Two pH-directed coordination polymers of cadmium isomers based on a semi-rigid polycarboxylic acid: crystal structure and fluorescence sensing of tetracyclines†

Two isomers of the coordination polymer [Cd₂(μ₆-dppta)(2,2′-bpy)₂(H₂O)]_n, namely, CP-1 and CP-2 (H₄dppta = 4-(2,5-dicarboxyphenoxy)phthalic acid and 2,2′-bpy = 2,2′-bipyridine), were successfully synthesized under similar solvothermal conditions by varying the pH values. Colorless irregular prismatic crystals of CP-1 were generated under strong acidic conditions (with a pH value of less than 4.4). These crystals were assembled from two varieties of Cd(II) binuclear clusters. Under mildly acidic conditions, when the pH exceeded 6.1, cuboid crystals of CP-2 possessing a linear Cd(II) chain was formed. Notably, both types of crystals co-existed in the pH ranges from 4.4 to 6.1. CP-1 and CP-2 exhibited remarkable chemical stability and outstanding tolerance in water and organic solvents. Significantly, the two CPs possessed the capacity to rapidly detect tetracyclines (including tetracycline (TC), chlortetracycline (CTC) and oxytetracycline (OTC)) in aqueous environments, manifesting favorable recyclability and a low detection limit. The underlying fluorescence quenching mechanism was disclosed through the comprehensive integration of experimental results and density functional theory computations.