Relationship between the ligand structure and the luminescent properties of water-soluble lanthanide complexes containing bis(bipyridine) anionic arms

A series of six new ligands (L-1-L-6) suitable for the formation of luminescent lanthanide complexes in water is described. Ligands V-L 4 are constructed from two 6'-carboxy-6methylene-2,2'-bipyridine chromophoric arms bonded to the amino function of a 2-aminomethylene-6-carboxy-pyridine (L-1), an N,N-diacetate-ethylene diamine (L-2), a serine (L-3), or an aminomalonic acid (L-4). For ligands L-5 and L-6, the linking amino function is provided by a glutamic acid, and the anionic functions at the 6-position of the bipyridyl arms are made of the sodium salts of monoethylphosphonicester (L-5) and phosphonic acid (L-6). The synthesis and characterisation of the ligands are described, together with the study of the formation of lanthanide complexes with europium and terbium. In the case of V, the europium complex obtained in acidic conditions was crystallised and the X-ray crystal structure is depicted. Photophysical properties of the complexes were studied by means of UV-visible absorption, and steady-state and time-resolved luminescence spectroscopy. Excited-state luminescence lifetimes of the complexes were determined in water and deuterated water to gain insight into the number of water molecules directly coordinated in the first coordination sphere of the complexes. The coordination behaviour of the series of ligands is questioned in the light of the spectroscopic data and discussed in terms of protection of the cation towards water molecules and their impact on the luminescence efficiency.