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.