108 research outputs found
Spin anisotropy effects in dimer single molecule magnets
We present a model of equal spin dimer single molecule magnets. The
spins within each dimer interact via the Heisenberg and the most general set of
four quadratic anisotropic spin interactions with respective strengths and
, and with the magnetic induction . We solve the model
exactly for , and for antiferromagnetic Heisenberg couplings
(), present curves at low for these cases. Low-
curves for and electron paramagnetic susceptibility
for are also provided. For weak anisotropy
interactions, we employ a perturbative treatment, and show that the Hartree and
extended Hartree approximations lead to reliable analytic results at low
and large for these quantities and for the inelastic neutron scattering
cross-section . Our results are discussed with
regard to existing experiments on Fe dimer
single molecule magnets, and suggest that one of them contains a substantial
amount of single-ion anisotropy, without any sizeable global spin anisotropy.
We urge further experiments of the above types on single crystals of Fe and
on some [Mn] dimers, in order to elucidate the precise values
of the various microscopic interactions.Comment: 30 pages, 25 figures, submitted to Phys. Rev.
Molecular switching in iron complexes bridged via tin-cyanides observed by Mössbauer and ESR spectroscopy
The precursor [FeIII(L)Cl] (LH2 N,N'-bis(2'-hydroxy- 3'-X-benzyliden)-1,6-diamino-3-N-hexane) is a high-spin (S 5/2) complex (with X -CH3, -O-CH3). This precursor is combined with the bridging unit [SnIV(CN)4] to yield star-shaped pentanuclear clusters, [(L-X-FeIII)4Sn(CN) 4]Cl4 57Fe-Mössbauer, 119mSn- Mössbauer, and ESR spectroscopy are used to study our samples. For X -CH3 the 57Fe-Mössbauer data show a multiple spin transition between iron(III) in the high-spin and low-spin state. Changing the functional group from X -CH3 to X -O-CH3 turns the switchability off
Single-ion and exchange anisotropy effects and multiferroic behavior in high-symmetry tetramer single molecule magnets
We study single-ion and exchange anisotropy effects in equal-spin
tetramer single molecule magnets exhibiting , , ,
, , or ionic point group symmetry. We first write the
group-invariant quadratic single-ion and symmetric anisotropic exchange
Hamiltonians in the appropriate local coordinates. We then rewrite these local
Hamiltonians in the molecular or laboratory representation, along with the
Dzyaloshinskii-Moriay (DM) and isotropic Heisenberg, biquadratic, and
three-center quartic Hamiltonians. Using our exact, compact forms for the
single-ion spin matrix elements, we evaluate the eigenstate energies
analytically to first order in the microscopic anisotropy interactions,
corresponding to the strong exchange limit, and provide tables of simple
formulas for the energies of the lowest four eigenstate manifolds of
ferromagnetic (FM) and anitiferromagnetic (AFM) tetramers with arbitrary .
For AFM tetramers, we illustrate the first-order level-crossing inductions for
, and obtain a preliminary estimate of the microscopic
parameters in a Ni from a fit to magnetization data.
Accurate analytic expressions for the thermodynamics, electron paramagnetic
resonance absorption and inelastic neutron scattering cross-section are given,
allowing for a determination of three of the microscopic anisotropy
interactions from the second excited state manifold of FM tetramers. We also
predict that tetramers with symmetries and should exhibit both
DM interactions and multiferroic states, and illustrate our predictions for
.Comment: 30 pages, 14 figures, submitted to Phys. Rev.
Modeling molecular crystals formed by spin-active metal complexes by atom-atom potentials
We apply the atom-atom potentials to molecular crystals of iron (II)
complexes with bulky organic ligands. The crystals under study are formed by
low-spin or high-spin molecules of Fe(phen)(NCS) (phen =
1,10-phenanthroline), Fe(btz)(NCS) (btz = 5,5,6,6-tetrahydro-4\textit{H},4\textit{H}-2,2-bi-1,3-thiazine), and Fe(bpz)(bipy) (bpz =
dihydrobis(1-pyrazolil)borate, and bipy = 2,2-bipyridine). All
molecular geometries are taken from the X-ray experimental data and assumed to
be frozen. The unit cell dimensions and angles, positions of the centers of
masses of molecules, and the orientations of molecules corresponding to the
minimum energy at 1 atm and 1 GPa are calculated. The optimized crystal
structures are in a good agreement with the experimental data. Sources of the
residual discrepancies between the calculated and experimental structures are
discussed. The intermolecular contributions to the enthalpy of the spin
transitions are found to be comparable with its total experimental values. It
demonstrates that the method of atom-atom potentials is very useful for
modeling organometalic crystals undergoing the spin transitions
Antisymmetric Magnetic Interactions in Oxo-Bridged Copper(II) Bimetallic Systems
The antisymmetric magnetic interaction is studied using correlated wave-function-based calculations in oxo-bridged copper bimetallic complexes. All of the anisotropic multispin Hamiltonian parameters are extracted using spin-orbit state interaction and effective Hamiltonian theory. It is shown that the methodology is accurate enough to calculate the antisymmetric terms, while the small symmetric anisotropic interactions require more sophisticated calculations. The origin of the antisymmetric anisotropy is analyzed, and the effect of geometrical deformations is addressed.
Theoretical investigation of the electronic structure of Fe(II) complexes at spin-state transitions
The electronic structure relevant to low spin (LS)high spin (HS) transitions in Fe(II) coordination compounds with a FeN6 core are studied. The selected [Fe(tz)6]2+(1) (tz=1H-tetrazole), [Fe(bipy)3]2+(2) (bipy=2,2’-bipyridine) and [Fe(terpy)2]2+ (3) (terpy=2,2’:6’,2’’-terpyridine) complexes have been actively studied experimentally, and with their respective mono-, bi-, and tridentate ligands, they constitute a comprehensive set for theoretical case studies. The methods in this work include density functional theory (DFT), time-dependent DFT (TD-DFT) and multiconfigurational second order perturbation theory (CASPT2). We determine the structural parameters as well as the energy splitting of the LS-HS states (ΔEHL) applying the above methods, and comparing their performance. We also determine the potential energy curves representing the ground and low-energy excited singlet, triplet, and quintet d6 states along the mode(s) that connect the LS and HS states. The results indicate that while DFT is well suited for the prediction of structural parameters, an accurate multiconfigurational approach is essential for the quantitative determination of ΔEHL. In addition, a good qualitative agreement is found between the TD-DFT and CASPT2 potential energy curves. Although the TD-DFT results might differ in some respect (in our case, we found a discrepancy at the triplet states), our results suggest that this approach, with due care, is very promising as an alternative for the very expensive CASPT2 method. Finally, the two dimensional (2D) potential energy surfaces above the plane spanned by the two relevant configuration coordinates in [Fe(terpy)2]2+ were computed both at the DFT and CASPT2 levels. These 2D surfaces indicate that the singlet-triplet and triplet-quintet states are separated along different coordinates, i.e. different vibration modes. Our results confirm that in contrast to the case of complexes with mono- and bidentate ligands, the singlet-quintet transitions in [Fe(terpy)2]2+ cannot be described using a single configuration coordinate
Magnetic and Photoluminescent Sensors Based on Metal-Organic Frameworks Built up from 2-aminoisonicotinate
Red Guipuzcoana de Ciencia, Tecnologia e Innovacion
OF218/2018
University of Basque Country
GIU 17/13
Basque Government
IT1005-16
IT1291-19
IT1310-19
Junta de Andalucia
FQM-394
Spanish Ministry of Science, Innovation and Universities (MCIU/AEI/FEDER, UE)
PGC2018-102052-A-C22
PGC2018-102052-B-C21
MAT2016-75883-C2-1-P
European Union (EU)
ESFIn this work, three isostructural metal-organic frameworks based on frst row transition metal ions
and 2-aminoisonicotinate (2ain) ligands, namely, {[M(μ-2ain)2]·DMF}n [MII=Co (1), Ni (2), Zn (3)], are
evaluated for their sensing capacity of various solvents and metal ions by monitoring the modulation
of their magnetic and photoluminescence properties. The crystal structure consists of an open
diamond-like topological 3D framework that leaves huge voids, which allows crystallizing two-fold
interpenetrated architecture that still retains large porosity. Magnetic measurements performed on 1
reveal the occurrence of feld-induced spin-glass behaviour characterized by a frequency-independent
relaxation. Solvent-exchange experiments lead successfully to the replacement of lattice molecules by
DMSO and MeOH, which, on its part, show dominating SIM behaviour with low blocking temperatures
but substantially high energy barriers for the reversal of the magnetization. Photoluminescence studied
at variable temperature on compound 3 show its capacity to provide bright blue emission under UV
excitation, which proceeds through a ligand-centred charge transfer mechanism as confrmed by timedependent DFT calculations. Turn-of and/or shift of the emission is observed for suspensions of 3 in
diferent solvents and aqueous solutions containing metal ions
Iron(II) complexes of tridentate indazolylpyridine ligands: enhanced spin-crossover hysteresis and ligand-based fluorescence.
Reaction of 2,6-difluoropyridine with 2 equiv of indazole and NaH at room temperature affords a mixture of 2,6-bis(indazol-1-yl)pyridine (1-bip), 2-(indazol-1-yl)-6-(indazol-2-yl)pyridine (1,2-bip), and 2,6-bis(indazol-2-yl)pyridine (2-bip), which can be separated by solvent extraction. A two-step procedure using the same conditions also affords both 2-(indazol-1-yl)-6-(pyrazol-1-yl)pyridine (1-ipp) and 2-(indazol-2-yl)-6-(pyrazol-1-yl)pyridine (2-ipp). These are all annelated analogues of 2,6-di(pyrazol-1-yl)pyridine, an important ligand for spin-crossover complexes. Iron(II) complexes [Fe(1-bip)2](2+), [Fe(1,2-bip)2](2+), and [Fe(1-ipp)2](2+) are low-spin at room temperature, reflecting sterically imposed conformational rigidity of the 1-indazolyl ligands. In contrast, the 2-indazolyl complexes [Fe(2-bip)2](2+) and [Fe(2-ipp)2](2+) are high-spin in solution at room temperature, whereas salts of [Fe(2-bip)2](2+) exhibit thermal spin transitions in the solid state. Notably, [Fe(2-bip)2][BF4]2·2MeNO2 adopts a terpyridine embrace lattice structure and undergoes a spin transition near room temperature after annealing, resulting in thermal hysteresis that is wider than previously observed for this structure type (T1/2 = 266 K, ΔT = 16-20 K). This reflects enhanced mechanical coupling between the cations in the lattice through interdigitation of their ligand arms, which supports a previously proposed structure/function relationship for spin-crossover materials with this form of crystal packing. All of the compounds in this work exhibit blue fluorescence in solution under ambient conditions. In most cases, the ligand-based emission maxima are slightly red shifted upon complexation, but there is no detectable correlation between the emission maximum and the spin state of the iron centers
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