Fe-doped diaquabis[malonato(1-)-??2O,O???]zinc(II) an evidence case of the synthesis of thermochromic materials by metal doping

Undoped and doped diaquabis[malonato(1-)-κ2O,O′]zinc(II) crystals with 1, 2, and 5 % of iron were successfully synthesized at room temperature with zinc carbonate, malonic acid, and iron (III) chloride. They were characterized by Fourier Transform Infrared (FTIR), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), Thermogravimetric analysis (TGA), Powder X-ray Diffraction (PXRD), Single Crystal X-ray Diffraction (SCXRD), X-ray Absorption Spectroscopy (XAS), and solid-state Ultra-Violet and Visible spectroscopy (UV–Vis). FTIR analysis revealed the presence of all the expected functional groups. ICP-AES confirmed the concentration of iron in doped materials. TGA showed that materials decomposed in three steps with the departure of water molecules in the first step at around 110 °C. It is observed in doped samples that iron ions in mixed +2 and +3 (with a great part in +3) oxidation states substitute Zn2+ ions in the octahedral sites, and those samples show a blue color with a strong absorption at 620 nm related to the charge-transfer transition from the malonate pπ to the half-filled dπ* orbital. After dehydration, all the materials change their structure and become colorless. This structural change is reversible and the blue color is re-obtained in the air with about 75 % relative Humidity. Fe-doped diaquabis[malonato(1-)-κ2O,O′]zinc(II) materials are then thermochromic with a reversible color change from blue to colorless, and they can be used in moisture sensing. The metal doping process of materials could then be an interesting method for the rational synthesis of chromic materials.