Photochromic Porous and Nonporous Viologen-Based Metal–Organic Frameworks for Visually Detecting Oxygen

The detection of the oxygen molecule is of particular interest due to various applications in chemical, biological, and environmental areas, and the typical materials of detecting O2 such as Clark-type electodes and optical oxygen sensors suffer from disadvantages such as insensitivity or usage of expensive instrument. In this paper, we present a nonporous two-dimensional viologen-based metal–organic frameworks consisting of pseudo-left- and right-handed helical chains [Cd­(CPBPY)­(o-BDC)­(H2O)]­·H2O (CPBPY = N-(3-carboxyphenyl)-4,4′-bipyridinium, o-BDC = o-benzenedicarboxylate) (1) that exhibits a slow photochromism under a vacuum, inert atmosphere, and even exposure to oxygen, and a three-dimensional porous 6-connected pcu topological metal–organic framework [Cd3(CPBPY)2­(BDC)3]­·DMF­·H2O (BDC = 1,4-benzenedicarboxylate) (2), which displays rapid photochromism only under a vacuum or inert atmosphere. The photochromic product 2′ can quickly and conveniently detect O2 by naked eye recognition of color change. The reason why 2 can rapidly detect oxygen might be the presence of one-dimensional channels providing a pathway for oxygen molecules.