F. Varret

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. We characterized the interplay between the electronic (HS <--> LS) and structural (order <--> disorder) transformations. Elastic stresses and mechanical effects (hopping, self-cleavage) generated by the volume change upon electronic transition are also illustrated, with their impact on the photoswitching properties of the crystals.

Examples of molecular switching in inorganic solids, due to temperature, light, pressure, and magnetic field

We describe various molecular switching processes occurring in several types of inorganic solids: spin cross-over (SC) compounds, photomagnetic Prussian blue analogs (PBAs), and valence-tautomeric system. Their thermo-, photo-, piezo-, and magneto-chromic properties are illustrated by recent examples. A common description of their static properties by a two-level model is given.

Thermo-chromism and photo-chromism in a Prussian blue analogue

Abstract We report on the photo-magnetic Prussian blue analogue series Cs x Co[Fe(CN) 6 ] y (Cs-PBA). They undergo both a valence tautomeric conversion at high temperature (∼250 K) and a photo-excitation at low temperature, according to the equation already established for K-PBA and Rb-PBA series: Fe II +Co III (LS)→Fe III +Co II (HS). The high-temperature state can be trapped by thermal quenching. The photo-magnetic data are not analysed quantitatively because of the strong effects of bulk absorption of light. However, the quantitative investigation could be completed by diffuse reflectivity, which only probes the top (=photo-excited) layers of the sample. We thus observed: (i) the bleaching effect of the photo-process, due to the weakening of the charge-transfer band in the red and near IR; (ii) the complete character of the photo-transformation, compared to the spontaneous tautomeric conversion; (iii) the similitude between the photo-excited and the high-temperature states, deduced from the comparison of absorption spectra and relaxation kinetics. We have focussed on the “optimised” composition Cs 0.175 Co[Fe(CN) 6 ] y . Generic features of photo-induced magnetic structures (e.g. magnetic metastability) are also illustrated. A promising feature is the lifetime of the electronic excited state, which up to ∼130 K remains long enough to allow sizeable photo-excitation experiments on a rather long time scale. This is a promising step on the route of futuristic applications to optical information storage.

Dynamical Ising-like model for the two-step spin-crossover systems

In order to reproduce the two-step relaxation observed experimentally in spin-crossover systems, we investigate analytically the static and the dynamic properties of a two-sublattice Ising-like Hamiltonian. The formalism is based on a stochastic master equation approach. It is solved in the mean-field approximation, and yields two coupled differential equations that correspond to the HS fractions of the sublattices A and B.

On bistability in molecular systems: Photomagnetic studies of inorganic complexes

Photo‐induced effects have been detected by magnetic measurements, Mössbauer spectroscopy and reflectivity. The LIESST effect has been achieved in the spin‐crossover system [FexCo1-x(btr)2(NCS)2]·H2O. We investigated the purely photo‐induced magnetism of a Prussian Blue analogue Rb0.52Co[Fe(CN)6]0.84, 2.31 H2O, involving an optical electron transfer from FeII to CoIII. Inherent aspects of photomagnetic experiments are described: bulk and surface effects, magnetic and electronic metastabilities of the photo‐excited state.

Dynamic spin-crossover in [FeII(TRIM)2]Cl2 investigated by Mössbauer spectroscopy and magnetic measurements

Abstract The magnetic and Mossbauer data of the new coordination complex [Fe II (TRIM) 2 ]CI 2 , where TRIM = 4′-(4-methylimidazole-2′- (2″-methylimidazole)-imidazole) afford evidence for a complete and progressive spin conversion between the 1 A 1 and 5 T 2 spin states. The interstate conversion frequencies measured in the 220–300 K temperature range follow an Arrhenius behavior. The thermal variation of the equilibrium constant over the whole temperature range approximately follows a linear Arrhenius plot, thus indicating a very low degree of cooperativity in this material; a small distortion from the linear plot suggests a vibrational effect.

Monte Carlo simulations of spin-crossover transitions using the two-level model. I: Mononuclear single sublattice case

We study by Monte-Carlo technique the Hamiltonian associated with the phenomenological two level Ising-like model proposed to explain the thermal behaviour of the spin-crossover compounds. We propose an adaptation of the Metropolis algorithm to take into account the different degeneracies of the two levels.

Thermally Induced Dilution of the Photo-Induced Magnetic State of Prussian Blue Analogues

We report on recent progress in the control of dilution of photo-induced magnetic structures, by using thermal activation of the steric relaxation of the photo-excited electronic state. Experimental data relative to Rb- and Cs- based Prussian Blue Analogues are reported in terms of dynamic phase diagrams: Curie temperature vs concentration in photo-excited species. The building-up of correlations, due to steric interactions during the thermal relaxation process, is discussed. photomagnetism relaxation diluted magnetic systems

Generic features of a photo-induced magnetic structure, exhibited by the Rb0.52Co[Fe(CN)6]0.84, 2.31H2O Prussian Blue analogue

Appealing generic features of photo-induced magnetic structures are illustrated on the example of the Prussian Blue analogue Rb0.52Co[Fe(CN)6]0.84, 2.31H2O: (i) the ability of controlling by light the concentration in magnetic moments. This is conveniently achieved by combining an initial complete photo-transformation of the material, and various subsequent thermal annealings in the range 80–100 K. The dilution phase diagram of the ferrimagnetic system is, for the first time, derived. (ii) The magnetic metastability of the ‘raw photo-induced’ (RPI) state obtained at low temperatures (below Tc) and in low magnetic fields (below Hc). The RPI state is characterised by a non-relaxed domain structure, due to the lack of thermal fluctuations during the building process of the ferrimagnetic domains. It evolves irreversibly under the effect of temperature or magnetic field. The thermal aging properties of the RPI state are characterised.

Monte Carlo simulations of spin-crossover transitions using the two level model. II: Binuclear case

Abstract We show preliminary results of the Monte Carlo simulations on spin-crossover binuclear systems. The two-level Ising-like model was used with two interaction parameters: intermolecular (‘ferro’) and intramolecular (‘antiferro’). The obtained two step transitions are analysed in term of LS-LS, LS-HS HS-1HS populations.