A novel photo- and hydrochromic europium metal–organic framework with good anion sensing properties

Crystalline chromic materials coupled with other functionalities are of particular interest due to their multiple external stimuli responsive properties. In this work, a new photochromic and hydrochromic metal–organic framework based on a bipyridinium carboxylate cationic 1-(3,5-dicarboxyphenyl)-4,4′-bipyridinium bromide (H2ipbpBr) ligand and a luminescent Eu3+ ion has been successfully designed and synthesized. [Eu(ipbp)2(H2O)3]·Br·6H2O (1) is photo-active under both UV and visible light irradiation accompanied by an obvious color change from light yellow to green owing to the formation of ipbp radicals under light stimulus. Meanwhile, compound 1 exhibits unique hydrochromic properties, and the reversible color change between light yellow and dark green caused by electron transfer can be induced by release/reabsorption of guest water molecules accompanied by a structure transformation. Moreover, the luminescence properties and two-dimensional layered cationic network of 1 endow it with excellent sensing ability for Cr2O72− and CrO42− anions. This work will provide a new perspective to design smart MOF materials that can serve as practical multi-responsive sensors to light, water, and chemicals.