Pressure effect investigations on spin-crossover coordination compounds

Abstract

The piezo-chromic properties of spin crossover complexes have been recognized for a long time, with increasing pressure favouring the low spin state due to its smaller volume and therefore shifting the spin equilibrium towards higher temperatures and accelerating the relaxation at a given temperature. However, the interpretation and quantification of pressure induced changes have been several times compromised by the relatively poor and incomplete spectral and structural information provided by the detection methods or due to the experimental difficulties related to the need for hydrostatic conditions at low temperatures. The present review is therefore primarily focused on these experimental aspects of high pressure spin crossover research providing an overview of methods of pressure generation and associated detection methods as well as on selected recent results.Résumé: Les propriétés piézo-chromiques de complexes à transition de spin ont été identifiées depuis longtemps. Du point de vue théorique, l'application d'une pression favorise l'état bas spin, de plus petit volume. Ainsi les températures de transition augmentent et les cinétiques de relaxation s'accélèrent sous pression. Toutefois, l'interprétation et la quantification des modifications induites par la pression restent un challenge en raison de la difficulté d'obtenir des données structurales et spectrales fiables dans des conditions de pression hydrostatique. Ceci est d'autant plus vrai lorsque les hautes pressions et les basses températures doivent être combinées. Ainsi, l'objectif principal du présent article est une mise au point sur les techniques utilisées pour l'étude des composés à transition de spin sous pressionillustrées par une sélection de résultats récents.the piston(s) in the desired position even after removing the cell from the press. These (relatively) small, autonomous cells are particularly popular because they can be easily combined with cryostats, magnetometers and other equipment [26,82,109]. While several designs, both home built and commercially available have been used in the study of SCO materials, the principle upon which they work is similar. In general the sample is placed in a Teflon capsule J Rotaru et al. have been analysed the thermal behaviour of the spin transition complexes [FexZn1-x(btr)2(NCS)2]H2O (for x=0.6 and x=1) by diffuse reflectivity measurements, under constant pressure in the range 1-1600 bars obtained with a gas pressure cell [186]. I J , as shown in Figure 24.

Single‐Crystal X‐Ray Diffraction Study of Pressure and Temperature‐Induced Spin Trapping in a Bistable Iron(II) Hofmann Framework

“…The equilibrium temperature, T 1/2 ( p ) (Δ G HL =0), at which the molar fractions γ HS and γ LS are equal to , increases following the Clausius–Clapyron equation: T 1/2 ( p )= T 1/2 + p [Δ V HL /Δ S HL ], where T 1/2 is the equilibrium temperature at ambient pressure. Besides this linear increase of T 1/2 ( p ), pressure imparts significant changes to the cooperative properties, such as modification of the hysteresis width, introduction of steps in the hysteresis loop, stabilisation of the HS state, or even induction of SCO in inert metal centres …”

“…Unlike thermally‐driven SCO studies, work on the effect of pressure on SCO complexes is relatively rare due to experimental constraints . This is particularly true for single crystal X‐ray diffraction studies because, in addition to intrinsic technical difficulties, molecular SCO materials are sensitive to low pressures (<0.1 GPa), which are difficult to control.…”

Room‐Temperature Bistability in a Ni–Fe Chain: Electron Transfer Controlled by Temperature, Pressure, Light, and Humidity

“…The 1 HT phase changes color from brownish to green upon grinding or pressing at RT, which prompted us to investigate the influence of pressure on the ETPT in 1 employing magnetic measurements (Figures 5 and S21). Similarly to many spin SCO and ET compounds, [5a,d, 17d–g] the application of pressure shifts the T 1/2 temperatures to higher values. The T 1/2↑ increases linearly from 309 K at ambient pressure to 339 K at 85 MPa.…”

“…Indeed, numerous examples of efficient photo‐switching in such materials have been reported [16] . Very rarely, the ETPT in CN‐bridged metal complexes have been induced or tuned by applied pressure, [17] which was widely investigated for SCO systems [5d] …”

Putting the Squeeze on Molecule-Based Magnets: Exploiting Pressure to Develop Magneto-Structural Correlations in Paramagnetic Coordination Compounds

“…One area where this approach has made a significant impact is in the field of magnetism. Herein, we highlight our efforts in developing magneto-structural correlations in transition metal molecule-based magnets; we do not attempt to cover examples from other researchers, which are many and varied [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. At the outset, what we hoped to observe was that applied pressure would change bond lengths and angles around the metal centres, and that that would lead to changes in magnetic exchange interactions and/or magneto-anisotropies.…”