Time-Resolved Spectroscopic Detection of High Energy Intermediates in Energy Conversion and Photoredox Reactions

Abstract

Dissertation Advisor: Professor Daniel G. Nocera Kristopher Glen Reynolds Time-Resolved Spectroscopic Detection of High Energy Intermediates in Energy Conversion and Photoredox Reactions Abstract All reactions of interest to the chemist occur via high energy transient intermediates. Understanding the nature and dynamics of these transient species is a key step in elucidating relevant reaction mechanisms. However, due to the often extremely short lifetime of those intermediates, their detection requires spectroscopic tools which are commensurate with the timescale of their formation and consumption. Herein, I employ transient absorption spectroscopy across a wide variety of timescales and spectral ranges to uncover the nature and dynamics of, elusive and sought after, high energy transient intermediates in both energy conversion and photoredox transformations. In Chapter 1, I provide a general overview of photochemistry and transient spectroscopy which is intended to set the broader context in which the remainder of this work exists. In Chapter 2, I briefly cover the theoretical foundations upon which this work rests. The aim of this chapter is to familiarize the reader with the key concepts of electronic spectroscopy as well as the photophysical and photochemical phenomena which will be used to interpret the spectroscopic data in the chapters to follow. In Chapter 3, I discuss the use of transient infrared spectroscopy to uncover the mechanism of the photochemical oxidation of bicarbonate. These studies reveal the first known spectroscopic evidence of the bicarbonate radical as a result of single electron oxidation and its subsequent evolution of CO2 and O2. The work presented here conclusively shows that the single electron oxidation of bicarbonate can serve as a means to clearing bicarbonate in CO2 electrolytic cells addressing the carbonate problem, a key bottleneck toward the realization of efficient low temperature CO2 reduction. In Chapter 4, ultrafast transient absorption spectroscopy is employed to uncover the mechanisms by which open-shell photoreagents engage redox recalcitrant substrates despite both thermodynamic and kinetic limitations. Transient absorption spectroscopy reveals that all radical ions studied herein exhibit extremely short excited state lifetimes when excited at their lowest energy optical transition in the red, too short in fact to engage in diffusion limited chemistry with substrates. However, blue light excitation of these radical ions revealed the universal formation of long lived (>6 ns) solvated electrons in the NIR, the lifetime of which was quenched by extremely hard to reduce substrates such as fluorobenzene. Furthermore, the formation of closed shell decomposition products originating from the radical ions under certain conditions leads to the formation of long lived and closed-shell photoreagents competent of driving challenging redox chemistry. Together this work demonstrates that the observed reactivity from radical ions to drive extremely challenging reactions originates from both the generation of solvated electrons and/or photodecomposition of the radicals to a closed shell species. In Chapter 5, ultrafast transient absorption spectroscopy in conjunction with high level quantum chemical calculations are employed to determine the origins of the ultra-short excited state lifetimes of radical ions. Ultrafast transient absorption spectroscopy on the radical anions of a series of structurally related dicyanoacenes as well as select radical cations reveal that the ground electronic state is repopulated in a few picoseconds after excitation. In such cases quantum chemical calculations reveal the presence of low-lying conical intersections between the lowest energy doublet excited state and the ground state which are responsible for the ultrashort lifetimes measured by transient absorption spectroscopy. Furthermore, this work provides general classifications of the types of structural distortions that are responsible for the conical intersections. In Chapter 6, additional studies of the dynamics of photo-induced reactivity of select photoredox platforms are discussed. Examples of both intramolecular transformations arising from LMCT or intramolecular energy transfer as well as intermolecular reactivity arising from the interaction of an excited state or photo-induced species with another are presented. Lastly, possible project directions for future students in the Nocera lab toward the generation and detection of the CO2 radical anion are discussed.