Energy Transfer Processes and Quenching Pathways in Lanthanide Complexes: The Role of Structural Changes

Abstract: Due to their unique magnetic and luminescent properties, trivalent lanthanides (Ln(III)) have been widely utilised in cellular imaging. Ln(III) luminescence is usually sensitized by a light-harvesting ‘antenna’, a chromophore that can transfer the excitation energy to the Ln(III). Our research focuses on the understanding and tuning of the luminescent properties of Ln(III) coordination compounds. In order to obtain bright emitters, the optimisation of energy transfer and minimisation of quenching processes are key challenges. In complexes of reducible Ln(III) ions photoinduced electron transfer (PeT) from the excited state antenna to the Ln(III) is possible, often quenching both antenna fluorescence and Ln(III) luminescence. Chapter 1 is a brief introduction to luminescent Ln(III) coordination compounds. In Chapter 2 intraligand PeT from the excited state antenna to the pyridines was studied in picolinate-containing TACN (1,4,7-triazacyclononane)-based Ln(III) complexes. Such PeT was found to be thermodynamically favoured and strongly dependent on the electron accepting ability of pyridines. Chapter 3 addresses the importance on how distant structural changes can lead to improved luminescent properties in Ln complexes. Two sets of complexes with identical coordination environments and antennae were compared. In the first set the antenna was connected to the metal-binding site via a secondary amide linker, while in the second the linker was replaced with a tertiary amide. All tertiary amide linked Ln(III) complexes showed increased quantum yields compared to their secondary amide analogues. Moreover, tertiary complexes showed improved photostability compared to the secondary-amide linked ones.In Chapter 4 and 5 we investigated the effects of fluorinated antennae in secondary and tertiary amide-linked DO3A (cyclen-1,4,7-triacetate)-based, and pyridyl-containing TACN-based Ln(III) complexes. Photophysical characterisation revealed that monofluorination yielded no significant improvement in the luminescent quantum yields compared to the non-fluorinated analogues. The fluorination of the carbostyril antenna in the 3 position drastically effected the excited state behaviour. The presence of several emissive species was observed possibly caused by changes in the coordination environment. 3-CF3 and the 4-fluoro carbostyrils were excellent sensitisers of Eu(III) and Tb(III) emission. Changes in the antenna T1 resulted in 5–12-fold increase in Tb(III) luminescent quantum yields in the 3-CF3 series compared to the 4-CF3 analogues. Eu(III) luminescent quantum yields and the sensitisation efficiencies were systematically lower in the 3-CF3 compounds vs the 4-CF3 emitters, which might be attributed to the differences in the PeT quenching in these complexes.