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.

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

Hofmann coordination polymers (CPs) with cationic ligands provide an innovative strategy for recognizing p-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 p-p 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.

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.

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.

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.

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.

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.

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.

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.

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.

A Vanadium Dioxide-PMMA Composite For Microwave Radiation Switching.

Reconfigurable radio-frequency components are in high demand for modern communication systems as they can be involved in multiband and multistandard electronic devices. The key part of such components is an active switching element. This work offers a way to obtain an efficient microwave switch using vanadium dioxide-poly (methyl methacrylate) composite. Differential scanning calorimetry, SQUID magnetometery, and impedance spectroscopy measurements were used to characterize the phase transition in the proposed composite. Temperature induced metal-insulator transition occurs at technologically attractive 341 K. The transition leads to a change of microwave transmission trough VO2 -PMMA composite from -4.9 dB for low-temperature monoclinic form to -5.8 dB for high-temperature rutile form. This provides an ability to tune the material's transparency in the microwave range, while the shaping polymer matrix provides the proper mechanical processability of the switching element.

Novel family of bis-pyrazole coordination complexes as potent antibacterial and antifungal agents.

A new pyrazole ligand, N,N-bis(2(1',5,5'-trimethyl-1H,1'H-[3,3'-bipyrazol]-1-yl)ethyl)propan-1-amine (L) was synthesized and characterized by 1H-NMR, 13C-NMR, FT-IR and HRMS. The coordination ability of the ligand has been employed for the construction of a new family of coordination complexes, namely: [Cu2LCl4] (1), [ML(CH3OH)(H2O)](ClO4)2 (MII = Ni (2), Co (3)) and [FeL(NCS)2] (4). The series of complexes were characterized using single-crystal X-ray diffraction, HRMS, FT-IR and UV-visible spectroscopy. Moreover, the iron(ii) analogue was investigated by 57Fe Mössbauer spectroscopy and SQUID magnetometry. Single-crystal X-ray structures of the prepared complexes are debated within the framework of the cooperative effect of the hydrogen bonding network and counter anions on the supramolecular formations observed. Furthermore, within the context of biological activity surveys, these compounds were reviewed against different types of bacteria to validate their efficiency, including both Gram-positive as well as Gram-negative bacteria. Enhanced behaviour towards Fusarium oxysporum f. sp. albedinis fungi, were detected for 1 and 4.

Increasing Permittivity and Mechanical Harvesting Response of PVDF-Based Flexible Composites by Using Ag Nanoparticles onto BaTiO3 Nanofillers.

The role of Ag addition on the structural, dielectric, and mechanical harvesting response of 20%(xAg - (1 - x)BaTiO3) - 80%PVDF (x = 0, 2, 5, 7 and 27 vol.%) flexible composites is investigated. The inorganic fillers were realized by precipitating fine (~3 nm) silver nanoparticles onto BaTiO3 nanoparticles (~60 nm average size). The hybrid admixtures with a total filling factor of 20 vol.% were embedded into the PVDF matrix. The presence of filler enhances the amount of ß-PVDF polar phase and the BaTiO3 filler induces an increase of the permittivity from 11 to 18 (1 kHz) in the flexible composites. The addition of increasing amounts of Ag is further beneficial for permittivity increase; with the maximum amount (x = 27 vol.%), permittivity is three times larger than in pure PVDF (εr ~ 33 at 1 kHz) with a similar level of tangent losses. This result is due to the local field enhancement in the regions close to the filler-PVDF interfaces which are additionally intensified by the presence of silver nanoparticles. The metallic addition is also beneficial for the mechanical harvesting ability of such composites: the amplitude of the maximum piezoelectric-triboelectric combined output collected in open circuit conditions increases from 0.2 V/cm2 (PVDF) to 30 V/cm2 for x = 27 vol.% Ag in a capacitive configuration. The role of ferroelectric and metallic nanoparticles on the increasing mechanical-electric conversion response is also been explained.

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.

Auxiliary alkyl chain modulated spin crossover behaviour of [Fe(H2Bpz2)2(Cn-bipy)] complexes.

Three new alkyl chain substituted complexes [Fe(H2Bpz2)2(Cn-bipy)] (pz = pyrazolyl, Cn-bipy = bipyridine alkyl chain diester, n = 3 (3), 4 (4) and 5 (5)) show versatile spin state switching behaviour with different "tail" lengths as revealed by structural and magnetic analyses. The most striking phenomenon is observed for 5 which undergoes an abrupt spin transition accompanied by thermal hysteresis of ca. 10 K, which is attributed to crystal packing effects derived from the competition between π⋯π and C-H⋯O interactions. Interestingly, each of the complexes exhibits similar gradual and complete spin crossover in methanol solution with a transition temperature around 249 K, as deduced from temperature-dependent UV-vis spectroscopy. This highlights the differences between the solid state (ligand field; crystal packing) and solution (ligand field; solvation) effects on spin crossover. This work demonstrates that the length of the complex's alkyl chain substituents on the complex can have a large impact on the transition temperature and profile of solid state spin crossover, offering a potential path to the fabrication of soft matter spin crossover materials.

Spin crossover in iron(II) Hofmann clathrates analogues with 1,2,3-triazole.

Hofmann-like cyanometallic complexes represent one of the biggest and well-known classes of FeII spin-crossover compounds. In this paper, we report on the first FeII Hofmann clathrate analogues with unsubstituted 1,2,3-triazole, which exhibit temperature induced spin transition. Two new coordination polymers with the general formula [FeII(1,2,3-triazole)2MII(CN)4] (M = Pt, Pd) undergo abrupt hysteretic spin crossover in the range of 190-225 K as revealed by magnetic susceptibility measurements. Two compounds are isostructural and are built of infinite cyanometallic layers which are supported by 1,2,3-triazole ligands. The thermal hysteresis loop is very stable at different scan rates from 0.5 to 10 K min-1. The compounds display strong thermochromic effect, changing their colour from pink in the low-spin state to white in the high-spin state. Our findings show that 1,2,3-triazole is suitable for elaboration of spin-crossover Hofmann clathrate analogues, and its use instead of more classical azines can advantageously expand this family of complexes.

Synthesis and cytotoxicity against tumor cells of pincer N-heterocyclic ligands and their transition metal complexes.

The complexes: [CoL2](ClO4)2 (1), [FeL2](ClO4)2 (2), [NiL2](ClO4)2 (3) and [MnLCl2] (4), with L = diethyl-1,1'-(pyridine-2,6-diyl)bis(5-methyl-1H-pyrazole-3-carboxylate), were synthesized and fully characterized. Structural analysis revealed two distinct patterns influenced by the counter ions where L acts as a tridentate chelating ligand. The in vitro antitumor activity of L and L' (diethyl 2,2'-(pyridine-2,6-diylbis(5-methyl-1H-pyrazole-3,1-diyl)) diacetate) as well as their metal complexes, was tested by the measurement of their cytostatic and cytotoxic properties towards the blood cancer mastocytoma cell line P815. We have also investigated their interactions with the antioxidant enzyme system. As a result, [MnL'Cl2] (1') exhibited the strongest activity compared to reference cis-platin with no cytotoxicity towards normal cells PBMCs (Peripheral Blood Mononuclear Cells). On the other hand, the antioxidant enzyme activity showed that the efficiency of metal complex 1' against P815 tumor cells was via the rise in the SOD activity and inhibition of CAT enzyme activity. This proof of concept study allows disclosure of a new class of molecules in cancer therapeutics.

Innovative Low-Cost Carbon/ZnO Hybrid Materials with Enhanced Photocatalytic Activity towards Organic Pollutant Dyes' Removal.

A new type of material based on carbon/ZnO nanostructures that possesses both adsorption and photocatalytic properties was obtained in three stages: cellulose acetate butyrate (CAB) microfiber mats prepared by the electrospinning method, ZnO nanostructures growth by dipping and hydrothermal methods, and finally thermal calcination at 600 °C in N2 for 30 min. X-ray diffraction (XRD) confirmed the structural characteristics. It was found that ZnO possesses a hexagonal wurtzite crystalline structure. The ZnO nanocrystals with star-like and nanorod shapes were evidenced by scanning electron microscopy (SEM) measurements. A significant decrease in Eg value was found for carbon/ZnO hybrid materials (2.51 eV) as compared to ZnO nanostructures (3.21 eV). The photocatalytic activity was evaluated by studying the degradation of three dyes, Methylene Blue (MB), Rhodamine B (RhB) and Congo Red (CR) under visible-light irradiation. Therefore, the maximum color removal efficiency (both adsorption and photocatalytic processes) was: 97.97% of MB (C0 = 10 mg/L), 98.34% of RhB (C0 = 5 mg/L), and 91.93% of CR (C0 = 10 mg/L). Moreover, the value of the rate constant (k) was found to be 0.29 × 10-2 min-1. The novelty of this study relies on obtaining new photocatalysts based on carbon/ZnO using cheap and accessible raw materials, and low-cost preparation techniques.