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Harnessing multi-photon excitation: from spectroscopy to catalysis

Pfund, Björn. Harnessing multi-photon excitation: from spectroscopy to catalysis. 2024, Doctoral Thesis, University of Basel, Faculty of Science.

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Abstract

Photoredox catalysis has emerged as a versatile method in organic synthesis for the mild activation of small molecules, thus enabling novel chemical transformations. However, initiating challenging chemical transformations with a single visible photon faces intrinsic energetic limitations, corresponding to the energy of blue photons, as well as additional cumulative energy losses of the photocatalyst itself. As a result, considerable attention has been directed towards multi-photonic mechanisms, aiming to extend the thermodynamic boundaries of photoredox catalysis by the cumulative energy of two visible photons per catalytic turnover. Although these multi-photon approaches have opened up numerous new chemical transformations, the underlying mechanisms present a significant challenge. Yet, enhanced understanding is needed for the rational design of novel and more efficient catalytic systems or for the development of novel excitation strategies. This thesis aims to address the increasing complexity of multi-photon excitation strategies through an in-depth study of their photocatalytic applications and mechanistic processes. By combining these approaches, the goal is to uncover the underlying mechanisms, that could facilitate better control over photocatalytic reactivity and open up new methodologies for photoredox catalysis.
The first part (Chapter 2) presents an overview of the photocatalytic scope of excited organic radicals as extremely potent photocatalysts, surpassing the current limits of classical photoredox catalysis. Further, this chapter aims to elucidate the proposed mechanism by comparing the theoretical and observed reactivity and summarizes methods for uncovering the main reaction pathway, fostering a deeper understanding, which can lead to improved catalytic systems. A kinetic analysis of the ultra-fast decay rate of the proposed excited organic radical photocatalysts indicates the possibility of anti-Kasha reactivity if pre-association with the substrate occurs.
This will be followed by the introduction of a novel excitation strategy mimicking photosystems I and II by introducing two different light absorbers for the discovery of novel highly reactive organic radical anions (Chapter 3). Using a consecutive excitation strategy with two visible photons leads to an exceptionally strong reductant, able to activate C(sp2)―F bonds, which were previously beyond the reach of excited organic radicals. Using time-resolved optical spectroscopy, we followed each light-dependent elementary step of the overall mechanism, including the reaction of the radical anion with the substrate. Further, the first direct evidence for the anticipated pre-association between radical ions and substrates was provided, revealing substantial free energy similar to that involved in template effects in supramolecular chemistry.
In the second research project (Chapter 4), novel organic triplet photosensitizers, the isoacridones, with simultaneously formed photoactive singlet- and triplet-excited states were developed. The uncommon photophysical behavior of these new isoacridones offers new perspectives for multiphotonic mechanisms, where parallel triplet-triplet energy transfer and electron transfer are required. To illustrate the potential applications of these new isoacridone dyes, proof-of-concept photoreactions such as Birch-type arene reductions and challenging C(sp2)―C(sp2) couplings, were achieved, and the tandem reactivity was spectroscopically analyzed.
In the last part of this thesis, rare water-soluble cyclometalated iridium(III) complexes with redox-active excited states, high triplet energies, and long excited state lifetimes were applied in multi-photonic mechanisms in the challenging solvent water. In Chapter 4, the high triplet excited state energies were employed for sensitized triplet-triplet annihilation upconversion, reaching unprecedented singlet excited state energies of almost 4 eV in water. The applied annihilators exhibit potent excited state reduction potentials capable of decomposing persistent tertiary ammonium compounds as typical water pollutants. In Chapter 5, the same iridium(III) based photosensitizers combined with a well-known rhodium co-catalyst were employed for the regioselective reduction of a NAD+ (NAD = nicotinamide adenine dinucleotide) mimic under physiological conditions. NADH is involved in many biologically relevant redox reactions and its regeneration is of interest in photobiocatalysis. Here, we used two sequential photoinduced electron transfers from the iridium(III) photosensitizer, followed by a proton transfer, ultimately generating the active rhodium co-catalyst. The electron transfer processes were analyzed based on the correlation between the electron transfer efficiency of the iridium(III) photosensitizer to the rhodium co-catalyst and the overall reaction's efficiency, providing insights into the overall mechanism.
These findings have the potential to introduce novel mechanistic concepts within the field of multi-photon catalysis, including tandem triplet-triplet energy transfer and electron transfer reactivity. Additionally, our findings on photocatalyst-substrate aggregations clarify one of the most controversial aspects of modern photocatalysis. Our work introduces new perspectives for photochemistry that go beyond current kinetic and thermodynamic constraints.
Advisors:Wenger, Oliver
Committee Members:De Roo, Jonathan and Ceroni, Paola
Faculties and Departments:05 Faculty of Science > Departement Chemie > Chemie > Anorganische Chemie (Wenger)
UniBasel Contributors:Wenger, Oliver and De Roo, Jonathan
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:15470
Thesis status:Complete
Number of Pages:V, 320
Language:English
Identification Number:
  • urn: urn:nbn:ch:bel-bau-diss154700
edoc DOI:
Last Modified:10 Sep 2024 04:30
Deposited On:09 Sep 2024 14:54

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