Solid-State Replacement in the Polyhedra of Cu, Ni, and Co 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(HO)](CHNOS)·2HO (), [Ni(HO)](CHNOS)·2HO (), and [Co(HO)](CHNOS)·2HO (), 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 to [Cu(HO)(CHNOS) ()]), and then, this grayish-red color becomes lighter after the second step, which is completed around 230 °C (conversion of to [Cu(CHNOS)] ()). The , , and 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 , , and to their partial dehydrated phases , , and , 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.