Unravelling Sensitisation and Quenching Pathways in Lanthanide Luminophores

Abstract: Lanthanide (Ln) ions find use in cellular detection and probing of many analytes, owing to their unique photophysical properties. However, to make the Ln emission efficient, one has to develop a light-harvesting antenna, which is an organic chromophore that absorbs and transfers energy to the Ln. By exciting the Ln complex, photoinduced electron transfer (PeT) from the antenna to the readily reducible Ln might occur. Usually, PeT is quenching Ln emission intensity. Thus, the work herein aims to evaluate and distinguish sensitising and quenching pathways in Ln emission to improve the brightness of Ln luminophores for biological applications.After presenting an introduction to Ln photophysics and sensitisation in chapter 1, chapter 2 describes our work on Ln octa- and nonacoordinate complexes bearing the same antenna. We demonstrated that the azide and alkyne reactive groups could be attached to the antenna to make Ln complexes bioconjugable. The saturation of the Ln coordination environment resulted in intraligand PeT, which cancelled out the benefits of eliminated quenching from the coordinated water molecule.To study the effect of photoredox quenching in Eu emission, we focused on tuning reduction potentials for a possible response in luminescence properties in chapter 3. Increasing positive charges of complexes destabilised Eu3+ and decreased Ln emission quantum yields. This behaviour was due to PeT quenching of the more readily reducible Eu under similar sensitisation conditions. Fluoride binding boosted Ln emission intensity in the least Eu3+-stabilising ligand field.In chapter 4, the role of a tertiary amide linker between the antenna and the metal binding site was examined. For poorly emissive coumarin-appended complexes quenching via photoinduced reduction of Eu from the antenna was unlikely, while in carbostyril-sensitised complexes PeT was efficient.Ln compounds with lower ligand denticity allowed better fluoride detection via the increased effect of PeT quenching and the larger number of water molecules, as shown in chapter 5. However, the presence of cyanide significantly quenched Eu emissions of the same complexes, making it possible to distinguish cyanide from fluoride.Chapters 6‒7 were devoted to the sensitisation of Yb luminescence. The NIR emission intensity increased marginally in the complexes with efficient PeT. The main sensitising effect originated from phonon-assisted energy transfer favourable in our Yb complexes. When picolinate donors stimulated intraligand PeT, detrimental to the ligand fluorescence in Yb compounds, Yb emission was noticeably more intense than in the first series without picolinates.