Hofmann Clathrates: A "Blue Box" Approach to Modulate Spin-Crossover Properties.

Hofmann coordination polymers (CPs) with cationic ligands provide an innovative strategy for recognizing π-electron-rich aromatic molecules - similar to the "little blue box". In this study, we demonstrate that hydroquinone molecules can be incorporated into these coordination polymers when redox-active bipyridinium derivatives are used as axial ligands. The insertion leads to a significant structural modification, resulting in a shift of the spin transition by 150 K and an approximate 23 % increase in volume, caused by the strong donor-acceptor π-π stacking interaction formed between the ligands and the guest molecule. These findings have been confirmed through temperature-dependent single crystal X-ray diffraction, magnetic measurements and optical reflectivity measurements.

Cooperative Spin Crossover above Room Temperature in the Iron(II) Cyanoborohydride-Pyrazine Complex.

Hysteretic spin crossover in coordination complexes of 3d-metal ions represents one of the most spectacular phenomena of molecular bistability. In this paper we describe a self-assembly of pyrazine (pz) and Fe(BH3CN)2 that afforded the new 2D coordination polymer [Fe(pz)2(BH3CN)2]∞. It undergoes an abrupt, hysteretic spin crossover (SCO) with a T1/2 of 338 K (heating) and 326 K (cooling) according to magnetic susceptibility measurements. Mössbauer spectroscopy revealed a complete transition between the low-spin (LS) and the high-spin (HS) states of the iron centers. This LS-to-HS transition induced an increase of the unit cell volume by 10.6%. Meanwhile, a modulation of multiple [C-Hδ+···Hδ--B] dihydrogen bonds stimulates a contraction in direction c (2.2%). The simplicity of the synthesis, mild temperatures of transition, a pronounced thermochromism, stability upon thermal cycling, a striking volume expansion upon SCO, and an easy processability to composite films make this new complex an attractive material for switchable components of diverse applications.

Two-Step Spin Crossover in Hofmann-Type Coordination Polymers [Fe(2-phenylpyrazine)2{M(CN)2}2] (M = Ag, Au).

Two 2D Hofmann-type complexes of the composition [Fe(Phpz)2{M(CN)2}2] (where Phpz = 2-phenylpyrazine; M = Ag, Au) have been synthesized, and their spin-crossover (SCO) behavior has been thoroughly characterized. Single-crystal X-ray analysis reveals that these complexes contain a crystallographically unique Fe(II) center surrounded by two axial Phpz ligands and four equatorial cyanide [M(CN)2]- bridges. It is shown that, using of a ligand with two aromatic rings, an advanced system of weak supramolecular interactions (metal-metal, C-H···M, and π···π stacking contacts) is realized. This ensures additional stabilization of the structures and the absence of solvent-accessible voids due to dense packing. Both complexes are characterized by a highly reproducible two-step SCO behavior, as revealed by different techniques (superconducting quantum interference device magnetometry, optical microscopy, etc.). Research shows the exceptional role of the presence of various supramolecular interactions in the structure and the influence of the bulky substituent in the ligand on SCO behavior. Moreover, the perspective of substituted pyrazines for the design of new switchable materials is supported by this work.

Investigating the Vibrational, Magnetic and Dielectric Properties, and Antioxidant Activity of Cerium Oxide Nanoparticles.

Dielectric, magnetic and Raman measurements of cerium oxide nanoparticles obtained by the precipitation method are discussed. Morphological study was performed by scanning electron microscopy, confirming the formation of nanoparticles of 5-27 nm. The Raman spectra exhibited a strong band around 465 cm-1, corresponding to the symmetrical stretching mode of the Ce-O8 vibrational unit. The nature of the room temperature ferromagnetism of cerium oxide nanoparticles was analyzed, taking into account the oxygen defects at the surface or interface of the nanoparticles. The evolution of dielectric constant, ε', and dielectric loss, ε″ was studied as a function of frequency at different temperatures. Additionally, the variation of the electric conductivity versus temperature was investigated. Finally, complex impedance study of the cerium oxide nanoparticles was performed.

Spin Crossover Behavior in a Homologous Series of Iron(II) Complexes Based on Functionalized Bipyridyl Ligands.

A series of bulky substituted bipyridine-related iron(II) complexes [Fe(H2Bpz2)2(L)] (pz = pyrazolyl) were prepared, where L = 5,5'-dimethyl-2,2'-bipyridine (bipy-CH3, 1), L = dimethyl-2,2'-bipyridyl-5,5'-dicarboxylate (MeObpydc, 2), L = diethyl-2,2'-bipyridyl-5,5'-dicarboxylate (EtObpydc, 3), or L = diisopropyl-2,2'-bipyridine-5,5'-dicarboxylate ( i-PrObpydc, 4). The crystal structures of five new iron(II) complexes were determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of 3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover (SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo gradual and incomplete SCO behaviors. Complexes 2 and 4 show a completely gradual and steep SCO, respectively. Such different SCO behaviors can be attributed to an electronic substituent effect in the bipyridyl ligand conformation and a crystal packing effect. Importantly, the electronic substituent effect of the isopropyl acetate group and C-H···O supramolecular interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition.

Fe(II) Spin Transition Materials Including an Amino-Ester 1,2,4-Triazole Derivative, Operating at, below, and above Room Temperature.

A new family of one-dimensional Fe(II) 1,2,4-triazole spin transition coordination polymers for which a modification of anion and crystallization solvent can tune the switching temperature over a wide range, including the room temperature region, is reported. This series of materials was prepared as powders after reaction of ethyl-4H-1,2,4-triazol-4-yl-acetate (αEtGlytrz) with an iron salt from a MeOH/H2O medium affording: [Fe(αEtGlytrz)3](ClO4)2 (1); [Fe(αEtGlytrz)3](ClO4)2·CH3OH (2); [Fe(αEtGlytrz)3](NO3)2·H2O (3); [Fe(αEtGlytrz)3](NO3)2 (4); [Fe(αEtGlytrz)3](BF4)2·0.5H2O (5); [Fe(αEtGlytrz)3](BF4)2 (6); and [Fe(αEtGlytrz)3](CF3SO3)2·2H2O (7). Their spin transition properties were investigated by (57)Fe Mossbauer spectroscopy, superconducting quantum interference device (SQUID) magnetometry, and differential scanning calorimetry (DSC). The temperature dependence of the high-spin molar fraction derived from (57)Fe Mössbauer spectroscopy in 1 reveals an abrupt single step transition between low-spin and high-spin states with a hysteresis loop of width 5 K (Tc(↑) = 296 K and Tc(↓) = 291 K). The properties drastically change with modification of anion and/or lattice solvent. The transition temperatures, deduced by SQUID magnetometry, shift to Tc(↑) = 273 K and Tc(↓) = 263 K for (2), Tc(↑) = 353 K and Tc(↓) = 333 K for (3), Tc(↑) = 338 K and Tc(↓) = 278 K for (4), T(↑) = 320 K and T(↓) = 305 K for (5), Tc(↑) = 106 K and Tc(↓) = 92 K for (6), and T(↑) = 325 K and T(↓) = 322 K for (7). Annealing experiments of 3 lead to a change of the morphology, texture, and magnetic properties of the sample. A dehydration/rehydration process associated with a spin state change was analyzed by a mean-field macroscopic master equation using a two-level Hamiltonian Ising-like model for 3. A new structural-property relationship was also identified for this series of materials [Fe(αEtGlytrz)3](anion)2·nSolvent based on Mössbauer and DSC measurements. The entropy gap associated with the spin transition and the volume of the inserted counteranion shows a linear trend, with decrease in entropy with increasing the size of the counteranion. The first materials of this substance class to display a complete spin transition in both spin states are also presented.

Pressure and Temperature Sensors Using Two Spin Crossover Materials.

The possibility of a new design concept for dual spin crossover based sensors for concomitant detection of both temperature and pressure is presented. It is conjectured from numerical results obtained by mean field approximation applied to a Ising-like model that using two different spin crossover compounds containing switching molecules with weak elastic interactions it is possible to simultaneously measure P and T. When the interaction parameters are optimized, the spin transition is gradual and for each spin crossover compounds, both temperature and pressure values being identified from their optical densities. This concept offers great perspectives for smart sensing devices.

Two-step spin transition in a 1D Fe(II) 1,2,4-triazole chain compound.

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(ßAlatrz)3](BF4)2⋅2 H2O (1⋅2 H2O), whose precursor ßAlatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a ß-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), (57)Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two-step spin crossover (T1/2(↓) = 230 K and T1/2(↑) = 235 K, and T1/2(↓) = 172 K and T1/2(↑) = 188 K, respectively) is registered for the first time for a 1,2,4-triazole-based Fe(II) 1D coordination polymer. The two-step SCO configuration is observed in a 1:2 ratio of low-spin/high-spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1⋅2 H2O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low-spin and high-spin states of 1⋅2 H2O. Vibrational spectra of 1⋅2 H2O reveal that the BF4(-) anions and water molecules play no significant role on the vibrational properties of the [Fe(ßAlatrz)3](2+) polymeric chains, although non-coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(ßAlatrz)3](BF4)2 (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated.

Light induced modulation of charge transport phenomena across the bistability region in [Fe(Htrz)2(trz)](BF4) spin crossover micro-rods.

We studied the effect of light irradiation on the electrical conductance of micro-rods of the spin crossover [Fe(Htrz)2(trz)](BF4) network, organized between interdigitated gold electrodes. By irradiating the sample with different wavelengths (between 295 and 655 nm) either in air or under a nitrogen atmosphere we observed both a reversible and an irreversible change of the current flowing in the device. The reversible process consists of an abrupt decrease of the current intensity (ca. 10-50%) upon light irradiation, while the irreversible process is characterized by a slow, but continuous increase in time of the current, which persists also in the dark. These photo-induced processes were only detected in the high conductance low-spin (LS) state of the complex. On switching the rods to the high spin (HS) state the conductance decreases two orders of magnitude (at the same temperature) and - as a consequence - the photo-effect vanishes.

Pressure sensor via optical detection based on a 1D spin transition coordination polymer.

We have investigated the suitability of using the 1D spin crossover coordination polymer [Fe(4-(2'-hydroxyethyl)-1,2,4-triazole)3]I2∙H2O, known to crossover around room temperature, as a pressure sensor via optical detection using various contact pressures up to 250 MPa. A dramatic persistent colour change is observed. The experimental data, obtained by calorimetric and Mössbauer measurements, have been used for a theoretical analysis, in the framework of the Ising-like model, of the thermal and pressure induced spin state switching. The pressure (P)-temperature (T) phase diagram calculated for this compound has been used to obtain the P-T bistability region.

Selective and reusable iron(II)-based molecular sensor for the vapor-phase detection of alcohols.

A mononuclear iron(II) neutral complex (1) is screened for sensing abilities for a wide spectrum of chemicals and to evaluate the response function toward physical perturbation like temperature and mechanical stress. Interestingly, 1 precisely detects methanol among an alcohol series. The sensing process is visually detectable, fatigue-resistant, highly selective, and reusable. The sensing ability is attributed to molecular sieving and subsequent spin-state change of iron centers, after a crystal-to-crystal transformation.

Combining electron transfer, spin crossover, and redox properties in metal-organic frameworks.

Hofmann coordination polymers (CPs) that couple the well-studied spin transition of the FeII central ion with electron-responsive ligands provide an innovative strategy toward multifunctional metal-organic frameworks (MOFs). Here, we developed a 2D planar network consisting of metal-cyanide-metal sheets in an unusual coordination mode, brought about by infinitely π-stacked redox-active bipyridinium derivatives as axial ligands. The obtained family of materials show vivid thermochromism attributed to electron transfer and/or electronic spin state change processes that can occur either independently or concomitantly. Importantly, the redox activity of the ligands within the structure leads to the quasi-reversible electrochemical reduction reaction on a spin-crossover complex at solid state. These observations have been confirmed via temperature-dependent single-crystal X-ray diffraction, magnetic measurements, Mössbauer, EPR, optical and vibrational spectroscopies as well as quantum chemical calculations.

Polysaccharides-Calcium Phosphates Composite Beads as Bone Substitutes for Fractures Repair and Regeneration.

The tendency of population aging is continuously increasing, which is directly correlated with a significative number of associated pathologies. Several metabolic bone diseases such as osteoporosis or chronic kidney disease-mineral and bone disorders involve a high risk of fractures. Due to the specific fragility, bones will not self-heal and supportive treatments are necessary. Implantable bone substitutes, a component of bone tissue engineering (BTE) strategy, proved to be an efficient solution for this issue. The aim of this study was to develop composites beads (CBs) with application in the complex field of BTE, by assembling the features of both biomaterials' classes: biopolymers (more specific, polysaccharides: alginate and two different concentrations of guar gum/carboxymethyl guar gum) and ceramics (more specific, calcium phosphates), in a combination described for the first time in the literature. The CBs prepared by double crosslinking (ionic and physically) showed adequate physico-chemical characteristics and capabilities (morphology, chemical structure and composition, mechanical strength, and in vitro behaviour in four different acellular simulated body fluids) for bone tissue repair. Moreover, preliminary in vitro studies on cell cultures highlighted that the CBs were free of cytotoxicity and did not affect the morphology and density of cells. The results indicated that the beads based on a higher concentration of guar gum have superior properties than those with carboxymetilated guar, especially in terms of mechanical properties and behaviour in simulated body fluids.

Enhanced energy storage performance in reaction-sintered AgNbO3 antiferroelectric ceramics.

In this research, AgNbO3 ceramics were produced by two sintering methods: reaction sintering (RS) and conventional solid-state sintering (CSSS). The process was similar for both methods, except that in RS, Ag2O and Nb2O5 precursors were mixed, then formed into pellets, skipping the calcination step, and sintered at 1100 °C for 6 hours. Both prepared ceramics had the same perovskite crystal structure with an orthorhombic crystal system and Pbcm and Pmc21 space groups with similar lattice dynamic vibration modes at room temperature. The average grain size of the polycrystalline samples prepared by RS and CSSS was found to be ∼2.03 ± 0.77 and ∼1.85 ± 0.96 µm, respectively. The relative bulk densities of the ceramics produced by RS and CSSS were found to be ∼94.0 ± 1.8 and ∼96.5 ± 1.3%, respectively. Ceramics prepared by both methods showed antiferroelectric behavior, and reaction-sintered AgNbO3 ceramics exhibited lower energy loss density than CSSS samples. In addition, a recoverable energy storage density (Wrec) of 3.1 J cm-3 and higher energy storage efficiency (η) for RS samples were measured at 175 kV cm-1. Moreover, the η values of 74.2% and 57.7% were measured for samples sintered by RS and CSSS, respectively. This energy storage efficiency is the highest ever reported for pure AgNbO3 ceramics. Furthermore, reaction-sintered samples showed good temperature stability for Wrec and η in the 30-80 °C temperature range.

Dielectric, pyroelectric, and ferroelectric studies in (1 - x)AgNbO3-xFeNbO4 lead-free ceramics.

In the present study, the effect of heterovalent Fe3+ ions on the dielectric, pyroelectric, and ferroelectric properties of the (1 - x)AgNbO3-xFeNbO4 (x = 0.005, 0.01, 0.025, 0.05, and 0.1) system was investigated. The substitution of smaller ionic radius Fe3+ in B-sites and the formation of FeNbO4 as a secondary phase contributed to improved dielectric performance, especially the pyroelectric effect, of (1 - x)AgNbO3-xFeNbO4 ceramics by generating electron-rich ceramics. The (1 - x)AgNbO3-xFeNbO4 ceramics were prepared by conventional solid-state sintering. Pure AgNbO3 had a perovskite crystal structure with an orthorhombic crystal system, but the FeNbO4 in (1 - x)AgNbO3-xFeNbO4 ceramics was formed as a secondary phase with a monoclinic structure. In addition, the XRD and Raman spectroscopy data showed that some Fe3+ was substituted into B-sites of AgNbO3. The introduction of FeNbO4 effectively reduced the average grain size from 1.85 ± 0.09 µm to 1.22 ± 0.03 µm for pure AgNbO3 and 0.9AgNbO3-0.1FeNbO4, respectively. In addition, the relative density of the (1 - x)AgNbO3-xFeNbO4 ceramics decreased from 97.96% ± 0.01 for x = 0 to 96.75% ± 0.03 for x = 0.1. The real part of the permittivity ε', at room temperature, increased from 186.6 for x = 0 to a value of 738.7 for x = 0.1. Additionally, the maximum pyroelectric coefficient increased fivefold, reaching values of 2270 nC cm-2 K-1 for x = 0.1. Furthermore, a harvested pyroelectric energy density (W) of 1140 µJ cm-3 for x = 0.025 was achieved, which is appreciably higher than the 840 µJ cm-3 value for x = 0.

Fabrication and Characterization of Dielectric ZnCr2O4 Nanopowders and Thin Films for Parallel-Plate Capacitor Applications.

We report here the successful shape-controlled synthesis of dielectric spinel-type ZnCr2O4 nanoparticles by using a simple sol-gel auto-combustion method followed by successive heat treatment steps of the resulting powders at temperatures from 500 to 900 °C and from 5 to 11 h, in air. A systematic study of the dependence of the morphology of the nanoparticles on the annealing time and temperature was performed by using field effect scanning electron microscopy (FE-SEM), powder X-ray diffraction (PXRD) and structure refinement by the Rietveld method, dynamic lattice analysis and broadband dielectric spectrometry, respectively. It was observed for the first time that when the aerobic post-synthesis heat treatment temperature increases progressively from 500 to 900 °C, the ZnCr2O4 nanoparticles: (i) increase in size from 10 to 350 nm and (ii) develop well-defined facets, changing their shape from shapeless to truncated octahedrons and eventually pseudo-octahedra. The samples were found to exhibit high dielectric constant values and low dielectric losses with the best dielectric performance characteristics displayed by the 350 nm pseudo-octahedral nanoparticles whose permittivity reaches a value of ε = 1500 and a dielectric loss tan δ = 5 × 10-4 at a frequency of 1 Hz. Nanoparticulate ZnCr2O4-based thin films with a thickness varying from 0.5 to 2 µm were fabricated by the drop-casting method and subsequently incorporated into planar capacitors whose dielectric performance was characterized. This study undoubtedly shows that the dielectric properties of nanostructured zinc chromite powders can be engineered by the rational control of their morphology upon the variation of the post-synthesis heat treatment process.

New triazole-based coordination complexes as antitumor agents against triple negative breast cancer MDA-MB-468 cell line.

The present work describes the synthesis of a new triazole based ligand 3-(3,5-dimethyl-1H-pyrazol-1-yl)-1-methyl-1H-1,2,4-triazole (LM) and demonstration of its coordination diversity giving rise to a family of seven new coordination complexes, namely: [Ni(LM)3](ClO4)2·C2H6OS (5), [Co2(LM)6](ClO4)4·(C2H5)O (6), [Cd(LM)2Cl2] (7), [Cu(LM)2NO3]NO3 (8), [Fe(LM)3](BF4)2 (9), [Zn(LM)3](BF4)2 (10) and [Zn(LM)2NO3]NO3 (11), whose crystal structure was determined by single-crystal X-ray diffraction. Cytotoxic activity was evaluated against the MDA-MB-468 cancer cell line, which serves as a model for triple-negative breast cancer, and compared to the precursor molecule (L), as well as their coordination complexes (H3O){[NiL3](ClO4)3} (1), [CoL3](ClO4)2·2H2O (2), [CdL2Cl2] (3) and [CuL3](NO3)2 (4), for which the crystal structure was earlier determined. Notably, cadmium complexes 3 and 7 exhibit remarkable cytotoxicity and demonstrated a high selectivity index towards cancer cells when compared to peripheral blood mononuclear cells. Such activity highlights their potential function as anticancer agents.

GHz-THz Dielectric Properties of Flexible Matrix-Embedded BTO Nanoparticles.

BaTiO3 (BTO) nanoparticles produced by wet chemistry methods were embedded in several types of flexible materials in order to fabricate flexible electronic devices. Starting from the produced nanoparticle dielectric properties, flexible material dielectric properties were tested for high electromagnetic frequencies (30 GHz-2 THz) using time domain spectroscopy. Dielectric performances of the different materials obtained with variable nanoparticle concentrations up to 40 wt.%, embedded in, gelatin, epoxy, and styrene-butadiene were compared at several working temperatures between 0 °C and 120 °C. Beside the general trend of ε' decrease with temperature and loses increase with the operating frequency, we were able to identify few matrix dependent optimal nanoparticle concentrations. The best composite performances were achieved by the BTO-SBS matrix, with filler concentration of 2 wt.%, where the losses have been of 1.5%, followed by BTO-gelatin matrix, with filler concentration of 40 wt.%, with higher losses percent of almost 10% for THz frequencies.

Influence of Ferroelectric Filler Size and Clustering on the Electrical Properties of (Ag-BaTiO3)-PVDF Sub-Percolative Hybrid Composites.

The paper presents a study concerning the role of ferroelectric filler size and clustering in the dielectric properties of 20%BaTiO3-80%PVDF and of 20% (2%Ag-98%BaTiO3)-PVDF hybrid nanocomposites. By finite element calculations, it was shown that using fillers with ε > 103 does not provide a permittivity rise in the composites and the effective dielectric constant tends to saturate to specific values determined by the filler size and agglomeration degree. Irrespective of the ferroelectric filler sizes, the addition of metallic ultrafine nanoparticles (Ag) results in permittivity intensification and the effect is even stronger if the metallic nanoparticles are connected to a higher degree with the ferroelectric particles' surfaces. When using coarse ferroelectric fillers, the probability of clustering is higher, thus favoring the permittivity increase by field concentration in small regions close to the interfaces separating dissimilar materials. The modeling results were validated by an experimental dielectric analysis performed in a series of PVDF-based thick films with the same amount of BaTiO3 fillers or with Ag-BaTiO3 hybrid fillers. Similar trends as predicted by simulations were found experimentally but with slightly higher permittivity values which were assigned to the modifications of the polymer phase composition due to the presence of nanofillers and the local sample inhomogeneity (the presence of clustering, in particular for coarse BaTiO3 grains), which create regions with enhanced local fields.

A non-porous Fe(II) complex for the colorimetric detection of hazardous gases and the monitoring of meat freshness.

Food quality monitoring and freshness assessment are critical for ensuring food safety at a large scale. Ammonia is used as an important indicator of protein rich food spoilage state. However, current ammonia gas sensors suffer from insufficient sensitivity and selectivity, or sophisticated instrumentation, hindering their practical application in in-situ and real-time food quality monitoring. To overcome such limitations, an innovative nonporous colorimetric complex 1 has been synthesized and investigated for the detection of NH3(g) and its volatile organic derivatives including aliphatic amines, 1,2-diaminopropane(g), isobutylamine(g) and ethylenediamine(g), etc. The sensor operates colorimetrically at room temperature without energy input, with a detection limit to ammonia(g) of 105 ppb, and show excellent reusability. The colorimetric detection mechanism involves a partial spin state change of Fe(II) ions upon exposure to amines in the gas phase. In addition, the complex was utilized as real-time monitoring of meat freshness using a smartphone. Thus, chemosensor 1 is considered as a ground breaking new-generation portable electronic nose for vapors of volatile organic compounds discrimination at room temperature.