Distinct Chromic and Magnetic Properties of Metal-Organic Frameworks with a Redox Ligand

An electron-deficient and potentially chromic ligand has been utilized to impart redox activity, photo- and hydrochromism, and solvotomagnetism to metal-organic frameworks (MOFs). A pair of MOFs were constructed from the flexible zwitterionic viologen-tethered tetracarboxylate linker N,N'-bis(3,5-dicarboxylatobenzyl)-4,4'-bipyridinium (L^2-): [Co₃(L)(N₃)₄] (1) and [Mn₂(L)(N₃)₂(H₂O)₂]·3H₂O (2). Both compounds show three-dimensional frameworks in which mixed azido- and carboxylato-bridged chains are connected through the electron-deficient viologen moieties. The chain in 1 is built from alternating bis(azide) and (azide)bis(carboxylate) bridges, while that in 2 contains uniform (azide) (carboxylate) bridges. The MOFs shows the characteristic redox properties of the viologen moieties. The redox activity affords the MOFs with different chromic properties, owing to subtle differences in chemical environments. 1 shows reversible photochromism, which is related to the radical formation through photoinduced electron transfer from azide-carboxylate to viologen according to UV-vis, X-ray photoelectron, and electron spin resonance spectroscopy and DFT calculations. 2 is nonphotochromic for lack of appropriate pathways for electron transfer. Unexpectedly, 2 shows a novel type of solid-state hydrochromism. Upon the removal and reabsorption of water, the compound shows remarkable color change because of reversible electron transfer accompanying a reversible structural transformation. The radical mechanism is distinct from those for traditional hydrochromic inorganic and organic materials. Magnetic studies indicate ferro- and antiferromagnetic coupling in 1 and 2, respectively. What's more, 2 shows marked magnetic response to the removal of water molecules owing to the formation of radicals. The compound illustrates a unique material exhibiting dual responses (color and magnetism) to water.