Solvent- and anion-controlled photochromism of viologen-based metal-organic hybrid materials

Research on the tunable photosensitive properties of soft metal organic hybrid materials via environmental stimulation is presented. A supramolecular coordination polymer accommodating guest water molecules and anions is synthesized in aqueous solution. The original photochemically inert framework exhibits thermochromism, and can be transferred into a photochromic state via dehydration treatment. Furthermore, the interlayer ClO ₄ ^- ions can be exchanged by halide and pseudohalide anions in a crystal to crystal manner. Although the favorable geometry for electron transfer from the carboxylate group to the bipyridinium moiety is created in transformed structures, the introduction of halide or pseudohalide anions leads to the formation of different charge transfer interactions with electron-accepting bipyridinium units in the ground state, which improves the light absorption of the compounds in the visible region and greatly affects the photoreduction efficiency of the bipyridinium unit. Consequently, the transformed crystals exhibit multicolor photochromic properties in solid states.