Thermal‐ and Photoinduced Spin‐State Switching in an Unprecedented Three‐Dimensional Bimetallic Coordination Polymer

Abstract

The compound {Fe(pmd)[Ag(CN)2][Ag2(CN)3]} (pmd=pyrimidine) was synthesized and characterized. Magnetic, calorimetric and single crystal visible spectroscopic studies demonstrate the occurrence of a two-step high-spin (HS) low-spin (LS) transition. The critical temperatures are Tc1=185 and Tc2=148 K. Each step involves 50 % of the iron centers, with the low-temperature step showing a hysteresis of 2.5 K. The enthalpy and entropy variations associated with the two steps are H1=3.6±0.4 kJ mol-1 and S1=19.5±3 J K-1 mol-1; H2=4.8±0.4 kJ mol-1 and S2=33.5±3 J K-1 mol-1. Photomagnetic and visible spectroscopy experiments show that below 50 K, where the LS state is the thermodynamically stable state, the compound can be switched quantitatively to the HS state using green-red light (550-650 nm). HS-to-LS relaxation experiments in the dark at temperatures between 15 and 55 K show that the relaxation takes place via a two-step cooperative process, which was analyzed in the context of the mean field theory. The crystal structure has been studied at 290, 220, 170, 90 and 30 K together with 30 K after irradiation. The compound adopts monoclinic symmetry (P21/c, Z=16) at all temperatures. There are five [FeN6] pseudo-octahedral sites linked by pmd, [Ag(CN)2]- and [Ag2(CN)3]- bridging ligands to form an unprecedented three-dimensional (6,6) topology. The structural analysis allows for an understanding of the microscopic mechanism of the two-step behavior of the thermally induced spin transition as well as the corresponding relaxation of the photoexcited compound based on the individual changes of the five sites. Synergy between metallophilic interactions and the spin transition is also shown by the variation of the AgAg distances. Correlations between the variation of the unit-cell volume and the change of AgAg interactions within each step with the asymmetric change of the anomalous heat capacity have also been inferred

The interaction between the spin transition and a crystallographic phase transition in the spin-crossover compound [Fe(bbtr)3](ClO4)2: Nucleation, formation of domains and fluctuations

“…7a shows a series of thermal hysteresis loops measured following different protocols. Basically they all are in agreement with the published curve as determined from magnetic susceptibility measurements on a polycrystalline powder [32] showing a hysteresis of approximately 15 K. However, for the single crystals they are much steeper, as previously observed for other spin transition compounds [38]. More importantly, the hysteresis width depends (i) on the temperature sweep rate, indicating that the kinetics of the associated phase transition are very slow indeed, and (ii) on the history of the crystal: when starting from room temperature the apparent transition temperature for the HS ?…”

Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound

“…7,8 Short-range correlations may also compete with long-range interactions and give rise to deviation from mean-field behaviors, 9 as evidenced in some cases by distinctive long relaxation tails from the LIESST state. 10 Although the vast majority of thermal spin transitions proceed in a single step, there has been an increasing number of two-step transitions reported for polymeric, [11][12][13][14][15][16][17] polynuclear, [18][19][20][21][22][23][24] or mononuclear SCO systems. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] Such transitions result most frequently from the existence of two crystallographically (symmetrically) independent SCO Fe I I sites with slightly different coordination environments, leading to dissimilar transition temperatures delimiting a plateau (or shoulder), which usually occurs with an approximately equal populations of HS and LS species.…”

State of the art and opportunities in probing photoinduced phase transitions in molecular materials by conventional and picosecond X-ray diffraction

“…Out of equilibrium processes also occur under continuous or pulsed light irradiation, in particular the LIESST effect (Light-Induced Excited Spin-State Trapping) at low temperature. Only recently the LIESST effect has begun to be thoroughly investigated by X-ray diffraction (Gütlich et al, 2004;Huby et al, 2004;Ichiyanagi et al, 2006;Kusz et al, 2005;Marchevie et al, 2002;Niel et al, 2005;Thompson et al, 2004;Pillet et al, 2006;Trzop et al, 2007 . .…”