Metal–organic frameworks based on bipyridinium carboxylate: photochromism and selective vapochromism

Two new metal–organic frameworks based on a cationic 1-(3,5-dicarboxyphenyl)-4,4′-bipyridinium bromide (H2ipbpBr) ligand have been designed and synthesized by varying the solvents and counterions to adjust the coordination modes between organic linkers and metal ions. Both of them possess two dimensional (2D) neutral layered frameworks, which show photochromic and photo-modulated luminescence properties. Compound 1 is photo-active in response to UV light, while 2 is sensitive to visible and UV light, accompanied by color transformation from light yellow to brown and green, respectively. Furthermore, compound 1 exhibits interesting selective vapochromism to diethylamine (DEA), and the vapochromic sample can be easily recovered when washed with MeOH, which offers a simple, convenient and recyclable vapor sensor for organic gas emissions. This work provides abundant scope to explore novel MOF materials with useful photo-active and colorimetric sensor properties.