Pressure and Temperature Spin Crossover Sensors with Optical Detection

Iron(II) spin crossover molecular materials are made of coordination centres switchable between two states by temperature, pressure or a visible light irradiation. The relevant macroscopic parameter which monitors the magnetic state of a given solid is the high-spin (HS) fraction denoted nHS, i.e., the relative population of HS molecules. Each spin crossover material is distinguished by a transition temperature T1/2 where 50% of active molecules have switched to the low-spin (LS) state. In strongly interacting systems, the thermal spin switching occurs abruptly at T1/2. Applying pressure induces a shift from HS to LS states, which is the direct consequence of the lower volume for the LS molecule. Each material has thus a well defined pressure value P1/2. In both cases the spin state change is easily detectable by optical means thanks to a thermo/piezochromic effect that is often encountered in these materials. In this contribution, we discuss potential use of spin crossover molecular materials as temperature and pressure sensors with optical detection. The ones presenting smooth transitions behaviour, which have not been seriously considered for any application, are spotlighted as potential sensors which should stimulate a large interest on this well investigated class of materials

Molecular and multidimensional polyoxotungstates functionalized by {Cu(bpy)}2+ groups

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Spin-transition in nearly cubic site in [Fe II (L) 3 ][PF 6 ] 2

Abstract The spin-transition ( has been investigated by 57 Fe Mössbauer spectroscopy. Analysis of the Mössbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mössbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom

Spin-transition in nearly cubic site in [FeII(L)3][PF6]2

The spin-transition (1A1 ↔ 5T2) behaviour of a new mononuclear iron(II) compound [FeII(L)3][PF6]2[L = 2-[3-(2´-pyridyl)pyrazole-1-ylmethyl]pyridine] has been investigated by 57Fe Mössbauer spectroscopy. Analysis of the Mössbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mössbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom

Photoexcitation and relaxation properties of a spin-crossover solid in the case of a stable high-spin state

The molecular solid [FeIIL2](ClO4)2·CH3CN where L is 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine provides a stable high-spin (HS) state at low temperature. Photoexcitation and subsequent relaxation have been studied using light-induced excited state spin trapping [LIESST(H → L)] in the 700−850 nm range, determination of TLIESST, relaxation curves at different temperatures, and temperature dependence of the light-induced spin equilibrium under constant irradiation. The measured photoinduced population of the metastable low-spin (LS) state (<30%) was drastically limited by the concomitant L → H photoprocess. The absence of static light-induced thermal hysteresis and the stretched exponential shape of the relaxation curves respectively revealed the absence of sizable interactions and a large spreading of the activation energies attributed to the ligand flexibility. The whole data set has been simulated using a linear rate equation, with a simplified correction for the bulk extinction of light in the powder sample

Spin-transition in nearly cubic site in [Fe<SUP>II</SUP>(L)<SUB>3</SUB>][PF<SUB>6</SUB>]<SUB>2</SUB>

The spin-transition (1A1&#8596;5T2) behaviour of a new mononuclear iron(II) compound [FeII(L)3][PF6]2[L = 2-[3-(2'-pyridyl)pyrazole-1-ylmethyl]pyridine] has been investigated by 57Fe Mossbauer spectroscopy. Analysis of the Mossbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mossbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom

Non-classical Fe(II) spin crossover behaviour leading to an unprecedented extremely large apparent thermal hysteresis of 270 K : application for displays

[Fe(hyetrz)3 ](anion)2 ·3H2O [hyetrz=4-(2′-hydroxyethyl)-1,2,4-triazole, anion=3-nitrophenylsulfonate] is a novel linear polynuclear FeII spin-crossover compound. The low-spin to high-spin transition accompanied by a pronounced thermochromic effect occurs at 370 K in a very abrupt way. Just before this temperature, the three non-coordinated water molecules are removed. The dehydrated high-spin form remains stable down to ca. 100 K, where it transforms into a new low-spin form, implying that this material shows an apparent thermal hysteresis width of 270 K. Applications of this material are discussed

Monitoring Spin-Crossover Properties by Diffused Reflectivity

International audienceIn this work we present a detailed study showing the importance of the Kubelka-Munk (KM) correction in the analysis of diffuse reflectivity measurements to characterize spin crossover compounds. Combined reflectance and magnetic susceptibility measurements are carried out as a function of temperature or time to highlight the conditions under which this correction becomes critical. In particular, we investigate the influence of the color contrast between the two spin states on the reflectance measurements. Interestingly, the samples’ contrast seems to play an important role on the spin-like domain structure as suggested by the symmetry of the FORC diagrams. These latest results are discussed within the framework of Classical Preisach model (CPM)