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- Pettersson Rimgard, Belinda, 1992-
(författare)
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Tailing Charges on New Paths : Ultrafast intramolecular charge transfer in chromophores
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- By tailing charges on their paths within a molecule, one can gain fundamental knowledge of their inherent reactivity. The charges, originally residing in their lowest energy configuration, can upon light absorption move across the system, or transfer in between them. The focus of this thesis has been to study how electrons and protons transfer between different fragments of a molecule, following an absorption of laser light. The relevance of this work is not only attributed to its mechanistic insights, but also that it might provide a foundation for future designs of renewable energy systems, such as solar cells and fuel cells. This thesis entails the investigation of a derivative of the famous N3 dye, developed for dye sensitized solar cells. This dye is a ruthenium-complex, with two bipyridyl and two isothiocyanate coordinated ligands. There was no real consensus in the literature whether the initial excitation of this complex, would cause the localization of an electron on one bipyridyl or, if it would delocalize over both of them. Moreover, if the charge did initially localize, would it, at some point in time, transfer in between the ligands, hence perform interligand electron transfer? The results confirm the existence of an initial localization onto one ligand, and that interligand electron transfer occurs on a sub-picosecond time scale. This diminishes the risk of e.g. slow electron injection into a semiconductor surface such as in a solar cell, due to a charge localization onto a surface unbound ligand. In photocatalysis, a concerted proton and electron transfer is sought after, as the total charge neutralizes thus avoids the formation of high energy intermediates. In a set of anthracene-phenol-pyridine triads a concerted mechanism was investigated, where the charges were shown to separate upon excitation, but later slowly recombine. The slow recombination followed a Marcus inverted region behavior, with a counter-intuitive decrease in rate with an increase in driving force. A concerted mechanism in the inverted region had previously never been observed. As the driving forces were altered, with the help of solvent and temperature, alternative reaction paths in the triads became visible. The anthracene-localized excitation was observed to transfer across the molecule, to the phenol-pyridine, where a concomitant proton transfer occurred. This proton-coupled energy transfer mechanism, is a new phenomenon that further adds to the knowledge of charge transfer mechanisms.
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| 2. |
- Tyburski, Robin, 1991-
(författare)
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Deciphering the Mechanistic Diversity of Proton-Coupled Electron Transfer Reactions
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Proton coupled electron transfer is ubiquitous in biological and artificial reaction systems. Much has been done in order to describe the occurrence of such reactions. However, PCET reactivity is often very complex. For instance, there are multiple (stepwise and concerted) mechanistic pathways through which PCET may occur. The aim of this thesis is to further describe factors and underlying principles governing PCET reactivity. The contents of this thesis can be summarized in three parts:In the first part (chapter 4), the competition between different PCET mechanisms is discussed. Considering all mathematical expressions for the dependence of the rate constants on The Gibbs free energy changes (driving forces) associated with electron and proton transfer, mechanistic Zone-Diagrams are constructed. These show which of the mechanistic pathways is dominant, given a certain electron and proton transfer driving force. It is shown, how these diagrams simplify discussion of PCET reactivity. Strategies for modifying the mechanistic landscape, suppressing or favoring a CEPT mechanism, are demonstrated in the PCET oxidation of 4-methoxyphenol by photogenerated Ru(III) oxidants in the presence of pyridine bases. These are discussed utilizing the zone-diagram methodology. Implications for catalytic applications are discussed.The second part (chapter 5) introduces pressure dependence as a tool for mechanistic characterization of the PCET reactions. The PCET oxidation of tungsten hydrides covalently linked to pyridine bases by photogenerated Ru(III) oxidants was studied, and contributions from multiple mechanistic pathways were uncovered. It is shown, how each pathway has a characteristic pressure dependence. These can be related to changes in electrostriction of the solvent modifying the volume profile of the reaction.Finally, the third part (Chapter 6) deals with the concerted pathway. The possibility of photo-EPT, where electronic excitation directly yields the PCET product state, in Phenol/N-Methyl-4,4’-bipyridine complexes is discussed. It is shown that the optical charge-transfer absorption in these complexes is not coupled to proton transfer, in spite of previous claims. Further, the pressure dependence of the CEPT quenching of excited state fac-Re(CO)3(2,2’-bpy)(4,4’-bpy)+ by substituted Phenols is monitored. It is shown that the observed pressure dependence cannot be rationalized using the electrostriction arguments outlined in chapter 5. Instead, a model relating the observations to pressure induced changes of contributing proton tunneling distances is constructed.
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