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Theses

Modélisation de Processus Photo induits du Photosystem II

Abstract : In natural photosynthesis, light energy is converted into chemical energy by photosynthetic reaction centers. This energy is stored in the form of high energy substances synthesized in the reductive branch of the photosynthetic process. The electrons needed for these processes are furnished by water upon its oxidation by the Oxygen Evolving Complex (OEC) in PSII.
Artificial photosynthesis aims to replicate the reactions that take place in natural organisms in order to i) gain a better understanding of the chemical processes taking place in the natural systems, and ii) strive towards the harnessing of sunlight in order to have access to clean, sustainable fuels. Processes undergone in nature such as light capture, energy transfer, electron transfer, charge separation, activation of catalyst, and reaction at the catalytic site must be accomplished within the framework of artificial systems.
With these concepts in mind we have designed, synthesized and characterized molecules that mimic the reactions performed by antennas and reaction centers present in Photosystem II. These molecules are able to undergo light-induced charge separation, electron transfer, and accumulation of oxidizing/reducing equivalents that mimic the processes occurring in natural systems. The artificial anntenas are composed of carotenoid and phthalocyanin groups. These molecules show large absoption profiles with high extinction coefficients, and are capable of ultra-fast energy transfer processes which lead up to charge separation states. Varying the conjugation length of the carotenoid molecules from 9 double bonds to 11 double bonds, we can show how these molecules may act as energy donors as well as energy dissipators, a process akin to the Non Photochemical Quenching (NPQ) processes which happen during the xanthophyll cycle. The donor side mimics of Photosystem II have also been studied. These supramolecular systems contain a photoactive component covalently linked through a spacer to a cavity where a metal ion or cluster is located. The photosensitizer used is a [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) analogue, a counterpart to P680, which absorbs light in the visible region and triggers an electron transfer process. The resulting RuIII species has a reversible oxidation potential of 1.30 V vs. SCE, similar to that of P680 (1.25 V vs NHE),17,18 and is, in theory, capable of oxidizing a Manganese cluster and an electron source. Among the molecules mimicking the donor side of PSII we have synthesized ruthenium-phenol pairs, as well as bimetallic Ruthenium-Manganese systems. Among the latter we have studied those with terpyridine coordination cavities since Mn-di-μ-oxo-Mn dimers of this kind have been reported to catalyze the oxidation of water into molecular oxygen. Other catalytic groups such as salens and salophens have also been studied. These have been reported to perform the two electron oxidation of organic substrates following the same oxygen atom transfer mechanism as the one thought to be responsible for the oxidation of water, and could therefore be useful in achieving our goal. Following the work on synthesis and characterization of molecules capable of harnessing light and using it to drive reduction/oxidation reactions will be presented.
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Submitted on : Wednesday, February 25, 2009 - 3:32:09 PM
Last modification on : Monday, February 10, 2020 - 6:12:52 PM
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  • HAL Id : tel-00364271, version 1

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Christian Herrero Moreno. Modélisation de Processus Photo induits du Photosystem II. Chimie. Université Paris Sud - Paris XI, 2007. Français. ⟨tel-00364271⟩

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