Jean-François Létard

Photomagnetism of iron(II) spin crossover complexes—the T(LIESST) approach

The Light-Induced Excited Spin State Trapping (LIESST) effect, encountered in some Spin-Crossover (SCO) complexes, is of major interest for the design of optical switches. Nevertheless, until now any applications have been prohibited, because the lifetimes of the photomagnetic states are long enough only at low temperatures. Hereby we review the recent progress made by using the T(LIESST) procedure, which consists of systematically measuring the limit temperature above which a photomagnetic effect in a material is erased by warming the material from 10 K at a rate of 0.3 K min−1. This method has been today applied to more than sixty SCO compounds and by comparing the various materials a relation between T(LIESST) and thermal spin transition (T1/2) temperatures has been obtained, i.e.T(LIESST) = T0 − 0.3T1/2. The second section reports part of works done to identify the parameters affecting the T0 factor; that is to find a guideline for the rational design of materials with long-lived photomagnetic lifetimes at working room temperature. Finally, we present the procedure used to simulate a T(LIESST) curve and illustrate it using the examples of a mononuclear SCO complex and of a binuclear SCO system displaying antiferromagnetic interactions.

Critical temperature of the LIESST effect in iron(II) spin crossover compounds

Abstract The light-induced crossover in a series of iron(II) compounds has been investigated by irradiating the sample at 10 K with a Kr + laser coupled to an optical fiber within a SQUID cavity. The temperature dependence of the molar fraction of the light-induced metastable HS state has been recorded for 22 compounds. The critical LIESST temperature, T c (LIESST), has been defined as the temperature for which the light-induced HS information was erased in the SQUID cavity. The dependence of T c (LIESST) as a function of the thermal spin crossover temperature, T 1/2 , has been discussed. The effect of cooperation has been analyzed and the influence of horizontal and vertical displacements of the LS and HS potential wells has been discussed on the basis of the non-adiabatic multiphonon theory.

Photo‐induced spin‐transition: the role of the iron(II) environment distortion

The [FeLn(NCS)2] iron(II) spin-crossover complexes cover a wide range of magnetic behaviour. Owing to the large number of known structural and magnetic data, this series is perfectly adapted to the investigation of the structure-magnetic properties relationship. In this paper we propose a new structural parameter, denoted Theta, which is used to correlate the features of the spin-crossover phenomena with the distortion of the iron environment. In particular, this parameter has shed light on the role of such distortion on the limiting temperature of photo-inscription, known as T(LIESST). A strong dependence of T(LIESST) on Theta is clearly demonstrated. The stronger the distortion the higher the T(LIESST) value. This structure-property dependence represents, for instance, a powerful tool to estimate the highest potential T(LIESST) value for a series of complexes. This limit in the [FeLn(NCS)2] series is estimated to be around 120 K, which probably prevents their use in any industrial application.

Nanoparticles of iron(II) spin-crossover

We report the synthesis of spin crossover 69 nm spherical nanoparticles of [Fe(NH2-trz)3](Br)2.3H2O.0.03(surfactant) (NH2trz = 4-amino-1,2,4-triazole, surfactant = Lauropal), prepared by the reverse micelle technique, which exhibit at room temperature a thermal hysteresis characterized by magnetic, diffuse reflectivity and Raman studies.

Nanoparticles of [Fe(NH2-trz)3]Br2.3H2O (NH2-trz=2-amino-1,2,4-triazole) prepared by the reverse micelle technique: influence of particle and coherent domain sizes on spin-crossover properties.

This paper describes the synthesis of iron(II) spin-crossover nanoparticles prepared by the reverse micelle technique by using the non-ionic surfactant Lauropal (Ifralan D0205) from the polyoxyethylenic family. By changing the surfactant/water ratio, the size of the particles of [Fe(NH2-trz)3]Br2.3H2O (with NH2trz=4-amino-1,2,4-triazole) can be controlled. On the macroscopic scale this complex exhibits cooperative thermal spin crossovers at 305 and 320 K. We find that when the size is reduced down to 50 nm, the spin transition becomes gradual and no hysteresis can be detected. For our data it seems that the critical size, for which the existence of a thermal hysteresis can be detected, is around 50 nm. Interestingly, the change of the particle size induces almost no change in the temperature of the thermal spin transition. A systematic determination of coherent domain size carried out on the nanoparticles by powder X-ray diffraction indicates that at approximately 30 nm individual particles consist of one coherent domain.

Critical temperature of the LIESST effect in a series of hydrated and anhydrous complex salts [Fe(bpp)2]X2

Abstract The magnetic properties of a series of hydrated and dehydrated spin crossover compounds derived from the 2,6-bis(pyrazol-3-yl)pyridine (bpp) ligand have been reinvestigated and the light-induced crossover studied at 10 K. The capacity of a compound to retain the light-induced HS information has been estimated through the determination of the T (LIESST). The position of each compound in the T (LIESST)– T 1/2 diagram has been analyzed and the effect of the nature of the salt, hydration degree and cooperativity have been discussed.

Co(II) molecular complexes as a reference for the spin crossover in Fe(II) analogues

The crystal structures of a series of cobalt(II) molecular complexes, [Co(PM-L)2(NCS)2] [PM = N-2-pyridylmethylene, L = 4-(aminobiphenyl) or 4-(phenylethynyl)aniline], are investigated and compared to the analogous iron(II) complexes, [Fe(PM-L)2(NCS)2], already known in the literature. At room temperature, the Co(II) complexes prove to be isostructural with the iron(II) complexes. An interesting point is that the iron complexes, unlike the cobalt complexes, undergo a spin crossover at low temperature. Hence, a comparison of the temperature dependence of the structural properties of the Co(II) and the Fe(II) complexes underlines some structural features of the spin crossover. Comparative deformation of the lattices and thermal expansion tensors are first discussed. Then, new parameters to estimate the distortion and the contraction at the spin crossover of the FeN6 coordination sphere are presented, thereby allowing the estimation of the reduction of the volume of the octahedron to around 3 A3 (25%). As well, comparative discussions on the intermolecular contact modifications with temperature are proposed. In the above considerations the cobalt series is therefore used as a reference to distinguish between the effects of the spin crossover and the purely thermal effects.

SPIN-CROSSOVER IN THE FE(ABPT)2(NCX)2 (X = S, SE) SYSTEM : STRUCTURAL, MAGNETIC, CALORIMETRIC AND PHOTOMAGNETIC STUDIES

The compounds [Fe(abpt) 2 (NCS) 2 ] ( 1 ) and [Fe(abpt) 2 (NCSe) 2 ] ( 2 ) with abpt=4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole have been synthesized. The X-ray structures have been determined at 293 K. 1 and 2 are isostructural and crystallize in the monoclinic space group P 2 1 / n with Z =2, a =8.538(8), b =10.246(8), c =16.45(2) A, β =93.98(9)° for 1 and a =8.623(2), b =10.243(3), c =16.585(3) A, β =93.19(2)° for 2 . In both complexes, the coordination core has a similar pseudo-octahedral geometry with the NCS − ( 1 ) and NCSe − ( 2 ) groups in the trans -position. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)↔high-spin (HS) conversion centered at T 1/2 around 180 and 224 K for 1 and 2 , respectively. The spin conversion takes place gradually, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Δ H =5.8±0.5 ( 1 ) and 8.6±0.8 kJ mol −1 ( 2 ); Δ S =32.5±3 ( 1 ) and 38±4 J mol −1 K −1 ( 2 ). At 10 K the light-induced excited spin state trapping (LIESST) effect has been observed within the SQUID magnetometer cavity, by irradiating powder samples with a Kr + laser coupled to an optical fiber. The critical LIESST temperatures T liesst are around 40 and 32 K for 1 and 2 , respectively. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect. The HS→LS relaxation kinetics have been investigated in the temperature range 6–40 K. A thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures can be observed for 1 . The very fast relaxation precludes the estimation of the kinetic parameters for 2 at temperatures higher than 10 K.

Multiple relaxation pathways in photoexcited dimethylaminonitro- and dimethylaminocyano-stilbenes

Abstract The photophysical behaviour of para-nitro-, para-cyano- and meta-nitro-4′-dimethylaminostilbenes is compared and explained. The increase of the fluorescence quantum yield with solvent polarity is the result of a negative solvatokinetic behaviour of the nonradiative channel leading to the ground state via the double-bond twisted species P* which is of low-polarity nature. The maximum in the fluorescence quantum yield at intermediate solvent polarities, observable for the nitro compounds, is due to a further nonradiative channel specifically linked with the nitro group and with positive solvatokinetic behaviour. This quenching channel becomes important for high-polarity solvents but is absent for compounds without the nitro group. TICT state formation (twisted nitro group) as a candidate for this channel is discussed.

Hysteretic three-step spin crossover in a thermo- and photochromic 3D pillared Hofmann-type metal-organic framework.

The integration of spin crossover (SCO) centers into porousframework materials is leading to the emergence of newfamilies of functional solids that display a range of interestingand potentially useful physicochemical properties. This mate-rialsdesignapproachgivesrisetoauniquemolecularscenarioin which factors that govern the spin switching response (e.g.,temperature, pressure, light, magnetic field, and chemicalenvironment) are newly intertwined with highly cooperativestructure–function relationships, and potentially also with thedynamic host–guest chemistry of the materials.