From Spin-Driven Dimerization to Redox Chemistry: Spectroscopic Studies of Neutral Radicals and Photocatalysts

Abstract

This thesis delves into the ultrafast photophysics of various photoredox catalysts and metal-ligand complexes, expanding our understanding of their dynamic behaviors and photophysical properties through meticulous optical spectroscopy studies. The core focus lies on the elucidation of complex electron and energy transfer mechanisms, which are fundamental to the application of these materials in photocatalytic processes and other relevant fields. The study explores the temperature-dependent spin-driven dimerization in a copper(II)-bound tripyrrindione radical, TD1-Cu. It reveals that at low temperatures, TD1-Cu radicals undergo reversible dimerization, mediated by multicenter interactions and antiferromagnetic coupling between the unpaired electrons on the ligands. These insights into spin-coupled, temperature-controlled molecular dynamics offer potential applications in spintronics, molecular switches, and catalytic systems. Further investigation focuses on a helical carbenium ion known as nPr-DMQA+, a red-light-activated organic photocatalyst, examining its role in the oxidative hydroxylation of phenylboronic acid. The longevity of the excited triplet state of nPr-DMQA+ and the critical involvement of a sacrificial amine, DIPEA, in facilitating electron transfer processes highlight the capabilities of organic photocatalysts in sustainable chemistry under red-light activation. Additionally, the research extends to the ultrafast dynamics of the nPr-DMQA• radical, a stable neutral organic radical formed by reduction of the nPr-DMQA+ cation. Through transient absorption spectroscopy, the study delineates how the radical participates in rapid internal conversion processes, essential for its activity as a photoredox catalyst. This work contributes significantly to our understanding of the stability and reactivity of organic radicals. Overall, this thesis bridges significant gaps in our understanding of the fundamental properties of photoredox systems and lays the foundation for the development of novel photocatalysts. The methodologies and insights obtained pave the way for future innovations in photochemistry and photophysics, potentially revolutionizing approaches to sustainable and efficient photocatalytic applications.