Synthesis and Solution Structure Analysis of a Bispyrenyl Bishydroxamate Calix[4]arene-Based Receptor, a Fluorescent Chemosensor for Cu²⁺ and Ni²⁺ Metal Ions

A bispyrenyl calix[4]arene-based receptor, 1, incorporating two hydroxamic acid functionalities has been designed for the optical detection of Cu 2+ and Ni 2+ metal ions in solution. The synthetic route is based on the utilization of pyrene-labeled, O -protected hydroxylamines that represent versatile building blocks for the generation of fluorescent siderophore-based chelators. Compound 1 is shown to exist in CDCl 3 solution at room temperature as a mixture of mainly two conformers, namely, the cone and partial-cone species (partial-cone/cone ratio ca. 57/43). The full assignment of the proton and carbon resonances of the calix[4]arene core for both conformers is described. Ligand 1 exhibited in solution a dual fluorescence emission spectrum composed of monomer and excimer contributions, the latter resulting from intramolecular interactions between pyrene nuclei in the excited state. In methanol/water (80/20 v/v), the fluorescence intensity was shown to be sensitive to proton concentration and to the presence of transition-metal ions. Addition of Cu 2+ and Ni 2+ metal cations induced a dramatic quenching of fluorescence of 1 depending on the value of −log[H + ]. The excimer-forming photoresponsive ligand 1 belongs to a new class of sensitive chemosensors for the selective detection of transition-metal species in aqueous media.