Metal-based contrast agents for magnetic resonance imaging present a promising avenue to image hypoxia. EuII-based contrast agents have a unique biologically relevant redox couple, EuII/III, that distinguishes this metal for use in hypoxia imaging. To that end, we investigated a strategy to enhance the contrast-enhancing capabilities of EuII-based cryptates in magnetic resonance imaging by controlling the rotational dynamics. Two dimetallic, EuII-containing cryptates were synthesized to test the efficacy of rigid versus flexible coupling strategies. A flexible strategy to dimerization led to a modest (114 %) increase in contrast enhancement per Eu ion (60 MHz, 298 K), but a rigid linking strategy led to an excellent (186 %) increase in contrast enhancement despite this compound′s having the smaller molecular mass of the two dimetallic complexes. We envision the rigid linking strategy to be useful in the future design of potent EuII-based contrast agents for magnetic resonance imaging.
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