Tuning the Coordination Sphere around Highly Luminescent Lanthanide Complexes

A series of three ligands designed for the formation of water-soluble luminescent lanthanide complexes is described. All ligands are based on a 6′′-carboxy-2,2′:6′,2′′-terpyridine framework linked via a methylene bridge to n-butylamine. The second negatively charged arm consists of a 6-carboxy-2-methylenepyridine for L1, a 6′-carboxy-6-methylene-2,2′-bipyridine for L2, and a 6′′-carboxy-6-methylene-2,2′:6′,2′′-terpyridine for L3. The photophysical properties of the Eu and Tb complexes were studied in aqueous solutions by means of absorption spectroscopy and steady-state and time-resolved luminescence spectroscopy. Luminescence excited-state lifetimes were recorded and led to the determination of two water molecules in the first coordination sphere. The europium complexes were characterized by means of ¹H NMR spectroscopy in D₂O and DFT calculations performed at the B3LYP level both in vacuo and in aqueous solution. Finally, the influence of different phosphorylated anions such as HPO₄²⁻, ATP⁴⁻, ADP³⁻, and AMP²⁻ on the luminescence properties of the [EuLX(H₂O)₂]⁺ complexes (X = 1−3) was investigated in buffered aqueous solutions (0.01 M TRIS, pH 7.0), showing a significant interaction of ATP⁴⁻ with [Eu(L2)(H₂O)₂]⁺. The coordination of anions was understood in terms of partial decomplexation of one arm of the ligands and water displacement, with formation of ternary species, and it was rationalized on the basis of the structural models of the complexes obtained from DFT calculations.