Photoinduced Electron Transfer in Luminescent Lanthanide Complexes

Luminescence from the trivalent lanthanides is used for detection in biological systems. Lanthanide luminescence is usually sensitized by a light-harvesting organic chromophore (’antenna’). An ideal emitter has an antenna that efficiently transfers energy to the lanthanide, and a ligand that shields the metal from quenching solvent molecules. Additional photophysical processes can impact the luminescence quantum yield. One such process is electron transfer from the excited state antenna to the lanthanide, yielding a lanthanide(II) and an antenna radical cation. Back electron transfer returns the antenna and a lanthanide(III), which, may or may not emit a photon. This thesis summarizes my work on the investigation of the photoinduced electron transfer in lanthanide complexes. Chapter 1 is a brief introduction. Chapter 2 describes a structure-activity relationship in a focused library of lanthanide complexes. PeT was observed with most antennae and in a larger of lanthanides than previously assumed. In Chapter 3, the role of the linker connecting the antenna and the metal binding site is investigated. Replacement of the secondary amide linker with a tertiary amide yielded unexpectedly high luminescence quantum yields. This was attributed to an improved sensitization efficiency due to a combination of factors (structural as well as electronic) in the Eu complexes, and a reduced back energy transfer in the Tb complexes. In Chapter 4, the role of the ligand in PeT was investigated. Stabilization of the lanthanide(III) state by encapsulation of the ion in a highly negatively charged ligand resulted in a recovery of some of the excitation energy lost to PeT. The addition of external a strongly coordinating anionic ligand, fluoride, which could replace the charge-neutral water ligand, had a similar effect. The changes in the lanthanide redox potential was confirmed by cyclic voltammetry. Finally, complexes equipped with azide and alkyne reactive handles are described. The introduction of bioconjugatable groups had only a small effect on the luminescent properties of the compounds. We attempted to improve the luminescence quantum yields by excluding the lanthanide-bound water molecule that has been occupying the ninth coordination. However, the nonadentate ligands did not yield appreciably better emitters due to a carbostyril-to-pyridine PeT.