Quasi-Reversible Water Vapochromism in a Pillared Layer Metal–Organic Framework Based on Cu(II) Nitrate and 4,4′-Bipyridine

Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are flexible solid materials constructed from organic ligands and metal nodes. These materials are well known for their ability to adsorb and desorb guest molecules, often accompanied by structural transformations and changes in the physical properties. In this study, we synthesized and characterized a Cu(II)-based MOF/PCP, [Cu(4,4′-bpy)2(NO3)][NO3]·solvent (1·solvent; 4,4′-bpy = 4,4′-bipyridine), using single-crystal X-ray diffraction analysis. The Cu(II) centers are coordinated by 4,4′-bpy ligands to form 2D square-grid layers, which are further interconnected by nitrate anions, resulting in a 3D pillared-layer framework. The crystals of this MOF exhibit pronounced structural changes upon natural crystal solvent evaporation. The original purple color is maintained when suspended in ethylene glycol (EG) or under carefully controlled dry conditions to keep EG in the pore. Under high humidity, however, the material undergoes a quasi-reversible color change from purple to blue, demonstrating water-induced vapochromism. Spectroscopic and quantum chemical studies reveal that the use of high-boiling-point solvents such as EG helps retain the framework integrity, enabling repeatable and quasi-reversible vapochromic behavior.