Exchange Bias Induced by the Spin-Glass-Like State in a Te-Rich FeGeTe van der Waals Ferromagnet.

We have experimentally investigated the mechanism of the exchange bias in 2D van der Waals (vdW) ferromagnets by means of the anomalous Hall effect (AHE) together with the dynamical magnetization property. The temperature dependence of the AC susceptibility with its frequency response indicates a glassy transition of the magnetic property for the Te-rich FeGeTe vdW ferromagnet. We also found that the irreversible temperature dependence in the anomalous Hall voltage follows the de Almeida-Thouless line. Moreover, the freezing temperature of the spin-glass-like phase is found to correlate with the disappearance temperature of the exchange bias. These important signatures suggest that the emergence of magnetic exchange bias in the 2D van der Waals ferromagnets is induced by the presence of the spin-glass-like state in FeGeTe. The unprecedented insights gained from these findings shed light on the underlying principles governing exchange bias in vdW ferromagnets, contributing to the advancement of our understanding.

Polar Crystals Using Molecular Chirality: Pseudosymmetric Crystallization toward Polarization Switching Materials.

Polar compounds with switchable polarization properties are applicable in various devices such as ferroelectric memory and pyroelectric sensors. However, a strategy to prepare polar compounds has not been established. We report a rational synthesis of a polar CoGa crystal using chiral cth ligands (SS-cth and RR-cth, cth = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Both the original homo metal Co crystal and Ga crystal exhibit a centrosymmetric isostructure, where the dipole moment of metal complexes with the SS-cth ligand and those with the RR-cth ligand are canceled out. To obtain a polar compound, the Co valence tautomeric complex with SS-cth in the homo metal Co crystal is replaced with the Ga complex with SS-cth by mixing Co valence tautomeric complexes with RR-cth and Ga complexes with SS-cth. The CoGa crystal exhibits polarization switching between the pseudononpolar state at a low temperature and the polar state at a high temperature because only Co complexes exhibit changes in electric dipole moment due to metal-to-ligand charge transfer. Following the same strategy, the polarization-switchable CoZn complex was synthesized. The CoZn crystal exhibits polarization switching between the polar state at a low temperature and the pseudononpolar state at a high temperature, which is the opposite temperature dependence to that of the CoGa crystal. These results revealed that the polar crystal can be synthesized by design, using a chiral ligand. Moreover, our method allows for the control of temperature-dependent polarization changes, which contrasts with typical ferroelectric compounds, in which the polar ferroelectric phase typically occurs at low temperatures.

Two-Dimensional Spin-Crossover Molecular Solid Solutions with Tunable Transition Temperatures across 90 K.

Spin-crossover (SCO) materials exhibit remarkable potential as bistable switches in molecular devices. However, the spin transition temperatures (Tc) of known compounds are unable to cover the entire ambient temperature spectrum, largely limiting their practical utility. This study reports an exemplary two-dimensional SCO solid solution system, [FeIII(H0.5LCl)2-2x(H0.5LF)2x]·H2O (H0.5LX = 5-X-2-hydroxybenzylidene-hydrazinecarbothioamide, X = F or Cl, x = 0 to 1), in which the adjacent layers are adhered via hydrogen bonding. Notably, the Tc of this system can be fine-tuned across 90 K (227-316 K) in a linear manner by modulating the fraction x of the LF ligand. Elevating x results in strengthened hydrogen bonding between adjacent layers, which leads to enhanced intermolecular interactions between adjacent SCO molecules. Single-crystal diffraction analysis and periodic density functional theory calculations revealed that such a special kind of alteration in interlayer interactions strengthens the FeIIIN2O2S2 ligand field and corresponding SCO energy barrier, consequently resulting in increased Tc. This work provides a new pathway for tuning the Tc of SCO materials through delicate manipulation of molecular interactions, which could expand the application of bistable molecular solids to a much wider temperature regime.

Electrically Detectable Photoinduced Polarization Switching in a Molecular Prussian Blue Analogue.

Light, a nondestructive and remotely controllable external stimulus, effectively triggers a variety of electron-transfer phenomena in metal complexes. One prime example includes using light in molecular cyanide-bridged [FeCo] bimetallic Prussian blue analogues, where it switches the system between the electron-transferred metastable state and the system's ground state. If this process is coupled to a ferroelectric-type phase transition, the generation and disappearance of macroscopic polarization, entirely under light control, become possible. In this research, we successfully executed a nonpolar-to-polar phase transition in a trinuclear cyanide-bridged [Fe2Co] complex crystal via directional electron transfer. Intriguingly, by exposing the crystal to the wavelength of light─785 nm─without any electric field─we can drive this ferroelectric phase transition to completely depolarize the crystal, during which a measurable electric current response can be detected. These discoveries signify an important step toward the realization of fully light-controlled ferroelectric memory devices.

Fully Optical Control of Polarization Current Direction in a Cyanide-Bridged Trinuclear Complex.

As a remote and non-contact stimulus, light offers the potential for manipulating the polarization of ferroelectric materials without physical contact. However, in current research, the non-contact write-read (erase) process lacks direct observation through the stable current as output signal. To address this limitation, we investigated the photoinduced polarization switching capabilities of the cyanide-bridged compound [Fe2Co] using visible light, leading to the achievement of rewritable polarization. By subjecting [Fe2Co] crystals to alternating irradiation with 785 nm and 532 nm light, the polarization changes exhibited a distinct square wave pattern, confirming the reliability of the writing and erasing processes. Initialization involved exposing specific crystal units to 532 nm light for storing "1" or "0" information, while reading was accomplished by scanning the units with 785 nm light, resulting in brief current pulses for "1" states and no current signal for "0" states. This research unveils new possibilities for optical storage systems, paving the way for efficient and rewritable data storage and retrieval technologies, such as the next-generation memories.

Magnetoelectricity Enhanced by Electron Redistribution in a Spin Crossover [FeCo] Complex.

Molecular-based magnetoelectric materials are among the most promising materials for next-generation magnetoelectric memory devices. However, practical application of existing molecular systems has proven difficult largely because the polarization change is far lower than the practical threshold of the ME memory devices. Herein, we successfully obtained an [FeCo] dinuclear complex that exhibits a magnetic field-induced spin crossover process, resulting in a significant polarization change of 0.45 µC cm-2. Mössbauer spectroscopy and theoretical calculations suggest that the asymmetric structural change, coupled with electron redistribution, leads to the observed polarization change. Our approach provides a new strategy toward rationally enhancing the polarization change.

Observation of proton-transfer-coupled spin transition by single-crystal neutron-diffraction measurement.

The application of single-crystal neutron diffraction (SCND) to observe proton-transfer phenomena in crystalline compounds exhibiting unusual protonation states or proton dynamics has garnered significant research interest in recent years. However, proton tautomerism, which results in different protonation states before and after proton transfer, has never been observed using the SCND technique. Thus, to observe the proton tautomerism phenomenon by SCND measurements, we developed an iron(II) complex that forms a large crystal and exhibits a proton-transfer-coupled spin transition (PCST). The presence of the two types of proton tautomers was determined by conventional analysis of the proton position by X-ray crystallography, infrared spectroscopy, and density functional theory calculations. Finally, our results confirmed that proton tautomerism was successfully observed for the first time using variable-temperature SCND measurements.

Discovery of Ingenane Diterpenoids from Euphorbia hylonoma as Antiadipogenic Agents.

Thirteen new Euphorbia diterpenoids, euphylonanes A-M (1-13), and eight known ones were isolated from the whole plants of Euphorbia hylonoma. Compounds 1 and 2 are two rearranged ingenanes bearing a rare 6/6/7/3-fused ring system. Compound 3 represents the first example of a 9,10-epoxy tigliane, while 4-21 are typical ingenanes varying with substituents. Structures were elucidated using a combination of spectroscopic, computational, and chemical methods. Most ingenanes exerted a significant antiadipogenic effect in 3T3-L1 adipocytes, among which 4 was the most active with an EC50 value of 0.60 ± 0.27 µM. Mechanistic study revealed that 4 inhibited the adipogenesis and lipogenesis in adipocytes via activation of the AMPK signaling pathway.

Discovering a New Okadaic Acid Derivative, a Potent HIV Latency Reversing Agent from Prorocentrum lima PL11: Isolation, Structural Modification, and Mechanistic Study.

Marine toxins (MTs) are a group of structurally complex natural products with unique toxicological and pharmacological activities. In the present study, two common shellfish toxins, okadaic acid (OA) (1) and OA methyl ester (2), were isolated from the cultured microalgae strain Prorocentrum lima PL11. OA can significantly activate the latent HIV but has severe toxicity. To obtain more tolerable and potent latency reversing agents (LRAs), we conducted the structural modification of OA by esterification, yielding one known compound (3) and four new derivatives (4-7). Flow cytometry-based HIV latency reversal activity screening showed that compound 7 possessed a stronger activity (EC50 = 46 ± 13.5 nM) but was less cytotoxic than OA. The preliminary structure-activity relationships (SARs) indicated that the carboxyl group in OA was essential for activity, while the esterification of carboxyl or free hydroxyls were beneficial for reducing cytotoxicity. A mechanistic study revealed that compound 7 promotes the dissociation of P-TEFb from the 7SK snRNP complex to reactivate latent HIV-1. Our study provides significant clues for OA-based HIV LRA discovery.

Tigliane and rhamnofolane glycosides from Euphorbia wallichii prevent oxidative stress-induced neuronal death in PC-12 cells.

Tigliane and rhamnofolane diterpenoids bearing glycosyl substituents are rarely found in nature. In the current study, seven new tigliane glycosides, euphorwallsides A - G (1-7), and five new rhamnofolane glycosides, euphorwallsides H - L (8-12), were isolated from the whole plants of Euphorbia wallichii. Their structures were elucidated by a combination of spectroscopic, computational, and chemical means. The aglycones of 1-5 represent a rare class of 13-deoxygenated tiglianes, while those of 8-12 represent the first examples of 4-deoxygenated rhamnofolanes. 2, 3, 5, 7, 8, and 12 showed significant neuroprotective effects on sodium nitroprusside (SNP)-induced neuronal death in pheochromocytoma cell line PC-12 at 10 µM, being more active than the clinical drug, edaravone. Mechanistic study revealed that the most active compound, 3, could inhibit reactive oxygen species (ROS) accumulation and restore the mitochondrial membrane potential via modulating the Nrf2 signaling pathway in PC-12 cells.

Photoinduced Persistent Polarization Change in a Spin Transition Crystal.

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

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.

Quenching and Restoration of Orbital Angular Momentum through a Dynamic Bond in a Cobalt(II) Complex.

Orbital angular momentum plays a vital role in various applications, especially magnetic and spintronic properties. Therefore, controlling orbital angular momentum is of paramount importance to both fundamental science and new technological applications. Many attempts have been made to modulate the ligand-field-induced quenching effects of orbital angular momentum to manipulate magnetic properties. However, to date, reported changes in the magnitude of orbital angular momentum are small in both molecular and solid-state magnetic materials. Moreover, no effective methods currently exist to modulate orbital angular momentum. Here we report a dynamic bond approach to realize a large change in orbital angular momentum. We have developed a Co(II) complex that exhibits coordination number switching between six and seven. This cooperative dynamic bond switching induces considerable modulation of the ligand field, thereby leading to substantial quenching and restoration of the orbital angular momentum. This switching mechanism is entirely different from those of spin-crossover and valence tautomeric compounds, which exhibit switching in spin multiplicity.

Effect of Axial Ligands on Easy-Axis Anisotropy and Field-Induced Slow Magnetic Relaxation in Heptacoordinated FeII Complexes.

A rational approach to modulating easy-axis magnetic anisotropy by varying the axial donor ligand in heptacoordinated FeII complexes has been explored. In this series of complexes with formulae of [Fe(H4 L)(NCS)2 ]⋅3 DMF⋅0.5 H2 O (1), [Fe(H4 L)(NCSe)2 ]⋅3 DMF⋅0.5 H2 O (2), and [Fe(H4 L)(NCNCN)2 ]⋅DMF⋅H2 O (3) [H4 L=2,2'-{pyridine-2,6-diylbis(ethan-1-yl-1-ylidene)}bis(N-phenylhydrazinecarboxamide)], the axial positions are successively occupied by different nitrogen-based π-donor ligands. Detailed dc and ac magnetic susceptibility measurements reveal the existence of easy-axis magnetic anisotropy for all of the complexes, with 1 [Ueff =21 K, τ0 =1.72×10-6  s] and 2 [Ueff =25 K, τ0 =2.25×10-6  s] showing field-induced slow magnetic relaxation behavior. However, both experimental studies and theoretical calculations indicate the magnitude of the D value of complex 3 to be larger than those of complexes 1 and 2 due to the axial bond angle being smaller than that for an ideal geometry. Detailed analysis of the field and temperature dependences of relaxation time for 1 and 2 has revealed that multiple relaxation processes (quantum tunneling of magnetization, direct, and Raman) are involved in slow magnetic relaxation for both of these complexes. Magnetic dilution experiments support the role of intermolecular short contacts.

Giant Single-Crystal Shape Transformation with Wide Thermal Hysteresis Actuated by Synergistic Motions of Molecular Cations and Anions.

Manipulating the collective molecular movements to implement macroscopic mechanical response of bulk material is attractive and challenging. Here, an organic-inorganic hybrid single crystal is synthesized, which exhibits a giant macroscopic shape transformation with a remarkable thermal hysteretic feature. The colossal anisotropic shape change, which manifests as an abrupt elongation of ca. 9 % along the crystallographic c-axis and a concomitant contraction of ca. 9 % in a perpendicular direction, is induced by a significant reorientation of imidazolium, accompanied with a substantial configurational variation in CuBr4 2- complex anions. The synergistic motions of both the molecular cations and anions engender a remarkable large thermal hysteresis (>30 K) in the shape transformation of the single crystal, implying that this material may play a role in alternating memory media. Furthermore, due to the stable crystal lattice, a single crystal that demonstrates naked-eye detectable large shape transformation was used as a thermal actuator to spontaneously control an electric circuit by temperature variation.

Structural Modulation of Fluorescent Rhodamine-Based Dysprosium(III) Single-Molecule Magnets.

Two rhodamine 6G-based mononuclear dysprosium complexes, [Dy(LR)(LA)2](ClO4)3·Et2O·1.5MeOH·0.5H2O (1) and [Dy(LR)(H2O)4(MeCN)](ClO4)3·2H2O·MeCN (2) (LR = salicylaldehyde rhodamine 6G hydrazone, LA = 2-pyridylcarboxaldehyde benzoyl hydrazone), are synthesized, aiming at improving the magnetic behavior by modulating their coordination environment. Both complexes own one exclusive short Dy-Ophenoxy coordination bond as the predominant bond and exhibit single-molecule magnet behavior under zero dc field with the energy barrier (Ueff/kB) of 90 K (1) and 320 K (2) and apparent hysteresis at 1.9 K. The ab initio calculations indicate that the short Dy-Ophenoxy bond determines the direction of magnetic anisotropic axis for 1 and 2. The quantum tunneling of magnetization (QTM) between the ground Kramers doublets (KDs) in 1 cannot be neglected, leading to an experimental Ueff/kB much lower than the calculated energy of the first excited state (318.2 K). For 2, the stronger magnetic anisotropy and negligible QTM between the ground KDs guarantees that the energy barrier is close to the calculated energy of first KDs (320.8 K). On the other hand, the presence of ring-opened xanthene moiety makes complexes 1 and 2 in the solid state emit red light with emission bands of 645 and 658 nm, respectively.

Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex.

An important technique to realize novel electron- and/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex-a phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

Electron-Transfer Activity in a Cyanide-Bridged Fe42 Nanomagnet.

The ability to switch a molecule between different magnetic states is of considerable importance for the development of new molecular electronic devices. Desirable properties for such applications include a large-spin ground state with an electronic structure that can be controlled via external stimuli. Fe42 is a cyanide-bridged stellated cuboctahedron of mixed-valence Fe ions that exhibits an extraordinarily large S = 45 spin ground state. We have found that the spin ground state of Fe42 can be altered by controlling the humidity and temperature. Dehydration results in a 15 µB reduction of the saturation magnetization that can be partially recovered upon rehydration. The complementary use of UV-vis, IR, L2,3-edge X-ray absorption spectroscopy and X-ray magnetic circular dichroism is applied to uncover the mechanism for the observed dynamic behavior. It is identified that dehydration is concurrent with metal-to-metal electron transfer between Fe pairs via a cyanide π hybridization. Upon dehydration, electron transfer occurs from low-spin {FeII(Tp)(CN)3} sites to high-spin FeIII centers. The observed reduction in magnetization upon dehydration of Fe42 is inconsistent with a ferrimagnetic ground state and is proposed to originate from a change in zero-field splitting at electron-reduced high-spin sites.

Rhodamine 6G-Labeled Pyridyl Aroylhydrazone Fe(II) Complex Exhibiting Synergetic Spin Crossover and Fluorescence.

Here, we use a pyridinecarbaldehyde rhodamine 6G hydrazone ligand (L) to synthesize an Fe(II) complex 1 for the search of new fluorescent-spin crossover (SCO) materials. Single-crystal structural determinations suggest that the Fe(II) ion is chelated by two ring-opened ligands (L-o) to form a FeN4O2 coordination environment, and intermolecular π---π contacts of the xanthene groups connect the adjacent molecules to form a supramolecular one-dimensional chain. Magnetic susceptibility measurements on complex 1 show that three-step SCO takes place in the temperature range of 120-350 K, and its desolvated form 1-d exhibits SCO around room temperature ( Tc↑ = 343 K and Tc↓ = 303 K) with a wide hysteresis loop of 40 K. Moreover, complex 1-d displays light-induced excited spin-state trapping phenomenon. Intriguingly, the fluorescence intensity of the maximum emission at 560 nm for complex 1-d displays discontinuous variation in the range of 250-400 K, indicative of the occurrence of synergetic fluorescence and SCO.