Fe(Me2-bpy)2(NCSe)2 spin-crossover micro- and nanoparticles showing spin-state switching above 250 K

Abstract

We present the study of nano- and microparticles of the Fe(Me₂-bpy)₂(NCSe)₂ spin-crossover complex prepared from the diamagnetic precursor [Fe(Me₂-bpy)₃](NCSe)₂·S. Two solvates of the latter were characterized by single-crystal X-ray structures at 100 K (S = 2MeOH or 3H₂O). The extraction of one Me₂-bpy per metal ion in [Fe(Me₂-bpy)₃](NCSe)₂·S was achieved either by thermolysis at temperature higher than 150 °C or by precipitation in an anti-solvent, leading to a polycrystalline or particulate powder of Fe(Me₂-bpy)₂(NCSe)₂. This chemical conversion was investigated by TGA, powder X-ray diffraction, IR, Raman and magnetic measurements. The S = 0 ↔ S = 2 spin-crossover of Fe(Me₂-bpy)₂(NCSe)₂ centered at ca. 340 K is almost complete at low temperature (HS residue ≤ 5% below 250 K) while at 370 K, the HS fraction can be estimated at ∼0.7. These features are essentially preserved whatever be the size of particles (56, 460 and 1200 nm) as a consequence of the weak cooperativity of the process occurring at high temperature, the crystallinity and the molecular nature of particles. This approach leading to dispersion of small particles in a polymer is suitable for the preparation of materials of optical quality, via the stabilization and processing of nanoparticles in convenient matrices to form thin films.