Thermochromic Properties of a Photochromic Layered Zirconium Bisphosphonate Viologen Compound: [Zr₂(O₃P-CH₂CH₂-NH₄C₅-C₅H₄N-CH₂CH₂-PO₃)F₄.4H₂O]FBr.2H₂O

Abstract

The zirconium bisphosphonate viologen compound, [Zr₂(O₃P-CH₂CH₂-NH₄C₅-C₅H₄N-CH₂CH₂-PO₃)F₄.4H₂O]FBr.2H₂O (ZPVFB), has been synthesized and studied by infrared, elemental and thermogravimetric analyses. These studies show the existence of reversible thermochromism (white-green) due to dehydration and rehydration and irreversible thermochromism (yellow/brown-black) consequential to loss of various components in the compound. Elemental and infrared analyses suggest that this microcrystalline compound represents the second independent structural type of a new class of layered zirconium bisphosphonate viologen compounds with the general formula Zr₂(O₃P-CH₂CH₂-NH₄C₅-C₅H₄N-CH₂CH₂-PO₃)F_6-nX_n.2H₂O (n ≤ 2.5 and X = Cl). In ZPVFB, X = Br, n = 1. The coordination environment of each zirconium atom consists of two water molecules, two fluoride ions and two phosphonate oxygen atoms arranged in an octahedron. Heating the photocolored (blue) compound leads to thermal bleaching prior to development of a green color. Conversely, irradiation of the green compound produces a blue-green species implying that the reversible thermochromic behavior does not affect its photochromic characteristics. A structure is proposed on the basis of the structure of a similar compound solved by X-ray technique. The suggested structure and mechanism satisfactorily account for the observed behavior of ZPVFB.