Calix[4]arene with lower-rim β-amino α,β-unsaturated ketones containing bis-chelating sites as a highly selective fluorescence turn-on chemosensor for two copper(II) ions

We report herein the synthesis of a fluorescence turn-on chemosensor, 25,27-bis{N-[1-(4-{[4-amino-4-(1-naphthyl)-2-oxo-3-butenyl]oxy}phenyl) aminocarbonyl]methoxy}-26,28-dihydroxycalix[4]arene (3b), which is highly selective toward Cu²⁺. The fluorescence intensity of 3b was enhanced upon adding [Cu(ClO₄)₂], which reached a maximum with approximately 4 equiv. of Cu²⁺ but then started to decrease in intensity at higher Cu²⁺ concentrations. Job plot experiments revealed a 1:2 binding stoichiometry of 3b with Cu²⁺. Based on ¹H NMR titration results, we infer that there are two possible binding sites for Cu²⁺ in 3b: one at the lower-rim phenolic-OH and amide groups, and the second at the β-amino α,β-unsaturated ketone groups. It is important to note that during the complexation of 3b with [Cu(ClO₄)₂], the Cu²⁺ ions were reduced to Cu⁺ by both the phenolic OH and the amines of the β-amino α,β-unsaturated ketones. Furthermore, control compounds 6 and 9b were synthesized to clarify the possible binding sites of Cu²⁺ in 3b. By comparing the binding constants of 3b, 6, and 9b with Cu²⁺, we found that 3b exhibited a positive allosteric behavior toward the coordination of two Cu²⁺ ions. Calix[4]arene 3b shows high binding selectivity toward Cu²⁺ ions, which causes a dramatic enhancement of the fluorescence intensities at 341 and 452 nm by 4- and 40-fold, respectively. Chemosensor 3b can recognized two Cu²⁺ ions; both ions are reduced to Cu⁺ by both the phenolic-OH and the amines of the β-amino α,β- unsaturated ketones.