Chloride selective calix[4]arene optical sensor combining urea functionality with pyrene excimer transduction

A neutral 2-site chloride selective compound has been developed (3), based on a 1,3-alternate tetrasubstituted calix[4]arene providing a preorganized supramolecular scaffold. The resultant supramolecular cavity is among the first to combine urea functional groups bridged with single methylene spacers to pyrene moieties. It combines a naturally and synthetically proven H-bonding system with the elegant ratiometric fluorescent signaling properties of an intramolecular pyrene excimer system, triggered by conformational changes upon anion coordination. The excimer emission of 3 is quenched, with a simultaneous rise in the monomer emission solely by the chloride anion among a wide variety of anions tested. 3 has an association constant of 2.4 × 10⁴ M^-1 with chloride. The suitability and advantages of ratiometric optical sensor compounds like 3 for use in practical sensor devices is discussed. 3 has an LOD of 8 × 10^-6 M with chloride in acetonitrile-chloroform (95:5 v/v). A dynamic fluorescence study revealed a response time of <3 s. A recently developed and simple HPLC-based purification method complimented conventional organic work up methods to yield pure product.