Eu(Ⅲ) luminescence study of the Eu(Ⅲ) binding sites in synthetic and natural polyelectrolytes

Abstract

The complexation properties of Eu(III) ion with simple and polyelectrolytic ligands was investigated using laser induced Eu(III) luminescence methods. From Eu(III) $^7\!$F$_0\to^5\!$D$_0$ excitation spectra and Eu(III) hydration number measurement data, we can get some useful information(ring size effect and role of heteroatom in chelation) about Eu(III) environments in solution state. Oxalate and malonate can form stable chelate while the other larger size ligands(e.g. succinate, maleate, glutarate and phthalate) form Eu(III) complex with only one of the two carboxylate in ligand molecule. However, larger size ligands (e.g. IMDA, ODA and DPA) containing hard heteroatom (oxygen and nitrogen) can also form stable tridentate chelate with Eu(III) ion while S-ODA, a similar ligand with soft heteroatom(sulfur) in molecule, is suspected to form Eu(III) complex that has similar structure with Eu(III)-glutarate complex. In $^7\!$F$_0\to^5\!$D$_0$ excitation spectra of Eu(III) ion bound to synthetic and natural polyelectrolyte ligands, MA and $MA_2$ type species are observed. For synthetic linear polyelectrolyte ligands(PAA and PVBA), the $MA_2$ type species are predominant at neutral solution pH. By contrast, the dominance of MA type species in natural polyelectrolyte such as $T_3FA$, was observed at neutral solution pH. These different properties are thought to be mainly arise from their differences in molecular flexibility. The dominance of $MA_2$ species in synthetic linear polyelectrolyte is mainly resulting from the easy intramolecular association of two carboxylate groups due to their flexible linear chain. The dominance of MA type species in natural polyelectrolyte containing various type of cross-linked aromatic structures, can be ascribed to their rigid structures that disturb intramolecular association of carboxylate groups.