Novel photo- and/or thermochromic MOFs derived from bipyridinium carboxylate ligands

Abstract

By utilizing the photo-active bipyridinium carboxylate ligand 1-(3,5-dicarboxyphenyl)-4,4′-bipyridinium bromide (H₂ipbpBr) and varying the solvents, two new Zn-MOFs with photo- and/or thermochromic as well as photocontrolled luminescent properties have been successfully synthesized. [Zn(ipbp)(H₂O)]·NO₃·H₂O (1) has a three-dimensional (3D) interpenetrated cationic framework, while [Zn(ipbp)(H₂O)₂]·NO₃·H₂O (2) possesses a 2D layered cationic network. Interestingly, 2 and its dehydrated phase 2′ can undergo reversible structure transformation upon removal and uptake of guest and coordinated water molecules. 1 and 2 are photo-active to UV and visible light, and undergo an obvious color transformation from light yellow to green as a result of the formation of ipbp radicals. Compared with 2, 1 is more photosensitive due to its close donor–acceptor interaction and dense arrangement of interpenetrated frameworks. Meanwhile, 1 is thermochromic with a lower thermal-response temperature (90 °C). These two compounds also exhibit tuneable luminescence in response to light. This work provides a facile method to synthesize novel chromic MOFs using the photosensitive organic linkers and the conformational adjustment.