Spin-crossover nanoparticles and nanocomposite materials

Spin crossover {[Fe(atrz)3](OTs)2}n monolith: a green synthesis approach for Robust switchable materials

This work presents a straightforward, room-temperature synthesis of a robust {[Fe(atrz)3](OTs)2}n monolith. This approach offers a green alternative to traditional nanoparticle synthesis for manipulating spin crossover (SCO) behaviour. The monolith exhibits a more gradual SCO transition at lower temperatures compared to the bulk material, aligning with observations in smaller particle systems. Notably, the synthesis employs a solvent- and surfactant-free approach, simplifying the process and potentially reducing environmental impact, aligning with the principles of green chemistry.

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.

Room temperature spin crossover properties in a series of mixed-anion Fe(NH2trz)3(BF4)2-x(SiF6)x/2 complexes

A series of mixed-anion Fe(NH2trz)3(BF4)2-x(SiF6)x/2 spin crossover complexes is obtained modifying the reaction time but also using an increase amount of tetraethyl orthosilicate as the source for the production and the incorporation of SiF62- competing with the BF42- anions present in the mother solution. The increase of the SiF62- anion inclusion to the detriment of the BF4- counterpart induces a shift of the temperature transition toward high temperatures leading to interesting bistability properties around room temperature with T1/2 spanning from 300 K to 325 K. Moreover, the implementation of a solid-liquid post synthetic modification approach from the Fe(NH2trz)3(BF4)2 parent complex with identical TEOS proportions and under certain experimental conditions lead systematically to the same Fe(NH2trz)3(BF4)1.2(SiF6)0.4 composition. This compound presents an abrupt spin crossover behaviour with a narrow hysteresis loop just above room temperature (320 K), which is stable under thermal cycling and along time with no specific storage conditions. Such crystalline powder sample incorporates homogeneous rod-shaped particles whose formation and physical properties can be followed simultaneously using infra-red spectroscopy, dynamic light scattering (DLS), transmission electronic microscopy (TEM) and optical reflectance. The observation of a stabilized single ca. 800 nm population of mixed-anion particles starting from insoluble various sizes (from nano- to microscale) Fe(NH2trz)3(BF4)2 particles supports the key role of the solvent (water molecules) on the separation, the reactivity and the reorganization of the 1D iron-triazole chains forming the packing of the structure.

Effects of the surface energy and surface stress on the phase stability of spin crossover nano-objects: a thermodynamic approach

Size-induced phase transformation at the nanoscale is a common phenomenon whose understanding is essential for potential applications. Here we investigate phase equilibria in thin films and nanoparticles of molecular spin crossover (SCO) materials. To calculate the size-temperature phase diagrams we have developed a new nano-thermodynamic core-shell model in which intermolecular interactions are described through the volume misfit between molecules of different spin states, while the contributions of surface energy and surface stress are explicitly included. Based on this model, we rationalize the emergence of previously-reported incomplete spin transitions and the shift of the transition temperature in finite size objects due to their large surface-to-volume ratio. The results reveal a competition between the elastic intermolecular interaction and the internal pressure induced by the surface stress. The predicted transition temperature of thin films of the SCO compound [Fe(pyrazine)][Ni(CN)4] follows a clear reciprocal relationship with respect to the film thickness and the transition behavior matches the available experimental data. Importantly, all input parameters of the present model are experimentally accessible physical quantities, thus providing a simple, yet powerful tool to analyze SCO properties in nano-scale objects.

Laser-Driven Transient Phase Oscillations in Individual Spin Crossover Particles

An unusual expansion dynamics of individual spin crossover nanoparticles is studied by ultrafast transmission electron microscopy. After exposure to nanosecond laser pulses, the particles exhibit considerable length oscillations during and after their expansion. The vibration period of 50-100 ns is of the same order of magnitude as the time that the particles need for a transition from the low-spin to the high-spin state. The observations are explained in Monte Carlo calculations using a model where elastic and thermal coupling between the molecules within a crystalline spin crossover particle govern the phase transition between the two spin states. The experimentally observed length oscillations are in agreement with the calculations, and it is shown that the system undergoes repeated transitions between the two spin states until relaxation in the high-spin state occurs due to energy dissipation. Spin crossover particles are therefore a unique system where a resonant transition between two phases occurs in a phase transformation of first order.

Observation of Two-Step Spin Transition in Graphene Oxide-Based Hybrids with Iron(II) 4-amino-1,2,4-triazole Spin Crossover Nanoparticles

A novel experimental protocol based on a reverse micellar method is presented for the synthesis of graphene oxide (GO)-based hybrids with spin crossover nanoparticles (SCO NPs) of the 1D iron(II) coordination polymer with the formula [Fe(NH2trz)3](Br2). By introducing different quantities of 0.5% and 1.0% of GO (according to iron(II)) into the aqueous phase, two hybrids, NP4 and NP5, were synthesized, respectively. The morphological homogeneity of the NPs on the surface of the GO flakes is greatly improved in comparison to the pristine [Fe(NH2trz)3](Br2) NPs. From the magnetic point of view and at a low magnetic sweep rate of 1 K/min, a two-step hysteretic behavior is observed for NP4 and NP5, where the onset of the low-temperature second step appeared at 40% and 30% of the HS fraction, respectively. For faster sweep rates of 5-10 K/min, the two steps from the cooling branch are progressively smeared out, and the critical temperatures observed are T1/2↑ = 343 K and T1/2↓ = 288 K, with a thermal width of 55 K for both NP4 and NP5. A Raman laser power-assisted protocol was used to monitor the thermal tolerance of the hybrids, while XPS analysis revealed electronic interactions between the SCO NPs and the GO flakes.

Spin crossover in mixed-anion Fe(NH2trz)3(BF4)(SiF6)0.5 crystalline rod-shaped particles: the strength of the solid-liquid post synthetic modification

A pure mixed-anion Fe(NH₂trz)₃(BF₄)(SiF₆)_0.5 spin crossover complex is obtained implementing a solid-liquid post synthetic modification approach from the Fe(NH₂trz)₃(BF₄)₂ parent complex. This method allows obtaining highly crystalline powder samples incorporating homogeneous micrometric (1 μm long) rod-shaped particles. This compound presents an abrupt spin crossover behaviour with a narrow (10 K) hysteresis loop centred just above room temperature (320 K) which makes it very interesting for future integration into devices for various applications.

Nanocomposites Based on Spin-Crossover Nanoparticles and Silica-Coated Gold Nanorods: A Nonlinear Optical Study

A nanocomposite based on silica-coated AuNRs with the aminated silica-covered spin-crossover nanoparticles (SCO NPs) of the 1D iron(II) coordination polymer with the formula [Fe(Htrz)₂(trz)](BF₄) is presented. For the synthesis of the SCO NPs, the reverse micelle method was used, while the gold nanorods (AuNRs) were prepared with the aspect ratio AR = 6.0 using the seeded-growth method and a binary surfactant mixture composed of cetyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). The final nanocomposite was prepared using the heteroaggregation method of combining different amounts of SCO NPs with the AuNRs. The nonlinear optical (NLO) properties of the hybrid AuNRs coated with different amounts of SCO NPs were studied in detail by means of the Z-scan technique, revealing that the third-order NLO properties of the AuNRs@SCO are dependent on the amount of SCO NPs grafted onto them. However, due to the resonant nature of the excitation, SCO-induced NLO switching was not observed.

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

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

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

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

Spontaneous Synthesis of [FeII (Atrz)3 ]SO4 and its Analogues Through Accelerated Ageing: New Insights from Small-Scale Reactions

Accelerated ageing reactions that take place between two solid materials on contact in the absence of added solvent have been used to synthesize two spin-crossover-active 1D coordination polymers and one of their Cu(II) analogues. The hygroscopy of the ligands and the relative humidity of the reaction chamber have been shown to be particularly important factors in the rate of reaction. Small-scale reactions between a few individual crystals have allowed observation of deliquescence of the 4-aminotriazole ligand at high humidity. The metal salt does not dissolve, and the ligand diffuses into the crystal of the metal salt during the reaction. In the case of the Cu analogue, the formation of the product causes the crystal of the metal salt to deform with the formation of pseudocrystals, which have a fibrous structure.

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.

Thermal hysteresis of stress and strain in spin-crossover@polymer composites: towards a rational design of actuator devices

Polymer composites of molecular spin crossover complexes have emerged as promising mechanical actuator materials, but their effective thermomechanical properties remain elusive. In this work, we investigated a series of iron(ii)-triazole@P(VDF-TrFE) particulate composites using a tensile testing stage with temperature control. From these measurements, we assessed the temperature dependence of the Young's modulus as well as the free deformation and blocking stress, associated with the thermally-induced spin transition. The results denote that the expansion of the particles at the spin transition is effectively transferred to the macroscopic composite material, providing ca. 1-3% axial strain for 25% particle load. This strain is in excess of the 'neat' particle strain, which we attribute to particle-matrix mechanical coupling. On the other hand, the blocking stress (∼1 MPa) appears reduced by the softening of the composite around the spin transition temperature.

Realizing shape and size control for the synthesis of coordination polymer nanoparticles templated by diblock copolymer micelles

The combination of polymers with nanoparticles offers the possibility to obtain customizable composite materials with additional properties such as sensing or bistability provided by a switchable spin crossover (SCO) core. For all applications, a precise control over size and shape of the nanomaterial is highly important as it will significantly influence its final properties. By confined synthesis of iron(II) SCO coordination polymers within the P4VP cores of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) micelles in THF we are able to control the size and also the shape of the resulting SCO nanocomposite particles by the composition of the PS-b-P4VP diblock copolymers (dBCPs) and the amount of complex employed. For the nanocomposite samples with the highest P4VP content, a morphological transition from spherical nanoparticles to worm-like structures was observed with increasing coordination polymer content, which can be explained with the impact of complex coordination on the self-assembly of the dBCP. Furthermore, the SCO nanocomposites showed transition temperatures of T_1/2 = 217 K, up to 27 K wide hysteresis loops and a decrease of the residual high-spin fraction down to γ_HS = 14% in the worm-like structures, as determined by magnetic susceptibility measurements and Mössbauer spectroscopy. Thus, SCO properties close or even better (hysteresis) to those of the bulk material can be obtained and furthermore tuned through size and shape control realized by tailoring the block length ratio of the PS-b-P4VP dBCPs.

Synthesis and Characterization of [Fe(Htrz)2(trz)](BF4)] Nanocubes

Compounds that exhibit spin-crossover (SCO) type behavior have been extensively investigated due to their ability to act as molecular switches. Depending on the coordinating ligand, in this case 1H-1,2,4-triazole, and the crystallite size of the SCO compound produced, the energy requirement for the spin state transition can vary. Here, SCO [Fe(Htrz)2(trz)](BF4)] nanoparticles were synthesized using modified reverse micelle methods. Reaction conditions and reagent ratios are strictly controlled to produce nanocubes of 40-50 nm in size. Decreases in energy requirements are seen in both thermal and magnetic transitions for the smaller sized crystallites, where, compared to bulk materials, a decrease of as much as 20 °C can be seen in low to high spin state transitions.

Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(Lnpdtz)2(NCX)2] Complexes

Molecular materials that exhibit stimuli-responsive bistability are promising candidates for the development of molecular switches and sensors. We herein report on the coexistence of a wide thermal hysteretic spin crossover (SCO) effect and a thermally inducible metastable high-spin state at low temperatures achieved with the two new complexes [Fe^II(L^npdtz)₂(NCX)₂] (X = S; Se), with L^npdtz being (2-naphthyl-5-pyridyl-1,2,4-thiadiazole) and X = S (1) and Se (2). Pronounced π-π-stacking of the aromatic side residues of the ligands enables strong intermolecular interactions, leading to abrupt SCO properties and broad magnetic hysteresis of 10 K for X = S and 58 K for X = Se. In this paper, we also present the pressure-induced spin-state switching around 0.8 GPa. A pronounced thermally induced excited spin state trapping (TIESST effect) is observed for the highly cooperative SCO compounds, which was experimentally followed by low-temperature single crystal structure analysis (20 K) and temperature-dependent Mössbauer spectroscopy.

Tuning the Spin-Crossover Behaviour in Fe(II) Polymeric Composites for Food Packaging Applications

Although the spin-crossover (SCO) phenomenon is well documented, tuning the SCO behaviour remains a challenging task. This could be mainly attributed to the ‘delicate’ nature of the phenomenon; cooperativity expressed through differences in particle size and morphologies, and electrostatic interactions could significantly affect the process. The goal of the present effort is dual bearing both scientific and technological interest. Firstly, to examine the technological potential of SCO complexes by incorporating them into polymers, and secondly—and most importantly—to investigate if polymer-SCO complex interactions could occur and could affect the SCO behaviour, depending on the structural properties of both the polymer matrix and the SCO complex. In this context, two different polymers, polylactic acid (PLA) and polysulphone (PSF), which are capable of developing different interactions with the inclusions, and the SCO complexes [Fe(abpt)2{N(CN)2}2] and [Fe(abpt)2(SCN)2] were examined; abpt is the N,N’-bidentate chelating ligand 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole. The composites were characterised through scanning electron microscopy (SEM), attenuated total reflectance infrared (ATR/FTIR), and Raman spectroscopy. In addition, the potential migration release of the SCO compounds from the polymeric matrices and their toxicity evaluation were also studied. In addition, the potential migration release of the SCO compounds from the polymeric matrices was evaluated, and their insignificant toxicity was also verified. Temperature-dependent Raman spectra were collected in situ for the monitoring of the SCO behaviour after the incorporation of the Fe(II) complexes into the polymers; an upshift of the T1/2 transition and a hysteretic behaviour was detected for PSF-SCO composites, compared with the non-hysteretic behaviour of the pristine SCO complexes.

Post-synthetic modification mechanism for 1D spin crossover coordination polymers

Suitable solvent os crucial to achieve a quantitative PSM reaction. Then, this method is not restricted to porous materials.

Wet-chemistry assembly of one-dimensional nanowires: switching characteristics of a known spin-crossover iron(II) complex through Raman spectroscopy

In this study, a simple, fast, one-pot approach for the isolation of nanowires (NWs) in coordination chemistry is reported. Nanowires (NWs) of spin-crossover (SCO) materials are extremely rare. Here, an innovative and easy synthetic process was developed to prepare NWs of a switchable polymorph of the known complex trans-[Fe(NCS)₂(abpt)₂] using a wet-chemistry approach for the first time; abpt is the bidentate chelating ligand 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole. The remarkable smoothness of the high-spin to low-spin transition, monitored through variable-temperature (300-80 K) Raman microscopy, compared with the sharp transition exhibited by the polycrystalline material, demonstrates the effect of the topological properties on the physical phenomena of the system.

Bioanalytical applications of Mössbauer spectroscopy

Data on the applications of Mössbauer spectroscopy in the transmission (mainly on 57Fe nuclei) and emission (on 57Co nuclei) variants for analytical studies at the molecular level of metal-containing components in a wide range of biological objects (from biocomplexes and biomacromolecules to supramolecular structures, cells, tissues and organisms) and of objects that are participants or products of biological processes, published in the last 15 years are discussed and systematized. The prospects of the technique in its biological applications, including the developing fields (emission variant, use of synchrotron radiation), are formulated.

The bibliography includes 248 references.

Photo-Thermal Switching of Individual Plasmonically Activated Spin Crossover Nanoparticle Imaged by Ultrafast Transmission Electron Microscopy

Spin crossover (SCO) is a promising switching phenomenon when implemented in electronic devices as molecules, thin films or nanoparticles. Among the properties modulated along this phenomenon, optically induced mechanical changes are of tremendous importance as they can work as fast light-induced mechanical switches or allow to investigate and control microstructural strains and fatigability. The development of characterization techniques probing nanoscopic behavior with high spatio-temporal resolution allows to trigger and visualize such mechanical changes of individual nanoscopic objects. Here, ultrafast transmission electron microscopy (UTEM) is used to precisely probe the length changes of individual switchable nanoparticles induced thermally by nanosecond laser pulses. This allows revealing of the mechanisms of spin switching, leading to the macroscopic expansion of SCO materials. This study is conducted on individual pure SCO nanoparticles and SCO nanoparticles encapsulating gold nanorods that serve for plasmonic heating under laser pulses. Length changes are compared with time-resolved optical measurements performed on an assembly of these particles.

Morphological Studies of Composite Spin Crossover@SiO2 Nanoparticles

Spin crossover (SCO) iron (II) 1,2,4-triazole-based coordination compounds in the form of composite SCO@SiO2 nanoparticles were prepared using a reverse microemulsion technique. The thickness of the silica shell and the morphology of the as obtained core@shell nanoparticles were studied by modifying the polar phase/surfactant ratio (ω), as well as the quantity and the insertion phase (organic, aqueous and micellar phases) of the tetraethylorthosilicate (TEOS) precursor, the quantity of ammonia and the reaction temperature. The morphology of the nanoparticles was monitored by transmission electron microscopy (TEM/HRTEM) while their composition probed by combined elemental analyses, thermogravimetry and EDX analyses. We report that not only the particle size can be controlled but also the size of the silica shell, allowing for interesting perspectives in post-synthetic modification of the shell. The evolution of the spin crossover properties associated with the change in morphology was investigated by variable temperature optical and magnetic measurements.

Room temperature optoelectronic devices operating with spin crossover nanoparticles

Molecular systems can exhibit multi-stimuli switching of their properties, with spin crossover materials having unique magnetic transition triggered by temperature and light, among others. Light-induced room temperature operation is however elusive, as optical changes between metastable spin states require cryogenic temperatures. Furthermore, electrical detection is hampered by the intrinsic low conductivity properties of these materials. We show here how a graphene underlayer reveals the light-induced heating that triggers a spin transition, paving the way for using these molecules for room temperature optoelectronic applications.

2-D spin crossover materials at the nanometric scale: the effects of the size-reduction on the magnetic properties

Spin Crossover (SCO) particles at the nanometric scale provide an alternative point of view and a new perspective concerning the development of a new generation of spintronic, electronic, photonic and mechanical devices. The coexistence of the SCO phenomenon with the accompanying hysteresis loop enhances the functionality of future devices for storing and processing information. Despite all promising facts, the SCO phenomena are greatly affected by cooperativity issues resulting in a direct relation between the decrease of the size of nanopatricle and the overall decrease of cooperativity towards more gradual spin transitions. This minireview aims to summarise the synthetic techniques for the synthesis of 2-D FeII SCO particles at the nanometric scale, an underexplored area of research, highlighting the effects of the size-reduction on the magnetic properties of the corresponding nanoparticles and hopefuly showcasing the importance of studying in the context of 2D limit the SCO phenomena.

Water Soluble Iron-Based Coordination Trimers as Synergistic Adjuvants for Pancreatic Cancer

Pancreatic cancer is a usually fatal disease that needs innovative therapeutic approaches since the current treatments are poorly effective. In this study, based on cell lines, triazole-based coordination trimers made with soluble Fe(II) in an aqueous media were explored for the first time as adjuvant agents for the treatment of this condition. These coordination complexes were effective at relatively high concentrations and led to an increase in reactive oxygen species (ROS) in two pancreatic cancer cell lines, PANC-1 and BXPC-3, and this effect was accompanied by a significant reduction in cell viability in the presence of gemcitabine (GEM). Importantly, the tested compounds enhanced the effect of GEM, an approved drug for pancreatic cancer, through apoptosis induction and downregulation of the mTOR pathway. Although further evaluation in animal-based models of pancreatic cancer is needed, these results open novel avenues for exploring these iron-based materials in biomedicine in general and in pancreatic cancer treatment.

Co-Processing of [Fe(NH2trz)3](2ns)2 and UHMWPE into Materials Combining Spin Crossover and High Mechanical Strength

The coordination polymer [Fe(NH2trz)3](2ns)2 (NH2trz = 4-amino-1,2,4-triazole and 2ns− = counterion 2-naphthalene sulfonate) exhibits the rare phenomenon of spin crossover in an attractive temperature range, i.e., somewhat above room temperature. Spin crossover in [Fe(NH2trz)3](2ns)2 is manifested by thermochromism, which is accompanied by a magnetic transition from diamagnetism to paramagnetism. However, [Fe(NH2trz)3](2ns)2 is brittle and difficult to process, which limits its use. In this study, we show that [Fe(NH2trz)3](2ns)2 can be co-processed with ultrahigh molecular weight polyethylene (UHMWPE), which possesses outstanding mechanical properties, particularly when tensile-drawn. Therefore, [Fe(NH2trz)3](2ns)2–UHMWPE blends were gel-processed by extrusion, employing a relatively poor solvent, which has recently been shown to offer advantages compared to good solvents. Uniform and flexible films, ribbons and fibers with [Fe(NH2trz)3](2ns)2 fractions as high as 33.3% m/m were obtained that could be readily drawn. Spin crossover in the coordination polymer is retained in these materials, as evident from their thermochromism. The tensile strength and Young’s modulus of the blends exceed those of typical commodity polymers. Thus, the films, ribbons and fibers constitute a special class of multifunctional materials that combine the flexibility and excellent mechanical properties of drawn UHMWPE with the spin crossover behavior of [Fe(NH2trz)3](2ns)2.

Fe(II) Spin Crossover/Polymer Hybrid Materials: Investigation of the SCO Behavior via Temperature-Dependent Raman Spectroscopy, Physicochemical Characterization and Migration Release Study

Polymeric composites constitute an appealing class of materials with applications in various fields. Spin crossover (SCO) coordination complexes are switchable materials with potential use in data storage and sensors. Their incorporation into polymers can be considered an effective method for their wider practical application. In this study, Fe(II) SCO/polylactic acid hybrid polymeric composites have been prepared by film casting. The mononuclear coordination complex [Fe{N(CN)₂}₂(abpt)₂] was incorporated into polylactic acid. The morphological, structural and thermoanalytical characterization of the composite films were performed via scanning electron microscopy (SEM), attenuated total reflectance (ATR/FTIR), Raman spectroscopy and differential scanning calorimetry (DSC). In addition, the migration release study (MRS) of the SCO compound from the polymeric matrix into the food simulant 50% v/v water/ethanol solution was also examined via UV/Vis absorption. Of particular interest was the investigation of the SCO behavior of the coordination complex after its incorporation into the polymer matrix; it was accomplished by temperature-dependent micro-Raman spectroscopy. The described attempt could be considered a preparatory step toward the development of SCO-based temperature sensors integrated into food packaging materials.

Rational direct synthesis of [Fe(Htrz)2(trz)](BF4) polymorphs: temperature and concentration effects

Rational design of [Fe(Htrz)₂trz](BF₄) spin crossover (SCO) polymorphs through the study of the phase diagram and structure refinement.

OctaDist: a tool for calculating distortion parameters in spin crossover and coordination complexes

OctaDist is an interactive and visual program for determination of structural distortion in octahedral coordination complexes such as spin crossover complexes, single-ion magnets, perovskites or metal-organic frameworks. OctaDist computes the octahedral distortion parameters initially designed in the context of the spin-crossover phenomenon and denoted ζ, Σ, and Θ from standard structural files. The program also provides additional tools for molecular analyses and visualization. It emphasizes performance, flexibility, ease of use, application programming interface (API) consistency, and clear documentation. The modules and classes in OctaDist can be easily customized to include new algorithms or analytical tools. OctaDist is cross-platform supported for modern operating systems and is available as open-source distributed under the GNU General Public License version 3.

Tuning of the phase transition between site selective SCO and intermetallic ET in trimetallic magnetic cyanido-bridged clusters

A series of crystalline phases composed of trimetallic 3d-5d-5d' {Fe₉[Re(CN)₈]_6-x[W(CN)₈]_x(MeOH)₂₄}·yMeOH (x = 1 (1), 2 (2), 3 (3), 4 (4) and 5 (5); y = 10-15) clusters were obtained by altering the octacyanidometalate composition. The temperature dependent studies involving SC XRD, SQUID magnetic measurements, IR spectroscopy and ⁵⁷Fe Mössbauer spectroscopy revealed reversible phase transition with the retention of single crystal character in each congener. The transition was assisted by reversible spin-crossover (SCO) ^HSFe^II↔^LSFe^II transition at the central Fe1(ii) site for Fe₉Re₅W₁ (1), Fe₉Re₄W₂ (2), Fe₉Re₃W₃ (3) and Fe₉Re₂W₄ (4). In contrast, the tungsten-rich congener Fe₉Re₁W₅ (5) exhibited nontrivial behavior with the SCO transition being stopped halfway through the cooling process, to be completed with single electron transfer (ET) from the external Fe2(ii) center towards one of the neighboring W(v) sites. The critical temperature T_c of SCO has been systematically increased from 193 K (1) to 247 K (4). All experimental data indicate the domination of the Fe(ii)-W(v) valence states in all crystals 1-5, however, with increasing quantity of [W(CN)₈]^3- (and decreasing quantity of [Re(CN)₈]^3-), the valence equilibrium Fe(ii)-W(v) ↔ Fe(iii)-W(iv) was systematically shifted to the right, starting from congener 3. The overall electronic configuration at low temperatures and variable amounts and location of spin carriers along the whole series suggest the remarkable competition between magnetic super-exchange Fe(ii)-CN-W(v) interactions and intermolecular interactions. The observed behavior is in line with the information collected previously for the bimetallic congeners Fe₉Re₆ and Fe₉W₆, to shed light on the role of the mixed tri-metallic composition in changing the properties observed for the relevant bimetallic cyanido-bridged skeletons.

One-Step Synthesis of Spin Crossover Nanoparticles Using Flow Chemistry and Supercritical CO2

Switchable materials are increasingly considered for implementation in devices or multifunctional composites leading to a strong need in terms of reliable synthetic productions of well-defined objects. Here, an innovative and robust template-free continuous process was developed to synthesize nanoparticles of a switchable coordination polymer, including the use of supercritical CO₂ , aiming at both quenching the particle growth and drying the powder. This all-in-one process offers a 12-fold size reduction in a few minutes while maintaining the switching properties of the selected spin crossover coordination polymer.

Synthesis of Zn-based 1D and 2D coordination polymer nanoparticles in block copolymer micelles

Nanoparticles of the 1D and 2D coordination polymers [Zn(OAc)₂(bipy)] _n and [Zn(TFA)₂(bppa)₂] _n were prepared, employing polystyrene-block-poly(4-vinylpyridine) diblock copolymers with different weight fractions of the 4-vinylpyridine (4VP) block and comparable overall molecular weights of M _n ≈ 155 kg mol^-1 as template (SV-15 and SV-42 with 15 and 42 wt% 4VP, respectively). [Zn(OAc)₂(bipy)] _n nanoparticles were successfully synthesised within the 4VP core of SV-42 micelles, showing a core size of D _core = 47 ± 5 nm and a hydrodynamic diameter of D _h = 157 ± 46 nm, determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The crystallinity of the composite is quite low, showing only low intensity reflexes in the powder X-ray diffraction (PXRD) pattern with the highest particle load. No indications for larger microcrystals were detected by scanning electron microscopy (SEM), proving the successful integration of the coordination polymer nanoparticles within the micellar cores. Nanocomposites of the 2D coordination network [Zn(TFA)₂(bppa)₂] _n were synthesised using both diblock copolymers. The particle core sizes (from TEM) and hydrodynamic diameters (from DLS) correlate with the 4VP fraction of the micelles, resulting in D _core = 46 ± 6 nm for SV-42 and 15 ± 2 nm for SV-15 and D _h = 340 ± 153 nm and 177 ± 57 nm, respectively. The successful synthesis was proven by PXRD and SEM images, confirming the absence of larger crystallites. Hence, it is possible to synthesise nanocomposites of Zn-based 1D and 2D coordination polymers by a direct approach utilizing diblock copolymer micelles as template.

Enhancing Magnetic Cooperativity in Fe(II) Triazole-based Spin-crossover Nanoparticles by Pluronic Matrix Confinement

Polymeric one-dimensional (1D) triazole-based Fe^II spin crossover nanoparticles have been entrapped in pluronic P123 matrix, forming nanorods in which the interaction between host (P123) and guest (Fe^II complex) promoted high reproducibility of the spin crossover process, significant shifts of the transition temperatures (T↑=370 K, T↓=338 K for the P123 entrapped material vs the literature values of T↑=358 K, T↓=341 K for the neat/polymer free system) and larger magnetic hysteresis width.

Design of Targeted Nanostructured Coordination Polymers (NCPs) for Cancer Therapy

Conventional cancer chemotherapy presents notable drug side effects due to non-selective action of the chemotherapeutics to normal cells. Nanoparticles decorated with receptor-specific ligands on the surface have shown an important role in improving site-selective binding, retention, and drug delivery to the cancer cells. This review summarizes the recent reported achievements using nanostructured coordination polymers (NCPs) with active targeting properties for cancer treatment in vitro and in vivo. Despite the controversy surrounding the effectivity of active targeting nanoparticles, several studies suggest that active targeting nanoparticles notably increase the selectivity and the cytotoxic effect in tumoral cells over the conventional anticancer drugs and non-targeted nanoparticle platform, which enhances drug efficacy and safety. In most cases, the nanocarriers have been endowed with remarkable capabilities such as stimuli-responsive properties, targeting abilities, or the possibility to be monitored by imaging techniques. Unfortunately, the lack of preclinical studies impedes the evaluation of these unique and promising findings for the translation of NCPs into clinical trials.

Effect of Mechanochemical Recrystallization on the Thermal Hysteresis of 1D FeII-triazole Spin Crossover Polymers

The thermal hysteresis in the cooperative spin crossover (SCO) polymer [Fe(trz)(Htrz)₂]_n[BF₄]_n (1) has been tuned by a simple ball milling grinding process. Mechanical treatment affects the size and morphology of the crystallite domains, as confirmed by multiple complementary techniques, including ESEM, DLS, and PXRD data. Upon milling, the regular cubic shape particles recrystallize with slightly different unit cell parameters and preferential orientation. This macroscopic change significantly modifies the thermally induced SCO behavior, studied by temperature-dependent magnetic susceptibility, X-ray diffraction, and DSC analysis. Transition temperatures downshift, closer to room temperature, while hysteresis widens, when particle sizes are actually decreasing. We relate this counterintuitive observation to subtle modifications in the unit cell, offering new alternatives to tune and enhance SCO properties in this class of 1D-cooperative polymers.

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.

Confined Crystallization of Spin-Crossover Nanoparticles in Block-Copolymer Micelles

Nanoparticles of the spin-crossover coordination polymer [FeL(bipy)]n were synthesized by confined crystallization within the core of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer micelles. The 4VP units in the micellar core act as coordination sites for the Fe complex. In the bulk material, the spin-crossover nanoparticles in the core are well isolated from each other allowing thermal treatment without disintegration of their structure. During annealing above the glass transition temperature of the PS block, the transition temperature is shifted gradually to higher temperatures from the as-synthesized product (T1/2 ↓=163 K and T1/2 ↑=170 K) to the annealed product (T1/2 ↓=203 K and T1/2 ↑=217 K) along with an increase in hysteresis width from 6 K to 14 K. Thus, the spin-crossover properties can be shifted towards the properties of the related bulk material. The stability of the nanocomposite allows further processing, such as electrospinning from solution.