A highly selective chromogenic and fluorogenic chemodosimeter for dual detection of Cu²⁺ based on a redox-active...

Calix[4]arene 1, with two lower-rim isoxazolylchloroanthracene groups, is shown to be not only a chromogenic but also a fluorogenic chemodosimeter for the selective sensing of Cu²⁺. The binding properties of ligand 1 and control compounds 2 and 3 toward metal ions in CH₃CN/CHCl₃ (v/v, 1 : 1) were investigated by UV-vis and fluorescence spectroscopy. The results showed that only ligand 1 was highly selective for Cu²⁺ ions. When complexed with Cu²⁺, ligand 1 displayed a new absorption band around 435 nm and the color of the solution changed from colorless to yellow. Furthermore, the fluorescence of ligand 1 was severely quenched by Cu²⁺ and the limit of detection (LOD) was determined to be 1.674 μM. Therefore, compound 1 is not only a chromogenic but also a fluorogenic sensor for the detection of Cu²⁺ ions over other metal ions examined. When complexed with ligand 1, Cu²⁺ was reduced to Cu⁺ by the free phenolic-OH of ligand 1 and concurrently the phenol was oxidized by Cu²⁺ to quinones. The ¹H NMR, EPR, and FTIR spectra provided evidence for the redox behavior of ligand 1 with Cu²⁺. The isolation of calix[4]diquinone 8 and Cu(CH₃CN)₄ClO₄ from the reaction of ligand 1 with Cu(ClO₄)₂ confirmed their redox reaction.