann-type spin-crossover coordination polymer [FeII(o-NTrz)2PtII(CN)4]·H2O (o-NTrz = 4-(o-nitrobenzyl)imino-1,2,4-triazole). Due to the remarkable configurational flexibility of triazole-based ligand, the porous structure of this compound can be reversibly regulated by the loss of guest water molecules as a consequence of rotation of o-NTrz. The 180° reorientation of the o-nitrobenzyl moiety not only induces a response of gate-closing/opening of the porous framework but also significantly modulates the spin transition temperature. The present investigation highlights the potential of Hofmann-type SCO compounds with flexible ligands in exploring unusual physical and chemical phenomena.

Molecular Twist-Induced Single-Crystal Isomerization and Valence Tautomeric Transitions in a Cobalt-Dioxolene Complex.

A mononuclear valence tautomeric (VT) complex, [Co(pycz)2(Sq)(Cat)] (1-trans), where pycz = 9-(pyridin-4-yl)-9H-carbazole, Sq⋅- = 3,5-di-tert-butyl-semiquinonato, and Cat2- = 3,5-di-tert-butyl-catecholato, is synthesized in the trans configuration, which undergoes one-step valence tautomeric transition above room temperature. Remarkably, 1-trans can transform into its isomeric structure, [Co(pycz)2(Sq)(Sq)] (1-cis), at temperature above 350 K in a single-crystal-to-single-crystal way by in situ molecular twist, and the resulting 1-cis exhibits a pronounced two-step VT transition during magnetic measurements that is rare for mononuclear VT complexes. Such drastic solid-state structural transformation is reported in VT compounds for the first time, which is actuated by a crystal surface's melting-recrystallization induced phase transition process. DFT calculations offer an underlying mechanism suggesting a concerted bond rotation during the structural transformation. The results demonstrate an unconventional approach that realizes structural transformation of VT complexes and the control of VT performance.

High-Temperature Metal-Free Molecular Ferroelectrics with Large Spontaneous Polarization.

In the past two decades, numerous molecular ferroelectrics have been reported. However, metal-free molecular ferroelectrics with high working temperatures and large spontaneous polarizations are still uncommon. Herein, we present two metal-free molecular ferroelectrics prepared from monoprotonated hexamethylenetetramine (HMTA), namely (HMTAH)Cl and (HMTAH)Br, which crystallize in a polar point group of 3m. In these crystals, the polar HMTAH+ organic cations can be reoriented 180° along the polar axis because of the quasispherical molecular geometry. As a result of the large shift of the positively charged protonated N atoms, these compounds demonstrate large spontaneous polarizations with values of 8.3 and 8.1 µC cm-2 and high working temperatures of 390 and 435 K, respectively. The ferroelectric property of these compounds is characterized with second-harmonic generation, ferroelectric hysteresis loop, and pyroelectric current measurements.

Inversion of Molecular Chirality Associated with Ferroelectric Switching in a High-Temperature Two-Dimensional Perovskite Ferroelectric.

Controlling molecular chirality by external stimuli is of great significance in both fundamental research and technological applications. Herein, we report a high-temperature (384 K) molecular ferroelectric of a Cu(II) complex whose spontaneous polarization can be switched associated with flipping of molecular chirality. In this two-dimensional perovskite structure, the inorganic layer is separated by (NH3(CH2)2SS(CH2)2NH3)2+ organic cations skewed in a chiral conformation (P- or M-helicity in an individual crystal). As the stereodynamic disulfide bridge determines the molecular dipole moment along the polar axis, the chiral organic cation can be converted to its enantiomer as a consequence of an electric field-induced shift of the S-S moiety relative to its screw axis during the ferroelectric switching. The variation of the molecular chirality is examined with single-crystal X-ray diffraction and circular dichroism spectra. The simultaneous switching of molecular chirality and spontaneous polarization in this perovskite ferroelectric may lead to novel chiral electronic phenomena.

Spin-Crossover Behaviors of Iron(II) Complexes Bearing Halogen Ligands in Solid State and Solution.

Numerous Fe(II) spin-crossover (SCO) compounds have been developed in the past decades, while the reports on the SCO materials with halogen atoms acting as coordinating ligands remain rare. In this study, we synthesize three iron(II) halide complexes with a general formula of [FeII(Py5Me2)X]+ (Py5Me2 = 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine, X = Cl- or Br-) that undergo complete SCO transitions at near room temperature. The SCO properties of these compounds are investigated in detail by magnetic measurements, variable-temperature single-crystal X-ray diffractions, and Mössbauer spectra analyses. Because of the good stability of the coordination structures and suitable ligand-field strength, these compounds show robust spin transitions in both solid state and solution.

Stress-Induced Inversion of Linear Dichroism by 4,4'-Bipyridine Rotation in a Superelastic Organic Single Crystal.

The molecular crystals that manifest unusual mechanical properties have attracted growing attention. Herein, we prepared an organic single crystal that shows bidirectional superelastic transformation in response to shear stress. Single-crystal X-ray diffractions revealed this crystal-twinning related shape change is owed to a stress-controlled 90° rotation of 4,4'-bipyridine around the hydrogen bonds of a chiral organic trimer. As a consequence of the 90° shift in the aromatic plane, an interconversion of crystallographic a-, b-axes (a→b' and b→a') was detected. The molecular rotations changed the anisotropic absorption of linearly polarized light. Therefore, a stress-induced inversion of linear dichroism spectra was demonstrated for the first time. This study reveals the superior mechanical flexibilities of single crystals can be realized by harnessing the molecular rotations and this superelastic crystal may find applications in optical switching and communications.

Magnetic and Dielectric Switchings Actuated by the Rotation of the Picoline Ligand in an Iron-Based Dinuclear Phase-Transition Complex.

Multifunctional materials with switchable magnetic and dielectric properties are crucial for the development of memory and sensor devices. Herein, we report a methoxy-bridged dinuclear iron-pyridyl complex [Fe2(4-picoline)4(NCS)4(µ-OCH3)2] (1), which shows simultaneous thermal-induced magnetic and dielectric switchings. Within the phase-transition temperature range, both magnetic switching and the dielectric anomaly were detected, in which the thermal hysteresis loops were 23 and 21 K, respectively. Detailed structural analyses revealed that these simultaneous switchings were rooted in the flexible rotatable ligands, which were actuated by readjusting the π-π intermolecular interactions between the pyridine ligands in the trans positions of the metal centers. These results were comprehensively investigated both experimentally and theoretically. This study presents a new guideline to control both the magnetic and dielectric properties of molecular complexes by external stimuli.

Two-Dimensional Coordination Polymer Showing Spin-Crossover Behavior with a 64 K Wide Hysteresis Loop.

A two-dimensional grid-like coordination polymer, [Fe(NCBH3)2(Py2ttz)2]·4CHCl3 (1·4CHCl3, Py2ttz = 2,5-di(pyridin-4-yl)thiazolo[5,4-d]thiazole), showed one-step complete spin crossover with unexpectedly large hysteresis loop of 64 K wide and temperature-induced excited spin-state trapping effect below 91 K.

Synergistic strategies for the synthesis of Fe(II)-based bifunctional fluorescent spin-crossover materials.

Spin-crossover (SCO) materials have been extensively investigated in the past few decades with promising applications in a wide range of fields including displays, sensors, and storage components. The introduction of secondary physical properties to SCO materials resulting in bifunctional materials is becoming a research hot spot. In particular, for the fluorescent SCO materials, fluorescence can serve as a sensitive probe that easily expresses the spin-state changes of SCO materials; therefore, fluorescent SCO bifunctional materials have promising applications in the fields of magneto-optical switching and information processing. In this perspective, we briefly review the main synthetic strategies and progresses in fluorescent SCO materials in the last twenty years.

A spin-crossover framework endowed with pore-adjustable behavior by slow structural dynamics.

Host-guest interactions play critical roles in achieving switchable structures and functionalities in porous materials, but design and control remain challenging. Here, we report a two-dimensional porous magnetic compound, [FeII(prentrz)2PdII(CN)4] (prentrz = (1E,2E)-3-phenyl-N-(4H-1,2,4-triazol-4-yl)prop-2-en-1-imine), which exhibits an atypical pore transformation that directly entangles with a spin state transition in response to water adsorption. In this material, the adsorption-induced, non-uniform pedal motion of the axial prentrz ligands and the crumpling/unfolding of the layer structure actuate a reversible narrow quasi-discrete pore (nqp) to large channel-type pore (lcp) change that leads to a pore rearrangement associated with simultaneous pore opening and closing. The unusual pore transformation results in programmable adsorption in which the lcp structure type must be achieved first by the long-time exposure of the nqp structure type in a steam-saturated atmosphere to accomplish the gate-opening adsorption. The structural transformation is accompanied by a variation in the spin-crossover (SCO) property of FeII, i.e., two-step SCO with a large plateau for the lcp phase and two-step SCO with no plateau for the nqp phase. The unusual adsorption-induced pore rearrangement and the related SCO property offer a way to design and control the pore structure and physical properties of dynamic frameworks.

A series of dynamic single crystals of [MII(en)3]SO4 (M = Ni, Mn, and Cd) shows tunable dielectric properties and anisotropic thermal expansion.

A series of dynamic single crystals with a chemical formula of [MII(en)3]SO4 (en = ethylene and MII = NiII, MnII, and CdII) was synthesized. As the temperature decreases, these materials exhibit dielectric switching in the vicinity of the phase transition point accompanied by anisotropic thermal expansion in the cell parameters as a consequence of the order-disorder structural change of SO2-4 in a cavity surrounded by five [MII(en)3]2+ complex cations. Because the variation of metal centers with different ionic radii changes the shape of the complex cation, which affects the distribution of hydrogen-bond interactions around the SO2-4, the dynamic motion of SO2-4 is substantially tuned. Correspondingly, the dielectric properties and anisotropic thermal expansion of materials were largely shifted, especially in the single crystals of [MnII(en)3]SO4, whose structural change is distinctly different from the crystals of Ni(II) and Cd(II). The detailed structural mechanism accounting for the different physical properties of these materials was discussed.

Discovery of Kinetic Effect in a Valence Tautomeric Cobalt-Dioxolene Complex.

Two isostructural valence tautomeric (VT) complexes with different critical temperatures were prepared and fully investigated through a series of magnetic, structural, spectral, and differential scanning calorimetry evidence. The kinetic effect in the VT complex was observed for the first time through scan-rate-dependent studies and further validated by annealing tests.

Giant single-crystal-to-single-crystal transformations associated with chiral interconversion induced by elimination of chelating ligands.

Numerous single crystals that exhibit single-crystal-to-single-crystal (SCSC) transformations have been reported, and some of them show great promise for application to advanced adsorption materials, magnetic switches, and smart actuators. However, the development of single crystals with super-adaptive crystal lattices capable of huge and reversible structural change remains a great challenge. In this study, we report a ZnII complex that undergoes giant SCSC transformation induced by a two-step thermal elimination of ethylene glycol chelating ligands. Although the structural change is exceptionally large (50% volume shrinkage and 36% weight loss), the single-crystal nature of the complex persists because of the multiple strong hydrogen bonds between the constituent molecules. This allows the reversible zero-dimensional to one-dimension and further to three-dimensional structural changes to be fully characterized by single-crystal X-ray diffraction analyses. The elimination of chelating ligands induces a chiral interconversion in the molecules that manifests as a centric-chiral-polar symmetric variation of the single crystal. The study not only presents a unique material, featuring both a periodic crystal lattice and gel-like super-ductility, but also reveals a possible solid-state reaction method for preparing chiral compounds via the elimination of chelating ligands.

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

Halogen-substituted Fe(III) compounds, [Fe(HphsalpmX)2]PTFB (HphsalpmX = 5-X-(R,S)-((phenyl(2-pyridyl)methylimino)methyl)phenol, PTFB = phenyltrifluoroborate; X = F for 1, Cl for 2, Br for 3) and [Fe(HphsalpmX)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 (TC) 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.

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

A mononuclear complex with long alkyl chains, [FeII(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.

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 FeII 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.

Adjusting Rotational Behavior of Molecular Rotors by a Rational Tuning of Molecular Structure.

Many crystalline molecular rotors have been developed in the past decades. However, manipulating the rotational gesture that intrinsically controls the physical performance of materials remains a challenge. Herein, we report a series of crystalline rotors whose rotational gestures can be modulated by modifying the structures of molecular stators. In these dynamic crystals, the ox2- (ox2- = oxalate anion) behave as molecular rotators performing axial-free rotation in cavities composed of five complex cations, [MII(en)3]2+ (en = ethylenediamine). The structure of [MII(en)3]2+ that serves as a molecular stator can be tuned by varying the metal center with different ionic radii, consequently altering the chemical environment around the molecular rotator. Owing to the quasi-transverse isotropy of ox2- and multiple hydrogen-bond interactions around it, the molecular rotator exhibits unusual motional malleability, i.e., it can rotate either longitudinally in the compound of ZnII, or with a tilt angle of 42° in the compound of FeII, or even laterally in the compound of CdII. The atypical dynamic behavior demonstrated here provides a new chance for the development of exquisite crystalline molecular rotors with advanced tunable functionalities.