Search results for "spin transition"
showing 10 items of 173 documents
High-Spin → Low-Spin Relaxation in [Fe(bpp)2](CF3SO3)2 H2O after LIESST and Thermal Spin-State Trapping—Dynamics of Spin Transition Versus Dynamics o…
1996
The iron(II) complex [Fe(bpp)2]-(CF3SO3)2 H2O (bpp = 2,6-bis(pyrazolyl-3-yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T1/2 (where the fraction of HS species γHS = 0.5) are 147 K and ≈285 K in the cooling and heating directions, respectively. The compound shows the LIESST and reverse-LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mossbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal …
Meltable Spin Transition Molecular Materials with Tunable Tc and Hysteresis Loop Width.
2015
Herein, we report a way to achieve abrupt high-spin to low-spin transition with controllable transition temperature and hysteresis width, relying not on solid-state cooperative interactions, but utilizing coherency between phase and spin transitions in neutral FeII meltable complexes
Spin crossover in metallomesogens
2009
Abstract In this review article are illustrated the strategies developed in order to achieve interplay/synergy between spin transition and liquid crystal transition. The synthesised Fe(II) metallomesogens exhibit different types of interplay between both phase transitions. A classification according to the analysis of the magnetic and structural data has led to the separation of three types of interplay, namely: type i systems with coupled phase transitions, subdivided into three groups a , b and c (in a the structural changes associated with the Cr ↔ LC drive the spin transition while in b these structural changes influence the spin state of the metallic centers but they are not the drivin…
Total free energy of a spin-crossover molecular system
2004
The free energy of spin-crossover molecular systems studied so far deal with the inner degrees of freedom of the spin-crossover molecules and a variety of interaction schemes between the molecules in the high spin (HS) and low spin (LS) states. Different types of transition curves, gradual, abrupt, hysteresis, and also two step transitions have been simulated or even satisfactorily fitted to experimental data. However, in the last decade spin transition curves were measured, especially under pressure, which could not be explained within these theoretical models. In this contribution the total free energy of an anharmonic lattice incorporating spin-crossover molecules which have a certain mi…
Spin Transition in Iron Compounds
1984
First-row transition metal complexes with d4 up to d7 electron configuration in octahedral ligand fields (and d8 electron configuration in six-coordinate complexes of lower symmetry) may undergo temperature dependent high spin (HS) ⇌ low spin (LS) transition, provided the ligand field strength (Δ), including low symmetry contributions, becomes comparable in magnitude with the mean spin pairing energy (P). At a critical field strength Δcrit = P, the energy levels of the two spin states cross. This is illustrated in Fig. 1, where a simplified Tanabe-Sugano type energy level diagram for d6 systems (e.g. Fe2+, Co3+) is shown as an example. For weak ligand fields, Δ Δcrit, the LS state1A1g(Oh) b…
Spin crossover properties of the [Fe(PM-BiA)2(NCS)2] complex - phases I and II
2003
International audience; In the present review, we reexamine the photomagnetic properties of the [Fe(PM-BiA)2(NCS)2], cis-bis(thiocyanato)-bis[(N-2'-pyridylmethylene)-4-(aminobiphenyl)]iron(II), compound which exhibits, depending on the synthetic method, an exceptionally abrupt spin transition (phase I) with a very narrow hysteresis (T1/2O = 168 K and T1/2N = 173 K) or a gradual spin conversion (phase II) occurring at 190 K. In both cases, light irradiation in the tail of the 1MLCT-LS absorption band, at 830 nm, results in the population of the high-spin state according to the light-induced excited spin-state trapping (LIESST) effect. The capacity of a compound to retain the light-induced HS…
Temperature- and pressure-dependent structural study of {Fe(pmd) 2[Ag(CN)2 ]2}n spin-crossover compound by neutron Laue diffraction
2014
The effect of pressure (up to 0.17GPa) on the spin-crossover compound {Fe(pmd)2[Ag(CN)2]2} n [orthorhombic isomer (II), pmd = pyrimidine] has been investigated by temperature- and pressure-dependent neutron Laue diffraction and magnetometry. The cooperative high-spin ↔ low-spin transition, centred at ca 180K at ambient pressure, is shifted to higher temperatures as pressure is applied, showing a moderate sensitivity of the compound to pressure, since the spin transition is displaced by ca 140KGPa-1. The space-group symmetry (orthorhombic Pccn) remains unchanged over the pressure-temperature (P-T) range studied. The main structural consequence of the high-spin to low-spin transition is the c…
Synergetic effect of host-guest chemistry and spin crossover in 3D Hofmann-like metal-organic frameworks [Fe(bpac)M(CN)4] (M=Pt, Pd, Ni).
2012
The synthesis and characterization of a series of three-dimensional (3D) Hofmann-like clathrate porous metal-organic framework (MOF) materials [Fe(bpac)M(CN) 4] (M=Pt, Pd, and Ni; bpac=bis(4-pyridyl)acetylene) that exhibit spin-crossover behavior is reported. The rigid bpac ligand is longer than the previously used azopyridine and pyrazine and has been selected with the aim to improve both the spin-crossover properties and the porosity of the corresponding porous coordination polymers (PCPs). The 3D network is composed of successive {Fe[M(CN) 4]} n planar layers bridged by the bis-monodentate bpac ligand linked in the apical positions of the iron center. The large void between the layers, w…
Precise Control and Consecutive Modulation of Spin Transition Temperature Using Chemical Migration in Porous Coordination Polymers
2011
Precise control of spin transition temperature (T(c)) is one of the most important challenges in molecular magnetism. A Hofmann-type porous coordination polymer {Fe(pz)[Pt(II)(CN)(4)]} (1; pz = pyrazine) exhibited cooperative spin transition near room temperature (T(c)(up) = 304 K and T(c)(down) = 284 K) and its iodine adduct {Fe(pz)[Pt(II/IV)(CN)(4)(I)]} (1-I), prepared by oxidative addition of iodine to the open metal sites of Pt(II), raised the T(c) by 100 K. DSC and microscopic Raman spectra of a solid mixture of 1-I and 1 revealed that iodine migrated from 1-I to 1 through the grain boundary after heating above 398 K. We have succeeded in precisely controlling the iodine content of {Fe…
On the Nature of the Plateau in Two-Step Dinuclear Spin-Crossover Complexes
2004
A remarkable feature of the spin-crossover process in several dinuclear iron(II) compounds is a plateau in the two-step transition curve. Up to now, it has not been possible to analyse the spin state of dinuclear pairs that constitute such a plateau, due to the relative high temperatures at which the transition takes place in complexes investigated so far. We solved this problem by experimentally studying a novel dinuclear spin-crossover compound [[Fe(phdia)(NCS)(2)](2)(phdia)] (phdia: 4,7-phenanthroline-5,6-diamine). We report here on the synthesis and characterisation of this system, which exhibits a two-step spin transition at T(c1)=108 K and T(c2)=80 K, displaying 2 K and 7 K wide therm…