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Title: Membrane-organized Chemical Photoredox Systems

Abstract

The key photoredox process in photosynthesis is the accumulation of oxidizing equivalents on a tetranuclear manganese cluster that then liberates electrons and protons from water and forms oxygen gas. Our primary goal in this project is to characterize inorganic systems that can perform this same water-splitting chemistry. One such species is the dinuclear ruthenium complex known as the blue dimer. Starting at the Ru(III,III) oxidation state, the blue dimer is oxidized up to a putative Ru(V,V) level prior to O-O bond formation. We employ electron paramagnetic resonance spectroscopy to characterize each step in this reaction cycle to gain insight into the molecular mechanism of water oxidation.

Authors:
 [1]
  1. Univ. of California, Davis, CA (United States)
Publication Date:
Research Org.:
Univ. of California, Davis, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1315212
Report Number(s):
DE-SC0004334
DOE Contract Number:
SC0004334
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Britt, R. David. Membrane-organized Chemical Photoredox Systems. United States: N. p., 2016. Web. doi:10.2172/1315212.
Britt, R. David. Membrane-organized Chemical Photoredox Systems. United States. doi:10.2172/1315212.
Britt, R. David. 2016. "Membrane-organized Chemical Photoredox Systems". United States. doi:10.2172/1315212. https://www.osti.gov/servlets/purl/1315212.
@article{osti_1315212,
title = {Membrane-organized Chemical Photoredox Systems},
author = {Britt, R. David},
abstractNote = {The key photoredox process in photosynthesis is the accumulation of oxidizing equivalents on a tetranuclear manganese cluster that then liberates electrons and protons from water and forms oxygen gas. Our primary goal in this project is to characterize inorganic systems that can perform this same water-splitting chemistry. One such species is the dinuclear ruthenium complex known as the blue dimer. Starting at the Ru(III,III) oxidation state, the blue dimer is oxidized up to a putative Ru(V,V) level prior to O-O bond formation. We employ electron paramagnetic resonance spectroscopy to characterize each step in this reaction cycle to gain insight into the molecular mechanism of water oxidation.},
doi = {10.2172/1315212},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 9
}

Technical Report:

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  • This project has three interrelated goals relevant to solar water photolysis, which are to develop: (1) vesicle-organized assemblies for H2 photoproduction that utilize pyrylium and structurally related compounds as combined photosensitizers and cyclic electroneutral transmembrane electron carriers; (2) transmembrane redox systems whose reaction rates can be modulated by light; and (3) homogeneous catalysts for water oxidation. In area (1), initial efforts to photogenerate H2 from vectorially-organized vesicles containing occluded colloidal Pt and commonly available pyrylium ions as transmembrane redox mediators were unsuccessful. New pyrylium compounds with significantly lower reduction potentials have been synthesized to address this problem and their apparentmore » redox potentials in functioning systems have been now evaluated by using a series of occluded viologens. These studies provide an estimate of thermodynamic constraints imposed by these assemblies on hydrogen photoproduction. In area (2), spirooxazine-quinone dyads have been synthesized and their capacity to function as redox mediators across bilayer membranes has been evaluated through continuous photolysis and transient spectrophotometric measurements. These studies provide information on how quinone pools transfer charge in biomimetic systems designed to store solar energy as transmembrane electrochemical gradients. Photoisomerization of the spiro moiety to the ring-open mero form caused net quantum yields to decrease significantly, providing a basis for photoregulation of transmembrane redox; unexpectedly, both electrogenic and electroneutral pathways were observed, which were dependent upon the isomeric state of the chromophore (mero vs. spiro) and quinone substituent groups. Research on water oxidation (area 3) has been directed at understanding mechanisms of catalysis by cis,cis-[(bpy)2Ru(OH2)]2O4+ and related polyimine complexes. Using a variety of physical techniques, we have: (i) identified the redox state of the complex ion that is catalytically active; (ii) shown using 18O isotopic labeling that there are two reaction pathways, both of which involve participation of solvent H2 O; and (iii) detected by EPR and resonance Raman spectroscopies new species which may be key intermediates in the catalytic cycle. Analogs containing substituted bipyridine ligands have been synthesized to probe molecular details of these reactions whose understanding is necessary for rational design of superior catalysts.« less
  • This project has three interrelated goals relevant to solar water photolysis, which are to develop: (1) vesicle-organized assemblies for H2 photoproduction that utilize pyrylium and structurally related compounds as combined photosensitizers and cyclic electroneutral transmembrane electron carriers; (2) transmembrane redox systems whose reaction rates can be modulated by light; and (3) homogeneous catalysts for water oxidation. . In area (1), initial efforts to photogenerate H2 from vectorially-organized vesicles containing occluded colloidal Pt and commonly available pyrylium ions as transmembrane redox mediators were unsuccessful. New pyrylium compounds with significantly lower reduction potentials have been synthesized to address this problem, their apparentmore » redox potentials in functioning systems have been now evaluated by using a series of occluded viologens, and H2 photoproduction has been demonstrated in continuous illumination experiments. In area (2), spirooxazine-quinone dyads have been synthesized and their capacity to function as redox mediators across bilayer membranes has been evaluated through continuous photolysis and transient spectrophotometric measurements. Photoisomerization of the spiro moiety to the ring-open mero form caused net quantum yields to decrease significantly, providing a basis for photoregulation of transmembrane redox. Research on water oxidation (area 3) has been directed at understanding mechanisms of catalysis by cis,cis-[(bpy)2Ru(OH2)]2O4+ and related polyimine complexes. Using a variety of physical techniques, we have: (i) identified the redox state of the complex ion that is catalytically active; (ii) shown using 18O isotopic labeling that there are two reaction pathways, both of which involve participation of solvent H2O; and (iii) detected and characterized by EPR and resonance Raman spectroscopies new species which may be key intermediates in the catalytic cycle.« less
  • We have developed a system for carrying out chlorophyll (chl)- photosensitized vectorial transbilayer electron transfer from reduced cytochrome c (cyt) in the inner aqueous compartment of negatively charged unilamellar lipid bilayer vesicles to oxidized ferridoxin (fd) in the outer aqueous phase, with the viologen analog propylene diquat in the outer phase as a mediator. This was investigated using both laser flash and steady state photolysis techniques. The results demonstrate that triplet chl is initially quenched by viologen at the outer membrane surface to form chl cation radical and viologen radical, followed by a biphasic recombination. The slow phase represents reversemore » electron transfer and could be suppressed by reduction of the chl radical by reduced cyt at the inner vesicle surface, following transbilayer electron transfer, and by electron transfer from viologen radical to oxidized fd. These reactions lead to charge separation across the vesicular membrane. The yields are limited by the formation of the transmembrane potential and accumulation of oxidized cyt in the lumen of the vesicle. Addition of the ionophore valinomycin will diminish the membrane potential, and double the reaction yield. It is important to note that this system mimics one of the key events in photosynthesis (Photosystem 1) and results in appreciable energy storage in the reaction products (about 0.7 V). Reduction of cyt has been investigated at a Pt electrode modified with a lipid bilayer membrane with immobilized vinyl ferrocene as a mediator. The current-voltage curves show that the direct reduction of cyt at the electrode occurs quite efficiently, allowing us to calculate an absorption equilibrium constant and an electron transfer rate constant. These results suggest that biomembrane-like'' electrode surfaces have potential for metalloprotein electrochemistry, as well as the development of biosensors.« less
  • Most of our effort during the past grant period has been directed towards investigating electron transfer processes involving redox proteins at lipid bilayer/aqueous interfaces. This theme, as was noted in our previous three year renewal proposal, is consistent with our goal of developing biomimetic solar energy conversion systems which utilize the unique properties of biological electron transfer molecules. Thus, small redox proteins such as cytochrome c, plastocyanin and ferredoxin function is biological photosynthesis as mediators of electron flow between the photochemical systems localized in the membrane, and more complex soluble or membrane-bound redox proteins which are designed to carry outmore » specific biological tasks such as transbilayer proton gradient formation, dinitrogen fixation, ATP synthesis, dihydrogen synthesis, generation of strong reductants, etc. In these studies, we have utilized two principal experimental techniques, laser flash photolysis and cyclic voltammetry, both of which permit direct measurements of electron transfer processes.« less
  • This project has received DOE support since July 1, 1978. During this period, 40 papers have been published dealing predominantly with chlorophyll-photosensitized electron transfer reactions in a variety of media (solutions, polymer films, lipid bilayer membranes). The overall theme of this work has been to develop mechanistic strategies f or photochemical energy storage via chlorophyll, using the green plant photosynthetic system as a paradigm for designing in vitro systems. Microheterogeneous lipid vesicle suspensions allow ready application of time-resolved optical spectroscopy to follow the course of light-induced electron transfer processes. Both the yields and the lifetimes of electron transfer products weremore » markedly improved in the vesicle systems. In subsequent studies, this compartmentalization was favorably manipulated by controlling the electrical charge on the membrane surface, by controlling the solubility properties of the acceptors, by varying the lipid composition, by using mediators to create a concentration gradient to carry electrons from within the bilayer to the aqueous medium, and by incorporating secondary electron acceptors into the aqueous phase.« less