Bichromophoric Naphthalene Derivatives of Ethylenediaminetetraacetate: Fluorescence from Intramolecular Excimer, Protonation and Complexation with Zn2+ and Cd2+

Abstract

Chelating water-soluble bichromophoric compounds were designed by linking two naphthalene rings with an ethylenediaminetetraacetate (edta) chain through amide linkages, and characterized by fluorescence and NMR spectroscopies: the bichromophores studied were 1,4-bis(methylenecarboxy)-1,4-bis(N-1-naphthylacetamide)-1,4-diazabutane, abbreviated as (edta1nap)H2, and the corresponding 2-naphthylacetamide, abbreviated as (edta2nap)H2. Completely deprotonated species M2 − that formed in basic solution exhibited an intense emission from intramolecular excimer at 440 nm for (edta1nap)2 − and 404 nm for (edta2nap)2 − , while emission from monomeric naphthyl group was comparatively very weak. Geometry optimization based on Density Functional Theory showed that two naphthyl groups in an M2 − molecule faced each other in such a way that an intramolecular excimer was readily formed by light excitation. Protonation on amino nitrogen affected monomer–excimer interconversion in different modes for the two compounds: the excimer emission was strengthened in the resulting (edta1nap)H− , but was weakened in (edta2nap)H− . Upon coordination with Zn2+, the excimer band of (edta1nap)2 − as well as (edta2nap)2 − was strengthened in a linear manner with [Zn]/[L]; concurrently monomer emission was weakened. Coordination of (edta2nap)2 − with Cd2+ resulted in a large decrease in the excimer emission, while the emission of (edta1nap)2 − was almost unchanged. The formation of intramolecular excimer, which is highly sensitive to protonation and complexation, is due to the strictly defined molecular conformation.