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)·2H₂O (<b>1</b>), [Ni(H₂O)₆](C₁₆H₈N₂O₈S₂)·2H₂O (<b>2</b>), and [Co(H₂O)₆](C₁₆H₈N₂O₈S₂)·2H₂O (<b>3</b>), 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 °C (conversion of <b>1</b> to [Cu(H₂O)₂(C₁₆H₈N₂O₈S₂) (<b>1'</b>)]), and then, this grayish-red color becomes lighter after the second step, which is completed around 230 °C (conversion of <b>1'</b> to [Cu(C₁₆H₈N₂O₈S₂)] (<b>1d</b>)). The <b>1</b>, <b>1'</b>, and <b>1d</b> 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 <b>1</b>, <b>2</b>, and <b>3</b> to their partial dehydrated phases <b>1'</b>, <b>2'</b>, and <b>3'</b>, 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.