Switchable Coordination Nanoparticles

Abstract : The research work in this thesis is focused on the preparation and the study of the physical properties of switchable nanocrystals based on spin crossover and photomagnetic materials that are potential candidates as nanosensors. The spin crossover nanocrystals belong to the coordination network family based on the Fe(II) Hoffman chlatrates [Fe(pyrazine)][Pt(CN)4]. The study is focused on controlling the size of self-standing nanorods and on the investigation of their thermal bistability in relation with their environment and their interaction with guest molecules. We show that the growth of the rod-shaped nanocrystals is obtained in the presence of preformed 2-3 nm nanoparticles of the Prussian Blue Analogue CsNiCr(CN)¬6 that is thought to direct to anisotropic growth of the objects. The formed rods can be stable for a long time due to their negative charge. They possess a rather wide thermal hysteresis centered around room temperature. The width and position in temperature of the thermal hysteresis depends on the nature of the environment of the nanocrystals. With very long ammonium salt (such as dodecyl trimetylammonium bromide or dexhyl trimethyl ammonium bromide), the thermal hysteresis becomes narrower and shifts to low temperature. While with shorter alkyl chains, the shift of the hysteresis is smaller without much change in its width. When the rods are embedded in a rigid silica matrix, the transition is shifted to very low temperature and the thermal hysteresis vanishes. The insertion of I2 molecules leads to a wider thermal hysteresis centered at a higher temperature.The photomagnetic nanoparticles belong to the bimetallic Prussian Blue Analogues (PBAs) family based on CoFe. This type of materials may undergo a thermal transition from the paramagnetic CoIIFeIII state to the diamagnetic CoIIIFeII one. The high temperature state can then be restored at low temperature by light illumination. We focused on three types of CoFe PBAs nanoparticles containing the CsI (11 nm) and the RbI alkali ions with two different sizes 30 and 80 nm for the latter. The size of the objects was controlled by different parameters such as the nature of the alkali ion, the concentration of the precursors and their stoichiometry. The combination of different techniques such as electronic and infra spectroscopies together with Electron Spin Resonance Spectroscopy and X-ray powder diffraction allows following the evolution of the nanoparticles’ composition with time in solution at room temperature. The evolution study leads to the general conclusion that the electron transfer phenomenon from CoII to FeIII occurs in solution during several days before the objects reach their thermodynamic stable state that was found to depend on the nature of the alkali ion and on the size of the objects. The (photo)magnetic behavior was then investigated in the solid state for different evolution time of the three types of particles, which allowed proposing a qualitative mechanism of their formation in solution.
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Thi Mai Linh Trinh. Switchable Coordination Nanoparticles. Inorganic chemistry. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLS171⟩. ⟨tel-02194933⟩

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