Dansylated Polyamines as Fluorescent Sensors for Metal Ions: Photophysical Properties and Stability of Copper(II) Complexes in Solution

Abstract

Protonation and the CuII complexation constants of the dansylated polyamines N-dansylethylenediamine (1), N-dansyldiethylenetriamine (2), N-dansyltriethylenetetramine (3), N′-[2-(dansylamino)ethyl]diethylenetriamine (4), and tris(2-dansylaminoethyl)amine (5) were determined by glass-electrode potentiometry in MeOH/H2O 9 : 1 (v/v) solution. For ligands 3 and 4, the determinations were also performed in aqueous solution. The complexes formed by these ligands in neutral form correspond to those observed for the analogous unsubstituted monoprotonated amines, whereas, when the ligands are deprotonated at the sulfonamide moiety, the species parallel those of the corresponding amines. The molecular structures of the complexes were deduced from the VIS absorption spectra. The crystal structure of the [CuL2H−2] complex 6 of ligand 1 (L) was determined by X-ray diffraction. The study of the photophysical properties of the ligands 3 – 5 showed that they are good fluorescent sensors for copper(II), which induced fluorescence quenching. Time-resolved fluorescence measurements allowed us to clarify the sensing mechanism. The pH dependence of the quenching effect demonstrated that it occurs for all Cu2+ complexes, even for species in which the sulfonamide moiety is not deprotonated. Sensing of Cu2+ was compared with that of other metal ions (Co2+, Ni2+, Zn2+, Cd2+, Hg2+), and selectivity was studied as a function of pH. Ligands 3 and 4 were found to be selective chemosensors for Cu2+ in weakly acidic solution (pH ca. 4 – 5).