Calix[4]arene with Lower-Rim beta-Amino alpha,beta-Unsaturated Ketones Containing Bis-Chelating Sites as a Highly Selective Fluorescence Turn-On Chemosensor for Two Copper(II) Ions

Abstract

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 Cu(2+). The fluorescence intensity of 3b was enhanced upon adding [Cu(ClO(4))(2)], which reached a maximum with approximately 4 equiv. of Cu(2+) but then started to decrease in intensity at higher Cu(2+) concentrations. Job plot experiments revealed a 1:2 binding stoichiometry of 3b with Cu(2+). Based on (1)H NMR titration results, we infer that there are two possible binding sites for Cu(2+) 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(ClO(4))(2)], the Cu(2+) 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 Cu(2+) in 3b. By comparing the binding constants of 3b, 6, and 9b with Cu(2+), we found that 3b exhibited a positive allosteric behavior toward the coordination of two Cu(2+) ions.