Structure:Function Relationships for Thermal and Light-Induced Spin-Crossover in Isomorphous Molecular Materials

Isomorphous [FeL2][BF4]2·solv and [FeL2][ClO4]2·solv (L = 4-{isopropylsulfanyl}-2,6-di{pyrazol-1-yl}pyridine; solv = MeNO2, MeCN, 0.67Me2CO or H2O) exhibit a variety of thermal spin-crossover (SCO) behaviours. This complexity extends to the light induced excited spin state trapping (LIESST) experiment where, uniquely, five members show the expected inverse relationship between their thermal SCO (T½) and LIESST relaxation (T(LIESST)) temperatures but a sixth compound ([FeL2][BF4]2·MeCN) does not. The structural basis of these observations has been probed by X-ray crystallography, photocrystallography and periodic DFT+U+D2 calculations. Among the compounds examined, more cooperative thermal SCO is strongly coupled to order/disorder transitions in the solvent and/or isopropyl substituents and vice versa. A series of symmetry breaking phase transitions in [FeL2][BF4]2·MeNO2, before and after photoexcitation, occurs 10-20 K below T(LIESST) and has no direct bearing on the T½/T(LIESST) relationship. These phase changes are not shown by other compounds in the study. The anomalous T(LIESST) in [FeL2][BF4]2·MeCN, and its observed negative lattice expansion during isothermal low→high-spin conversion, are not reproduced computationally which implies those properties are unconnected to its spin state energetics. Its minimised high- and low-spin structures also deviate more from experiment than the other compounds investigated, in the most plastic region of the lattice which includes the solvent molecule. We conclude that reorientation of the linear MeCN molecule contributes a temperature-dependent lattice activation barrier to the spin-transition in [FeL2][BF4]2·MeCN, leading to the higher T(LIESST) value observed

The role of symmetry breaking in the structural trapping of light-induced excited spin states

Light-Induced Excited Spin State Trapping (LIESST) data are reported for seven isostructural solvate salts from the iron(II)/2,6-di(pyrazol-1-yl)pyridine family. A complicated relationship between their spin-crossover T1/2 and T(LIESST) values may reflect low-temperature thermal and light-induced symmetry breaking, which is shown by one of the compounds but not by two others

Electronic vs structural ordering in a manganese(III) spin crossover complex

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Spin crossover behaviour in a homologous series of iron(II) complexes based on functionalized-bipyridyl ligands

International audienceA series of bulky substituted bipyridine-related iron(II) complexes [Fe(HBpz)(L)] (pz = pyrazolyl) were prepared, where L = 5,5'-dimethyl-2,2'-bipyridine (bipy-CH, 1), L = dimethyl-2,2'-bipyridyl-5,5'-dicarboxylate (MeObpydc, 2), L = diethyl-2,2'-bipyridyl-5,5'-dicarboxylate (EtObpydc, 3), or L = diisopropyl-2,2'-bipyridine-5,5'-dicarboxylate ( i-PrObpydc, 4). The crystal structures of five new iron(II) complexes were determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of 3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover (SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo gradual and incomplete SCO behaviors. Complexes 2 and 4 show a completely gradual and steep SCO, respectively. Such different SCO behaviors can be attributed to an electronic substituent effect in the bipyridyl ligand conformation and a crystal packing effect. Importantly, the electronic substituent effect of the isopropyl acetate group and C-H···O supramolecular interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition

Stress-Induced Domain Wall Motion in a Ferroelastic Mn³⁺ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin‐state ordered phase of a spin crossover complex. Single crystal X‐ray diffraction and resonant ultrasonic spectroscopy (RUS) revealed two distinct symmetry‐breaking phase transitions in the mononuclear Mn 3+ compound [Mn(3,5‐diBr‐sal 2 (323))]BPh 4 , 1. The first at 250 K, involves the space group change Cc → Pc and is thermodynamically continuous, while the second, Pc → P1 at 85 K, is discontinuous and related to spin crossover and spin‐state ordering. Stress‐induced domain wall mobility was detected as softening of the phonon modes at the Pc → P1 transition

From molecular switching to material transformation: revisiting the spin crossover with ultrafast pump-probe techniques

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