Effect of Magnetic Anisotropy on the 1 H NMR Paramagnetic Shifts and Relaxation Rates of Small Dysprosium(III) Complexes.

We present a detailed analysis of the 1 H NMR chemical shifts and transverse relaxation rates of three small Dy(III) complexes having different symmetries ( C 3 , D 2 or C 2 ). The complexes show sizeable emission in the visible region due to 4 F 9/2 → 6 H J transitions ( J = 15/2 to 11/2). Additionally, NIR emission is observed at ca. 850 ( 4 F 9/2 → 6 H 7/2 ), 930 ( 4 F 9/2 → 6 H 5/2 ), 1010 ( 4 F 9/2 → 6 F 9/2 ), and 1175 nm ( 4 F 9/2 → 6 F 7/2 ). Emission quantum yields of 1-2% were determined in aqueous solutions. The emission lifetimes indicate that no water molecules are present in the inner coordination sphere of Dy(III), which in the case of [Dy(CB-TE2PA)] + was confirmed through the X-ray crystal structure. The 1 H NMR paramagnetic shifts induced by Dy(III) were found to be dominated by the pseudocontact mechanism, though, for some protons, contact shifts are not negligible. The analysis of the pseudocontact shifts provided the magnetic susceptibility tensors of the three complexes, which were also investigated using CASSCF calculations. The transverse 1 H relaxation data follow a good linear correlation with 1/ r 6 , where r is the distance between the Dy(III) ion and the observed proton. This indicates that magnetic anisotropy is not significantly affecting the relaxation of 1 H nuclei in the family of complexes investigated here.