The advantage of reversible coordination polymers in producing visible light sensitized Eu(III) emissions over EDTA via excluding water from the coordination sphere

Abstract

In this paper, we report on the impact of the structure of ligands on the luminescence enhancement of Eu(III) by directly exciting Eu(III) with visible light in aqueous media. Upon replacing the water molecules that coordinated around a Eu³⁺ ion with a ditopic ligand 1,11-bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane (L₂EO₄) or ethylenediaminetetraacetic acid disodium salt (EDTA), significant luminescence can be obtained. L₂EO₄ may occupy all 9 coordinating sites of a Eu³⁺ ion at proper L₂EO₄/Eu ratios, whereas EDTA only occupies 6 of them with 3 sites left for water at various EDTA/Eu ratios. These coordinated water molecules quench the fluorescence of EDTA–Eu complexes drastically so that the luminescence is about 30 times lower than that of the L₂EO₄–Eu system. Furthermore, the negatively charged L₂EO₄/Eu = 3/2 coordinated complex can be further transformed into coordination ‘polymers’ by mixing with a positively charged block polyelectrolyte, which forms electrostatic micelles with further enhanced luminescence. The emission of the EDTA–Eu complex is not influenced by the addition of polymers due to the formation of stable small 1 : 1 EDTA–Eu complex which doesn't change with increasing concentration. Our work points out that the L₂EO₄–Eu system is superior to the EDTA–Eu system in creating visible light sensitized Eu(III) luminescence, and the emission of Eu(III) can be indeed significantly enhanced to an applicable level by excluding all the water molecules in the coordination sphere of Eu(III).