Antoine Goujon

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.

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.

Structure, Magnetic Properties, Polarized Neutron Diffraction, and Theoretical Study of a Copper(II) Cubane

The paper reports the synthesis, X-ray and neutron diffraction crystal structures, magnetic properties, high field-high frequency EPR (HF-EPR), spin density and theoretical description of the tetranuclear CuII complex [Cu4L4] with cubane-like structure (LH2=1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one). The simulation of the magnetic behavior gives a predominant ferromagnetic interaction J1 (+30.5 cm(-1)) and a weak antiferromagnetic interaction J2 (-5.5 cm(-1)), which correspond to short and long Cu-Cu distances, respectively, as evidence from the crystal structure [see formulate in text]. It is in agreement with DFT calculations and with the saturation magnetization value of an S=2 ground spin state. HF-EPR measurements at low temperatures (5 to 30 K) provide evidence for a negative axial zero-field splitting parameter D (-0.25+/-0.01 cm(-1)) plus a small rhombic term E (0.025+/-0.001 cm(-1), E/D = 0.1). The experimental spin distribution from polarized neutron diffraction is mainly located in the basal plane of the CuII ion with a distortion of yz-type for one CuII ion. Delocalization on the ligand (L) is observed but to a smaller extent than expected from DFT calculations.

A helium-gas-pressure apparatus with optical-reflectivity detection tested with a spin-transition solid

We have constructed a low-temperature helium pressure device that incorporates optical-reflectivity detection and is useful for the study of thermochromic materials. We have tested the device up to 1600 bar (0.16 GPa) and down to 10 K. The performance of the device is illustrated by the results of constant-pressure and constant-temperature runs for the spin-transition solid [Fe(btr)2(NCS)2]·H2O. The data establish a consistent pressure-temperature phase diagram, which is discussed.

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.

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.

Photo-Induced Electron Transfer and Magnetic Switching in CoFe Cyanides: Study of the Metastable State

Abstract Photo-induced magnetisation has been recently evidenced by Hashimoto et al. in CoFe cyanide[1]. We synthesized a diamagnetic RbFeCo cyanide able to present the same photomagnetic effect. The magnetic properties of the excited state, the electronic structure and the local structure of the ground and the excited states have been investigated. The conditions required to observe the phenomenon and the mechanism of the electron transfer are discussed.

Quasi-static nature of the light induced thermal hysteresis in [Fe(ptz)6](BF4)2 spin-transition solid

The quasi-static nature of a light induced thermal hysteresis was studied on the spin-transition compound [Fe(ptz)6](BF4)2 ,b y means of optical spectroscopy and magnetic measurements in the temperature interval between 10 and 80 K. Various experimental procedures are discussed in relation to the competition between the two processes considered, namely the photoexitation and the high-spinlow-spin relaxation. A detailed discussion of the experimental parameters, which should be considered in order to avoid erroneous interpretations of LITH, is given.