Near Room-Temperature Memory Devices Based on Hybrid Spin-Crossover@SiO2 Nanoparticles Coupled to Single-Layer Graphene Nanoelectrodes

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.

Unlocking room-temperature bistable spin transition at the nanoscale: the synthesis of core@shell [Fe(NH2trz)3(NO3)2]@SiO2 nanoparticles

In this work, we address the synthesis of stable spin-crossover nanoparticles capable of undergoing a hysteretic spin transition at room temperature. For this purpose, we use the reverse-micelle protocol to prepare naked [Fe(NH2trz)3](NO3)2 and core@shell [Fe(NH2trz)3](NO3)2@SiO2 nanoparticles. Through meticulous adjustment of synthetic parameters, we achieved nanoparticle sizes ranging from approximately 40 nm to 60 nm. Our findings highlight that [Fe(NH2trz)3](NO3)2 presents a modest thermal hysteresis of 7 K, which decreases by downsizing. Conversely, silica-coated nanoparticles with sizes of ca. 60 and 40 nm demonstrate a remarkable hysteretic response of approximately 30 K, switching their spin state around room temperature. Moreover, the presence of a SiO2 shell substantially enhances the nanoparticles' stability against oxidation. In this context, the larger 60 nm [Fe(NH2trz)3](NO3)2@SiO2 hybrid remains stable in water for up to two hours, enabling the observation of an unreported water-induced spin transition after 30 min. Therefore, this work also introduces an intriguing avenue for inducing spin transitions through solvent exchange, underscoring the versatility and potential of these nanoparticles.

Bistable Spin-Crossover Nanoparticles for Molecular Electronics

The field of spin-crossover complexes is rapidly evolving from the study of the spin transition phenomenon to its exploitation in molecular electronics. Such spin transition is gradual in a single-molecule, while in bulk it can be abrupt, showing sometimes thermal hysteresis and thus a memory effect. A convenient way to keep this bistability while reducing the size of the spin-crossover material is to process it as nanoparticles (NPs). Here, the most recent advances in the chemical design of these NPs and their integration into electronic devices, paying particular attention to optimizing the switching ratio are reviewed. Then, integrating spin-crossover NPs over 2D materials is focused to improve the endurance, performance, and detection of the spin state in these hybrid devices.

Desolvation-Induced Highly Symmetrical Terbium(III) Single-Molecule Magnet Exhibiting Luminescent Self-Monitoring of Temperature

A conjunction of Single-Molecule Magnet (SMM) behavior and luminescence thermometry is an emerging research line aiming at contactless read-out of temperature in future SMM-based devices. The shared working range between slow magnetic relaxation and the thermometric response is typically narrow or absent. We report TbIII -based emissive SMMs formed in a cyanido-bridged framework whose properties are governed by the reversible structural transformation from [TbIII (H2 O)2 ][CoIII (CN)6 ] ⋅ 2.7H2 O (1) to its dehydrated phase, TbIII [CoIII (CN)6 ] (2). The 8-coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal-prismatic TbIII complexes in 2, showing the SMM features up to 42 K. They are governed by the combination of QTM, Raman, and Orbach relaxation with the energy barrier of 594(18) cm-1 (854(26) K), one of the highest among the TbIII -based molecular nanomagnets. Both systems exhibit emission related to the f-f electronic transitions, with the temperature variations resulting in the optical thermometry below 100 K. The dehydration leads to a wide temperature overlap between the SMM behavior and thermometry, from 6 K to 42 K. These functionalities are further enriched after the magnetic dilution. The role of post-synthetic formation of high-symmetry TbIII complexes in achieving the SMM effect and hot-bands-based optical thermometry is discussed.

Dynamical Behavior of Pure Spin Current in Organic Materials

Growing concentration on the novel information processing technology and low-cost, flexible materials make the spintronics and organic materials appealing for the future interdisciplinary investigations. Organic spintronics, in this context, has arisen and witnessed great advances during the past two decades owing to the continuous innovative exploitation of the charge-contained spin polarized current. Albeit with such inspiring facts, charge-absent spin angular momentum flow, namely pure spin currents (PSCs) are less probed in organic functional solids. In this review, the past exploring journey of PSC phenomenon in organic materials are retrospected, including non-magnetic semiconductors and molecular magnets. Starting with the basic concepts and the generation mechanism for PSC, the representative experimental observations of PSC in the organic-based networks are subsequently demonstrated and summarized, by accompanying explicit discussion over the propagating mechanism of net spin itself in the organic media. Finally, future perspectives on PSC in organic materials are illustrated mainly from the material point of view, including single molecule magnets, complexes for the organic ligands framework as well as the lanthanide metal complexes, organic radicals, and the emerging 2D organic magnets.

Electro-Elastic Modeling of Thermal Spin Transition in Diluted Spin-Crossover Single Crystals

Spin-crossover solids have been studied for many years for their promising applications as optical switches and reversible high-density memories for information storage. This study reports the effect of random metal dilution on the thermal and structural properties of a spin-crossover single crystal. The analysis is performed on a 2D rectangular lattice using an electro-elastic model. The lattice is made of sites that can switch thermally between the low-spin and high-spin states, accompanied by local volume changes. The model is solved by Monte Carlo simulations, running on the spin states and atomic positions of this compressible 2D lattice. A detailed analysis of metal dilution on the magneto-structural properties allows us to address the following issues: (i) at low dilution rates, the transition is of the first order; (ii) increasing the concentration of dopant results in a decrease in cooperativity and leads to gradual transformations above a threshold concentration, while incomplete spin transitions are obtained for big dopant sizes. The effects of the metal dilution on the spatiotemporal aspects of the spin transition along the thermal transition and on the low-temperature relaxation of the photo-induced metastable high-spin states are also studied. Significant changes in the organization of the spin states are observed for the thermal transition, where the single-domain nucleation caused by the long-range elastic interactions is replaced by a multi-droplet nucleation. As to the issue of the relaxation curves: their shape transforms from a sigmoidal shape, characteristic of strong cooperative systems, into stretched exponentials for high dilution rates, which is the signature of a disordered system.

Electrical Sensing of the Thermal and Light-Induced Spin Transition in Robust Contactless Spin-Crossover/Graphene Hybrid Devices

Hybrid devices based on spin-crossover (SCO)/2D heterostructures grant a highly sensitive platform to detect the spin transition in the molecular SCO component and tune the properties of the 2D material. However, the fragility of the SCO materials upon thermal treatment, light irradiation, or contact with surfaces and the methodologies used for their processing have limited their applicability. Here, an easily processable and robust SCO/2D hybrid device with outstanding performance based on the sublimable SCO [Fe(Pyrz)₂ ] molecule deposited over chemical vapor deposition (CVD) graphene is reported, which is fully compatible with electronics industry protocols. Thus, a novel methodology based on growing an elusive polymorph of [Fe(Pyrz)₂ ] (tetragonal phase) over graphene is developed that allows a fast and effective light-induced spin transition in the devices (≈50% yield in 5 min) to be detected electrically. Such performance can be enhanced even more when a flexible polymeric layer of poly(methyl methacrylate) is inserted in between the two active components in a contactless configuration, reaching a ≈100% yield in 5 min.

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.

Proposal for All-Electrical Spin Manipulation and Detection for a Single Molecule on Boron-Substituted Graphene

All-electrical writing and reading of spin states attract considerable attention for their promising applications in energy-efficient spintronics devices. Here we show, based on rigorous first-principles calculations, that the spin properties can be manipulated and detected in molecular spinterfaces, where an iron tetraphenyl porphyrin (FeTPP) molecule is deposited on boron-substituted graphene (BG). Notably, a reversible spin switching between the S=1 and S=3/2 states is achieved by a gate electrode. We can trace the origin to a strong hybridization between the Fe-d_{z^{2}} and B-p_{z} orbitals. Combining density functional theory with nonequilibrium Green's function formalism, we propose an experimentally feasible three-terminal setup to probe the spin state. Furthermore, we show how the in-plane quantum transport for the BG, which is non-spin polarized, can be modified by FeTPP, yielding a significant transport spin polarization near the Fermi energy (>10% for typical coverage). Our work paves the way to realize all-electrical spintronics devices using molecular spinterfaces.

Enlightening the Alkali Ion Role in the Photomagnetic Effect of FeCo Prussian Blue Analogues

FeCo Prussian blue analogues of general formula A_xCo_y[Fe(CN)₆]_z are responsive, non-stoichiometric materials whose magnetic and optical properties can be reversibly switched by light irradiation. However, elucidating the critical influence of the inserted alkali ion, A⁺, on the material's properties remains complicated due to their complex local structure. Here, by investigating soluble A ⊂ [Fe₄-Co₄] cyanido cubes (A = K, Rb, and Cs), both accurate structural and electronic information could be obtained. First, X-ray diffraction analyses reveal distinct interactions between the inserted A⁺ ions and the {Fe₄-Co₄} box, which impacts the structural distortion in the cubic framework. These distortions vanish, and a displacement of the small K⁺ ion from a corner toward the center is observed, as a cobalt corner Co^II_HS is oxidized to Co^III_LS. Second, cyclic voltammetry experiments performed at variable temperatures show distinct splitting of the Co^II_HS ⇔ Co^III_LS peak potentials for the different A⁺ cations, which can be qualitatively linked to different thermodynamic (standard potentials) and kinetic (energy barriers) parameters associated with the structural reorganization accompanying this redox-coupled spin state change. Moreover, for the first time, photomagnetism was investigated in frozen solution to avoid effects of intermolecular interactions. The results show that the metastable state is stabilized following the trend K > Rb > Cs. The outcome of these studies suggests that the interaction of the inserted alkali ions with the cyanide cage and the structural changes accompanying the electron transfer impact the stability of the photoinduced state and the relaxation temperature: the smaller the cation, the higher the structural reorganization and the associated energy barrier, and the more stable the metastable state.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

Spin-crossover nanoparticles anchored on MoS2 layers for heterostructures with tunable strain driven by thermal or light-induced spin switching

In the past few years, the effect of strain on the optical and electronic properties of MoS₂ layers has attracted particular attention as it can improve the performance of optoelectronic and spintronic devices. Although several approaches have been explored, strain is typically externally applied on the two-dimensional material. In this work, we describe the preparation of a reversible 'self-strainable' system in which the strain is generated at the molecular level by one component of a MoS₂-based composite material. Spin-crossover nanoparticles were covalently grafted onto functionalized layers of semiconducting MoS₂ to form a hybrid heterostructure. Their ability to switch between two spin states on applying an external stimulus (light irradiation or temperature change) serves to generate strain over the MoS₂ layer. A volume change accompanies this spin crossover, and the created strain induces a substantial and reversible change of the electrical and optical properties of the heterostructure.

Plasmon-assisted spin transition in gold nanostar@spin crossover heterostructures

Herein we report the design of core@shell nanoparticles formed by a metallic Au nanostar core and a spin-crossover shell based on the coordination polymer [Fe(Htrz)₂(trz)](BF₄). This procedure is general and has been extended to other metallic morphologies (nanorods, nanotriangles). Thanks to the photothermal effect arising from the plasmonic properties of the Au nanostar, 60% of iron centers undergo a thermal spin transition inside the thermal hysteresis triggered by a 808 nm laser low intensity irradiation. Compared to other Au morphologies, the great advantage of the nanostar shape arises from the hot spots created at the branches of the nanostar. These hot spots give rise to large NIR absorptions, making them ideal nanostructures for efficiently converting light into heat using low energy light, like that provided by a 808 nm laser.

Thermal Bistability of an Ultrathin Film of Iron(II) Spin-Crossover Molecules Directly Adsorbed on a Metal Surface

Spin-crossover molecules are very attractive compounds to realize multifunctional spintronic devices. Understanding their properties when deposited on metals is therefore crucial for their future rational implementation as ultrathin films in such devices. Using X-ray absorption spectroscopy, we study the thermal transition of the spin-crossover compound Fe^II((3,5-(CH₃)₂Pz)₃BH)₂ from submonolayer to multilayers on a Cu(111) substrate. We determine how the residual fraction of high spin molecules at low temperature, as well as the bistability range and the temperature of switching, depends on the layer thickness. The most spectacular effect is the clear opening of a 35 ± 9 K thermal hysteresis loop for a 3.0 ± 0.7 monolayers thick film. To better understand the role played by the substrate and the dimensionality on the thermal bistability, we have performed Monte Carlo Arrhenius simulations in the framework of a mechanoelastic model that include a molecule-substrate interaction. This model reproduces well the main features observed experimentally and can predict how the spin-crossover transition is modified by the thickness and the substrate interaction.

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.

Graphene nanogaps for the directed assembly of single-nanoparticle devices

Significant advances in the synthesis of low-dimensional materials with unique and tuneable electrical, optical and magnetic properties has led to an explosion of possibilities for realising hybrid nanomaterial devices with unconventional and desirable characteristics. However, the lack of ability to precisely integrate individual nanoparticles into devices at scale limits their technological application. Here, we report on a graphene nanogap based platform which employs the large electric fields generated around the point-like, atomically sharp nanogap electrodes to capture single nanoparticles from solution at predefined locations. We demonstrate how gold nanoparticles can be trapped and contacted to form single-electron transistors with a large coupling to a buried electrostatic gate. This platform offers a route to the creation of novel low-dimensional devices, nano- and optoelectronic applications, and the study of fundamental transport phenomena.

A Reversible Hydrogen-Bond Isomerization Triggered by an Abrupt Spin Crossover near Room Temperature

The spin crossover salt [Fe(bpp)₂ ](isonicNO)₂ ⋅ 2.4 H₂ O (1⋅2.4 H₂ O) (bpp=2,6-bis(pyrazol-3-yl)pyridine; isonicNO=isonicotinate N-oxide anion) exhibits a very abrupt spin crossover at T_1/2 =274.4 K. This triggers a supramolecular linkage (H-bond) isomerization that responds reversibly towards light irradiation or temperature change. Isotopic effects in the thermomagnetic behavior reveal the importance of hydrogen bonds in defining the magnetic state. Further, the title compound can be reversibly dehydrated to afford 1, a material that also exhibits spin crossover coupled to H-bond isomerization, leading to strong kinetic effects in the thermomagnetic properties.

Bifunctional Fe(II) spin crossover-complexes based on ω-(1H-tetrazol-1-yl) carboxylic acids

To increase the supramolecular cooperativity in Fe(ii) spin crossover materials based on N1-substituted tetrazoles, a series of ω-(1H-tetrazol-1-yl) carboxylic acids with chain-lengths of C2-C4 were synthesized. Structural characterization confirmed the formation of a strong hydrogen-bond network, responsible for enhanced cooperativity in the materials and thus largely complete spin-state transitions for the ligands with chain lenghts of C2 and C4. To complement the structural and magnetic investigation, electronic spectroscopy was used to investigate the spin-state transition. An initial attempt to utilize the bifunctional coordination ability of the ω-(1H-tetrazol-1-yl) carboxylic acids for preparation of mixed-metallic 3d-4f coordination polymers resulted in a novel one-dimensional gadolinium-oxo chain system with the ω-(1H-tetrazol-1-yl) carboxylic acid acting as μ2-η2:η1 chelating-bridging ligand.

Octacyanidometallates for multifunctional molecule-based materials

Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)8]n- ions comprise a series of complexes of heavy transition metals in high oxidation states, including NbIV, MoIV/V, WIV/V, and ReV. Since the discovery of the pioneering bimetallic {MnII4[MIV(CN)8]2} and {MnII9[MV(CN)8]6} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d-d or d-f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)8]n--based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)8]n- ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

Ligand substitution effects on the charge transport properties of the spin crossover complex [Fe(Htrz)1+y-x (trz)2-y (NH2trz) x ](BF4)y·nH2O

The complex dielectric permittivity of a series of spin crossover complexes, with variable ligand stoichiometry [Fe(Htrz)_1+y-x (trz)_2-y (NH₂trz) _x ](BF₄) _y ·nH₂O, has been investigated as a function of temperature in a wide frequency range. In each compound, a substantial drop of the conductivity and permittivity is evidenced when going from the low spin to the high spin state, albeit with decreasing amplitude for increasing ligand substitution (i.e. for increasing x). The deconvolution of the dielectric spectra using the Havriliak-Negami equation allowed to extract the dipole and conductivity relaxation times, their distributions as well as the dielectric strengths in both spin states. Remarkably, no clear correlation appears between the conductivity changes and the lattice properties (Debye temperature) in the dilution series. We rationalize these results by considering the dimensionality of the system (1D), wherein the charge transport occurs most likely by hopping along the [Fe(Rtrz)₃] _n ⁿ⁺ chains.

Magneto-elastic properties of a spin crossover membrane deposited on a deformable substrate

Spin-crossover (SCO) solids have been studied for several years due to their fascinating physical properties and their potential applications as optical switches and reversible high-density memories for information storage. Through this article, we will examine in details the effects of substrate's lattice parameters, on a deformable spin crossover membrane, simulated using an electro-elastic model taking into account the volume change at the transition. The molecules of the membrane can be either in the low spin state (LS) or the high spin state (HS), while those of the substrate are electronically neutral. Magnetic properties of the SCO membrane and the pressure distribution as a function of the lattice parameter of the substrate have been investigated. We demonstrated that the thermally induced first-order spin transition is significantly affected by the structural properties of the substrate, where a rise in the lattice parameter of the latter lowers the transition temperature and reduces the width of the thermal hysteresis loop. The investigations on the spatiotemporal aspects of the spin transition in the membrane demonstrates that the nucleation and growth processes are sensitive to the structural properties of the elastic misfit between the substrate and the SCO membrane.

Resistance switching in large-area vertical junctions of the molecular spin crossover complex [Fe(HB(tz)3)2]: ON/OFF ratios and device stability

Multilayer crossbar junctions composed of ITO/[Fe(HB(1,2,4-triazol-1-yl)₃)₂]/M (with M  =  Al or Ca) were fabricated and investigated for their resistance switching properties. Current-voltage-temperature maps revealed ON/OFF resistance ratios as high as 400, with the ON and OFF states defined, respectively, as the low-resistance, low spin state and the high-resistance, high spin state of the spin crossover layer. Similar results were obtained with Al and Ca cathodes indicating that the charge transport in the insulating spin crossover film is at the origin of the resistance switching instead of electron injection at the electrodes. The reproducibility and stability of the device properties were also studied.

Spin crossover in Fe(triazole)-Pt nanoparticle self-assembly structured at the sub-5 nm scale

A main goal of molecular electronics is to relate the performance of devices to the structure and electronic state of molecules. Among the variety of possibilities that organic, organometallic and coordination chemistries offer to tune the energy levels of molecular components, spin crossover phenomenon is a perfect candidate for elaboration of molecular switches. The reorganization of the electronic state population of the molecules associated to the spin crossover can indeed lead to a significant change in conductivity. However, molecular spin crossover is very sensitive to the environment and can disappear once the molecules are integrated into devices. Here, we show that the association of ultra-small 1.2 nm platinum nanoparticles with Fe^II triazole-based spin crossover coordination polymers leads to self-assemblies, extremely well organized at the sub-3 nm scale. The quasi-perfect alignment of nanoparticles observed by transmission electron microscopy, in addition to specific signature in infrared spectroscopy, demonstrates the coordination of the long-chain molecules with the nanoparticles. Spin crossover is confirmed in such assemblies by X-ray absorption spectroscopic measurements and shows unambiguous characteristics both in magnetic and charge transport measurements. Coordinating polymers are therefore ideal candidates for the elaboration of robust, well-organized, hybrid self-assemblies with metallic nanoparticles, while maintaining sensitive functional properties, such as spin crossover.

Electrical conductivity and magnetic bistability in metal–organic frameworks and coordination polymers: charge transport and spin crossover at the nanoscale

This review aims to reassess the progress, issues and opportunities in the path towards integrating conductive and magnetically bistable coordination polymers and metal–organic frameworks as active components in electronic devices.

Broad-Band Dielectric Spectroscopy Reveals Peak Values of Conductivity and Permittivity Switching upon Spin Crossover

We use broad-band dielectric spectroscopy to investigate the spin-state dependence of electrical properties of the [Fe(Htrz)₂(trz)](BF₄) spin crossover complex. We show that the Havriliak-Negami theory can fully describe the variation of the complex dielectric permittivity of the material across the pressure-temperature phase diagram. The analysis reveals three dielectric relaxation processes, which we attribute to electrode/interface polarization, dipole relaxation, and charge transport relaxation. The contribution of the latter appears significant to the dielectric strength. Remarkably, the permittivity and conductivity changes between the high spin and low spin states are amplified at the corresponding relaxation frequencies.

Downsizing of robust Fe-triazole@SiO2 spin-crossover nanoparticles with ultrathin shells

A chemical protocol to design robust hybrid [Fe(Htrz)₂(trz)](BF₄)@SiO₂ nanoparticles (NPs) with sizes as small as 28 nm and ultrathin silica shells below 3 nm has been developed. These NPs present a characteristic abrupt spin transition with a subsequent decrease in the width of the thermal hysteresis upon reducing the NP size.

Selective Isomer Formation and Crystallization-Directed Magnetic Behavior in Nitrogen-Confused C-Scorpionate Complexes of Fe(O3SCF3)2

The complex [Fe(^HL*)₂](OTf)₂, 1, where ^HL* = bis(3,5-dimethylpyrazol-1-yl)(3-1H-pyrazole)methane, was prepared in order to compare its magnetic properties with those of the analogous parent complex, [Fe(^HL)₂](OTf)₂, that lacks methyl groups on pyrazolyl rings and that undergoes spin crossover (SCO) from the low spin (LS) to the high spin (HS) form above room temperature. It was anticipated that this new semibulky derivative should favor the HS state and undergo SCO at a lower temperature range. During this study, six crystalline forms of 1 were prepared by controlling the crystallization conditions. Thus, when reagents are combined in CH₃CN, an equilibrium mixture of cis and trans isomers is established that favors the latter below 311 K. The trans isomer can be isolated exclusively as a mixture of solvates, LS trans-1·2CH₃CN and HS trans-1·4CH₃CN, by cooling CH₃CN solutions to -20 °C with the former being favored at high concentrations and short crystallization times. Subsequently, vapor diffusion of Et₂O into CH₃CN solutions of pure trans-1·2CH₃CN gives solvate-free HS trans-1. Subjecting trans-1·2CH₃CN to vacuum at room temperature gives microcrystalline trans-1·CH₃CN, identified by elemental analysis and its distinct powder X-ray diffraction pattern. If an isomeric mixture of 1 is subject to room-temperature vapor diffusion, then a crystalline mixture of HS isomers cis-1 and trans-1 is obtained. Finally, slowly cooling hot acetonitrile solutions of isomeric mixtures of 1 to room temperature gives large prisms of HS co-1, a species with both cis and trans isomers in the unit cell. The complexes trans-1, trans-1·CH₃CN, cis-1, and co-1 undergo SCO below 250 K while trans-1·xCH₃CN (x = 2, 4) solvates do not undergo SCO before desolvation.

Design of Bistable Gold@Spin-Crossover Core-Shell Nanoparticles Showing Large Electrical Responses for the Spin Switching

A simple chemical protocol to prepare core-shell gold@spin-crossover (Au@SCO) nanoparticles (NPs) based on the 1D spin-crossover [Fe(Htrz)₂ (trz)](BF₄ ) coordination polymer is reported. The synthesis relies on a two-step approach consisting of a partial surface ligand substitution of the citrate-stabilized Au NPs followed by the controlled growth of a very thin layer of the SCO polymer. As a result, colloidally stable core@shell spherical NPs with a Au core of ca. 12 nm and a thin SCO shell 4 nm thick, are obtained, exhibiting a narrow distribution in sizes. Differential scanning calorimetry proves that a cooperative spin transition in the range 340-360 K is maintained in these Au@SCO NPs, in full agreement with the values reported for pristine 4 nm SCO NPs. Temperature-dependent charge-transport measurements of an electrical device based on assemblies of these Au@SCO NPs also support this spin transition. Thus, a large change in conductance upon spin state switching, as compared with other memory devices based on the pristine SCO NPs, is detected. This results in a large improvement in the sensitivity of the device to the spin transition, with values for the ON/OFF ratio which are an order of magnitude better than the best ones obtained in previous SCO devices.

Facile preparation of hybrid thin films composed of spin-crossover nanoparticles and carbon nanotubes for electrical memory devices

In this study, organic solvent-dispersible nanoparticles of an FeII-1,2,4-triazole spin-crossover complex were synthesized. On mixing the suspension of the spin-crossover nanoparticles with a solution of single-walled carbon nanotubes (SWCNTs), the nanoparticles were strongly adsorbed on the hydrophobic SWCNT bundles, resulting in hybrid network structures. Variable temperature DC electrical conductivity measurements of the hybrid network thin films demonstrated that the conductivities of the composite films were switched by the spin transition of the nanoparticles.

In Situ Ligand Transformation for Two-Step Spin Crossover in FeII[MIV(CN)8]4- (M = Mo, Nb) Cyanido-Bridged Frameworks

We report a unique synthetic route toward the multistep spin crossover (SCO) effect induced by utilizing the partial ligand transformation during the crystallization process, which leads to the incorporation of three different Fe^II complexes into a single coordination framework. The 3-acetoxypyridine (3-OAcpy) molecules were introduced to the self-assembled Fe^II-[M^IV(CN)₈]^4- (M = Mo, Nb) system in the aqueous solution which results in the partial hydrolysis of the ligand into 3-hydroxypyridine (3-OHpy). It gives two novel isostructural three-dimensional {Fe^II₂(3-OAcpy)₅(3-OHpy)₃[M^IV(CN)₈]}· nH₂O (M = Mo, n = 0, FeMo; M = Nb, n = 1, FeNb) coordination frameworks. They exhibit an unprecedented cyanido-bridged skeleton composed of {Fe₃M₂} _n coordination nanotubes bonded by additional Fe complexes. These frameworks contain three types of Fe sites differing in the attached organic ligands, [Fe1(3-OAcpy)₄(μ-NC)₂], [Fe2(3-OHpy)₄(μ-NC)₂], and [Fe3(3-OAcpy)₃(3-OHpy)(μ-NC)₂], which lead to the thermal two-step Fe^II SCO, as proven by X-ray diffraction, magnetic susceptibility, UV-vis-NIR optical absorption, and ⁵⁷Fe Mössbauer spectroscopy studies. The first step of SCO, going from room temperature to the 150-170 K range with transition temperatures of 245(5) and 283(5) K for FeMo and FeNb, respectively, is related to Fe1 sites, while the second step, occurring at the 50-140 K range with transition temperatures of 70(5) and 80(5) K for FeMo and FeNb, respectively, is related to Fe2 sites. The Fe3 site with both 3-OAcpy and 3-OHpy ligands does not undergo the SCO at all. The observed two-step SCO phenomenon is explained by the differences in the ligand field strength of the Fe complexes and the role of their alignment in the coordination framework. The simultaneous application of two related pyridine derivatives is the efficient synthetic route for the multistep Fe^II SCO in the cyanido-bridged framework which is a promising step toward rational design of advanced spin transition molecular switches.

Pressure Effects with Incorporated Particle Size Dependency in Graphene Oxide Layers through Observing Spin Crossover Temperature

This research highlights the pressure effects with the particle size dependency incorporated in two-dimensional graphene oxide (GO)/reduced graphene oxide (rGO). GO and rGO composites employing nanorods (NRs) of type [Fe(Htrz)2(trz)](BF4) have been prepared, and their pressure effects in the interlayer spaces through observing the changes of the spin crossover (SCO) temperature (T1/2) have been discussed. The composites show the decrease of interlayer spaces from 8.7 Å to 3.5 Å that is associated with GO to rGO transformation. The shorter interlayer spaces were induced by pressure effects, resulting in the increment of T1/2 from 357 K to 364 K. The pressure effects in the interlayers spaces estimated from the T1/2 value correspond to 24 MPa in pristine [Fe(Htrz)2(trz)](BF4) NRs under hydrostatic pressure. The pressure observed in the composites incorporating NRs (30 × 200 nm) is smaller than that observed in the composite incorporating nanoparticles (NPs) (30 nm). These results clearly demonstrated that the incorporated particle size and shape influenced the pressure effects between the GO/rGO layer.

Effect of nanostructuration on the spin crossover transition in crystalline ultrathin films

Film thickness and microstructure critically affect the spin crossover transition of a 2D coordination polymer.

Mastering the nanostructuration of molecular materials onto solid surfaces and understanding how this process affects their properties are of utmost importance for their integration into solid-state electronic devices. This is even more important for spin crossover (SCO) systems, in which the spin transition is extremely sensitive to size reduction effects. These bi-stable materials have great potential for the development of nanotechnological applications provided their intrinsic properties can be successfully implemented in nanometric films, amenable to the fabrication of functional nanodevices. Here we report the fabrication of crystalline ultrathin films (<1–43 nm) of two-dimensional Hofmann-type coordination polymers by using an improved layer-by-layer strategy and a close examination of their SCO properties at the nanoscale. X-ray absorption spectroscopy data in combination with extensive atomic force microscopy analysis reveal critical dependence of the SCO transition on the number of layers and the microstructure of the films. This originates from the formation of segregated nanocrystals in early stages of the growth process that coalesce into a continuous film with an increasing number of growth cycles for an overall behaviour reminiscent of the bulk. As a result, the completeness of the high spin/low spin transition is dramatically hindered for films of less than 15 layers revealing serious limitations to the ultimate thickness that might be representative of the performance of the bulk when processing SCO materials as ultrathin films. This unprecedented exploration of the particularities of the growth of SCO thin films at the nanoscale should encourage researchers to put a spotlight on these issues when contemplating their integration into devices.

Substituent effects on the fluorescent spin-crossover Fe(ii) complexes of rhodamine 6G hydrazones

The magnetic properties of Fe(ii) pyridine-2-carbaldehyde rhodamine 6G hydrazone complexes are modulated by the substituents. The desolvated complex displays the correlation between the spin crossover and the fluorescence.

A heterotrimetallic synthetic approach in versatile functionalization of nanosized {MxCu13–xW7}3+ and {M1Cu8W6} (M = Co, Ni, Mn, Fe) metal–cyanide magnetic clusters

The addition of 3d metal heteroatom into Cu^II(Me₃tacn)[W(CN)₈]ⁿ⁻ cluster system governs its structural and magnetic features.

An electro-elastic theory for the mechanically-assisted photo-induced spin transition in core-shell spin-crossover nanoparticles

The development of heterostructure materials may lead to new features that cannot be obtained with natural materials. Here we simulate a model structurally hybrid core-shell nanoparticle with different lattice parameters between an electronically inert shell and an active spin crossover core. The nanoparticle consists of a 2D core with 20 × 20 size with square symmetry, surrounded by a shell made of 10 atomic layers. The low temperature photoexcitation of the core shows a significant environment-dependent behavior. In particular, we demonstrate that a shell with a large lattice parameter accelerates the low-spin to high-spin photoexcitation process of the core through the single domain nucleation mechanism while a moderate shell lattice parameter leads to spatially-homogeneous growth of the high-spin fraction. We found that the mechanical retro-action of the shell may cause elastic instability of the core leading to efficient control and manipulation of its photo-conversion.

Charge Mobility and Dynamics in Spin-Crossover Nanoparticles Studied by Time-Resolved Microwave Conductivity

We use the electrodeless time-resolved microwave conductivity (TRMC) technique to characterize spin-crossover (SCO) nanoparticles. We show that TRMC is a simple and accurate means for simultaneously assessing the magnetic state of SCO compounds and charge transport information on the nanometer length scale. In the low-spin state from liquid nitrogen temperature up to 360 K the TRMC measurements present two well-defined regimes in the mobility and in the half-life times, in which the former transition temperature T_R occurs near 225 K. Below T_R, we propose that an activationless regime taking place associated with short lifetimes of the charge carriers points at the presence of shallow-trap states. Above T_R, these states are thermally released, yielding a thermally activated hopping regime where longer hops increase the mobility and, concomitantly, the barrier energy. The activation energy could originate not only from intricate contributions such as polaronic self-localizations but also from dynamic disorder due to phonons and/or thermal fluctuations of SCO moieties.

Evolution of cooperativity in the spin transition of an iron(II) complex on a graphite surface

Cooperative effects determine the spin-state bistability of spin-crossover molecules (SCMs). Herein, the ultimate scale limit at which cooperative spin switching becomes effective is investigated in a complex [Fe(H2B(pz)2)2(bipy)] deposited on a highly oriented pyrolytic graphite surface, using x-ray absorption spectroscopy. This system exhibits a complete thermal- and light-induced spin transition at thicknesses ranging from submonolayers to multilayers. On increasing the coverage from 0.35(4) to 10(1) monolayers, the width of the temperature-induced spin transition curve narrows significantly, evidencing the buildup of cooperative effects. While the molecules at the submonolayers exhibit an apparent anticooperative behavior, the multilayers starting from a double-layer exhibit a distinctly cooperative spin switching, with a free-molecule-like behavior indicated at around a monolayer. These observations will serve as useful guidelines in designing SCM-based devices.