Magnetism and Molecular Nonlinear Optical Second-Order Response Meet in a Spin Crossover Complex

The quadratic hyperpolarizability of two inorganic Schiff base metal complexes which differ from each other by the nature of the central metal ion (Fe^II or Zn^II) is estimated using hyper-Rayleigh light-scattering (HRS) measurements. The investigated Fe^II microcrystals exhibit a thermal spin-crossover (SCO) from a diamagnetic to a paramagnetic state centered at <i>T</i>_1/2 = 233 K that can be reproduced by the HRS signal whose modest intensity is mainly due to their centrosymmetric packing structure. Diamagnetic Zn^II microcrystals even lead to much weaker (∼400 times) HRS intensities which are in addition temperature-independent. These observations allow us to ascribe the change in HRS of the Fe^II complex to two contributions, namely, the molecular SCO phenomenon and the crystal orientation with respect to the light polarization. A connection between the SCO and a nonlinear optical property has thus been demonstrated for the first time, with potential future applications in photonics

Metal Substitution Effects on the Charge Transport and Spin Crossover Properties of [Fe_1–<i>x</i>Zn_<i>x</i>(Htrz)₂(trz)](BF₄) (trz = Triazole)

In this study we analyze the metal substitution effects on the structural, morphological, charge transport, and spin transition properties of the [Fe_1–<i>x</i>Zn_<i>x</i>(Htrz)₂(trz)]­(BF₄) (trz = triazole, <i>x</i> = 0, 0.26, or 0.43) compound using electron microscopy, powder X-ray diffraction, optical reflectivity, Raman, FTIR, ⁵⁷Fe Mössbauer, and broadband (10^–2–10⁶ Hz) dielectric spectroscopies. The crystal structure and the morphology of the objects remain nearly unaffected, whereas the thermal spin transition shifts from 362 to 316 K and the thermal hysteresis width decreases from 45 to 8 K for increasing values of <i>x</i>. For each compound the electrical conductivity drops when the iron­(II) electronic configuration is switched from the low-spin to the high-spin state. A strong overall decrease in conductivity with increasing Zn concentration is also observed in both spin states. These results, together with the analysis of the charge carrier dynamics, suggest that the ferrous ions participate directly in the charge transport mechanism, explaining the strong spin-state dependence of the electrical properties in this compound

Fe^II(pap-5NO₂)₂ and Fe^II(qsal-5NO₂)₂ Schiff-Base Spin-Crossover Complexes: A Rare Example with Photomagnetism and Room-Temperature Bistability

We focus here on the properties of Fe complexes formed with Schiff bases involved in the chemistry of Fe^III spin-transition archetypes. The neutral Fe­(pap-5NO₂)₂ (<b>1</b>) and Fe­(qsal-5NO₂)₂·Solv (<b>2</b> and <b>2·Solv</b>) compounds (<b>Solv</b> = 2H₂O) derive from the reaction of Fe^II salts with the condensation products of pyridine-2-carbaldehyde with 2-hydroxy-5-nitroaniline (Hpap-5NO₂) or 5-nitrosalicylaldehyde with quinolin-8-amine (Hqsal-5NO₂), respectively. While the Fe­(qsal-5NO₂)₂·Solv solid is essentially low spin (S = 0) and requires temperatures above 300 K to undergo a S = 0 ↔ S = 2 spin-state switching, the Fe­(pap-5NO₂)₂ one presents a strongly cooperative first-order transition (<i>T</i>↓ = 291 K, <i>T</i>↑ = 308 K) centered at room temperature associated with a photomagnetic effect at 10 K (<i>T</i>_LIESST = 58 K). The investigation of these magnetic behaviors was conducted with single-crystal X-ray diffraction (<b>1</b>, 100 and 320 K; <b>2</b>, 100 K), Mössbauer, IR, UV–vis (<b>1</b> and <b>2·Solv</b>), and differential scanning calorimetry (<b>1</b>) measurements. The Mössbauer analysis supports a description of these compounds as Fe^II Schiff-base complexes and the occurrence of a metal-centered spin crossover process. In comparison with Fe^III analogues, it appears that an expanded coordination sphere stabilizes the valence 2+ state of the Fe ion in both complexes. Strong hydrogen-bonding interactions that implicate the phenolato group bound to Fe^II promote the required extra-stabilization of the S = 2 state and thus determines the spin transition of <b>1</b> centered at room temperature. In the lattice, the hydrogen-bonded sites form infinite chains interconnected via a three-dimensional network of intermolecular van der Waals contacts and π–π interactions. Therefore, the spin transition of <b>1</b> involves the synergetic influence of electrostatic and elastic interactions, which cause the enhancement of cooperativity and result in the bistability at room temperature

Synthesis of Nanoscale Coordination Polymers in Femtoliter Reactors on Surfaces

In the present work, AFM-assisted lithography was used to perform the synthesis of a coordination polymer inside femtoliter droplets deposited on surfaces. For this, solutions of the metal salt and the organic ligand were independently transferred to adjacent tips of the same AFM probe array and were sequentially delivered on the same position of the surface, creating femtoliter-sized reaction vessels where the coordination reaction and particle growth occurred. Alternatively, the two reagents were mixed in the cantilever array by loading an excess of the inks, and transferred to the surface immediately after, before the precipitation of the coordination polymer took place. The <i>in situ</i> synthesis allowed the reproducible obtaining of round-shaped coordination polymer nanostructures with control over their <i>XY</i> positioning on the surface, as characterized by microscopy and spectroscopy techniques

Unprecedented Size Effect on the Phase Stability of Molecular Thin Films Displaying a Spin Transition

An unexpected upshift of the spin transition temperature by ca. 3 K is observed in thermally evaporated films of the [Fe^II(HB­(tz)₃)₂] (tz = 1,2,4-triazol-1-yl) complex when reducing the film thickness from ca. 200 to 45 nm. Fitting the experimental data to continuum mechanics and thermodynamical models allows us to propose an explanation based on the anisotropy of the transformation strain leading to ∼5 mJ/m² higher 00<i>l</i> surface energy in the high-spin phase

Piezoresistive Effect in the [Fe(Htrz)₂(trz)](BF₄) Spin Crossover Complex

We report on the effect of hydrostatic pressure on the electrical conductivity and dielectric permittivity of the [Fe­(Htrz)₂(trz)]­(BF₄) (Htrz = 1<i>H</i>-1,2,4,-triazole) spin crossover complex. Variable-temperature and -pressure broad-band impedance spectrometry revealed a piezoresistive effect of more than 1 order of magnitude for pressures as low as 500 bar, associated with a large pressure-induced hysteresis of 1700 bar. The origin of the piezoresistive effect has been attributed to the pressure-induced spin state switching in the complex, and the associated <i>P</i>,<i>T</i> phase diagram was determined

Spectroscopic and Magnetic Properties of the Metastable States in the Coordination Network [{Co(prm)₂}₂{Co(H₂O)₂}{W(CN)₈}₂]·4H₂O (prm = pyrimidine)

The study of the metastable states, obtained by thermal quenching or by light irradiation in the [{Co­(prm)₂}₂{Co­(H₂O)₂}­{W­(CN)₈}₂]·4H₂O complex, is reported using powder X-ray diffraction, Raman spectroscopy, optical reflectivity, and magnetic measurements. This compound is characterized by a electron-transfer (ET) phase transition occurring between a high-temperature phase (HT phase) formed by paramagnetic Co^II–W^V units and a low-temperature phase (LT phase) formed by diamagnetic Co^III–W^IV units. Metastable phases can be induced at low temperature either by thermal quenching rapidly cooling phase named RC or by irradiation photo-induced phase named PI similar to the well-known Light-Induced Excited Spin State Trapping effect. The relaxation dynamics of the metastable phases have been studied and revealed some differences between the RC and PI phases. The sigmoidal shape of the relaxation curves in the RC phase is in agreement with the cooperative nature of the process. Thermodynamic parameters that govern the relaxation have been determined and used to reproduce the experimental Thermal-Induced Excited Spin State Trapping curve

Homoleptic Iron(II) Complexes with the Ionogenic Ligand 6,6′-Bis(1<i>H</i>‑tetrazol-5-yl)-2,2′-bipyridine: Spin Crossover Behavior in a Singular 2D Spin Crossover Coordination Polymer

Deprotonation of the ionogenic tetradentate ligand 6,6′-bis­(1<i>H</i>-tetrazol-5-yl)-2,2′-bipyridine [H₂bipy­(ttr)₂] in the presence of Fe^II in solution has afforded an anionic mononuclear complex and a neutral two-dimensional coordination polymer formulated as, respectively, NEt₃H­{Fe­[bipy­(ttr)₂]­[Hbipy­(ttr)₂]}·3MeOH (<b>1</b>) and {Fe­[bipy­(ttr)₂]}<i>_n</i> (<b>2</b>). The anions [Hbipy­(ttr)₂]⁻ and [bipy­(ttr)₂]^2– embrace the Fe^II centers defining discrete molecular units <b>1</b> with the Fe^II ion lying in a distorted bisdisphenoid dodecahedron, a rare example of octacoordination in the coordination environment of this cation. The magnetic behavior of <b>1</b> shows that the Fe^II is high-spin, and its Mössbauer spectrum is characterized by a relatively large average quadrupole splitting, Δ<i>E</i>_Q = 3.42 mm s^–1. Compound <b>2</b> defines a strongly distorted octahedral environment for Fe^II in which one [bipy­(ttr)₂]⁻ anion coordinates the equatorial positions of the Fe^II center, while the axial positions are occupied by peripheral <i>N</i>-tetrazole atoms of two adjacent {Fe­[bipy­(ttr)₂]}⁰ moieties thereby generating an infinite double-layer sheet. Compound <b>2</b> undergoes an almost complete spin crossover transition between the high-spin and low-spin states centered at about 221 K characterized by an average variation of enthalpy and entropy Δ<i>H</i>^av = 8.27 kJ mol^–1, Δ<i>S</i>^av = 37.5 J K^–1 mol^–1, obtained from calorimetric DSC measurements. Photomagnetic measurements of <b>2</b> at 10 K show an almost complete light-induced spin state trapping (LIESST) effect which denotes occurrence of antiferromagnetic coupling between the excited high-spin species and <i>T</i>_LIESST = 52 K. The crystal structure of <b>2</b> has been investigated in detail at various temperatures and discussed

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