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-benzenedicar- boxylate)  (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.