Formation of local spin-state concentration waves during the relaxation from a photoinduced state in a spin-crossover polymer

Abstract

The complex relaxation from the photoinduced high-spin phase (PIHS) to the low-spin phase of the bimetallic two-dimensional coordination spin-crossover polymer [Fe[(Hg(SCN)3)2](4,4′-bipy)2]nis reported. During the thermal relaxation, commensurate and incommensurate spin-state concentration waves (SSCWs) form. However, contrary to the steps forming at thermal equilibrium, associated with long-range SSCW order, the SSCWs forming during the relaxation from the PIHS phase correspond to short-range order, revealed by diffuse X-ray scattering. This is interpreted as resulting from the competition between the two types of SSCW order and another structural symmetry breaking, due to ligand ordering, occurring at low temperature and precluding long-range SSCW order.

Spin-crossover materials: Getting the most from x-ray crystallography

“…Incommensurate spatially modulated structures have been reported in a limited number of SCO materials, taking place either during the thermal transition or the photoinduced transition. [68][69][70][71][72] A. Short-range ordering characterized from diffuse X-ray scattering X-ray diffraction from a perfect periodic single crystal is characterized by sharp Bragg peaks, plus satellite reflections in the case of long-range ordered modulations. Bragg peaks scattering reflects the mean electron density distribution in the unit cells of the crystal.…”

“…The dynamics of the phase transition has been interpreted using a structural model built in a wide variety of long-range ordered patterns of HS and LS molecules. [75][76][77][78][79][80]70 Up to now, spin state transitions with up to eight more or less clearly identified steps have been reported. In most of the cases, the crystal structure has been solved for all the steps, allowing a clear description of the HS/LS supramolecular ordering.…”

“…The spin state ordering component is described by the spatial probability for a site at position to be in the HS state, which in the harmonic approximation writes 70,79 :…”

“…Alternative situations have been encountered for which the wavevector is incommensurate, leading to aperiodic crystal structures, for which three dimensional periodicity is lost. 68,[70][71][72] The corresponding X-ray diffraction pattern displays the presence of incommensurate satellite reflections at positions :…”

Symmetry breakings in a metal organic framework with a confined guest

“…Combining both MOFs and SCO properties greatly enhance the richness in terms of instabilities both from the electronic and structural points of view, as reported recently. [16][17][18][19][20][21][22] The influence of a series of guest molecules, such as benzonitrile, nitrobenzene, benzaldehyde, acetonitrile/water, dimethylsulfoxide/water, on the SCO properties of this MOF-SCO was investigated more recently [16]. It was shown through the thermal dependence of χ M T (χ M = molar magnetic susceptibility, T = temperature) that the [F e(tvp) 2 (N CS) 2 ] n (tvp = trans(4,4-vinylenedipyridine)) system with no guest remains in the high spin state (HS, S=2) down to low temperature.…”