Tunable Bistability in a Three‐Dimensional Spin‐Crossover Sensory‐ and Memory‐Functional Material

Abstract

In the search for new functional materials, sensory and memory are important functions that require switchable components. Iron(II) spin-crossover (SCO) complexes have been demonstrated to be particularly suitable to this end. [1][2][3] They reversibly switch between a diamagnetic (S = 0) low-spin state (LS) and a paramagnetic (S = 2) high-spin state (HS) under an external stimulus like temperature, pressure, or light. A sharp variation of the structure, magnetism, color, or dielectric constant of the system in response to these stimuli may occur in the solid state. Furthermore, hysteresis accompanies the firstorder spin transition (ST) when the structural changes are transmitted cooperatively through the whole solid. Due to their switching properties the SCO materials are potentially useful for rewritable optical, thermal, or pressure memories at the nanometer scale. To be useful in practice, a demanding set of material requirements must be met, such as room-temperature operation, non-destructive writing, and readout of the information. Nowadays, prototypes of thermal displays based on the one-dimensional SCO polymeric systems ) have been described. [4,5] This family of compounds has been the focus of much attention because they present steep STs with large, stable hysteresis loops and color changes (white and violet for the HS and LS states, respectively) around room temperature for a given x value. The effect of pressure on the ST in some of these complexes has also been studied. ) exhibiting similar cooperative SCO behavior accompanied by a drastic color change from yellow-orange (HS) to deep red (LS) has been synthesized. [7,8] A Raman spectroscopic study of the pressure effects on the SCO coordination polymer {Fe(pz)[Ni(CN) 4 ]}·2H 2 O demonstrated the occurrence of a piezohysteresis of 0.07 GPa for this material at room temperature. [9] Unfortunately, this ST was found to be rather incomplete; indeed, only 50 % of the SCO iron(II) ions were involved in the ST and the hysteresis loop was strongly distorted. However, these results have created solid expectancies for the creation of pressure displays at room temperature. As a continuation of our research devoted to the study of new cyanide-bridged bimetallic polymers with strong cooperative SCO properties, [7][8][9][10][11][12][13][14] we decided to investigate the influence of pressure on the magnetic and optical properties of the 3D coordination polymer {Fe(pmd)(H 2 O)[Ag(CN) 2 ] 2 }·H 2 O (pmd = pyrimidine) (1). Herein, we report an unprecedented observation of pressure-tunable thermal bistability and piezochromic bistability at room temperature in 1.The crystal structure and magnetic behavior at ambient pressure of 1 were reported previously.[ (Fig. 1). At atmospheric pressure, the compound undergoes a thermally induced first-order ST with hys- COMMUNICATIONSAdv. Mater.

Pressure Effect Investigations on the Spin Crossover Systems{Fe[H2B(pz)2]2(bipy)} and {Fe[H2B(pz)2]2(phen)}

“…The measurements have been performed at the rate of 1 K·min -1 on single-microcrystals. At 300 K and 10 5 Pa, a χ M T value of 3.52 cm 3 ·K·mol -1 indicates a S = 2 spin state configuration for the iron(II) ions. χ M T remains constant down to 220 K, where it progressively diminishes as a consequence of the spin transition to the S = 0 spin state and attains a value of 0.06 cm 3 ·K·mol -1 at 100 K. The transition is complete and relatively abrupt.…”

“…increase in the hysteresis width, nonlinear behaviour of T c (P) versus P). [5,7,8,16] Figure 4. T 1/2 versus P plots for 1 (filled circles) and T av versus P plot for 2 (filled squares).…”

Cooperative Spin‐Crossover Behaviour in Polymeric 1D FeII Coordination Compounds: [{Fe(tba)3}X2]·nH2O

“…The unit cell contains four crystallographically independent copper ions with an occupancy factor of 0. 5 Figure 2 illustrates the crystal packing of 7 in the xy plane. The molecular crystal packing of 7 can be described as an infinite hexagonal array of [Cu(tba) 3 ] 2+ linear chains along the z direction, with water molecules and CF 3 …”

“…[5][6][7] The necessary conditions that an SCO material must fulfil for the construction of devices for thermal displays or recording data include abruptness of spin transition with hysteresis, room-temperature operation, change of colour and chemical stability. , where x Յ 1 and n Ն 0, have been described.…”

Ein thermisch induzierter Spinübergang an einem Eisen(II)‐Komplex mit einer 70 K breiten Hystereseschleife

“…[1] Um für technische Anwendungen geeignet zu sein, sollten solche Verbindungen einen kooperativen Spinübergang mit einer breiten Hystereseschleife bei Raumtemperatur aufweisen. [2] In den vergangenen Jahren wurde viel darüber diskutiert, wie kooperative Wechselwirkungen effizient innerhalb eines Kristallgitters übertragen werden können, da dies der Schlüssel für das Auftreten möglichst breiter, thermisch induzierter Hystereseschleifen ist. Eine vielversprechende Strategie wurde von Kahn et al [2a, 3] vorgeschlagen und basiert auf einer Verknüpfung der Komplexzentren über kovalente Brückenliganden, wobei Koordinationspolymere erhalten werden.…”