Euan T. Sarson

We show that europium trisbipyridyl cryptate exhibits an electrochemically reversible Eu^III/II redox couple at -360 mV vs the standard hydrogen electrode (SHE) and is susceptible to reduction or oxidation by a range of redox enzymes or biological small molecule redox agents. Modulation of the europium oxidation state can be used to achieve dramatic changes in the T₁ relaxation time of bulk water in solutions containing these complexes: in the reduced form, the complex exhibits high relaxivity (3.08 mM^-1 s^-1 at 499.9 MHz) comparable with that of commercial MRI contrast media, while the oxidized form has negligible relaxivity, illustrating the scope for application in imaging reducing environments in biological systems.

We report a family of lanthanide(III) complexes that act as redox probes via both magnetic resonance (MR) and luminescence outputs. The ligands are functionalized with nitro, azobenzene and azide groups which are reduced to a common aniline product, and each responds to both chemical and biocatalytic reductive conditions at different cathodic onset potentials. By judicious choice of complexed Ln(III), the probes can be optimized either for use in MR imaging (Ln = Gd), or as highly efficient turn‐on luminescence probes (Ln = Tb). The Tb(III) analogues are essentially nonemissive, until reductive generation of the aniline affords a complex which when excited by visible light (488 nm) emits green light with a quantum yield of 45% and millisecond long luminescent lifetimes (ms). The tunable redox response and imaging modalities of these versatile complexes have the potential to open up new approaches to redox sensing, such as the imaging of hypoxic environments in biology.

Lanthanide complexes of DOTA monoesters bearing nitrobenzyl and nitroimidazole groups are shown to be converted to the corresponding DOTA complexes under chemical and enzymatic conditions, giving rise to favorable changes in the luminescence properties of the europium and terbium complexes and relaxometric properties of the gadolinium complexes. The nitroimidazole complexes are converted more rapidly than their nitrobenzyl and benzyl analogues. We propose that activation of these complexes may occur by ester cleavage rather than nitro reduction and fragmentation since complexes bearing a simple benzyl group may also be cleaved under the same conditions, albeit more slowly.