Decay of a metastable high-spin state in spin-crossover compounds: mean first passage time analysis

We here focus on the effect of a transition from metastable high-spin to stable low-spin states in spin-crossover compounds in contact with heat bath as external realistic (colored) noises. The systems are described by the way of stochastic methods using a phenomenological model of photoexcitation and relaxation in spin-crossover compounds. It is shown that such transitions can be controlled by a parameter that governs relaxation flow, intensity of light and noise intensity.     

Light-induced formation of metastable high-spin states in [Fe(mtz)6](CiO4)2

[Fe(mtz)₆](CiO₄)₂ (mtz=1-methyltetrazole) is a spin crossover compound with two different iron(II) lattice sites. Only one of them (lattice site A) shows thermally induced high spin (HS) low spin (LS) spin transition. The LIESST effect (Light-Induced Excited Spin State Trapping) can be observed below 50 K. Complex molecules in B-sites remain in HS state at all temperatures. At 20 K irradiation with red light causes a partial conversion to another HS species, HS(C), with also practically infinite lifetime.In partial fulfilment of the Ph.D. thesis.     

Fe(II) spin-crossover compounds based on the extremely versatile ligand system of 1-substituted tetrazoles: a comparative study

Bulk measurements of magnetic susceptibility χ and ⁵⁷Fe-Mössbauer spectroscopy were used to investigate the low spin (LS) to high spin (HS) transition caused by temperature changes for different mono- and polynuclear Fe(II) complexes based on the extremely versatile class of 1-substituted tetrazoles. The influence of length and rigidity of different terminating and bridging ligands is shown.     

Controlling cooperativity of a metastable open system coupled weakly to a noisy environment

The notion of cooperativity comprises a specific characteristic of a multipartite system concerning its ability to demonstrate a sigmoidal-type response of varying sensitivities to input stimuli in transitions between states under controlled conditions.From a statistical physics viewpoint,in this work we attempt to describe the cooperativity by the stability of a metastable open system with respect to irreversibility.To treat the evolution of a system weakly coupled to the environment in a kinetic framework,we consider two fluctuating energy levels of different dimensionalities,initial population of one level,reversible transitions of population between the levels,and irreversible depopulation of another level.An average is made over level fluctuations and environment vibrations so that an inter-level transition rate can be obtained accounting for the influences of external control on level position and dimensionality.It is found that the cooperativity of the two-level system is bounded approximately between 0.736 and unity,with the lower bound indicating worsening system stability.     

EPR of the first Fe(III)-containing spin-crossover metallomesogens

The low-spin (LS) to high-spin (HS) transition in two mesogenic Fe(III) complexes with alkyloxysalicyliden-N^′-ethyl-N-ethylendiamine as ligands is studied by electron paramagnetic resonance (EPR). The symmetrization of the crystal field around the Fe(III) ion under first heating of a polycrystalline sample from the room temperature was observed in the spectra of HS complexes. The line narrowing (from 45 to 15 mT) under the crystal-smectic phase transition is explained by the strengthening of the intermolecular exchange interaction, as a result of the structural reorganization of layers in the smectic phase. A feature of both mesogenic spin-transition systemsis an unusual field-induced spin instability which leads to the hysteresis of the HS-LS composition. This instability is detected by EPR and magnetic susceptibility methods. The alignment of systems by magnetic field in the mesophase and a complete or partial orientational order under cooling the mesophase to the glass state are the main reason for this hysteresis. The influence of EPR-silent LS complexes reveals itself in the line broadening of HS compounds’ when the temperature is lowered from 220 K.     

Positive muons in spin-crossover complex [Fe(NCS)2(phen)2]

Diamagnetic muon species in a spin-crossover complex diisothiocyanatobisphenanthrolineiron(II) were investigated: two species with different spin relaxation functions and rates were observed over the range from 21 to 293 K. The sum of their yields changed at about the spin transition temperature of the complex, where the spin state of the complex changes from paramagnetic to diamagnetic when it is cooled down across the temperature. The results suggest that paramagnetic muon species could exhibit diamagnetic muon-like SR signals in paramagnetic environment.     

Analysis of NMR shifts of high-spin cobalt(II) pyrazolylborate complexes

We present a detailed study on the correlation of the nuclear magnetic resonance (NMR) parameters with the results of density functional theory (DFT) calculations performed for paramagnetic high-spin cobalt(II) complexes with trispyrazolylborate ligands. This work is a first attempt to calculate dipolar and contact shifts in high-spin cobalt(II) pyrazolylborate systems. The calculation results show frontier orbitals that may be responsible for the contact shift. The contact shift values are compared with the dipolar shift ones. The latter shift values were both obtained from experimental data and calculated using DFT methods. We attempt to explain the dipolar effect, which may have a great share in NMR chemical shift of paramagnetic compounds.     

Precipitation of a metastable Fe(Ag) solid solution upon annealing of supersaturated Fe(Ag) thin film prepared by pulsed laser deposition

Upon annealing, a single-phase, bcc Fe_1-xAg_x (x=1.8 or 3.2 at. %) thin film, prepared by pulsed laser deposition, simultaneously undergoes grain growth and precipitation of a bcc Fe(Ag,9-10 at. %) solid solution at the grain boundaries. At a high annealing temperature or after a long annealing time the metastable state transforms to the equilibrium state, pure Fe and pure Ag. A quantitative kinetic growth model has been proposed to explain that this non-equilibrium phenomenon results from surface diffusion. PACS 66.30.Pa; 68.35.Fx; 64.60.My     

Metamagnetism and weak ferromagnetism in nickel (II) oxalate crystals

Microcrystals of orthorhombic nickel (II) oxalate dihydrate were synthesized through a precipitation reaction of aqueous solutions of nickel chloride and oxalic acid. Magnetic susceptibility exhibits a sharp peak at 3.3 K and a broad rounded maximum near 43 K. We associated the lower maximum with a metamagnetic transition that occurs when the magnetic field is about ≥3.5 T. The maximum at 43 K is typical of 1D antiferromagnets, whereas weak ferromagnetism behavior was observed in the range of 3.3–43 K.     

57Fe Mössbauer spectra and spin-crossover behavior of [Fe(mbpN)(lut)]BPh4

New spin-crossover complexes [Fe(III)L(lut)]BPh₄ (L = mbpN or bpN) with N₄O₂ moiety are reported. The structure of the mbpN (N₃O₂ quinquedentate ligand) complex is determined at 293 K, and the temperature dependences of the⁵⁷Fe Mössbauer spectra and magnetic susceptibilities for both complexes have been examined.     

On/off spin-crossover phenomenon and control of the transition temperature in assembled Iron(II) complexes

The present study reveals the on/off of spin-crossover (SCO) phenomenon in assembled Fe(II) complexes bridged by bis(pyridyl) type ligand. Whether SCO phenomenon occurs or not in assembled Fe(II) complexes bridged by bis(pyridyl) type ligand is determined by local structure around iron atom. SCO phenomenon occurs when the coordinating pyridines facing to each other across the iron atom are propeller type, while the phenomenon does not occur when they are parallel type or distorted propeller type. DFT calculation explained that, in the shortening of Fe-pyridine bonds when changing from high-spin state to low-spin state, the pyridines of propeller type can approach the iron atom with smaller steric hindrance than those of parallel and distorted propeller type complexes. The local structure is controlled by introducing methyl substituent and introducing π-system, changing SCO phenomenon. And the transition temperature of SCO is also controlled in assembled complexes bridged by 1,2-bis(4-pyridyl)ethane by mixing anionic ligand.     

Dynamics of BF 4 - anion reorientation in the spin-crossover compound [Fe(1-n-propyl-1H-tetrazole) 6 ](BF 4 ) 2 and in its Zn II analogue

¹⁹ F and ¹¹ B spin-lattice relaxation times were measured in [ Zn ( ptz ) ₆ ] ( BF ₄ ) ₂ and in the spin-crossover compound [ Fe ( ptz ) ₆ ] ( BF ₄ ) ₂ . For both compounds BF ₄ ^- anion reorientation is active above 50 K. For [ Zn ( ptz ) ₆ ] ( BF ₄ ) ₂ , the anion-reorientation dynamics is different in the temperature regions of 50-90 K, 90-120 K, and above 150 K; between 120 and 150 K it changes rapidly reflecting a structural change. In [ Fe ( ptz ) ₆ ] ( BF ₄ ) ₂ the mechanism for the paramagnetic relaxation involving the ¹⁹ F nuclei is found to be of the diffusion-limited type according to the theory of Lowe and Tse. The present results prove that the spin-crossover takes place in a dynamic surrounding and not in a static crystal lattice. Received 09 February 1999 and Received in final form 14 June 1999     

Vibration Spectra of Coprecipitated Fe(III)-Ni(II) and Fe(III)-Cu(II) Hydroxides and of the Products of Their Thermolysis

A range of coprecipitated Fe(III)-Me(II) (Me(II) = Ni, Cu) hydroxides and of the products of their thermolysis have been investigated by the IR spectroscopy method. The basic absorption bands has been assigned, and the chemistry of the process of formation of coprecipitated hydroxides has been discussed from the standpoint of generation of heteronuclear hydrococomplexes.     

Colorimetric Detection of Fe(II) and Co(II) by Using Terpyridine-Based Derivative

Journal of Applied Spectroscopy - A multi-ion chromogenic sensor based on a terpyridine moiety was developed for the semiquantitative, visual, and sensitive speciation analysis of Fe2+ and Co2+...     

Structural origin of the gradual spin transition in a mononuclear iron(II) complex

Variable temperature single crystal X-ray diffraction and SQUID magnetometry experiments have revealed a gradual spin transition in [Fe^II(L)](ClO₄)₂ (where L=1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane), centred around room temperature. The gradual nature of the spin transition has been attributed to the lack of significant intermolecular interactions between iron centres and the propensity of the counter ions to accommodate the internal strain in the crystal caused by spin crossover.