Monika Tomar: Photochemical generation of divalent lanthanides: Applications in organic synthesis and small molecule activation

Abstract

Trivalent lanthanide (Ln(III)) compounds are known for their luminescent properties. Luminescence sensitization often happens via the so-called "antenna effect": a light-harvesting chromophore which transfers its excitation energy to Ln(III) ions. These compounds have widespread applications in biological detection and as security inks due to their unique and robust photophysical characteristics. However, their luminescence quantum yields are often diminished by several quenching processes, with one of the primary processes being photoinduced electron transfer (PeT) from the excited antenna to the Ln(III) ion. PeT generates a Ln(II) species and an antenna radical cation, which typically quenches both the fluorescence of the antenna and the luminescence of the Ln(III) ion.

This thesis explores inter- and -intramolecular photoinduced electron transfers in Ln(III) complexes, with a particular focus on harnessing the transiently formed reactive Ln(II) species in organic synthesis. Chapter 1 is a brief introduction to lanthanide complexes and their applications in organic synthesis and photocatalysis.

Chapters 2 and 3 focus on the synthesis of novel Ln(III) coordination compounds, specifically those containing reducible Eu(III) and Sm(III) ions. The physicochemical and redox properties as well as the excited-state behavior of these compounds are also examined. The resulting Ln(II) species are utilized in catalytic settings, enabling the development of a practical catalytic system that could replace the well-established SmI2-mediated reduction reactions.

Chapter 4 shifts focus to the modification of ligands designed for the selective reduction of carbon dioxide using a photocatalyst. The mechanisms underlying these reduction processes are explored.

Chapters 5 and 6 involve the interactions between various commercially available chromophores and Ln(III) ions in both their ground and excited states. Additionally, the electron transfer process from different chromophores to Ln(III) ions was investigated. These results show that it does not require the synthesis of complex ligand design to develop an effective lanthanide photocatalyst for Ln(II)-mediated reduction reactions.