Solution structures of fluorescent Zn(II) complexes with bis(naphthyl amide)-EDTA

Two EDTA-based bichromophoric isomers form Zn(II) complexes that exhibit distinct fluorescent behaviors; the ligands are abbreviated as (edta1nap)H₂ and (edta2nap)H₂, each of which consists of an EDTA chain linked to two 1-naphthyl or 2-naphthyl groups. The coordination chemistry of these complexes was studied by UV-Vis, fluorescence and ¹H NMR. The formation constants and the inherent emission intensities were determined by the pH dependence of the emission spectra; the species of the most intense emission is ML(OH) for L = (edta1nap)²⁻, and ML for (edta2nap)²⁻. The ¹H NMR of Zn-(edta1nap)²⁻ exhibits two sets of signals due to a slow exchange between two equivalent coordination geometries, whereas Zn-(edta2nap)²⁻ undergoes a fast exchange to show a single set of NMR signals. The spin-lattice relaxation time T₁ determined for the isostructural Mn(II) complexes shows that the naphthyl proton closest to the metal ion is H(8) in the (edta1nap)²⁻ complex, and H(3) in the (edta2nap)²⁻ complex. The two ligands differ only in the substitution position of the naphthyl group, but this apparently small difference leads to the notable difference in structural, dynamic, and consequent emission-spectral properties of their metal complexes as a result of the steric and size effects of the aromatic rings.