Jorge Linares

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.

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.

Physical properties of the spin-crossover compound hexakis(1-methyltetrazole-N4)iron(II) triflate, steady state and relaxation studies, X-ray structure of the isomorphic Ni(II) compound

Abstract The synthesis and characterization of the spin transition compound hexakis(1-methyltetrazole-N4)iron(II) trifluoromethanesulfonate obtained from the reaction of 1-methyltetrazole and iron(II) triflate are described. The spin transition of [Fe(mtz)6](CF3SO3)2 (1) presents a hysteresis loop in magnetism with T↓=157.0 and T↑=179.5 K. Only one-third of the Fe(II) ions undergo this transition, the other two-thirds of the ions remain in the high-spin state. Mossbauer spectroscopy and relaxation studies have shown that the actual width of the hysteresis is only 12 K, with T↓=167.5 and T↑=179.5 K. Thermal spin-state trapping appears to be possible. The trapped high-spin state is stable up to 103 K. The relaxation is not first-order. After irradiation with red light, reverse light-induced excited spin state trapping (LIESST) is observed. The new low-spin state is stable up to 42 K. Also, its relaxation to HS is non-first-order. A reverse light-induced thermal hysteresis loop of approximately 7 K width is present too. No LIESST effect could be observed. Finally, a ferromagnetic type of coupling has been recognized, which is temperature and field dependent.

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.

Effect of hydrostatic pressure on phase transitions in spin-crossover 1D systems

Abstract A model is developed to elucidate the role of hydrostatic pressure on the spin-crossover phase transitions in one-dimensional systems. The model involves long-range electron-deformational and short-range interactions. The conditions when pressure suppresses the thermal hysteresis are discussed. The pressure-induced system’s bistability is predicted. The qualitative explanation of experimental data is given.

Spin crossover in a heptanuclear mixed-valence iron complex

The complex [Fe(II){(CN)Fe(III)L(5)}(6)]Cl(2) consists of a mixed-valence heptanuclear cyanide-bridged unit formed of a Schiff-base pentadentate ligand L(5) and it shows a spin crossover of the peripheral Fe(III) centres.

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.

Comparison of static and light-induced thermal hystereses of a spin-crossover solid, in a mean-field approach†

We investigate the relationship between the two mean-field models which have been proposed independently for characterizing the static and the light-induced thermal hystereses of spin-transition solids, respectively involving an interaction parameter J and a relaxation self-acceleration parameter α. We unify these two models into a unique - static and dynamic - mean-field description based on Arrhenius thermal activation processes. The model is used to analyse the experimental hysteresis loops, the relaxation curves in the dark, and the light-induced instability in the spin-crossover system [FexCo1-x(btr)2(NCS)2]H2O. A linear relationship between J and αT is obtained, which is in qualitative agreement with the available theories.

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.