Solid-State Replacement in the Polyhedra of Cu2+, Ni2+, and Co2+ upon Dehydration/Rehydration Processes Associated with a Color Change in Three Isomorphous Materials with 5,5′-Indigodisulfonate

The significance of thermochromic materials in a wide range of applications makes their research and development highly desirable. Herein, three new thermochromic isomorphous supramolecular materials based on the 5,5 '-indigodisulfonate ion of the indigo carmine dye, with formulas [Cu(H2O)6](C16H8N2O8S2)<middle dot>2H2O (1), [Ni(H2O)6](C16H8N2O8S2)<middle dot>2H2O (2), and [Co(H2O)6](C16H8N2O8S2)<middle dot>2H2O (3), are reported. These materials exhibit thermochromic properties, with a color change from red-bronze to grayish-red after the first step of their dehydration process completed around 80 degrees C (conversion of 1 to [Cu(H2O)2(C16H8N2O8S2) (1 ')]), and then, this grayish-red color becomes lighter after the second step, which is completed around 230 degrees C (conversion of 1 ' to [Cu(C16H8N2O8S2)] (1d)). The 1, 1 ', and 1d phases have different crystal structures but similar tetragonal bipyramidal geometries around the copper ions. These dehydrated phases rehydrate by exposure to a higher humidity level. Solid-state UV-visible spectroscopy, thermogravimetric analysis, infrared spectroscopy, and X-ray powder diffraction analyses shed light on the structural and chemical changes accompanying the color change of 1, 2, and 3 to their partial dehydrated phases 1 ', 2 ', and 3 ', respectively. These transformations occur with the small modification of the charge transfer energy in the 5,5 '-indigodisulfonate ions, indicating the potential of dye-based metall-organic materials for chromic properties.