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Synthesis of Biomimetic Systems for Proton and Electron Transfer Reactions in the Ground and Excited State
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A detailed understanding of natural photosynthesis provides inspiration for the development of sustainable and renewable energy sources, i.e. a technology that is capable of converting solar energy directly into chemical fuels. This concept is called artificial photosynthesis. The work described in this thesis contains contributions to the development of artificial photosynthesis in two separate areas.

The first one relates to light harvesting with a focus on the question of how electronic properties of photosensitizers can be tuned to allow for efficient photo-induced electron transfer processes. The study is based on a series of bis(tridentate)ruthenium(II) polypyridyl complexes, the geometric properties of which make them highly appealing for the construction of linear donor-photosensitizer-acceptor arrangements for efficient vectorial photo-induced electron transfer reactions. The chromophores possess remarkably long lived 3MLCT excited states and it is shown that their excited-state oxidation strength can be altered by variations of the ligand scaffold over a remarkably large range of 900 mV.

The second area of relevance to natural and artificial photosynthesis that is discussed in this thesis relates to the coupled movement of protons and electrons. The delicate interplay between these two charged particles regulates thermodynamic and kinetic aspects in many key elementary steps of natural photosynthesis, and further studies are needed to fully understand this concept. The studies are based on redox active phenols with intramolecular hydrogen bonds to quinolines. The compounds thus bear a strong resemblance to the tyrosine/histidine couple in photosystem II, i.e. the water-plastoquinone oxidoreductase enzyme in photosynthesis. The design of the biomimetic models is such that the distance between the proton donor and acceptor is varied, enabling studies on the effect the proton transfer distance has on the rate of proton-coupled electron transfer reactions. The results of the studies have implications for the development of artificial photosynthesis, in particular in connection with redox leveling, charge accumulation, as well as electron and proton transfer.

In addition to these two contributions, the excited-state dynamics of the intramolecular hydrogen-bonded phenols was investigated, thereby revealing design principles for technological applications based on excited-state intramolecular proton transfer and photoinduced tautomerization.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 104 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1258
Keyword [en]
Solar energy conversion, artificial photosynthesis, hydrogen bonds, excited state intramolecular proton transfer, tautomerization, proton-coupled electron transfer, photosensitizer, ruthenium complex.
National Category
Organic Chemistry Physical Chemistry Inorganic Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-251471ISBN: 978-91-554-9253-3 (print)OAI: oai:DiVA.org:uu-251471DiVA: diva2:806239
Public defence
2015-06-12, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2015-05-22 Created: 2015-04-19 Last updated: 2015-07-07
List of papers
1. Tuning the Electronics of Bis(tridentate)ruthenium(II) Complexes with Long-Lived Excited States: Modifications to the Ligand Skeleton beyond Classical Electron Donor or Electron Withdrawing Group Decorations
Open this publication in new window or tab >>Tuning the Electronics of Bis(tridentate)ruthenium(II) Complexes with Long-Lived Excited States: Modifications to the Ligand Skeleton beyond Classical Electron Donor or Electron Withdrawing Group Decorations
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2013 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 52, no 9, 5128-5137 p.Article in journal (Refereed) Published
Abstract [en]

A series of homoleptic bis(tridentate) [Ru-(L)(2)](2+) (1, 3) and heteroleptic [Ru(L)(dqp)](2+) complexes (2, 4) [L = dqxp (1, 2) or dNinp (3, 4); dqxp = 2,6-di(quinoxalin-5-yl)pyridine, dNinp = 2,6-di(N-7-azaindol-1-yl)pyridine, dqp = 2,6-di(quinolin-8-yl)pyridine) was prepared and in the case of 2 and 4 structurally characterized. The presence of dqxp and dNinp in 1-4 result in anodically shifted oxidation potentials of the Ru3+/2+ couple compared to that of the archetypical [Ru(dqp)(2)](2+) (5), most pronounced for [Ru(dqxp)(2)](2+) (1) with a shift of +470 mV. These experimental findings are corroborated by DFT calculations, which show contributions to the complexes' HOMOs by the polypyridine ligands, thereby stabilizing the HOMOs and impeding electron extraction. Complex 3 exhibits an unusual electronic absorption spectrum with its lowest energy maximum at 382 nm. TD-DFT calculations suggest that this high-energy transition is caused by a localization of the LUMO on the central pyridine fragments of the dNinp ligands in 3, leaving the lateral azaindole units merely spectator fragments. The opposite is the case in 1, where the LUMO experiences large stabilization by the lateral quinorralines. Owing to the differences in LUMO energies, the complexes' reduction potentials differ by about 900 mV [E-1/2(1(2+/1+)) = -1.17 V, E-c,E-p(3(2+/1+)) = -2.06 V vs Fc(+/0)]. As complexes 1-4 exhibit similar excited state energies of around 1.80 V, the variations of the lateral heterocycles allow the tuning of the complexes' excited state oxidation strengths over a range of 900 mV. Complex 1 is the strongest excited state oxidant of the series, exceeding even [Ru(bpy)(3)](2+) by more than 200 mV. At room temperature, complex 3 is nonemissive, whereas complexes 1, 2, and 4 exhibit excited state lifetimes of 255, 120, and 1570 ns, respectively. The excited state lifetimes are thus somewhat shortened compared to that of 5 (3000 ns) but still acceptable to qualify the complexes as photosensitizers in light-induced charge-transfer schemes, especially for those that require high oxidative power.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-202924 (URN)10.1021/ic400009m (DOI)000318669400050 ()
Available from: 2013-07-01 Created: 2013-07-01 Last updated: 2017-12-06Bibliographically approved
2. Synthesis of Phenol-Quinolines with Intramolecular Hydrogen-Bonds of Different Strength
Open this publication in new window or tab >>Synthesis of Phenol-Quinolines with Intramolecular Hydrogen-Bonds of Different Strength
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(English)Manuscript (preprint) (Other academic)
Keyword
Hydrogen-bond strength, phenols, stereochemistry, Friedländer annulation, proton-coupled electron transfer
National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-251469 (URN)
Available from: 2015-04-19 Created: 2015-04-19 Last updated: 2015-07-07
3. Control over Excited State Intramolecular Proton Transfer and Photoinduced Tautomerization: Influence of the Hydrogen-Bond Geometry
Open this publication in new window or tab >>Control over Excited State Intramolecular Proton Transfer and Photoinduced Tautomerization: Influence of the Hydrogen-Bond Geometry
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2015 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 21, no 17, 6362-6366 p.Article in journal (Refereed) Published
Abstract [en]

The influence of H-bond geometry on thedynamics of excited state intramolecular proton transfer(ESIPT) and photoinduced tautomerization in a series ofphenol-quinoline compounds is investigated. Control overthe proton donor–acceptor distance (dDA) and dihedralangle between the proton donor–acceptor subunits isachieved by introducing methylene backbone straps ofincreasing lengths to link the phenol and quinoline. Wedemonstrate that a long dDA correlates with a higher barrierfor ESIPT, while a large dihedral angle opens highlyefficient deactivation channels after ESIPT, preventing theformation of the fully relaxed tautomer photoproduct.

Keyword
excited state, fluorescence, hydrogen bonds, intramolecular proton transfer, phenols, tautomerism
National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-251468 (URN)10.1002/chem.201500244 (DOI)000352796100008 ()
Available from: 2015-04-19 Created: 2015-04-19 Last updated: 2017-12-04Bibliographically approved
4. A Study of Concerted Proton-Coupled Electron Transfer as a Function of Intramolecular Proton Tunneling Distance
Open this publication in new window or tab >>A Study of Concerted Proton-Coupled Electron Transfer as a Function of Intramolecular Proton Tunneling Distance
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(English)Manuscript (preprint) (Other academic)
Keyword
proton-coupled electron transfer, phenols, quinolines
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-251470 (URN)
Available from: 2015-04-19 Created: 2015-04-19 Last updated: 2015-07-07

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