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 Eu^III and Tb^III 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 (φ H 2O=0.01 for [Eu₂(L³)] and 0.50 for [Tb₂(L³)] in 0.01 M 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 Gd^III complex containing a 4,4'-dimethyl-2,2'-bipyridyl spacer ([Gd₂(L¹)]). The relaxivity of [Gd₂(L ³)] recorded at 0.5 T and 298 K (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(L ³)], 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. Ln^III-based nanosized materials: A bis-macrocyclic ligand containing two 2,2',2''-(1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid chelating sites linked by a 2,6-bis(1H-pyrazol-1-yl)pyridine spacer forms binuclear complexes [Ln₂(L)] giving stable nanosized aggregates in water with a mean size of about 40 nm (see figure). These aggregates provide a very efficient sensitization of Eu^III, and particularly Tb ^III, and high relaxivities for the Gd^III complex, which make them potential candidates as bimodal probes for optical and magnetic resonance imaging.