A new pyridine-containing ligand, N,N'-bis(6-carboxy-2-pyridylmethyl)ethylenediamine-N,N'-diacetic acid (H(4)L), has been designed for the complexation of lanthanide ions. (1)H and (13)C NMR studies in D(2)O solutions show octadentate binding of the ligand to the Ln(III) ions through the nitrogen atoms of two amine groups, the oxygen atoms of four carboxylates, and the two nitrogen atoms of the pyridine rings. Luminescence measurements demonstrate that both Eu(III) and Tb(III) complexes are nine-coordinate, whereby a water molecule completes the Ln(III) coordination sphere. Ligand L can sensitize both the Eu(III) and Tb(III) luminescence; however, the quantum yields of the Eu(III)- and Tb(III)-centered luminescence remain modest. This is explained in terms of energy differences between the singlet and triplet states on the one hand, and between the 0-phonon transition of the triplet state and the excited metal ion states on the other. The anionic [Ln(L)(H2O)]- complexes (Ln=La, Pr, and Gd) were also characterized by theoretical calculations both in vacuo and in aqueous solution (PCM model) at the HF level by means of the 3-21G* basis set for the ligand atoms and a 46+4 f(n) effective core potential for the lanthanides. The structures obtained from these theoretical calculations are in very good agreement with the experimental solution structures, as demonstrated by paramagnetic NMR measurements (lanthanide-induced shifts and relaxation-rate enhancements). Data sets obtained from variable-temperature (17)O NMR at 7.05 T and variable-temperature (1)H nuclear magnetic relaxation dispersion (NMRD) on the Gd(III) complex were fitted simultaneously to give insight into the parameters that govern the water (1)H relaxivity. The water exchange rate (k(298)(ex)=5.0 x 10(6) s(-1)) is slightly faster than in [Gd(dota)(H2O)]- (DOTA=1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane). Fast rotation limits the relaxivity under the usual MRI conditions.

Responsive or smart magnetic resonance imaging (MRI) contrast agents are molecular sensors that alter the MRI signal upon changes in a particular parameter in their microenvironment. Consequently, they could be exploited for visualization of various biochemical events that take place at molecular and cellular levels. In this study, a set of dual-frequency calcium-responsive MRI agents are reported. These are paramagnetic, fluorine-containing complexes that produce remarkably high MRI signal changes at the (1)H and (19)F frequencies at varying Ca(2+) concentrations. The nature of the processes triggered by Ca(2+) was revealed, allowing a better understanding of these complex systems and their further improvement. The findings indicate that these double-frequency tracers hold great promise for development of novel functional MRI methods.

Lanthanide complexes (Ln=Eu, Tb, and Yb) that are based on a C2 -symmetric cyclen scaffold were prepared and characterized. The addition of fluoride anions to aqueous solutions of the complexes resulted in the formation of dinuclear supramolecular compounds in which the anion is confined into the cavity that is formed by the two complexes. The supramolecular assembly process was monitored by UV/Vis absorption, luminescence, and NMR spectroscopy and high-resolution mass spectrometry. The X-ray crystal structure of the europium dimer revealed that the architecture of the scaffold is stabilized by synergistic effects of the EuFEu bridging motive, π stacking interactions, and a four-component hydrogen-bonding network, which control the assembly of the two [EuL] entities around the fluoride ion. The strong association in water allowed for the luminescence sensing of fluoride down to a detection limit of 24 nM.