Epiphane Codjovi

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.

IRON(II)-1,2,4-TRIAZOLE SPIN TRANSITION MOLECULAR MATERIALS

The phenomenon of spin transition is probably one of the most spectacular examples of molecular bistability. Our main goal along this line concerns the design of molecular materials exhibiting abrupt spin transitions accompanied by thermochromic and large thermal hysteresis effects. The ideal situation, in terms of possible application of these compounds as active elements of memory devices, is realized when room temperature falls in the middle of the thermal hysteresis loop. In this context, we review our work concerning iron(II)-l,2,4-triazole compounds. We first introduce the idea that the cooperativity should be more pronounced in polymeric than in mononuclear compounds. Then we present some results concerning trinuclear iron(II)-1,2,4- triazole species. The heart of the paper is devoted to the polymeric compounds of formulae [Fe(Htrz)2(trz)](BF4) and [Fe(Htrz)3](BF4)2 • H2O. We report first on the magnetic, optical and calorimetric, then on the structural properties of these compounds. Afterwards, we introduce the concept of spin transition molecular alloy, and emphasize that it is possible to fine tune the spin transition regime of these alloys through their chemical composition. For some alloys, room temperature falls within the thermal hysteresis loop. In the conclusion, the mechanism of cooperativity in the spin transition polymeric compounds is briefly discussed.

Prediction of the Spin Transition Temperature in Fe(II) One-Dimensional Coordination Polymers: an Anion Based Database.

One-dimensional (1D) coordination polymers of formula [Fe(NH(2)trz)(3)]A.nH(2)O, {A = TiF(6)(2-), n = 0.5 (1) and n = 1 (2); A = ZrF(6)(2-), n = 0.5 (3) and n = 0 (4); A = SnF(6)(2-), n = 0.5 (5) and n = 1 (6); A = TaF(7)(2-), n = 3 (7) and n = 2.5 (8); A = GeF(6)(2-), n = 1 (9) and n = 0.5 (10), NH(2)trz = 4-amino-1,2,4-triazole} have been synthesized, fully characterized, and their spin crossover behavior carefully studied by SQUID magnetometry, Mossbauer spectroscopy, and differential scanning calorimetry. These materials display an abrupt and hysteretic spin transition around 200 K on cooling, as well as a reversible thermochromic effect. Accurate spin transition curves were derived by (57)Fe Mossbauer spectroscopy considering the corrected f factors for the high-spin and low-spin states determined employing the Debye model. The unusual hysteresis width of 3 (28 K), was attributed to a dense hydrogen bonding network involving the ZrF(6)(2-) counteranion and the 1D chains, an organization which is also revealed in [Cu(NH(2)trz)(3)]ZrF(6).H(2)O (11). Trinuclear spin crossover compounds of formula [Fe(3)(NH(2)trz)(10)(H(2)O)(2)](SbF(6))(6).S {S = 1.5CH(3)OH (12), 0.5C(2)H(5)OH (13)} were also obtained. A structural property relationship was derived between the volume of the inserted counteranion and the transition temperature T(1/2) of the 1D chains. Two linear size regimes were identified for monovalent anions (0.04 <or= V (nm(3)) <or= 0.09) and for divalent anions (above V >or= 0.11 nm(3)) with saturation around T(1/2) = 200 K. These characteristics allowed us to derive an anion based database that is of interest for the prediction of the transition temperature of such functional switchable materials. Diffuse reflectivity measurements under hydrostatic pressure for 3,4 combined with calorimetric data allow an estimation of the electrostatic pressure between cationic chains and counteranions in the crystal lattice of these materials. The chain length distribution that ranges between 1 and 4 nm was also derived.

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 ·3H 2 O [hyetrz=4-(2′-hydroxyethyl)-1,2,4-triazole, anion=3-nitrophenylsulfonate] is a novel linear polynuclear Fe II 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.

Non-classical FeII spin-crossover behaviour in polymeric iron(II) compounds of formula [Fe(NH2trz)3]X2xH2O (NH2trz=4-amino-1,2,4-triazole; X=derivatives of naphthalene sulfonate)

A new series of iron(ii) spin-crossover materials of formula [Fe(NH2trz)3 ]X2xH2O [X=1-naphthalene sulfonate (1ns), 2-naphthalene sulfonate (2ns), 4-hydroxy-1-naphthalene sulfonate (4OH-1ns), 4-amino-1-naphthalene sulfonate (4NH2-1ns) and 6-hydroxy-2-naphthalene sulfonate (6OH-2ns)] are reported. The structure of these compounds consists of linear chains in which the Fe(ii) ions are linked by triple N1 ,N2-1,2,4-triazole bridges. All compounds show non-classical spin-crossover behaviour. Optical and magnetic measurements recorded upon heating show an abrupt low-spin to high-spin transition accompanied by a pronounced thermochromic effect occuring between 330 and 340 K depending on the anion. Thermogravimetric analyses show that the transition is induced by the removal of the two lattice water molecules, which initially stabilized the low-spin state. The dynamical character of this transition has been monitored by extended 57Fe Mossbauer spectroscopic studies on [Fe(NH2trz)3 ](2ns)2xH2O. Upon cooling, the dehydrated modifications show classical spin-crossover behaviour with hysteresis at much lower temperatures, ranging from 229 to 297 K depending on the anion. [Fe(NH2trz)3 ](2ns)2 represents one of the first iron(ii) spin-crossover materials showing a spin transition in the close vicinity of room temperature (290 K) accompanied by hysteresis (14 K).

Examples of molecular switching in inorganic solids, due to temperature, light, pressure, and magnetic field

We describe various molecular switching processes occurring in several types of inorganic solids: spin cross-over (SC) compounds, photomagnetic Prussian blue analogs (PBAs), and valence-tautomeric system. Their thermo-, photo-, piezo-, and magneto-chromic properties are illustrated by recent examples. A common description of their static properties by a two-level model is given.

Dynamical Ising-like model for the two-step spin-crossover systems

In order to reproduce the two-step relaxation observed experimentally in spin-crossover systems, we investigate analytically the static and the dynamic properties of a two-sublattice Ising-like Hamiltonian. The formalism is based on a stochastic master equation approach. It is solved in the mean-field approximation, and yields two coupled differential equations that correspond to the HS fractions of the sublattices A and B.

Direct access to the photo-excitation and relaxation terms in photo-switchable solids : non-linear aspects

Abstract We investigate the photo-excitation process and the relaxation of the photo-excited state in photo-switchable solids, such as spin transitions systems. We follow the competition between the relaxation and the photo-excitation (up and eventually down) processes. The kinetic experimental curves are treated so as to derive the various contributions to the master equation, in terms of a single macroscopic variable: the population of the photo-excited state. The experimental examples are taken from [Fe x M 1− x (btr) 2 (NCS) 2 ].H 2 0, with M=Co, Ni, Zn. They give evidence for a non-linear character of the relaxation at all temperatures, including the tunnelling regime, and for non-linear behaviours of the photo-excitation rates.

A helium-gas-pressure apparatus with optical-reflectivity detection tested with a spin-transition solid

We have constructed a low-temperature helium pressure device that incorporates optical-reflectivity detection and is useful for the study of thermochromic materials. We have tested the device up to 1600 bar (0.16 GPa) and down to 10 K. The performance of the device is illustrated by the results of constant-pressure and constant-temperature runs for the spin-transition solid [Fe(btr)2(NCS)2]·H2O. The data establish a consistent pressure-temperature phase diagram, which is discussed.

Triggering the spin-crossover of Fe(phen)2(NCS)2 by a pressure pulse. Pressure and magnetic field induce ‘mirror effects’

Abstract We have studied the static and dynamic effects of pressure on the spin-transition in the temperature range of the thermal hysteresis loop for the compound Fe(phen)2(NCS)2. The high-spin fraction (nHS) as a function of pressure and temperature has been determined by optical reflectivity. In this compound, the pressure was found to upward shift the spin-transition temperature by 23 K per kbar. During the dynamic pressure pulse, a decrease in nHS is observed, with an irreversible (reversible) character in the descending (ascending) branch of the hysteresis loop. In this respect, pressure has a ‘mirror effect’ compared to the application of an intense and pulsed magnetic field, for which – as reported previously – an increase in nHS is observed, with an irreversible (reversible) character in the ascending (descending) branch of the hysteresis loop. To cite this article: A. Bousseksou et al., C. R. Chimie 6 (2003).