François Varret

How Do Redox Groups Behave around a Rigid Molecular Platform? Hexa(ferrocenylethynyl)benzenes and Their Electrostatic Redox Chemistry

A new family of hexakis(ferrocenylethynyl)benzenes was synthesized by Negishi coupling from ethynylferrocenes and C(6)Br(6) and can be reversibly oxidized to stable hexaferrocenium salts (see picture, Ar(F)=[3,5-C(6)H(3)(CF(3))(2)]). Their cyclic voltammograms show a single six-electron wave, three distinct two-electron waves, or a cascade of six single-electron waves, depending on the electrolyte counterion and number of methyl substituents on the ferrocenyl groups.

Raman spectroscopy of the high- and low-spin states of the spin crossover complex Fe(phen)2(NCS)2: an initial approach to estimation of vibrational contributions to the associated entropy change

Abstract Raman spectra of the spin-crossover complex Fe(phen) 2 (NCS) 2 in the solid state have been recorded at 785 nm as a function of temperature to investigate the contribution of intramolecular vibrations to the entropy change, Δ S , associated with spin crossover. The modes of major interest for estimating the contribution lie in the range 100–500 cm −1 , where the largest qualitative changes with temperature in the Raman spectra were observed. Analysis of these data, with the working assumption of an average frequency in this range as representative of the 15 distortion modes of an idealised FeN 6 octahedron, leads to the conclusion that the intramolecular vibrations represent a primary contribution to the total Δ S .

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.

Structures of Metastable States in Phase Transitions with a High-Spin Low-Spin Degree of Freedom

The difference in the degeneracy of low-spin (LS) and high-spin (HS) states causes interesting entropy effects on spin-crossover phase transitions and charge transfer phase transitions in materials composed of spin-crossover atoms. Mechanisms of the spin-crossover (SC) phase transitions have been studied using the Wajnflasz model, where the degeneracy of the spin states (HS or LS) is taken into account, and the cooperative nature of the spin-crossover phase transitions has been well described. Recently, a charge transfer (CT) phase transition due to electron hopping between LS and HS sites has been studied using a generalized Wajnflasz model. Systems with SC and CT both have a high temperature structure (HT) and a low temperature structure (LT), and the transition between them can be a smooth crossover or a discontinuous first-order phase transition, depending on the parameter values of the systems. Although, apparently, the standard SC system and the CT system are very different, it lias been shown that the two models are equivalent under a certain transformation of variables. In both systems, the structure of the metastable state at low temperatures is a matter of interest. We study the temperature dependence of the fraction of HT systematically in a, unified model and find several structures of equilibrium and metastable states of the model as functions of the system parameters. In particular, we find a reentrant-type metastable branch of HT in a low temperature region, which sould play an important role in the study of the photo-irradiated processes of related materials.

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 transition with a large thermal hysteresis near room temperature in a water solvate of an iron(III) thiosemicarbazone complex

The magnetic properties of the monohydrated ferric complex Li[Fe(5Brthsa)2]·H2O (H2-5Brthsa = 5-bromosalicylaldehyde thiosemicarbazone) have been investigated by SQUID and Mossbauer measurements. The S = 1/2 ↔ S = 5/2 spin transition of the ferric ion is accompanied by a quite broad hysteresis (ΔT = 39 K) centred around 313 K. The spin states involved are characterised by the quadrupole splittings ΔEQ(2T2) = 2.584 ± 0.002 mm s−1 and ΔEQ(6A1) = 0.338 ± 0.006 mm s−1 and the isomer shifts δIS (2T2) =+0.262 ± 0.001 mm s−1 and δIS (6A1) =+0.294 ± 0.002 mm s−1 at 77 and 360 K, respectively. A powder X-ray diffraction study at various temperatures demonstrates the occurrence of a crystallographic first-order phase transition of the lattice coupled to the spin conversion. The enthalpy and entropy variations associated with the transition have been estimated from DSC measurements at ΔH = 5.7 ± 0.5 kJ mol−1 and ΔS = 18 ± 2 J mol−1 K−1. The existence of a crystallographic first-order phase transition associated to the spin crossover is consistent with the cooperative character of the process. This phase transformation might originate from the modification of the extended hydrogen-bond network.

Intrinsic effects of the boundary condition on switching processes in effective long-range interactions originating from local structural change

Masamichi Nishino, Cristian Enachescu, Seiji Miyashita, Kamel Boukheddaden, and François Varret Computational Materials Science Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Japan Department of Physics, Alexandru Ioan Cuza University, Iasi, Romania Department of Physics, Graduate School of Science, The University of Tokyo, Bunkyo-Ku, Tokyo, Japan Groupe d’Etudes de la Matière Condensée, CNRS-Université de Versailles/St. Quentin en Yvelines, 45 Avenue des Etats Unis, F78035 Versailles Cedex, France CREST, JST, 4-1-8 Honcho Kawaguchi, Saitama, 332-0012, Japan (Dated: December 29, 2009)

Light-induced phase separation in the [Fe(ptz)6] (BF4)2 spin-crossover single crystal

We present novel insight on like-spin domains (LSD) in cooperative spin transition solids by following the photo-transformation and the subsequent relaxation of a [ Fe(ptz)6] (BF4)2 single crystal in the vicinity of the light-induced instability. Self-organization under light is observed, accompanied by Barkhausen-like noise and jumps which reveal the presence of elastic interactions between LSDs. The light-induced phase separation process is discussed in terms of a dynamic potential providing spinodal instability in the corresponding temperature range. This useful concept is applicable to all types of switchable molecular solids.

Caracterisation d'une variete amorphie de FeF3 : Etude thermique, magnetique et Mossbauer

Abstract Amorphous FeF3 has been prepared by fast vaporisation of the bulk material and condensation : it has been characterized by X-ray diffraction and differential thermal analysis. Strong antiferromagnetic interactions, but low spin-freezing temperature are observed. The magnetic arrangment is speromagnetic. The spin glass behaviour is questionable; the Mossbauer study reveals an electric field gradient distribution which, in average, has no definite sign.

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.