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Electron Transfer in Ruthenium-Manganese Complexes for Artificial Photosynthesis: Studies in Solution and on Electrode Surfaces
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Chemistry, Department of Physical Chemistry.
2001 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In today’s society there is an increasing need for energy, an increase which for the most part is supplied by the use of fossil fuels. Fossil fuel resources are limited and their use has harmful effects on the environment, therefore the development of technologies that produce clean energy sources is very appealing. Natural photosynthesis is capable of converting solar energy into chemical energy through a series of efficient energy and electron transfer reactions with water as the only electron source. Thus, constructing an artificial system that uses the same principles to convert sunlight into electricity or storable fuels like hydrogen is one of the major forces driving artificial photosynthesis research.

This thesis describes supramolecular complexes with the intention of mimicking the electron transfer reactions of the donor side in Photosystem II, where a manganese cluster together with a tyrosine catalyses the oxidation of water. All complexes are based on Ru(II)-trisbipyridine as a photosensitizer that is covalently linked to electron donors like tyrosine or manganese. Photochemical reactions are studied with time-resolved transient absorption and emission measurements. Electrochemical techniques are used to study the electrochemical behavior, and different photoelectrochemical techniques are used to investigate the complexes adsorbed onto titanium dioxide surfaces. In all complexes, intramolecular electron transfer occurs from the linked donor to photo-oxidized Ru(III). It is also observed that coordinated Mn(II) quenches the excited state of Ru(II), a reaction that is found to be distance dependent. However, by modifying one of the complexes, its excited state properties can be tuned in a way that decreases the quenching and keeps the electron transfer properties. The obtained results are of significance for the development of multinuclear Ru-Mn complexes that are capable of multi-electron transfer.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2001. , p. 69
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 669
Keywords [en]
Physics, Artificial photosynthesis, electron transfer, energy transfer, ruthenium, manganese, titanium dioxide
Keywords [sv]
Fysik
National Category
Physical Sciences
Research subject
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-1468ISBN: 91-554-5154-3 (print)OAI: oai:DiVA.org:uu-1468DiVA, id: diva2:161011
Public defence
2001-11-09, The Svedberg Lecture Hall, Institute of Chemistry, Uppsala University, Uppsala, 10:15
Opponent
Available from: 2001-10-19 Created: 2001-10-19Bibliographically approved
List of papers
1. Ruthenium-Manganese Complexes for Artificial Photosynthesis: Factors Controlling Intramolecular Electron Transfer and Excited State Quenching Reactions
Open this publication in new window or tab >>Ruthenium-Manganese Complexes for Artificial Photosynthesis: Factors Controlling Intramolecular Electron Transfer and Excited State Quenching Reactions
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2002 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 41, no 6, p. 1534-1544Article in journal (Refereed) Published
Abstract [en]

Continuing our work toward a system mimicking the electron-transfer steps from manganese to P(680)(+) in photosystem II (PS II), we report a series of ruthenium(II)-manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (k(ET)) values span a large range, 1 x 10(5)-2 x 10(7) s(-1), and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally (lambda = 1.5-2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically lambda approximately 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)-ruthenium(III) electron-transfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-89491 (URN)10.1021/ic0107227 (DOI)11896722 (PubMedID)
Available from: 2001-10-19 Created: 2001-10-19 Last updated: 2017-12-14Bibliographically approved
2. A Biomimetic Model System for the Water Oxidizing Triad in Photosystem II
Open this publication in new window or tab >>A Biomimetic Model System for the Water Oxidizing Triad in Photosystem II
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1999 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 121, no 1, p. 89-96Article in journal (Refereed) Published
Abstract [en]

In plants, solar energy is used to extract electrons from water, producing atmospheric oxygen. This is conducted by Photosystem II, where a redox ”triad” consisting of chlorophyll, a tyrosine, and a manganese cluster, governs an essential part of the process. Photooxidation of the chlorophylls produces electron transfer from the tyrosine, which forms a radical. The radical and the manganese cluster together extract electrons from water, providing the biosphere with an unlimited electron source. As a partial model for this system we constructed a ruthenium(II) complex with a covalently attached tyrosine, where the photooxidized ruthenium was rereduced by the tyrosine. In this study we show that the tyrosyl radical, which gives a transient EPR signal under illumination, can oxidize a manganese complex. The dinuclear manganese complex, which initially is in the Mn(III)/(III) state, is oxidized by the photogenerated tyrosyl radical to the Mn(III)/(IV) state. The redox potentials in our system are comparable to those in Photosystem II. Thus, our synthetic redox “triad” mimics important elements in the electron donor ”triad” in Photosystem II, significantly advancing the development of systems for artificial photosynthesis based on ruthenium−manganese complexes.

Keywords
Electron-Transfer, Y-Z, Photosynthesis, Mechanism, Complexes, Tyrosine, Oxygen, Light
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-89492 (URN)10.1021/ja981494r (DOI)
Available from: 2001-10-19 Created: 2001-10-19 Last updated: 2017-12-14
3. Hydrogen-Bond Promoted Intramolecular Electron Transfer to Photogenerated Ru(III): A Functional Mimic of TyrosineZ and Histidine 190 in Photosystem II
Open this publication in new window or tab >>Hydrogen-Bond Promoted Intramolecular Electron Transfer to Photogenerated Ru(III): A Functional Mimic of TyrosineZ and Histidine 190 in Photosystem II
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1999 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 121, no 29, p. 6834-6842Article in journal (Refereed) Published
Abstract [en]

As a model for redox components on the donor side of photosystem II (PS II) in green plants, a supramolecular complex 4 has been prepared. In this, a ruthenium(II) tris-bipyridyl complex which mimics the function of P680 in PS II, has been covalently linked to a tyrosine unit which bears two hydrogen-bonding substituents, dipicolylamine (dpa) ligands. Our aim is to mimic the interaction between tyrosineZ and a basic histidine residue, namely His190 in PSII, and also to use the dpa ligands for coordination of manganese. Two different routes for the synthesis of the compound 4 are presented. Its structure was fully characterized by 1H NMR, COSY, NOESY, 13C NMR, IR, and mass spectrometry. 1H NMR and NOESY gave evidence for the existence of intramolecular hydrogen bonding in 4. The interaction between the ruthenium and the substituted tyrosine unit was probed by steady-state and time-resolved emission measurements as well as by chemical oxidation. Flash photolysis and EPR measurements on 4 in the presence of an electron acceptor (methylviologen, MV2+, or cobalt pentaminechloride, Co3+) showed that an intermolecular electron transfer from the excited state of Ru(II) in 4 to the electron acceptor took place, forming Ru(III) and the methylviologen radical MV+ or Co2+. This was followed by intramolecular electron transfer from the substituted tyrosine moiety to the photogenerated Ru(III), regenerating Ru(II) and forming a tyrosyl radical. In water, the radical has a g value of 2.0044, indicative of a deprotonated tyrosyl radical. In acetonitrile, a radical with a g value of 2.0029 was formed, which can be assigned to the tyrosine radical cation. In both solvents the electron transfer is intramolecular with a rate constant kET > 1 × 107 s-1. This is 2 orders of magnitude greater than the one for a similar compound 3, in which no dpa arm is attached to the tyrosine unit. Therefore the hydrogen bonding between the substituted tyrosine and the dpa arms in 4 is proposed to be responsible for the fast electron transfer. This interaction mimics the proposed His190 and tyrosineZ interaction in the donor side of PS II.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-89493 (URN)10.1021/ja984048c (DOI)
Available from: 2001-10-19 Created: 2001-10-19 Last updated: 2017-12-14
4. Towards an artificial model for Photosystem II: A manganese(II,II) dimer covalently linked to ruthenium(II) tris-bipyridine via a tyrosine derivative
Open this publication in new window or tab >>Towards an artificial model for Photosystem II: A manganese(II,II) dimer covalently linked to ruthenium(II) tris-bipyridine via a tyrosine derivative
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2000 (English)In: Journal of Inorganic Biochemistry, ISSN 0162-0134, E-ISSN 1873-3344, Vol. 78, no 1, p. 15-22Article in journal (Refereed) Published
Abstract [en]

In order to model the individual electron transfer steps from the manganese cluster to the photooxidized sensitizer P680+ in Photosystem II (PS II) in green plants, the supramolecular complex 4 has been synthesized. In this complex, a ruthenium(II) tris-bipyridine type photosensitizer has been linked to a manganese(II) dimer via a substituted L-tyrosine, which bridges the manganese ions. The trinuclear complex 4 was characterized by electron paramagnetic resonance (EPR) and electrospray ionization mass spectrometry (ESI-MS). The excited state lifetime of the ruthenium tris-bipyridine moiety in 4 was found to be about 110 ns in acetonitrile. Using flash photolysis in the presence of an electron acceptor (methylviologen), it was demonstrated that in the supramolecular complex 4 an electron was transferred from the excited state of the ruthenium tris-bipyridine moiety to methylviologen, forming a methylviologen radical and a ruthenium(III) tris-bipyridine moiety. Next, the Ru(III) species retrieved the electron from the manganese(II/II) dimer in an intramolecular electron transfer reaction with a rate constant kET > 1.0 x 10(7) s(-1), generating a manganese(II/III) oxidation state and regenerating the ruthenium(II) photosensitizer. This is the first example of intramolecular electron transfer in a supramolecular complex, in which a manganese dimer is covalently linked to a photosensitizer via a tyrosine unit, in a process which mimics the electron transfer on the donor side of PS II.

Keywords
ruthenium complexes, manganese dimer complexes, artificial photosynthesis, electron transfer, photosystem II, ELECTRON-TRANSFER, MNIIMNIII COMPLEXES, PHOTOSYNTHESIS, PROTEINS, CATALASE, SPECTRA, ENZYMES, SYSTEMS, REDOX
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-89494 (URN)10.1016/S0162-0134(99)00200-7 (DOI)10714701 (PubMedID)
Available from: 2001-10-19 Created: 2001-10-19 Last updated: 2017-12-14
5. Ruthenium Trisbipyridyl Complexes Covalently Linked to Phenolate Ligands that Coordinates Manganese
Open this publication in new window or tab >>Ruthenium Trisbipyridyl Complexes Covalently Linked to Phenolate Ligands that Coordinates Manganese
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Article in journal (Refereed) In press
Identifiers
urn:nbn:se:uu:diva-89495 (URN)
Available from: 2001-10-19 Created: 2001-10-19 Last updated: 2010-01-14Bibliographically approved
6. Electron Transfer Kinetics for Ruthenium-Manganese Complexes Adsorbed onto Nanocrystalline TiO2 Films
Open this publication in new window or tab >>Electron Transfer Kinetics for Ruthenium-Manganese Complexes Adsorbed onto Nanocrystalline TiO2 Films
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2001 (English)Article in journal (Refereed) In press
Identifiers
urn:nbn:se:uu:diva-89496 (URN)
Available from: 2001-10-19 Created: 2001-10-19 Last updated: 2014-01-24Bibliographically approved

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