A series of four ligands based on a 5'-methyl-2,2'-bipyridyl framework substituted in the 6 position by a carboxylic acid, a phosphonic acid, a monoethyl ester phosphonic acid, or a diethyl ester phosphonic acid are described. The pK(a) values of all ligands and their assignments are determined by a combination of UV-vis absorption spectroscopy and H-1 and P-31 NMR spectroscopy. The ability of the tridentate ligands to form complexes with trivalent lanthanide cations (Ln = La, Nd, Eu, and Lu) in buffered water solutions (Tris-HCI, pH = 7.4) is studied by UV-vis absorption spectroscopy and H-1 NMR. While the two ester ligands display a weak coordination ability toward lanthanide cations, the acid ligands form stable complexes with 1:1, 1:2, and 1:3 Ln/L ratios. A weak selectivity is observed for the middle of the lanthanide series, and the complexes of the phosphonic acid derivative are up to 2 orders of magnitude more stable than those of the carboxylic acid ligand. Photophysical properties of the free phosphonic and carboxylic acid ligands and of their complexes with La, Eu, Gd, Tb, and Lu are investigated in buffered aqueous solutions both at room temperature and 77 K. An efficient ligand-to-metal energy transfer is observed for both the Eu and Tb complexes. Despite a relatively large energy gap between the ligand-centered (3)pipi(*) and the Eu(D-5(0)) or Tb(D-5(4)) emitting states, the metal-centered luminescence is well sensitized with quantum yields reaching up to 45.5 and 42.2% for the Tb 1:3 complexes with carboxylic and phosphonic acid ligands, respectively.