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The coordination compounds of the trivalent lanthanide ions (Ln(III)) have unique photophysical properties. Ln(III) excitation is usually performed through a light-harvesting antenna. To enable Ln(III)-based emitters to reach their full potential, an understanding of how complex structure affects sensitization and quenching processes is necessary. Here, the role of the linker between the antenna and the metal binding fragment was studied. Four macrocyclic ligands carrying coumarin 2 or 4-methoxymethylcarbostyril sensitizing antennae linked to an octadentate macrocyclic ligand binding site were synthesized. Complexation with Ln(III) (Ln = La, Sm, Eu, Gd, Tb, Yb and Lu) yielded species with overall &minus, 1, 0, or +2 and +3-charge. Paramagnetic 1H NMR spectroscopy indicated subtle differences between the coumarin- and carbostyril-carrying Eu(III) and Yb(III) complexes. Cyclic voltammetry showed that the effect of the linker on the Eu(III)/Eu(II) apparent reduction potential was dependent on the electronic properties of the N-substituent. The Eu(III), Tb(III) and Sm(III) complexes were all luminescent. Coumarin-sensitized complexes were poorly emissive, photoinduced electron transfer was not a major quenching pathway in these species. These results show that seemingly similar emitters can undergo very different photophysical processes, and highlight the crucial role the linker can play.

The quenching of sensitized Eu(III) luminescence by photoinduced electron transfer from the excited light-harvesting antenna to Eu(III) was investigated. A series of complexes incorporating different metal binding sites and thus having varying Eu(III)/Eu(II) reduction potentials were prepared. The complexes were fully characterized using a combination of single-crystal X-ray crystallography and paramagnetic 1H NMR spectroscopy, the results of which support the structural similarity of the complexes. The redox and photophysical behavior of the Eu(III) center and the light-harvesting antenna were studied using cyclic voltammetry and steady-state and time-resolved emission spectroscopy on the nanosecond and millisecond time scales. The contribution of photoinduced electron transfer to the overall reduction of the Eu(III) luminescence quantum yield was found to be comparable and, in many cases, larger than the quenching caused by well-established processes such as coupling to X-H oscillators. These results suggest that the elimination or mitigation of photoinduced electron transfer could substantially improve the emissive properties of the widely used Eu(III)-based emitters.

Biological probes constructed from lanthanides can provide a variety of readout signals, such as the luminescence of Eu(III), Tb(III), Yb(III), Sm(III) and Dy(III), and the proton relaxation enhancement of Gd(III) and Eu(II). For numerous applications the intracellular delivery of the lanthanide probe is essential. Here, we review the methods for the intracellular delivery of non-targeted complexes (i.e. where the overall complex structure enhances cellular uptake), as well as complexes attached to a targeting unit (i.e. to a peptide or a small molecule) that facilitates delivery. The cellular applications of lanthanide-based supramolecules (dendrimers, metal organic frameworks) are covered briefly. Throughout, we emphasize the techniques that can confirm the intracellular localization of the lanthanides and those that enable the determination of the fate of the probes once inside the cell. Finally, we highlight methods that have not yet been applied in the context of lanthanide-based probes, but have been successful in the intracellular delivery of other metal-based probes.