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Anionic Heptadentate Complexes as Building Blocks for Bimodal Imaging
[Thesis]. Manchester, UK: The University of Manchester; 2010.
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Abstract
Contrast agents are used in over 35% of MRI scans today and their use is increasing as they become ever more sophisticated. The challenge for researchers in this field is to synthesise smart imaging agents, which can be addressed using more than one imaging modality. The ultimate goal of these types of contrast agents is to be able to increase the contrast between normal and diseased tissue with smaller amounts of the agent being administered. This thesis describes the synthesis of complexes that are designed to have increased hydration numbers and rapid solvent exchange. The key to designing clinically effective contrast agents of this type is to minimize the affinity the contrast agent for endogenous anions such as phosphate that can exclude water from the inner coordination sphere, inhibiting exchange and reducing the relaxivity. A hexaanionic ligand was synthesized by tri-alkylation of cyclen with the dimethyl ester of N-isophthaloylchloroacetamide and subsequent hydrolysis. Lanthanide complexes of the ester and acid form of the ligand were synthesized and were shown to be luminescent. Relaxometric and time–resolved studies were used to establish that phosphate binding is not significantly inhibited in the anionic complex, suggesting that proximate negative charge is essential to inhibition of anion binding. Lanthanide complexes of DO3P and a bisDO3P analogue containing a bipyridyl bridge were also prepared, and shown to be more effective at inhibiting phosphate binding, though their properties exhibit dependence on pH. The bipyridyl derivative was used to prepare rhenium containing d-f hybrid complexes. These exhibited luminescence from both the rhenium MLCT and the lanthanide, though sensitized emission from the ytterbium is inefficient as a consequence of poor spectral overlap, Routes towards preparation of ligands bearing pyridylphosphinate pendent arms have also been explored.