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)-2oxo- 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 H-1 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 beta-amino alpha,beta-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 beta-amino alpha,beta-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.