Fluorescent Metal–Organic Frameworks for Selective Sensing of Toxic Cations (Tl3+, Hg2+) and Highly Oxidizing Anions ((CrO4)2−, (Cr2O7)2−, (MnO4)−)

The ligand 4,4'-(benzothiadiazole-4,7-diyl)dibenzoic acid was synthesized and employed for the synthesis of two metal-organic framework (MOF) compounds, [Mn4 (C20 H10 N2 O4 S)2 (HCOO)4 (DEF)2 ] (I; DEF=N,N'-diethylformamide) and [Pb(C20 H10 N2 O4 S)(DMF)] (II; DMF=N,N'-dimethylformamide). Single-crystal structure studies revealed that compound I has a three-dimensional structure and compound II has a two-dimensional structure. The luminescent nature of the MOFs was gainfully employed as a probe for the detection of highly toxic metal ions such as Tl3+ and Hg2+ . The detection limits were found to be in the parts per billion (ppb) level. The presence of an unsubstituted thiadiazole ring in the ligand appears to help in the detection of these highly toxic metal ions in solution as the metal ions interact with S, which was revealed by Raman spectroscopic studies. The compounds were also found to be good candidates for the detection of highly oxidizing anions such as chromate, dichromate, and permanganate, again in ppb levels of concentration in solution. Magnetic studies on compound I indicate antiferromagnetic behavior. The variable-temperature electrical conductivity studies indicate a semiconducting nature with comparable behavior to well-known semiconductors such as CdSe, ZnTe, and GaP.