Abstract:

Solar energy conversion harnesses electromagnetic radiation from the sun (i.e. light) offering long lasting solutions for sunlight-to-electronic or -ionic conversion to power. However, the fundamental understanding of sunlight-to-ionic processes via transport and reactivity of protons and hydroxides, have not been fully explored. This knowledge gap in the basic understanding of photo-induced ionic processes has not only slowed progress and innovation in light-to-ionic power conversion, but also in the fields of solar fuels, photovoltaics, iontronics, and more. Photoacids are a class of molecules that upon light absorption shows light induced excited-state proton transfer (ESPT), where a change in electron density alters the energetics of a protic bond that results in an increase in photoacid acidity resulting in proton transfer. This Dissertation deduces the thermodynamic and kinetic properties of reversible excited-state photoacids to understand what governs proton-transfer dynamics under different nonequilibrium conditions via addition of inert salt and excited-state electronic redistribution. In this presentation I will show that the addition of inert salt perturbs the reaction equilibrium due to changes in pH, pK_a, and pH₂O that are observable in absorption and photoluminescence spectra from ground-state proton transfer (GSPT) and excited-state proton transfer (ESPT) reactions. Unlike typical studies that assume pK_a and pH₂O to be constant, here it is shown that it is important to include effects due to changes in pK_a and pH₂O in the presence of high concentrations of protic or inert salt. I will also show the investigation of the ESPT behavior of reversible photoacids, 9-hydroxyphenanthrene-3-sulfonate (HPhenMS) and 9-hydroxyphenanthrene-3,10-disulfonate (HPhenDS). Results indicate surprisingly large reorganization energies of 1.1 eV, which is significantly larger than generally observed for proton-transfer reactions, which typically exhibit near-zero reorganization energy (i.e. < 0.1 eV). This behavior is unequivocal, where instead of observing a typical linear free energy relationship there is a parabolic dependence of the logarithm of the reaction rate constant on thermodynamic driving force, consistent with semiclassical Marcus theory. My data is consistent with extensive electronic delocalization in the electronic excited state. The research presented aims to contribute to the fundamental photochemical knowledge of light induced proton transfer reactions.