Showing papers by "Jean-François Létard published in 2004"

Towards spin crossover applications

Abstract: In this chapter we attempt to review the potential for the application of the spin crossover (SCO) phenomenon in various domains, such as molecular electronics, data storage, display devices. It is evident that SCO properties, such as room-temperature working range, chemical stability, low addressing power, short addressing time, full reversibility, are of promising value in the context of the stringent limits necessary in the future development of information technology, due to the unceasing miniaturization of the components. Of course, many requirements must be fulfilled before any use in a genuine device becomes feasible. Some of these are emphasized and discussed here. Additionally, this review reports recent progress in non-linear optics and photomagnetism of SCO materials.

Structural aspects of spin crossover. Example of the [FeIILn(NCS)2] complexes

Abstract: The interplay between the spin crossover and the structural properties of the complexes in the solid state is still under investigation. In particular the following questions may be asked. What are the structural modifications of the metal coordination sphere at the spin crossover? How are the dimensions and the symmetry of the crystallographic unit cell affected by the spin crossover? Conversely, how may structural properties influence the spin crossover behavior? Do intramolecular parameters account for the features of the spin crossover? What are the relevant characteristics of the crystal packing for the cooperativity? Do the above questions have general answers that can be used for all the spin crossover compounds? This contribution tries to give answers to these questions. The discussion is based on a large structural data set provided in the literature for the six-coordinated iron(II) mononuclear complexes of general formula [FeL n (NCS) 2 ]. The effects of temperature, light and pressure on the X-ray diffraction crystal structures are reviewed. The structural modifications due to the spin crossover are first estimated, these include the expansion and the distortion of the FeN 6 octahedron, the isotropic and the anisotropic changes of the unit cell. The influence of the structural properties on the features of the spin crossover is then discussed. For example, intramolecular properties such as Fe-N bond lengths are in general not relevant to account for the spin crossover features. In contrast, hydrogen bonds play a paramount role in the propagation of the spin conversion throughout the crystal lattice.

Atypical photomagnetic properties in a series of binuclear iron(II) spin crossover complexes

Abstract: In this work, we have re-examined the photomagnetic properties of a series of binuclear iron(II) complexes of formula [{Fe(L)(NCX)2}2bpym], where L is bpym = 2,2′-bipyrimidine and bt = 2,2′-bithiazoline and X = S, Se, by light irradiation at 647.1–676.4 nm and at 830 nm and we confirm that after optical excitation the magnetic behaviors of [{Fe(bpym)(NCSe)2}2bpym] (2), [{Fe(bt)(NCS)2}2bpym] (3) and [{Fe(bt)(NCSe)2}2bpym] (4) are close to the antiferromagnetic response of [{Fe(bpym)(NCS)2}2bpym] (1) characterized by two iron(II) metal ions in the HS electronic configuration. This hypothesis of quantitative HS–HS photoconversion is also supported by the reflectivity experiments and the infrared spectroscopies performed on 3. We have also investigated the relaxation process of 2–4 by using a SQUID magnetometer. Atypical magnetic behavior is found for compounds 3 and 4. More precisely, while the magnetic response of 2versus time continuously decreases whatever the temperature, the signal of 3 increases below 17 K, becomes stable at around 18–20 K and decreases at higher temperature. We thus propose a direct HS–HS→LS–HS relaxation process for 2 and an indirect HS–HS→LS–HS→LS–LS relaxation process for 3 and 4. The agreement found between the experimental T(LIESST) and the simulated T(LIESST) curves, calculated using the measured kinetic parameters, guarantees the validity of our approach.

Heat- and Light-Induced Spin Transition of an Iron(II) Polymer Containing the 1,2,4,5-Tetrakis(diphenylphosphanyl)benzene Ligand

Abstract: A new iron(II) spin-crossover compound containing a phosphane ligand has been synthesized and characterized by magnetic susceptibility, Mossbauer spectroscopy, and heat capacity and photomagnetic experiments...