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 (?=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(L³)] 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(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.