We describe a systematic study of the influence of halides and solvents on the optical properties of EuII-containing complexes in solution starting from well-defined crystalline precursors. Anionic halides, chloride and bromide, blue-shift the spectroscopic properties of EuII, whereas neutral ligands, methanol and acetonitrile, cause a red shift. This system provides evidence that EuII has a stronger affinity for chloride, and to some extent bromide, relative to acetonitrile but not methanol. We also describe a simple procedure using an ion-exchange resin for the exchange of iodide counterions to hexafluorophosphate. These findings are a step toward designing ligands that can tune the optical properties of EuII-containing complexes for solution-based applications.

We report a screening procedure to predict ligand coordination to EuII and EuIII using magnetic resonance imaging in which bright images indicate complexation and dark images indicate no complexation. Here, paramagnetic GdIII is used as a surrogate for EuIII in the screening procedure to enable detection with magnetic resonance imaging. The screening procedure was tested using a set of eight ligands with known coordination to EuII and EuIII, and results were found to be consistent with expected binding. Validation of the screening procedure with known coordination chemistry enables use with new ligands in the future.

Considerable research effort has focused on the in vivo use of responsive imaging probes that change imaging properties upon reacting with oxygen because hypoxia is relevant to diagnosing, treating, and monitoring diseases. One promising class of compounds for oxygen-responsive imaging is Eu-II-containing complexes because the Eu-II/III redox couple enables imaging with multiple modalities including magnetic resonance and photoacoustic imaging. The use of Eu-II requires care in handling to avoid unintended oxidation during synthesis and characterization. This review describes recent advances in the field of imaging agents based on discrete Eu-II-containing complexes with specific focus on the synthesis, characterization, and handling of aqueous Eu-II-containing complexes.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

We report, for the first time, a multimodal, oxidation-responsive contrast agent for magnetic resonance imaging and photoacoustic imaging that uses the differences in the properties between Eu in the +2 and +3 oxidation states. The enhancement of contrast in T1-weighted magnetic resonance and photoacoustic imaging was observed in the +2 but not in the +3 oxidation state, and the complex is a known chemical exchange saturation transfer agent for magnetic resonance imaging in the +3 oxidation state.

The Eu-II/III redox couple offers metal-based oxidation-sensing with magnetic resonance imaging making the study of Eu-II oxidation chemistry important in the design of new probes. Accordingly, we explored oxidation reactions with a set of Eu-II-containing complexes. Superoxide formation from the reaction between Eu-II and dioxygen was observed using electron paramagnetic resonance spectroscopy. Additionally, oxidation kinetics of three Eu-II-containing complexes with bromate and glutathione disulfide at pH values, including 5 and 7, is reported. In the reaction with bromate, the oxidation rate of two of the complexes increased by 7.3 and 6.7 times upon decreasing pH from 7 to 5, but the rate increased by 17 times for a complex containing amide functional groups over the same pH range. The oxidation rate of a fluorobenzo-functionalized cryptate was relatively slow, indicating that the ligand used to impart thermodynamic oxidative stability might also be useful for controlling oxidation kinetics.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Mechanistically unique 19F-EuII/III complex reports redox in vivo using both 1H- and 19F-MRI and displays temperature-dependent contrast enhancement.