Graduate Thesis Or Dissertation
Tuning Iron(II) Excited States with Bulky Ligands and Investigating Chromium(III) Photocatalytic Mechanisms for Earth-Abundant Photocatalysis Public Deposited
Photocatalysis opens up new synthetic pathways and is a potential method for storing solar energy in chemical bonds. Many transition metal photocatalysts utilize long-lived metal-to-ligand charge transfer (MLCT) states as the catalytically active state. However, earth-abundant Fe(II) analogues of successful Ru(II) photocatalysts are plagued by ultrafast relaxation. Decreased ligand-field splitting in these first-row metal complexes opens new relaxation pathways via low-lying metal-centered states. To design potential photocatalysts with Fe(II) centers we chose to exploit the decreased ligand-field splitting by destabilizing the singlet state until the lowest energy quintet state became the ground state, opening up a distinct dynamical picture and lengthening the MLCT excited-state lifetime. In general, this class of complexes displays a > 100-fold improvement relative to the ~100 fs MLCT lifetime of the low-spin parent, Fe(II) bis-terpyridine. This is accomplished using a sterically demanding bis-terpyridyl ligand framework in which interligand repulsion destabilizes lower spin states to force the quintet ground state. Furthermore, this framework is easily altered synthetically by employing substituents with either steric or electronic effects, allowing for substantial tunability. We have systematically synthesized a range of these compounds and investigated them using x-ray crystallography, electrochemistry, magnetic measurements, and transient absorption spectroscopy. Through this effort, we highlight a novel approach to controlling excited-state dynamics and ground-state absorption in Fe(II) polypyridines with potential photocatalysis applications.
A secondary project explores the mechanisms of Cr(III) photocatalyzed [4+2] cycloadditions. Cr(III) polypyridyl complexes have been found to serve as replacements for more expensive Ru(II) photocatalysts in these reactions. However, their application to a wider scope of reactions is limited by a lack of mechanistic understanding. With static and time-resolved photoluminescence spectroscopy and Stern-Volmer quenching studies, we show that [Cr(Ph2phen)3]3+ (Ph2phen = 4,7-diphenyl-phenanthroline) is a potent photooxidant for promoting radical cation Diels-Alder reactions. Further experiments show that atmospheric oxygen is critical for turning over the catalyst. Finally, the mechanistic study of a related reaction with the same catalyst suggests that an energy transfer mechanism is also feasible in certain cases. This broader understanding of mechanistic pathways uncovers a rich vein for further mechanistic studies and Cr(III) catalyst development.
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