Colorimetric and Luminescent Sensors for Chloride: Hydrogen Bonding vs Deprotonation

The synthesis and photophysical properties of four squaramide based fluorescent anion sensors (1-4) are described. These luminescent compounds showed selectivity for Cl- over various other anions with concomitant changes in both their UV/visible and fluorescence properties upon Cl- addition, attributed to initial H-bonding followed by NH deprotonation in the presence of excess Cl-, signaled by a color change. The nature of the electron withdrawing aryl substituents is directly related to the H-bonding ability/acidity of the squaramide protons and can be used to tune the deprotonation behavior.