Anion binding studies with anthraimidazoledione-based positional isomers: A comprehensive analysis of different strategies for improved selectivity.

In the present work, we have reported the design of three different positional isomers of anthraimidazoledione-based charge transfer probes and their anion-binding properties under various conditions. In the acetonitrile medium, the meta isomer showed ratiometric optical response towards basic anions, such as F, CN, AcO and HPO. Based on the differences in the hydrogen bonding ability of these anions, we observed distinguishable output signal, particularly in fluorescence. Though meta and para isomers showed effective interaction with anions, the response was relatively weak for the ortho isomer. We suspected that the presence intramolecular hydrogen bond between pyridine nitrogen and imidazole -NH group might be responsible for such poor performance. Further, we have employed two different strategies to improve the selectivity towards anions. In the first case, selective recognition of anions was achieved using suitable metal ions (Cu, Zn, Ca, and Al) as masking agents. On the contrary, we have varied the water content (0, 10, 30 and 50% v/v) in the acetonitrile-water mixture in the second case. The anions with relatively large hydration enthalpy showed no detectable interaction with probe in presence of water. Finally, we used the present system to detect cyanide ion in various natural water samples (tap, pond, and seawater). Also, low-cost reusable paper strips were developed as an alternative method for rapid, on-location detection of CN ions.