Metal-Organic Frameworks with Extended Viologen Units: Metal-Dependent Photochromism, Photomodulable Fluorescence, and Sensing Properties

The photoresponsive viologen unit has been widely used to endow metal-organic materials with photochromic and other photomodulable properties. Herein we report the first examples of the metal-organic materials functionalized by extended viologens (ExVs), of general formula [ML]·2H₂O (M = Zn for 1, M = Mn for 2, M = Co for 3, and L is a tetracarboxylate ligand with the p-phenylene-extended viologen spacer). Of the three isomorphic metal-organic frameworks, only 1 is photochromic owing to formation of extended viologen radicals through photoinduced electron transfer (PET). The incapability of 2 and 3 to undergo photochromism can be ascribed to longer intermolecular donor-acceptor contacts, emphasizing the sensitivity of solid-state PET to structural changes. 1 also shows strong fluorescence owing to interligand charge transfer, and the fluorescence can be reversibly modulated and switched on/off in the photochromic process. Furthermore, 1 shows excellent hydrolytic stability and can be used as a sensitive, selective, and recyclable fluorescence sensor for detecting Fe³⁺ in water. The results demonstrated the great potential of extended viologens as functional units for the design of novel responsive metal-organic materials.