Paramagnetic Susceptibility Measurements Research Articles

Large Room-Temperature Magnetoresistance in a High-Spin Donor-Acceptor Conjugated Polymer.

Open-shell conjugated polymers (CPs) offer new opportunities for the development of emerging technologies that utilize the spin degree of freedom. Their light-element composition, weak spin-orbit coupling, synthetic modularity, high chemical stability, and solution-processability offer attributes that are unavailable from other semiconducting materials. However, developing an understanding of how electronic structure correlates with emerging transport phenomena remains central to their application. Here, the first connections between molecular, electronic, and solid-state transport in a high-spin donor-acceptor CP, poly(4-(4-(3,5-didodecylbenzylidene)-4H-cyclopenta[2,1-b:3,4-b']dithiophen-2-yl)-6,7-dimethyl-[1,2,5]-thiadiazolo[3,4-g]quinoxaline), are provided. At low temperatures (T < 180 K), a giant negative magnetoresistance (MR) is achieved in a thin-film device with a value of -98% at 10 K, which surpasses the performance of all other organic materials. The thermal depopulation of the high-spin manifold and negative MR decrease as temperature increases and at T > 180 K, the MR becomes positive with a relatively large MR of 13.5% at room temperature. Variable temperature electron paramagnetic resonance spectroscopy and magnetic susceptibility measurements demonstrate that modulation of both the sign and magnitude of the MR correlates with the electronic and spin structure of the CP. These results indicate that donor-acceptor CPs with open-shell and high-spin ground states offer new opportunities for emerging spin-based applications.

A Water-Soluble Highly Oxidizing Cobalt Molecular Catalyst Designed for Bioinspired Water Oxidation.

Herein, we present a stable water-soluble cobalt complex supported by a dianionic 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol) ligand scaffold, which is a rare example of a high-oxidation species, as demonstrated by structural, spectroscopic and theoretical data. Electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements revealed that the CoIV center of the mononuclear complex in the solid state resides in the high spin state (sextet, S=5/2). The complex can effectively catalyze water oxidation via a single-site water nucleophilic attack pathway with an overpotential of only 360 mV in a phosphate buffer with a pH of 6. The key intermediate toward water oxidation was speculated based on theoretical calculations and was identified by in situ spectroelectrochemical experiments. The results are important regarding the accessibility of high-oxidation state metal species in synthetic models for achieving robust and reactive oxidation catalysis.

A Water‐Soluble Highly Oxidizing Cobalt Molecular Catalyst Designed for Bioinspired Water Oxidation

Herein, we present a stable water-soluble cobalt complex supported by a dianionic 2,2′-([2,2′-bipyridine]-6,6′-diyl)bis(propan-2-ol) ligand scaffold, which is a rare example of a high-oxidation species, as demonstrated by structural, spectroscopic and theoretical data. Electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements revealed that the CoIV center of the mononuclear complex in the solid state resides in the high spin state (sextet, S=5/2). The complex can effectively catalyze water oxidation via a single-site water nucleophilic attack pathway with an overpotential of only 360 mV in a phosphate buffer with a pH of 6. The key intermediate toward water oxidation was speculated based on theoretical calculations and was identified by in situ spectroelectrochemical experiments. The results are important regarding the accessibility of high-oxidation state metal species in synthetic models for achieving robust and reactive oxidation catalysis.

Effect of R substitution in spin glass RFeTi2O7 compounds

Zirconolite oxides R3+Fe3+Ti2O7 (R rare earth element) are known to exhibit spin glass behaviour at low temperatures. Here we present a detailed study of these compounds for R = Eu, Gd, Dy, Ho, and Er, together with reviewed previous measurements on Sm, Tb, Tm, Yb and Lu, with the scope of determining the role played by the rare earth on their magnetic properties. They have been investigated using X-ray powder diffraction, and further characterized by magnetization, frequency dependent ac susceptibility and heat capacity measurements. RFeTi2O7 compounds are all isostructural showing orthorhombic structure, space group Pcnb at 300 K. Disorder of the magnetic ions in the RFeTi2O7 lattice induces spin glass behaviour at low temperatures, mainly due to the Fe sublattice. We show that magnetic rare earth ions participate in the spin glass state tuning its properties. The single ion anisotropy of the R3+ ions, excluding exchange interaction with other magnetic ions, has been calculated by ab initio methods, and expressed in terms of the g tensor of the ground doublet or quasi-doublet in Kramers (Sm, Dy, Er, Yb) and non-Kramers (Tb, Ho) ions, respectively, in an effective spin S* = 1/2 model. In the case of R with a singlet ground state (Eu, Tm) or a multiplet state (Gd), the ion is isotropic. We show that the relative increase in the spin-glass temperature ΔTSGR/TSGFe with respect to the LuFeTi2O7, where Lu is non-magnetic, correlates qualitatively with the product of the ratio gz/gJ (R = Tb, Dy, Ho, and Er), or g⊥/gJ (R = Sm), times the ratio of exchange interactions J~R,FeJFe,Fe determined from the paramagnetic room temperature susceptibility measurements. Besides, for increasing anisotropy the spin glass transition dynamics slows down to values typical of cluster glass. The coercive field below the transition is increased in the same trend. This paper explains the effect of the R-Fe exchange interaction and R single ion anisotropy on the spin-glass behaviour of these compounds.

Structural Transformations in the Thermal Dehydration of [Cu2(bpa)(btec)(H2O)4]n Coordination Polymer.

Reactions between pyridinic ligands such as 1,2-bis(4-pyridyl)ethane (bpa) and transition metal cations are a very widespread technique to produce extended coordination polymers such as Metal-Organic Frameworks. In combination with a second ligand these systems could present different topologies and behaviors. In this context, the use of 1,2,4,5-benzenetetracarboxylic acid (H4btec) gave us a novel 2D compound, [Cu2(bpa)(btec)(H2O)4]n (1), which was prepared by microwave-assisted synthesis and structurally characterized by means of single crystal X-ray diffraction. Its thermal behavior was analyzed through thermogravimetric analysis and variable temperature powder X-ray diffraction, concluding that thermal stability is influenced by the coordination water molecules, allowing two sequential thermochromic phase transformations to take place. These transformations were monitored by electronic paramagnetic resonance spectroscopy and magnetic susceptibility measurements. In addition, the crystal structure of the anhydrous compound [Cu2(bpa)(btec)]n (1.ah) was determined. Finally, a topological study was carried out for the bpa ligand considering all the structures deposited in the Cambridge Structural Databased. More than 1000 structures were analyzed and classified into 17 different topologies, according to the role of the ligand.

A high-spin ground-state donor-acceptor conjugated polymer.

Interest in high-spin organic materials is driven by opportunities to enable far-reaching fundamental science and develop technologies that integrate light element spin, magnetic, and quantum functionalities. Although extensively studied, the intrinsic instability of these materials complicates synthesis and precludes an understanding of how fundamental properties associated with the nature of the chemical bond and electron pairing in organic materials systems manifest in practical applications. Here, we demonstrate a conjugated polymer semiconductor, based on alternating cyclopentadithiophene and thiadiazoloquinoxaline units, that is a ground-state triplet in its neutral form. Electron paramagnetic resonance and magnetic susceptibility measurements are consistent with a high-to-low spin energy gap of 9.30 × 10-3 kcal mol-1. The strongly correlated electronic structure, very narrow bandgap, intramolecular ferromagnetic coupling, high electrical conductivity, solution processability, and robust stability open access to a broad variety of technologically relevant applications once thought of as beyond the current scope of organic semiconductors.

Realization of an excellent two-dimensional Heisenberg ferromagnetic system: the synthesis, structure, and thermodynamic properties of piperazinediium tetrabromocuprate

A 2D Heisenberg ferromagnet with ultra-small interlayer coupling realized in a metal–organic crystal.

Ceric Ammonium Nitrate and Ceric Ammonium Chloride as Precursors for Ceric Siloxides: Ammonia and Ammonium Inclusion

Treatment of ceric ammonium nitrate (CAN) with sodium tris(tert‐butoxy)siloxide in THF afforded crystalline complexes [Ce{OSi(OtBu)3}3(NO3)(NH3)2(THF)] and [Ce{OSi(OtBu)3}4(NH3)2] featuring strong ammonia coordination. Conducting the CAN/NaOSi(OtBu)3 (1:4) reaction in acetonitrile gave ceric ate complex [Ce{OSi(OtBu)3}3(NO3)2(NH4)(NCCH3)] where the ammonium cation is embedded in the siloxy ligand sphere via hydrogen bonding. Salt‐metathetic conversion of ceric organo(R)ammonium hexachlorides (CACR, R = Me, Et) with NaOSi(OtBu)3 and sodium siloxides NaOSiR3 (R = Et, iPr) in THF allowed the isolation of ceric siloxide complexes [Ce{OSi(OtBu)3}4], [Ce(OSiEt3)4(THF)0.5], and [Ce(OSiiPr3)4]. Although the reaction products are prone to ate complex formation with the organoammonium chloride side products, the salt‐metathesis route gave good yields for homoleptic [Ce{OSi(OtBu)3}4]. The CACR/alkylsiloxide reaction mixtures also suffer from (photo)redox processes as revealed by the formation of trivalent [Ce(OSiiPr3)4(NMe4)], but can be further stabilized by pyridine coordination as shown for the solid‐state structure of [Ce(OSiiPr3)4(py)]. The siloxide complexes were characterized by NMR, DRIFT, UV/Vis spectroscopy and cyclic voltammetry, as well as paramagnetic susceptibility measurements, X‐ray structure, and elemental analysis. The electrochemical studies revealed an effective stabilization of the CeIV ion within the siloxy ligand environments.

Direct Observation of Node-to-Node Communication in Zeolitic Imidazolate Frameworks.

Zeolitic imidazolate frameworks (ZIFs) with open-shell transition metal nodes represent a promising class of highly ordered light harvesting antennas for photoenergy applications. However, their charge transport properties within the framework, the key criterion to achieve efficient photoenergy conversion, are not yet explored. Herein, we report the first direct evidence of a charge transport pathway through node-to-node communication in both ground state and excited state ZIFs using the combination of paramagnetic susceptibility measurements and time-resolved optical and X-ray absorption spectroscopy. These findings provide unprecedented new insights into the photoactivity and charge transport nature of ZIF frameworks, paving the way for their novel application as light harvesting arrays in diverse photoenergy conversion devices.

Magnetic characterization of microcrystalline Na3Ln0.99–xEr0.01Crx(PO4)2 orthophosphates synthesized by Pechini method (Ln = La, Gd)

Abstract Na3Ln(PO4)2 orthophosphates (Ln = La, Gd) doped with Er3+ and co-doped with Cr3+ ions were synthesized by Pechini method and characterized by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Low temperature EPR spectra were detected and analyzed in terms of temperature dependence and the structure of the obtained materials. They show that erbium and chromium ions substitute Ln3+ and also Na+ ions or Na+ channels forming complex EPR spectra. Both kinds of ions reveal ferromagnetic type of interaction which shows some anomaly at the temperature between 10 K and 15 K. Magnetic susceptibility reveals a weak antiferromagnetic kind of interaction dominating in the whole temperature range, from 3.5 to 300 K.

Co(II), Ni(II), Cu(II) and Zn(II) complexes of aminothiazole‐derived Schiff base ligands: Synthesis, characterization, antibacterial and cytotoxicity evaluation, bovine serum albumin binding and density functional theory studies

Transition metal complexes of type M(L)2(H2O)x were synthesized, where L is deprotonated Schiff base 2,4‐dihalo‐6‐(substituted thiazol‐2‐ylimino)methylphenol derived from the condensation of aminothiazole or its derivatives with 2‐hydroxy‐3‐halobenzaldehyde and M = Co2+, Ni2+, Cu2+ and Zn2+ (x = 0 for Cu2+ and Zn2+; x = 2 for Co2+ and Ni2+). The synthesized Schiff bases and their metal complexes were thoroughly characterized using infrared, 1H NMR, electronic and electron paramagnetic resonance spectroscopies, elemental analysis, molar conductance and magnetic susceptibility measurements, thermogravimetric analysis and scanning electron microscopy. The results reveal that the bidentate ligands form complexes having octahedral geometry around Co2+ and Ni2+ metal ions while the geometry around Cu2+ and Zn2+ metal ions is four‐coordinated. The geometries of newly synthesized Schiff bases and their metal complexes were fully optimized in Gaussian 09 using 6–31 + g(d,p) basis set. Fluorescence quenching data reveal that Zn(II) and Cu(II) complexes bind more strongly to bovine serum albumin in comparison to Co(II) and Ni(II) complexes. The ligands and their complexes were evaluated for in vitro antibacterial activity against Escherichia coli ATCC 25922 (Gram negative) and Staphylococcus aureus ATCC 29213 (Gram positive) and cytotoxicity against lever hepatocellular cell line HepG2.

Cerium(IV) Neopentoxide Complexes.

Treatment of ceric ammonium nitrate (CAN) with varying amounts of sodium neopentoxide led to the isolation of crystalline cerium(IV) complexes [Ce(OCH2tBu)2(NO3)2(HOCH2tBu)2], [Ce(OCH2tBu)3(NO3)-(NCCH3)]2, [Ce2(OCH2tBu)7(NO3)]2, and [Ce2(OCH2tBu)9Na-(THF)] featuring CeIV/(OR) ratios of 1:2, 1:3, 1:3.5, and 1:4.5, respectively. The complexes are light-sensitive and prone to ligand redistribution as evidenced by multicomponent NMR spectra as well as the formation of [{Ce(OCH2tBu)4}2(THF)] and the mixed-valent complex [Ce3(OCH2tBu)9(NO3)2]. The CAN protocol also gave access to the isopropoxide derivative [Ce(OiPr)3(NO3)(THF)]2. The reaction of [Et4N]2[CeCl6] (CAC, ceric organoammonium chloride) with different equivalents of Na(OCH2tBu) was also impaired by ligand reorganization and ate complexation as detected for the tetravalent cerium complex [Ce2(OCH2tBu)7Cl2][Et4N]. Protonolysis of [Ce{N(SiHMe2)2}4] with 4 equiv of HOCH2tBu afforded donor-free homoleptic [Ce(OCH2tBu)4]3 in quantitative yield. All complexes were characterized by NMR, DRIFT, and UV-vis spectroscopy, as well as paramagnetic susceptibility measurements, X-ray structure analysis, and elemental analysis.

Unusual ferroelectric and magnetic phases in multiferroic 2H−BaMnO3 ceramics

The structural phase transition in hexagonal BaMnO$_3$ occurring at $T_c$=130 K was studied in ceramic samples using electron and X-ray diffraction, second harmonic generation as well as by dielectric and lattice dynamic spectroscopies. The low-temperature phase (space group $P6_{3}cm$) is ferroelectric with a triplicated unit cell. The phase transition is driven by an optical soft mode from the Brillouin-zone boundary [$q = (\frac{1}{3},\frac{1}{3},0)$]; this mode activates in infrared and Raman spectra below $T_c$ and it hardens according to the Cochran law. Upon cooling below $T_c$, the permittivity exhibits an unusual linear increase with temperature; below 60 K, in turn, a frequency-dependent decrease is observed, which can be explained by slowing-down of ferroelectric domain wall motions. Based on our data we could not distinguish whether the high-temperature phase is paraelectric or polar (space groups $P6_{3}/mmc$ or $P6_{3}mc$, respectively). Both variants of the phase transition to the ferroelectric phase are discussed based on the Landau theory. Electron paramagnetic resonance and magnetic susceptibility measurements reveal an onset of one-dimensional antiferromagnetic ordering below $\approx220\,\rm K$ which develops fully near 140 K and, below $T_{n} \approx 59\,\rm K$, it transforms into a three-dimensional antiferromagnetic order.

Temperature Dependent Magnetic Susceptibility and Equilibrium Studies in Solution: Evans’s Method

The chemical shift in a solvent caused by the presence of a paramagnetic species and the temperature dependence of the chemical shift difference to determine the temperature dependent magnetic susceptibility can be studied using Evans method. The method is very useful because a common NMR instrument, easily accessible in a department of chemistry, allows the accurate measurement of paramagnetic susceptibilities. The present review highlights planar (S = 0) and octahedral (S = 1) forms of Ni(II) complexes are in equilibrium and has been followed in the temperature range 298 − 338 K by 1H NMR technique using the protocol of Evans’s method.

Investigations on magnetic properties of La3Ga5.5Ta0.5O14 single crystals doped with Er3+ ions

La3Ga5.5Ta0.5O14 (LGT) single crystals doped with 3.8 at. % of Er3+ ions have been investigated by performing Electron Paramagnetic Resonance (EPR) and magnetic susceptibility measurements. The results of EPR measurements show a large anisotropy of spin Hamiltonian parameters belonging to isolated erbium centers. In addition to the isolated erbium centers a small number of pairs of erbium ions was also observed. Based on the obtained spin Hamiltonian parameters we have determined the symmetry of isolated and paired erbium ions as being at least C4. From magnetization measurements it does not result strong coupling between Er3+ ions and the lattice of LGT crystals.

Thermal and Magnetic Diversity in the Behaviour of the CuII‐bdc‐bpa System: 1D, 2D and Interpenetrated 3D Frameworks

Metal organic frameworks (MOFs) are crystalline materials based on connections between metal ions through organic ligands. The combination of polycarboxylate anions and dipyridyl ligands is an effective strategy for producing extended structures. In this sense, the combination of bpa with m‐bdc [bpa = 1,2‐bis(4‐pyridyl)ethane, m‐bdc = 1,3‐benzenedicarboxylate] has produced three new CuII‐based networks with different dimensionalities, namely the 1D [Cu(m‐bdc)(bpa)0.5(H2O)dmf]·0.5H2O (1, synthesis based on sonication and slow evaporation), the 2D [Cu6(m‐bdc)6(bpa)6(dmf)3]·8dmf (2, microwave synthesis) and the 3D [Cu4(m‐bdc)4(bpa)2dmf]·dmf (3, microwave synthesis). The three compounds have been structurally characterized by means of single‐crystal X‐ray diffraction, IR and UV/Vis spectroscopy and elemental analysis. An unprecedented CuII dimer has been observed for compound 3. The thermal behaviour of the compounds was analysed by thermogravimetry (TG/DSC) and X‐ray thermodifractometry (TDX), and it was concluded that the dimensionality plays an important role in their thermal properties. Electronic paramagnetic resonance and magnetic susceptibility measurements for compounds 1 and 3 showed that antiferromagnetic interactions exist between the metal ions.

Magnetic properties of La3Ga5.5Ta0.5O14 single crystals doped with Sm3+

Electron Paramagnetic Resonance (EPR) and magnetic susceptibility measurements have been performed for La3Ga5.5Ta0.5O14 single crystal doped intentionally with 3 wt% of Sm3+ ions. X-band EPR measurements have been carried out with covering the field range 0–1.4 T and the temperature range 3–300 K. It has been found that two magnetically inequivalent sites having at least C2 symmetry are responsible for the EPR spectra. The static magnetization measurements as a function of temperature and magnetic field have been done using a vibrating sample magnetometer (VSM). The field and temperature dependences of magnetization are measured in zero-field cooled (ZFC) and field cooled (FC) regimes in an applied field of 100 Oe. It is seen from magnetization measurements that the strong coupling regime is due to the presence of a strong coupling of the Sm3+ ions to the lattice of LGT crystals.

The magnetic properties, DNA/HSA binding and nuclease activity of manganese N-confused porphyrin

The first oxidative cleavage of DNA by manganese [Formula: see text]-confused porphyrin [chloro(2-aza-2-methyl-5,10,15,20-tetrakis([Formula: see text]-chlorophenyl)-21-carbaporphyrin)manganese(III), 1] using H2O2 as oxidant agent and its magnetic, calf thymus DNA(ct-DNA)- and human serum albumin (HSA) binding properties were investigated. The magnitude of the axial (D) zero-field splitting for the mononuclear Mn(III) center in 1 was determined to be approximately 2.71 cm[Formula: see text] by paramagnetic susceptibility measurements. The DNA- and HSA binding experimental results showed that 1 bound to ct-DNA via an outside groove binding mode and the hydrophobic cavity located in subdomain IIA of HSA with the binding constant of 4.144 × 105 M[Formula: see text] and [Formula: see text] 106 M[Formula: see text], respectively. Thermodynamic parameters revealed that both DNA- and HSA binding were spontaneous process. The main driven forces were the hydrogen bond and van der Waals for the former, but hydrophobic interaction for the latter, which were further confirmed by molecular docking modeling. Manganese [Formula: see text]-confused porphyrin 1 could cleave the supercoiled plasmid DNA efficiently in the presence of hydrogen peroxide. Hydroxyl radical ([Formula: see text]OH) was found the active species for oxidative damage of DNA.

A Vanadium Chalcogenide Dicubane

AbstractThe discovery of novel transition metal cubane clusters with different electron counts and topologies is of considerable importance due to the prevalence of this structure type in catalytic proteins. Here, we report the solvothermal synthesis and full characterization of [V7S8Cl2(en)8]Cl4 (en = ethylenediamine), the first early transition metal dicubane cluster. Electron paramagnetic resonance and magnetic susceptibility measurements show that the cluster has one unpaired electron localized on a single V4+, which according to DFT simulations is the central bridging vanadium. The remaining V3+ atoms feature significant metal–metal bonding.

Effects of Gd3+: Ag co-doping on structural and magnetic properties of lead tellurite glass ceramics

Lead tellurite glass ceramics doped with variable amounts of Gd3+ ions and co-doped with fixed amounts of silver (Ag2O or Ag metallic nanoparticles, AgNPs) have been prepared and investigated by X-ray diffraction (XRD), density, electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The EPR spectra of the studied samples exhibit the typical ‘‘U’’-type shape with effective g-values of ≈6, 2.8, 2.0 and a weaker feature at g≈4.8. At higher contents of Gd3+ ions the spectral features are broadened and finally are dominated by a broad absorption line located at g≈2.0 and a weaker feature at g≈6. The broad EPR line centered at g≈2.0 is associated to the Gd3+ ions present predominantly as clustered species. The paramagnetic susceptibility (χ) was measured at various temperatures and from the 1/χ versus T dependence the molar Curie constant (CM) and the paramagnetic Curie temperature (θp) have been evaluated. Magnetic susceptibility data show that for low Gd2O3 contents, x≤3mol%, the Gd3+ ions are randomly distributed in the host glass ceramics matrix and are present only as isolated species. For higher Gd2O3 contents, x>3mol%, the Gd3+ ions appear as both isolated and antiferromagnetically coupled species.