Structural Features of Europium(II)‐Containing Cryptates That Influence Relaxivity

Eu II ‐containing complexes were studied with respect to properties relevant to their use as contrast agents for magnetic resonance imaging. The influences of molecular parameters and field strength on relaxivity were studied for a series of Eu II ‐containing cryptates and their adducts with β‐cyclodextrins, poly‐β‐cyclodextrins, and human serum albumin. Solid‐ and solution‐phase characterization of Eu II ‐containing complexes is presented that demonstrates the presence of inner‐sphere molecules of water. Additionally, relaxivity, water‐exchange rate, rotational correlation time, and electronic relaxation times were determined using variable‐temperature 17 O NMR, nuclear magnetic relaxation dispersion, and electron paramagnetic resonance spectroscopic techniques. These results are expected to be instrumental in the design of future Eu II ‐based contrast agents.