A new ligand, L-C, bis-[(6'-carboxy-2,2'-bipyridine-6-yl)]phenylphosphine oxide, in which the tridentate 6-carboxy2,2'-bipyridyl arms are directly linked to a phenylphosphine oxide fragment, has been synthesized. The corresponding [Ln center dot Lc]Cl center dot xH(2)O complexes (Ln = Eu, x = 4, and Tb, x = 3) were isolated from solutions containing equimolar amounts of Lc and hydrated LnCl(3) salts and characterized by elemental analysis, mass spectrometry, and infrared spectroscopy. The interactions of the Eu complex with various anions (AMP(2-), ADP(3-), ATP,(4-) HPO42-, and NO3-) were studied by titration experiments, using UV-vis, luminescence spectroscopy, and excited-state lifetime measurements. The results are in keeping with strong interactions with the ADP(3-), ATP(4-), and phosphate anions in TRIS/HCI buffer (0.01 M, pH = 7.0), as revealed by the determination of the conditional stepwise association constants. These values are higher than the one determined for ligand La, bis[(6'-carboxy-2,2'-bipyridine-6-methylyl)]-n-butylamine (Delta log K approximate to 1-2). The interaction of complexes [Ln center dot L-B](+) and [Ln-L-C ](+) with nitrate, monohydrogenophosphate, methyl phosphate (MeP2-), methyldiphosphate (MeDP3-), and methyltriphosphate (MeTp(4-)) anions was investigated by means of quantum mechanical (QM) calculations. The results, combined with data on the photophysical impact of the sequential competitive binding of anions to the Eu complexes in water, suggest that LB is too flexible to ensure a good coordination pocket, while the molecular structure of ligand Lc stabilizes both the formation of the lanthanide complexes and its adducts with ATP.