Interesting pH-Responsive Behavior in Benzothiadiazole-Derived Coordination Polymer Constructed via an in Situ Click Synthesis

Abstract

In this work, two new benzothiadiazole-derived complexes, namely, [Zn0.5(TBC)(H2O)] (1, TBC = 4-cyano-7-(1H-tetrazol-5-yl)benzothiadiazole) and [Cd(TBC)(N3)(H2O)]n (2), were successfully obtained by the in situ click synthetic methods in hydrothermal conditions. While complex 1 shows a mononuclear structure that can be extended to the three-dimensional (3D) supramolecular network by the intermolecular hydrogen bonds and N–S interactions, for 2, it is a coordination polymer comprised of undulated two-dimensional layers with a (8)(84•122) topological network, where the adjacent layers are stacked to form a 3D supramolecular framework sustained by interlayer N–S interactions. Comparing to the single emission of 1 centered at 570 nm, 2 showed the dual emissions, which were centered at 490 and 568 nm, respectively. Such phenomenon resulted from the different decay procedures for the π → π* states of monomeric benzothiadiazole species (ππ* state) between 1 and 2. Significantly, on the one hand, because of the deprotonated tetrazole group in the organic linker, 1 is capable of sensing the changes of pH, showing the reversible fluorescence on–off responses in acidic ambience. On the other hand, in comparison with in 1, the coordinated tetrazole group was close to neutral; therefore, 2 exhibited the self-referencing fluorescence response in an alkaline environment.