Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Mayer, James
Abstract
Proton-coupled electron transfer (PCET) reactions are integral to numerous energy conversion processes, including photosynthesis, fuel cells, and cellular respiration. The coupling of proton transfer (PT) with electron transfer (ET) influences the overall energetic landscape and mechanism of the PCET reaction. Developing a better understanding of the thermodynamics and kinetics of PCET reactions could enable the design of more efficient technologies for energy storage and conversion. However, it is often challenging to study PCET within naturally occurring systems, as many of these reactions are part of larger, complex biological processes. Well-defined, small molecule model systems, where variables such as reaction driving force can be systematically modulated, are therefore ideal for uncovering the fundamentals of PCET reactivity.Marcus Theory was developed to explain the rate of ET reactions and extensions have been made to apply an analogous theory to PCET reactions. One of the more surprising conclusions of Marcus Theory is the existence of an inverted region, where highly exergonic ET reactions are predicted to have slower rates as the driving force is increased. This is counterintuitive to chemical intuition and was debated until the first experimental evidence was reported nearly three decades later. A similar inverted region was not expected for PCET reactions due to participation of transitions to excited proton vibrational states. Despite these initial predictions, our group reported a concerted PCET reaction that exhibits inverted region behavior using a series of molecular triads. The first part of this thesis further explores the PCET reactivity of anthracene-phenol- pyridine triads. These compounds undergo a photoinduced PCET charge separation (CS) reaction to form a highly reactive charge separated state (CSS). For three of eight triads, the CSS can then undergo a follow up PCET charge recombination (CR) reaction, which exhibits an inverted dependence on reaction driving force. The solvent dependence of both CS and CR for two of the inverted triads was examined in polar (butyronitrile, acetonitrile) and non-polar (toluene) solvents. A significant solvent effect is observed for the inverted CR reaction, consistent with expectations from Marcus Theory and reports for inverted ET-only systems. Rates were also measured over a large range in temperature (~180 K to 298 K), revealing a shallow dependence for both the CS and CR reactions. Various models were applied to explain the observed variations in rate as a function of changes in driving force and reorganization energy. A decay mechanism is also proposed for the five triads that do not show inverted kinetics. Transient absorption spectroscopy of these triads did not show accumulation of the CSS and inverted CR. As CR of these triads should be even deeper into the Marcus inverted region, this result was puzzling and unresolved by prior studies. Chapter 4 presents evidence for a reaction pathway via a local electron-proton transfer state, which was originally predicted by theoretical calculations. The assigned reaction mechanism is via proton-coupled energy transfer, a newly discovered photochemical mechanism. The final two chapters of this thesis detail additional spectroscopic studies of energy transfer and charge transfer reactions. Chapter 5 focuses on a photochemical cycloaddition reaction with high levels of stereocontrol. Stern-Volmer analysis of time-resolved emission experiments were used to provide insight into the reaction mechanism. Finally, Chapter 6 summarizes preliminary work using stopped-flow IR spectroscopy to examine PCET reactions of phenol-bases. In summary, this thesis applies various spectroscopic approaches to explore energy and charge transfer reactions in small molecule, model systems. These findings provide fundamental insights into PCET and can be used in the future design of more efficient technologies for energy conversion.
Recommended Citation
Cotter, Laura Fei, "Spectroscopic Studies of Photochemical Proton-Coupled Electron Transfer Reactions" (2023). Yale Graduate School of Arts and Sciences Dissertations. 936.
https://elischolar.library.yale.edu/gsas_dissertations/936
