Zwitterion detection with a fluorescent squaramide cryptand: a study on size-dependent salt recognition and sensing

Abstract

We report the synthesis of a novel squaramide-based cryptand incorporating both anion and cation binding domains, and a fluorescent reporter designed to monitor the complexation process. The preorganization of the binding domains in the cryptand structure favored stronger binding of anions as compared with the open chain counterpart. X-ray structural analysis of the cryptand complex with ion pairs confirmed the interaction of ions with binding sites in the solid state. Employing a ball milling technique resolved solubility issues, facilitating the study of interactions between the cryptand and salts. We found that the cryptand exhibits stronger binding affinity for carboxylates than for chloride anions. This property, coupled with the closed structure of the cryptand, was utilized for the selective recognition of zwitterions differing in chain length. Mechanochemical methods allow for qualitative binding of amino acids within the receptor cavity. Fluorescence studies revealed the behavior of cryptand 1 as a sensor enabling the selective recognition of zwitterions, a capability not observed in the sensors with an open structure.