Reversible Control by Light of the High-Spin Low-Spin Elastic Interface inside the Bistable Region of a Robust Spin-Transition Single Crystal.

By using a weak modulated laser intensity we have succeeded in reversibly controlling the dynamics of the spin-crossover (SC) single crystal [{Fe(NCSe)(py)2 }2 (m-bpypz)] inside the thermal hysteresis. The experiment could be repeated several times with a reproducible response of the high-spin low-spin interface and without crystal damage. In-depth investigations as a function of the amplitude and frequency of the excitation brought to light the existence of a cut-off frequency ca.

Structure-driven orientation of the high-spin-low-spin interface in a spin-crossover single crystal.

The orientation of the high-spin (HS)-low-spin (LS) macroscopic interface at the thermal transition of thin [{Fe(NCSe)(py)2}2(m-bpypz)] crystals is explained by considering the possible vanishing of the structural mismatch between the coexisting phases. The structural property which allows mismatch-free interfaces is characterized. The observed orientations of the interface and the tilt angle between the HS and LS domains are accurately reproduced by a two-dimensional continuous medium model, based on the structural data.

Properties of the low-spin high-spin interface during the relaxation of spin-crossover materials, investigated through an electro-elastic model.

The present work is devoted to the spatio-temporal investigations of spin-crossover lattices during their thermal relaxation from high- to low-spin state. The analysis is performed using Monte Carlo simulations on a distortable 2D lattice the sites of which are occupied by high-spin (HS) or low-spin (LS) atoms. The lattice is circular in shape and the HS to LS transformation results in single domain nucleation followed by growth and propagation processes.

Velocity of the high-spin low-spin interface inside the thermal hysteresis loop of a spin-crossover crystal, via photothermal control of the interface motion.

We investigated by optical microscopy the thermal transition of the spin-crossover dinuclear iron(II) compound [(Fe(NCSe)(py)(2))(2)(m-bpypz)]. In a high-quality crystal the high-spin (HS) low-spin (LS) thermal transition took place with a sizable hysteresis, at ~108 K and ~116 K on cooling and heating, respectively, through the growth of a single macroscopic domain with a straight LS and HS interface. The interface orientation was almost constant and its propagation velocity was close to ~6 and 26 μ m s(-1) for the on-cooling and on-heating processes, respectively.

Macroscopic nucleation phenomena in continuum media with long-range interactions.

Nucleation, commonly associated with discontinuous transformations between metastable and stable phases, is crucial in fields as diverse as atmospheric science and nanoscale electronics. Traditionally, it is considered a microscopic process (at most nano-meter), implying the formation of a microscopic nucleus of the stable phase. Here we show for the first time, that considering long-range interactions mediated by elastic distortions, nucleation can be a macroscopic process, with the size of the critical nucleus proportional to the total system size.

Electronic and structural aspects of spin transitions observed by optical microscopy. The case of [Fe(ptz)6](BF4)2.

The colorimetric analysis of images recorded with an optical microscope during the onset of the spin crossover transformation allows monitoring separately the involved electronic and structural aspects, through the separation of resonant absorption and scattering effects. Complementary information can also be obtained by using the polarized modes of the microscope. These potentialities are illustrated by the observation of [Fe(ptz)(6)](BF(4))(2) single crystals during the onset of the thermal transitions in the 110-140 K range.

Spin-transition in [Fe(II)(L5)2][ClO4]2 [L5 = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)]: a further example of coexistence of features typical for disorder and cooperativity.

An iron(II) complex [Fe(L5)2][ClO4]2 of a new nonplanar tridentate ligand [L5 = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)] has been synthesized and its spin-transition properties have been investigated. X-ray structural analysis of the solvated compound [Fe(L5)2][ClO4]2.CH3CN at 100 K and at 298 K revealed a mononuclear Fe(II)N6 pseudooctahedron with the tridentate ligand in the meridional coordination mode.

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.

Temperature-dependent interactions and disorder in the spin-transition compound [Fe(II)(L)2][ClO4]2.C7H8 through structural, calorimetric, magnetic, photomagnetic, and diffuse reflectance investigations.

The title compound [Fe (II)(L) 2][ClO 4] 2.C 7H 8 (L = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine) has been isolated while attempting to grow single crystals of the spin-transition (continuous-type) compound [Fe (II)(L) 2][ClO 4] 2, published earlier ( Dalton Trans. 2003, 3392-3397).

Magnetic and Mössbauer investigation of the photomagnetic Prussian blue analogue Na(0.32)Co[Fe(CN)6](0.74).3.4H2O: cooperative relaxation of the thermally quenched state.

Thanks to thermal quenching we investigated the relaxation of the metastable state of Na(0.32)Co[Fe(CN)6](0.74).

Photoexcitation and relaxation properties of a spin-crossover solid in the case of a stable high-spin state.

The molecular solid [Fe(II)L(2)](ClO(4))(2).CH(3)CN where L is 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine provides a stable high-spin (HS) state at low temperature. Photoexcitation and subsequent relaxation have been studied using light-induced excited state spin trapping [LIESST(H --> L)] in the 700-850 nm range, determination of T(LIESST), relaxation curves at different temperatures, and temperature dependence of the light-induced spin equilibrium under constant irradiation.

SOM assembly of hydroxynaphthoquinone and its oxime: polymorphic X-ray structures and EPR studies.

Investigation on solvent-induced polymorphism in X-ray structures of 2-hydroxy-1,4-naphthoquinone (Lawsone) 1, is carried out. In protic methanol, 1 crystallizes in monoclinic space group P2(1)/c (1a) comprising of 2D hydrogen bonded network via cyclic dimers. In aprotic solvent such as acetone on the other hand, 1 exhibits orthorhombic space group Pna 2(1) (1b) and emerges with 1D catemeric chain.

Nonexponential relaxation of the metastable state of the spin-crossover system [Fe(L)2](ClO4)2.H2O [L = 2,6-Bis(pyrazol-1-ylmethyl)pyridine].

The relaxation of the metastable state of the spin-crossover compound [Fe(L)(2)](ClO(4))(2).H(2)O, with L = 2,6-bis(pyrazol-1-ylmethyl)pyridine, populated by the LIESST (light induced excited spin state trapping) effect, has been investigated by magnetic measurements. The time dependence of the relaxation curve at several temperatures, starting from different initial states, is in the shape of stretched exponentials, and the thermal variation of the photostationary state under constant photoexcitation is progressive and reversible.

Air-stable, electron-deficient Fe(II) catalytic porphyrins. Characterization and molecular structures of rare high spin Fe(II) hexacoordinated porphyrins.

The synthesis, characterization, and molecular structures of the first air-stable, hexacoordinated high spin Fe(II) porphyrins (1) with axial alcohols are reported (1 = Fe(II) meso-tetrakis(o-dichlorophenyl)-beta-octanitroporphyrin). The structure of 1 with two axial waters is also presented. The very different conformations and metrics observed with the two types of ligands illustrate the acute interplay between ligands, conformations, and spin states in Fe porphyrins.

Molecular batteries: synthesis and characterization of a dendritic 19-electron FeI complex that reduces C60 to its mono-anion.

Both redox forms of the dendrimer [dendr-64-NHCOCpFeII(eta 6-C6Me6)]64+/0, 6/7, are synthesized and characterized, and the 19-electron form reduces C60 to [dendr-64-NHCOCpFeII(eta 6-C6Me6)]64+(C60-.)64, 8.

Metastable states of an Ising-like thermally bistable system.

The lifetimes of the metastable states are investigated in an Ising-like model associated with thermally bistable systems. A discrete mesoscopic Markovian dynamic is established using an optimized version of the previously presented Monte Carlo entropic sampling method. This is well suited to an extensive study of the role of the physical parameters: temperature, interaction parameter, electronic energy gap.

Photoinduced ferrimagnetic systems in Prussian blue analogues C(I)xCo4[Fe(CN)6]y (C(I) = alkali cation). 3. Control of the photo- and thermally induced electron transfer by the [Fe(CN)6] vacancies in cesium derivatives.

We synthesized a series of CoFe Prussian blue analogues along which we tuned the amount of cesium cations inserted in the tetrahedral sites of the structure. Structure and electronic structure have been investigated, combining XANES, infrared spectroscopy, powder X-ray diffraction experiments, and magnetization measurements. The change of the magnetization induced by light along the series shows that the efficiency of the photoinduced magnetization, evidenced a few years ago in similar compounds by Hashimoto et al.

[Fe(II)(TRIM)(2)]F(2), the First Example of Spin Conversion Monitored by Molecular Vibrations.

The new [Fe(II)(TRIM)(2)]F(2) spin-crossover complex (TRIM = 4-(4-imidazolylmethyl)-2-(2-imidazolylmethyl)imidazole) has been synthesized, crystallizing in the monoclinic system, space group P2/n, with Z = 2, a = 9.798(2) Å, b = 8.433(2) Å, c = 14.

Spin crossover in a catenane supramolecular system.

The compound [Fe(tvp)(2)(NCS)(2)] . CH(3)OH, where tvp is 1,2-di-(4-pyridyl)-ethylene, has been synthesized and characterized by x-ray single-crystal diffraction. It consists of two perpendicular, two-dimensional networks organized in parallel stacks of sheets made up of edge-shared [Fe(II)](4) rhombuses.