Understanding Proton-Transfer Dynamics of Reversible Excited-State Photoacids Using Driving-Force-Dependent Kinetic Theories

This thesis investigates the thermodynamic and kinetic properties of reversible excited-state photoacids using steady-state spectroscopic techniques and explores the relationship between the Brønsted-Lowry excited-state acidities (thermodynamics) and the rate constants (kinetics) for excited-state proton-transfer reactions using driving-force-dependent kinetic theories. Chapter 1 presents methods to synthesize donor-π-acceptor reversible excited-state weak photoacids whose ground-state acidity is ideal to deprotonate water. The excited-state acidities (pKa* values) determined using the Hill equation analysis of steady-state photoluminescence data were close to the ground-state acidities (pKa values) but differed by several orders of magnitude from pKa* values calculated using the Förster cycle analysis. Initial efforts to resolve these discrepancies were aimed at understanding the pathways that result in the loss of excitation energy using time-dependent density function theory. However, non-ideal Hill coefficient values and ultrafast transient absorption spectroscopy data confirmed that the most dominant factors contributing to the determination of incorrect pKa* values using Hill equation analysis were the short lifetimes of the electronic excited-state species and the absence of a chemical quasi-equilibrium for the ESPT reaction to hydroxide, which is often the case for weak photoacids. Chapter 2 reports the experimental determination of driving-force-dependent rate constants for intermolecular excited-state proton-transfer reactions from reversible excited-state weak photoacids to a series of proton-accepting quenchers with varied and known pKa values using the Stern–Volmer equation. This chapter also discusses the dependence of pseudo-pKa* values of weak photoacids obtained using the Hill or Henderson-Hasselbalch equation on their excited-state lifetimes. Using a driving-force-dependent kinetic theory developed by Marcus and Cohen based on the bond-energy–bond-order, the true pKa* of these photoacids was determined from data measured using readily-available steady-state photoluminescence spectroscopy. Chapter 3 discloses the reasons for discrepancies in the previously reported values of intrinsic activation energies for proton-transfer reactions obtained from driving-force-dependence studies and sets forth guidelines for correctly applying driving-force-dependent kinetic theories to proton-transfer reactions involving reversible excited-state photoacids. Chapter 4 discusses the applicability of a modified Stern–Volmer equation to extract kinetic information from steady-state photoluminescence data for reversible excited-state strong photoacids with longer excited-state lifetimes of both protonated and deprotonated excited-state species than the excited-state lifetimes of weak photoacids. In such cases, the experimentally observed overall excited-state proton-transfer rate constants have significant contributions from rate constants for the forward and the reverse reactions, which can be deconvoluted using a modified Stern–Volmer equation

Direct observation of the local microenvironment in inhomogeneous CO 2 reduction gas diffusion electrodes via versatile pOH imaging

We report how the micrometer-scale morphology of a carbon dioxide reduction (CO2R) gas diffusion electrode (GDE) affects the mass transport properties and with it, the local CO2R performance. We developed a technique to probe the microenvironment in a CO2R GDE via local pOH imaging with time- and three-dimensional spatial, micrometer-scale resolution. The local activity of hydroxide anions (OH−), represented by the pOH value, around a GDE in contact with an aqueous electrolyte is a crucial parameter that governs the catalytic activity and CO2R selectivity. Here, we use fluorescence confocal laser scanning microscopy (CLSM) to create maps of the local pOH around a copper GDE by combining two ratiometric fluorescent dyes, one of which is demonstrated as a pOH sensor for the first time in this work. We observe that the local pOH decreases when current is applied due to the creation of OH− as a byproduct of CO2R. Interestingly, the pOH is lower inside microtrenches compared to the electrode surface and decreases further as trenches become more narrow due to enhanced trapping of OH−. We support our experimental results with multiphysics simulations that correlate exceptionally well with measurements. These simulations additionally suggest that the decreased pOH inside microcavities in the surface of a CO2R GDE leads to locally enhanced selectivity towards multicarbon (C2+) products. This study suggests that narrow microstructures on the length scale of 5 μm in a GDE surface serve as local CO2R hotspots, and thus highlights the importance of a GDE's micromorphology on the CO2R performance