The jungle of experimental behaviors of spin-crossover materials contains a tremendous number of unexpected behaviors, among which, the unsymmetrical hysteresis loops having different shapes on heating and cooling, that we often encounter in literature. Excluding an extra effect of crystallographic phase transitions, we study here these phenomena from the point of view of elastic modeling and we demonstrate that a simple model accounting for the bond lengths misfits between the high-spin and low-spin states is sufficient to describe the situation of unsymmetrical hysteresis showing plateaus at the transition only on cooling or on heating branches. The idea behind this effect relates to the existence of a discriminant elastic frustration in the lattice, which expresses only along the high-spin to low-spin transition or in the opposite side. The obtained two-step transitions showed characteristics of self-organization of the spin states under the form of stripes, which we explain as an emergence process of antagonist directional elastic interactions inside the lattice. The analysis of the spin state transformation inside the plateau on cooling in terms of two sublattices demonstrated that the elastic-driven self-organization of the spin states is accompanied with a symmetry breaking.

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A porous 3D Fe II coordination polymer made of 4,4′bipyridine ligands (bipy) and [N(CN) 2 ] − (dca) organic bridges, [Fe(bipy) 2 dca]ClO 4 •CHCl 3 •CH 3 OH (1•CHCl 3 •CH 3 OH), has been synthesized as single crystals, by a slow diffusion technique at room temperature. Single-crystal X-ray diffraction analysis has shown that the structure of 1•CHCl 3 •CH 3 OH presents large volume values of porosity (1790 Å 3 per iron atom). It crystallizes in the orthorhombic Cmc2 1 space group but undergoes a phase transition to Cmca at 298 K, presumably due to solvent release. According to magnetic data, this material displays a gradual spin crossover behavior, along with a thermal hysteresis loop of 15 K around 190 K. However, when lattice solvent molecules are removed, the magnetic behavior changes drastically resulting in more gradual spin conversion or even silencing the spin crossover behavior of the as-synthesized complex (1•CHCl 3 •CH 3 OH), evidencing the influence of guest molecules on the magnetic properties by host−guest interactions. This property was confirmed by spatiotemporal optical microscopy studies, performed on several single crystals. The generation of a host lattice that interacts with exchangeable guest species in a switchable fashion has implications for the generation of previously undeveloped advanced materials with applications in areas such as molecular sensing.