Thermal and Light-Induced Spin Crossover Phenomena in New 3D Hofmann-Like Microporous Metalorganic Frameworks Produced As Bulk Materials and Nanopatterned Thin Films

Three novel three-dimensional coordination polymers, {Fe II (azpy)[(M II (CN) 4 )]}· n H 2 O (azpy = 4,4′-azopyridine; M = Ni, Pd, and Pt), have been synthesized as polycrystalline bulk materials and as continuous as well as nanopatterned thin films on gold substrates. Single crystals of the Pt derivative have also been isolated and the corresponding X-ray diffraction analysis at 140 K indicates that it crystallizes in the tetragonal P 4/ mmm space group with a = b = 7.1670(5) Å, c = 13.0330(13) Å, V = 669.40(9) Å 3 and Z = 1. Four square-planar [Pt(CN) 4 ] 2− anions occupy the equatorial positions of [Fe II N 6 ] octahedron and four Fe(II) atoms are coordinated to each [Pt(CN) 4 ] 2− anion, thereby defining an infinite set of 2D planar layers pillared by the azpy ligands which bridge through the axial positions consecutive Fe(II) atoms. Magnetic susceptibility, calorimetric, and Mössbauer measurements reveal the occurrence of a thermal spin crossover phenomenon in the powder samples, which depends strongly on the water-content of the lattice in the case of Pd and Pt derivatives. The spin crossover is less cooperative with T c = 245 K in the case of Ni, but the Pd (Pt) analogues display almost complete spin transitions with a hysteresis loop centered around 292 (280) and 191 (183) K, in the hydrated and dehydrated forms, respectively. Thin films of the three compounds were grown via a sequential assembly method using coordination reactions. Raman spectroscopic study proves that thermal spin crossover, similar to the bulk forms, occurs in the films. With the exception of the bulk Ni complex, a virtually complete light-induced spin conversion (LIESST effect) is detected by Raman spectroscopy in the thin films as well as in the powders below ca. 130 K.