The Influence of Non-Covalent Interactions on Photoinduced Electron Transfer at Interfaces and in Fluid Solution

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  • March 21, 2019
Creator
  • Swords, Wesley
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Photoinduced electron transfer is an essential reaction in biology and artificial solar energy conversion. In the dye-sensitized solar cell (DSSC), regeneration of a semiconductor-anchored sensitizer through electron transfer from a solution-based redox mediator is a crucial reaction that is not optimized under operational conditions. Non-covalent interactions offer a way to control and enhance regeneration. This work details efforts in the development of finite control over electron transfer reactions, both at solid-liquid interfaces and in solution. The DSSC is introduced in the context of non-covalent interactions and pertinent electron transfer theory and techniques in Chapter 1. Chapters 2 and 3 focus on non-covalent enhancement of the regeneration reaction. Chapter 2 describes how a sensitizer’s increased propensity to form halogen bonds with the iodide redox mediator leads to an increase in the regeneration rate constant. The increased rate constant correlated with larger DSSC photovoltages. Chapter 3 demonstrates the importance of frontier molecular orbital overlap for electron transfer. Variation of a single chalcogen atom on a sensitizer showed an order of magnitude increase in regeneration only when the frontier molecular orbital localized on the chalcogen atom. Chapters 4-8 investigate electron transfer within electrostatic ion pairs. In chapter 4, bromide formed two-consecutive ion pairs with a dicationic ruthenium sensitizer, that yielded a drastic shut-off and then recovery of photoluminescence, which aligns with the Gibbs free energy for electron transfer. Chapters 5-8 utilize a dicationic bipyridyl ligand to increase the cationic charge of ruthenium polypyridyl compounds. In chapter 5, three ruthenium compounds, with 4+, 6+, or 8+ charge, show enhanced ion pairing with iodide in acetonitrile. Chapter 6 characterizes a rare example of ion pairing at the sensitized interface. Regeneration occurred rapidly, k > 108 s-1, within an ion pair between a hexacationic ruthenium and an anionic cobalt redox mediator. Chapter 7 determines the excited-state quenching mechanism of similar ruthenium-cobalt ion pairs through ultrafast transient absorption spectroscopy. Finally, chapter 8 extends the use of ion pairs to excited-state proton-coupled electron transfer (ES-PCET). Direct measurement of the ES-PCET rate constants within the ion pair facilitated the determination of a sequential proton, electron transfer (PT-ET) mechanism.
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  • In Copyright
Advisor
  • Dempsey, Jillian
  • Meyer, Gerald
  • Miller, Alexander
  • Lockett, Matthew
  • You, Wei
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2018
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