Showing papers by "Jean-François Létard published in 2012"

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

Abstract: 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.

Spin crossover materials evaporated under clean high vacuum and ultra-high vacuum conditions: from thin films to single molecules

Abstract: We report clean evaporation under ultra-high vacuum conditions of two spin crossover materials, yielding either microcrystallites or homogeneous thin films. Magnetic and photomagnetic studies show that thermal and light-induced spin crossover properties are preserved. Preliminary STM imaging of sub-monolayers indicates that the deposited molecules remain intact on the surface.

Intermolecular control of thermoswitching and photoswitching phenomena in two spin-crossover polymorphs

Abstract: We discuss here the important role of intermolecular coupling for the thermal- and light-induced molecular state switching in the solid state. Investigations were performed on the two crystalline polymorphs of the spin-crossover [Fe-(PM-BIA)${}_{2}$(NCS)${}_{2}$] material. In addition to structural studies at thermal equilibrium, light-induced phenomena were investigated through photocrystallography, photomagnetic, and dynamical optical measurements. Strong similarities between the thermal-equilibrium and the out-of-equilibrium light-induced transformations are observed in each polymorph: strong cooperative phenomena in one polymorph versus weak cooperative ones in the second polymorph. These different responses of the two crystalline forms of the compound to external perturbations are discussed at the microscopic level in terms of Ising-like model and two-mode description of on-site molecular potentials.

Photomagnetism of a sym-cis-dithiocyanato iron(II) complex with a tetradentate N,N'-bis(2-pyridylmethyl)1,2-ethanediamine ligand.

Abstract: A comprehensive study of the magnetic and photomagnetic behaviors of cis-[Fe(picen)(NCS) 2] (picen=N,N'-bis(2-pyridylmethyl)1,2- ethanediamine) was carried out. The spin-equilibration was extremely slow in the vicinity of the thermal spin-transition. When the cooling speed was slower than 0.1 K min -1, this complex was characterized by an abrupt thermal spin-transition at about 70 K. Measurement of the kinetics in the range 60-70 K was performed to approach the quasi-static hysteresis loop. At low temperatures, the metastable HS state was quenched by a rapid freezing process and the critical T(TIESST) temperature, which was associated with the thermally induced excited spin-state-trapping (TIESST) effect, was measured. At 10 K, this complex also exhibited the well-known light-induced excited spin-state-trapping (LIESST) effect and the T(LIESST) temperature was determined. The kinetics of the metastable HS states, which were generated from the freezing effect and from the light-induced excitation, was studied. Single-crystal X-ray diffraction as a function of speed-cooling and light conditions at 30 K revealed the mechanism of the spin-crossover in this complex as well as some direct relationships between its structural properties and its spin state. This spin-crossover (SCO) material represents a fascinating example in which the metastability of the HS state is in close vicinity to the thermal spin-transition region. Moreover, it is a beautiful example of a complex in which the metastable HS states can be generated, and then compared, either by the freezing effect or by the LIESST effect. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

When Stable and Metastable HS States Meet in Spin‐Crossover Compounds

Abstract: With the ultimate goal of rationally inducing overlap between the stable high spin state (HSSS) and the metastable high spin state (HSMS) generated by a rapid frozen effect, a series of [FexMn1-x(bpp)2](BF4)2 [bpp = 2,6-bis(pyrazol-3-yl)pyridine] complexes was prepared. For all the materials, the thermal spin crossover (SCO) properties were measured and the stability of the metastable HSMS state was probed through the determination of the T(TIESST) value associated with the Thermally-Induced Excited Spin-State Trapping effect. The overlap between the two systems was observed for x ≤ 0.90 and the consequences on the shape and the completeness of the SCO behavior were discussed.

Photomagnetic studies on spin-crossover solid solutions containing two different metal complexes, [Fe(1-bpp)2]x[M(terpy)2]1−x[BF4]2 (M = Ru or Co)

Abstract: The photomagnetic properties of two series of spin-crossover solid solutions, [Fe(1-bpp)2]x[Ru(terpy)2]1−x(BF4)2 and [Fe(1-bpp)2]x[Co(terpy)2]1−x(BF4)2 (1-bpp = 2,6-bis[pyrazol-1-yl]pyridine), have been investigated. For all the materials, the evolution of the T(LIESST) value, the high-spin → low-spin relaxation parameters and the LITH loops were thoroughly studied. Interestingly in the Fe : Co series, along the photo-excitation, cobalt ions are concomitantly converted from low-spin to high-spin states with the iron centres, and also fully relax after light excitation.

A Family of Discrete Magnetically Switchable Nanoballs

Abstract: The thermal and light-induced magnetic properties of a family of discrete magnetically switchable 'nanoball' species (3 nm in diameter) is reported. The self-assembly of these materials is accomplished by the use of the metallo building block, [Cu([Tp{sup 4-py}])(NCCH{sub 3})] ([Tp{sup 4-py}]=tris-[3-(4{prime}-pyridyl)pyrazol-1-yl]hydroborate), combined with a [Fe(NCX){sub 2}] (X = S, Se and BH{sub 3}) species. We previously showed that the thiocyanate analogue (Fe(NCS)-nano) undergoes a thermal and light-induced spin crossover (SCO) - the largest such discrete SCO material reported. Now included in this family are the Se and BH{sub 3} analogues, Fe(NCSe)-nano and Fe(NCBH{sub 3})-nano, which show increased thermal transition temperatures (T{sub 1/2} = 124 K, 162 and 173 K). This variation in transition temperature over the series S < Se < BH{sub 3} results in diverse photomagnetic properties, such that the light-induced excited spin state trapping (LIESST) effect is exhibited to varying degrees and at different temperatures by the S, Se and BH{sub 3} analogues.

One-Step Vs Stepwise Immobilization of 1-D Coordination-Based Rh–Rh Molecular Wires on Gold Surfaces

Abstract: Reaction of dimeric [Rh(II)(2)(phen)(2)(μ-OAc)(2)(MeCN)(2)](BF(4))(2) (phen =1,10-phenanthroline) with pyrazine (pz) in a 1:2 ratio leads to the new 1-D metal-metal-bonded coordination oligomer {[Rh(II)(2)(phen)(2)(μ-OAc)(2)(pz)](BF(4))(2)}(n) (Rh-Rhpz)(n) (1), where each Rh atom of the dimeric unit (Rh-Rh) is coordinated in the equatorial plane to a nitrogen atom of a rigid and linear bifunctionalized organic linker (pz). Single X-ray diffraction analysis reveals the 1-D straight oligomeric chain structure (molecular wire, MW) consists of alternating (Rh-Rh) units and pz linking ligands with free BF(4)(-) as counteranions, and each metal center has a slightly distorted octahedral arrangement. The presence of accessible labile MeCN groups on both ends of these MWs ("free ends") enables functionalization of a 4-mercaptopyridine-gold coordinating platform (Au/MP) to form in one step a layer of coordination oligomer (Au/MP(Rh-Rhpz)(n); n ≈ 50). Furthermore (Rh-Rhpz)(n) (n = 1-6) MWs were grafted to Au/MP surfaces by a conventional step-by-step assembly construction involving coordination reactions between the Rh dimer ([Rh(2)(phen)(2)(μ-OAc)(2)(MeCN)(2)](BF(4))(2) (2)) and pz. A detailed physicochemical study (UV-vis, RAIR, QCM-D, ellipsometry, contact angle measurements, as well as impedance spectroscopy and cyclic voltammetry) has been made during both assembly methods to characterize the resulting surface-anchored coordination molecular wire (CMW) layers (Au/MP(Rh-Rhpz)(n)). The results indicate that the immobilized molecular assemblies (MAs) were successfully fabricated using both methods of assembly. The efficiency of the two methods is discussed.

Optimised arrangement of triazole particles

Abstract: An electrical device in provided having two electrodes separated from one another, wherein one temperature controlled electronic spin-state transition particle is in direct contact with each of the two electrodes, the particle being of the ionic type and containing a transition metal bearing a cationic charge.

Agencement optimisé de particules de triazole

Abstract: L'invention concerne un dispositif electrique comportant deux electrodes separees l'une de l'autre dans lequel au moins une particule a transition d'etat de spin electronique commande en temperature est en contact direct avec chacune des deux electrodes, la particule etant de type ionique et comportant un metal de transition qui porte une charge cationique.