UC Irvine

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.