6 research outputs found
Metal Substitution Effects on the Charge Transport and Spin Crossover Properties of [Fe<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>(Htrz)<sub>2</sub>(trz)](BF<sub>4</sub>) (trz = Triazole)
In
this study we analyze the metal substitution effects on the
structural, morphological, charge transport, and spin transition properties
of the [Fe<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>(Htrz)<sub>2</sub>(trz)](BF<sub>4</sub>) (trz = triazole, <i>x</i> = 0, 0.26, or 0.43) compound using electron microscopy,
powder X-ray diffraction, optical reflectivity, Raman, FTIR, <sup>57</sup>Fe Mössbauer, and broadband (10<sup>–2</sup>–10<sup>6</sup> 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
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
Piezoresistive Effect in the [Fe(Htrz)<sub>2</sub>(trz)](BF<sub>4</sub>) Spin Crossover Complex
We
report on the effect of hydrostatic pressure on the electrical
conductivity and dielectric permittivity of the [Fe(Htrz)<sub>2</sub>(trz)](BF<sub>4</sub>) (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
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<sup>II</sup>(HB(tz)<sub>3</sub>)<sub>2</sub>] (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/m<sup>2</sup> higher
00<i>l</i> surface energy in the high-spin phase
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 [H<sub>2</sub>bipy(ttr)<sub>2</sub>] in the presence of Fe<sup>II</sup> in solution has afforded
an anionic mononuclear complex and a neutral two-dimensional coordination
polymer formulated as, respectively, NEt<sub>3</sub>H{Fe[bipy(ttr)<sub>2</sub>][Hbipy(ttr)<sub>2</sub>]}·3MeOH (<b>1</b>) and
{Fe[bipy(ttr)<sub>2</sub>]}<i><sub>n</sub></i> (<b>2</b>). The anions [Hbipy(ttr)<sub>2</sub>]<sup>−</sup> and [bipy(ttr)<sub>2</sub>]<sup>2–</sup> embrace the Fe<sup>II</sup> centers
defining discrete molecular units <b>1</b> with the Fe<sup>II</sup> 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<sup>II</sup> is high-spin, and its Mössbauer spectrum is characterized
by a relatively large average quadrupole splitting, Δ<i>E</i><sub>Q</sub> = 3.42 mm s<sup>–1</sup>. Compound <b>2</b> defines a strongly distorted octahedral environment for
Fe<sup>II</sup> in which one [bipy(ttr)<sub>2</sub>]<sup>−</sup> anion coordinates the equatorial positions of the Fe<sup>II</sup> center, while the axial positions are occupied by peripheral <i>N</i>-tetrazole atoms of two adjacent {Fe[bipy(ttr)<sub>2</sub>]}<sup>0</sup> 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><sup>av</sup> = 8.27 kJ mol<sup>–1</sup>, Δ<i>S</i><sup>av</sup> = 37.5 J K<sup>–1</sup> mol<sup>–1</sup>, 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><sub>LIESST</sub> = 52 K. The crystal
structure of <b>2</b> has been investigated in detail at various
temperatures and discussed
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