Crystalline coordination polymers are a newer class of organic-inorganic hybrid nanomaterials created by infinitely extending metal-ligand coordination interactions. These polymers show promise in a broad range of applications, including gas storage, molecular sieves, ion exchange, sensing, magnetism, and catalysis. [1, 2] Recently, a rational-design strategy for supramolecular gels based on the concept of coordination polymers is attracting interest. [3][4][5][6] In particular, recent studies have demonstrated that simple bridging organic units can facilitate the formation of coordination polymer gels in the absence of auxiliary moieties (e.g., urea, sugar, cholesterol, long alkyl chains), offering new possibilities to produce functional soft materials from structurally simple building blocks. [7] Although the literature includes several reports on the synthesis and characterization of bulk coordination polymer gels, there are only a few examples of such gels used in practical catalysis applications.[8] Also, to best of our knowledge, there is no report on the usage of coordination polymer gels as chemosensors or adsorbents for toxic gases. Coordination polymer gels feature easy preparation and handling, good stability, and recyclability. As such, they will act as very useful chemosensors if they can bind to specific toxic gases.To design a chemosensor, we chose tetrazole-appended benzene as the framework to attach ligands because of its well-defined and extensively developed metal-organic framework (MOF).[9] A tetrazole derivative, with multibinding sites, promotes cross-linking to form a polymer network, which is a critical condition for gelation. Herein, we describe the preparation, morphology, fluorescence, and lifetime of a tetrazole-appended coordination polymer gel incorporating Co 2 + ions. In addition, we tested the CoBr 2 coordination polymer gel (1) as a selective chemosensor for chloride gas, such as HCl, SOCl 2 , (COCl) 2 , and COCl 2 . Upon the addition of several gases containing chloride atoms, gel 1 changed from red to blue in the gel state through interconversion. This is a new phenomenon that has never before been reported for coordination polymer gels.In a typical experiment, 1,2,4,5-tetra(2H-tetrazole-5-yl)-benzene (TTB) was dissolved in organic solvent and the Co 2 + salt was dissolved in the same solvent. The Co 2 + solution was then added to the TTB solution without heating. The molar ratios of metal ions to TTB was in the range of 1-5:1, and the amount of the coordination polymer in the gel ranged from 1 to 5 wt %, corresponding to a molar ratio of gelator/solvent of % 1:10 4