EuII-cryptate with optimal water exchange and electronic relaxation: a synthon for potential pO2 responsive macromolecular MRI contrast agents.

The cryptate [EuII(2.2.2)(H2O)2]2+ displays several interesting features with respect to pO2 responsive MRI contrast agent applications: it is relatively stable against oxidation, it has two inner sphere water molecules, and the water exchange and electron spin relaxation rates are in the optimal range to attain high proton relaxivities, provided the rotation is also optimized.