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 Cu2+ and Ni2+ 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 CDCl3 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 Cu2+ and Ni2+ 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.