Gadolinium(III)‐Based Dual ¹H/¹⁹F Magnetic Resonance Imaging Probes

We present two novel octadentate cyclen‐based ligands bearing one (L 1 ) or two (L 2 ) phenylacetamide pendants with two CF 3 groups either at positions 3 and 5 (L 1 ) or 4 (L 2 ). The corresponding Gd 3+ complexes possess one coordinated water molecule, as confirmed by luminescence lifetime measurements on the Eu III and Tb III analogues. A detailed 1 H and 17 O relaxometric characterization has revealed the parameters that govern the relaxivities of these complexes. The water‐exchange rate of the mono‐amide derivative GdL 1 ( k ex 298 =1.52×10 6 s −1 ) is faster than that determined for the bis‐amide complex GdL 2 ( k ex 298 =0.73×10 6 s −1 ). 1 H and 19 F NMR studies have indicated that the complexes are present in solution almost exclusively as the square‐antiprismatic (SAP) isomers. 19 F NMR relaxation studies indicated Gd ⋅⋅⋅ F distances of 7.4±0.1 and 9.1±0.1 Å for GdL 1 and GdL 2 , respectively. Phantom MRI studies revealed the favorable properties of GdL 2 as a dual 1 H/ 19 F magnetic resonance imaging (MRI) probe, whereas the shorter Gd ⋅⋅⋅ F distance of GdL 1 reduces the signal‐to‐noise ratio due to the very short transverse relaxation time of the 19 F NMR signal.