Self-Aggregated Dinuclear Lanthanide(III) Complexes as Potential Bimodal Probes for Magnetic Resonance and Optical Imaging

Homodinuclear lanthanide complexes (Ln=La, Eu, Gd, Tb, Yb and Lu) derived from a bis-macrocyclic ligand featuring two 2,2’,2’’-(1,4,7,10- tetraazacyclododecane-1,4,7-triyl)triacetic acid chelating sites linked by a 2,6-bis(pyrazol-1-yl)pyridine spacer (H2L3) were prepared and characterized. Luminescence lifetime measurements recorded on solutions of the EuIII and TbIII complexes indicate the presence of one inner-sphere water molecule coordinated to each metal ion in these complexes. The overall luminescence quantum yields were determined (fH2O=0.01 for [Eu2(L3)] and 0.50 for [Tb2(L3)] in 0.01m TRIS/HCl, pH 7.4; TRIS=tris(hydroxymethyl)- aminomethane), pointing to an effective sensitization of the metal ion by the bispyrazolylpyridyl unit of the ligand, especially with Tb. The nuclear magnetic relaxation dispersion (NMRD) profiles recorded for [Gd2(L3)] are characteristic of slowly tumbling systems, showing a low-field plateau and a broad maximum around 30 MHz. This suggests the occurrence of aggregation of the complexes giving rise to slowly rotating species. A similar behavior is observed for the analogous GdIII complex containing a 4,4’-dimethyl-2,2’-bipyridyl spacer ([Gd2(L1)]). The relaxivity of [Gd2(L3)] recorded at 0.5 T and 298K (pH 6.9) amounts to 13.7 mm 1 s 1. The formation of aggregates has been confirmed by dynamic light scattering (DLS) experiments, which provided mean particle sizes of 114 and 38 nm for [Gd2(L1)] and [Gd2(L3)], respectively. TEM images of [Gd2(L3)] indicate the formation of nearly spherical nanosized aggregates with a mean diameter of about 41 nm, together with some nonspherical particles with larger size.