Spin Crossover Phenomenon in Nanocrystals and Nanoparticles of [Fe(3-Fpy)2M(CN)4] (MII = Ni, Pd, Pt) Two-Dimensional Coordination Polymers

The two-dimensional spin crossover (SCO) polymers [Fe(3-Fpy)(2)M(CN)(4)] (M-II = Ni(1), Pd(2), Pt(3)) were nanostructured in the form of nanocrystals and nanoparticles, and their chemical, structural, and physical characterization was carried out using different experimental methods (powder X-ray diffraction, magnetic susceptibility measurements, Mossbauer and infrared spectroscopy, transmission and scanning electronic microscopy, etc.). Surfactant-free nanocrystals of average dimensions 400 x 400 x 30 nm (1a, 1a*, 2a, 3a) were synthesized from water in oil microemulsions (w/o) while nanoparticles of average size 200 x 100, 100 x 60, and 70 x 30 nm were obtained in poly(vinylpyrrolidone) (PVP) coating polymer (1b-1d, 2b, 2b', and 3b). The spin crossover process is drastically influenced by the dimensions of the crystal/particle. Nanocrystals of 1-3 exhibit a quasi complete first-order spin transition centered within the interval 200-225 K, while PVP-coated nanoparticles undergo continuous second order spin transition at much lower temperatures (ca. 160 K).