Spectroscopic characterization of the water-oxidation intermediates in the Ru-based catalysts for artificial photosynthesis

Utilization of sunlight requires solar capture, light-to-energy conversion and storage. One effective way to store energy is to convert it into chemical energy by fuel-forming reactions, such as water splitting into hydrogen and oxygen. Ruthenium complexes are among few molecular-defined catalysts capable of water splitting. Insight into the mechanism of their action will help to design future robust and economically feasible catalysts for light-to-energy conversion. Mechanistic insights about the design of such catalysts can be acquired through spectroscopic analysis of short-lived intermediates of catalytic water oxidation. Development of time-resolved approaches through stopped flow UV-Vis Spectroscopy to follow the catalysis of water oxidation is critical for understanding the dynamics of this reaction. In addition, use of techniques sensitive to the electronic states of molecules such as EPR and X-ray absorption spectroscopy (XAS) is implemented to determine the electronic requirements of catalytic water oxidation.;We demonstrate that correlated UV-Vis stopped flow, EPR, X-Ray Absorption Spectroscopy and O2 evolution measurements on the blue dimer water oxidation catalyst oxidized through single and multiple catalytic turnovers result in characterization of a new reactives intermediate denoted as BD[3,4]4+-prime and BD[4,5]4+. EXAFS analysis demonstrated a considerably modified ligand environment in those intermediates as compared to stable intermediates BD[3,3] and BD[3,4]. During O2 evolution at pH 1, most of the blue dimer catalyst exists of BD[3,4]-prime suggesting that it is a key oxygen evolving intermediate in the catalytic cycle where oxidation is a rate limiting reaction under the conditions of the experiments. Furthermore, Raman measurements gave strong support for the presence of a Ru-O stretch in the Ru-OOH peroxide fragment of BD[3,4]-prime.;The H2O/D2O kinetic isotopic effects in water oxidation by the blue dimer are investigated and a kinetic isotope effect for blue dimer water oxidation reaction in D2O determined to be between 2.1-2.5 by combined UV-Vis stopped flow and EPR analysis. This study showed that the mechanism of O-O bond formation is atom proton transfer and revealed that the rate limiting steps in the overall catalytic cycle is not the O-O bond forming step consistent with the rate limiting oxidation of the oxygen evolving intermediate BD[3,4]-prime by Ce(IV). We also show that electron transfer processes in blue dimer water oxidation catalyst can be optimized by use of the photosensitizer [Ru(bpy)3]2+. This photosensitizer can work by redox shuttle mechanism and speed up several oxidation steps. These results are significant as they demonstrate that a redox mediator such as [Ru(bpy)3]3+ can shorten the lifetime of the BD[3,4]-prime oxygen evolving intermediate while enhancing the catalyst's oxygen evolution rate. Information obtained here about the physical, chemical, structural and electronic states of the reactive intermediates in the blue dimer catalytic cycle is critical and can contribute to the catalyst's optimization for better performance, as demonstrated by use of the electron-transfer mediator [Ru(bpy)3]3+.;Lastly, we demonstrate that chlorination of the photosensitizer [Ru(bpy)3]2+ result in a chlorinated product which show possible configuration of Ru(bpy)3(ClO3)3OOH peroxo product from EXAFS analysis. EXAFS fits indicate that addition of a Ru-peroxo distance improve the quality of the fit considerably. Such results proved to be extremely interesting as the chlorinated product had the same EPR signal as the peroxo species observed in monomeric Ru complexes [RuII(L)(4-pic)2(OH2)]2+ and [Ru(bda)(isoq)2]. Structure of this chlorinated product through XRD will undeniably shed more understanding on the catalytic cycle of monomeric Ru complexes which are by far efficient than the blue dimer.;In summary, this thesis outlines the structure and electronic configurations of the critical intermediates of water oxidation by catalytically active ruthenium complexes. Proposed spectroscopic approaches shown in this project have outstanding potential of uncovering the mechanism of the water splitting reaction and allow identification of the critical requirements for catalytic water oxidation paving the way for a light to fuel device. (Abstract shortened by UMI.).