A Computational And Conceptual Study On The Effect Of Molecular Structure On The Excited State Lifetime And Redox Energetics Of Iridium And Ruthenium Photoredox Catalysts

This work investigates the effect of ligand structure on the photophysical and redox properties of iridium and ruthenium photoredox catalysts. Using density functional theory (DFT) and intuitive theoretical tools, we systematically analyze how structural isomerism, cyclometallation, nitrogen-containing heterocycles, and ligand substitution impact the performance of [Ir(ppy)2(bpy)]+ and Ir(ppy)3-based catalysts. First, this dissertation explores structural isomerism, revealing that while ground-state and excited-state redox potentials remain stable across isomers, variations in transition dipole moments and spin-orbit coupling lead to significant changes in excited-state lifetimes. These findings highlight the importance of isomeric orientation for controlling decay rates. Then, this study investigates the role of cyclometallation in extending the excited-state lifetimes of iridium and ruthenium complexes, demonstrating the role of metal-carbon bonds in radiative and non-radiative decay pathways. This work also studies the effect of nitrogen-containing heterocycles, showing that incorporating pyrazine and pyrimidine groups can increase lifetimes and shift reduction potentials, thus improving photoredox efficiency. Furthermore, this study examines how electron-withdrawing and electron-donating substituents, located on redox-active and cyclometallating ligands, shift redox potentials and extend excited-state lifetimes, depending on their position. Finally, this dissertation investigates temperature-dependent decay pathways, demonstrating how modifying the ligand environment around the metal center can change the energy barriers throughout the 3MLCT → 3MC → S0 pathway, thus reducing non-radiative decay and improving catalyst stability. Collectively, this work provides a framework for the rational design of iridium photoredox catalysts, paving the way for more efficient applications in synthetic chemistry and sustainable energy.