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Title
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A STUDY OF THE PHOTOINDUCED REDOX CHEMISTRY OF TRIS(2,2'-BIPYRIDINE)RUTHENIUM(II) IN: (I) AN EDTA / PARAQUAT SOLAR ENERGY CONVERSION SYSTEM AND (II) A POROUS GLASS MATRIX.
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Identifier
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AAI8023713
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identifier
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8023713
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Creator
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KENNELLY, TERESA.
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Contributor
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Harry D. Gafney
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Date
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1980
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemistry, Inorganic | Energy
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Abstract
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The complex tris(2,2'-bipyridine)ruthenium(II)('2+), Ru(bpy)(,3)('2+), absorbs strongly in the visible region to form an excited state, 2.1 eV above its ground state, which has sufficient potential to oxidize and reduce water. Despite this strong driving force, however, photolysis of aqueous solutions of Ru(bpy)(,3)('2+) does not lead to the photodissociation of water. The use of excited state quenchers, which oxidize or reduce water in the presence of catalyst, is impeded by a rapid back reaction. Modification of the reactants by ion pairing Ru(bpy)(,3)('2+) with an electron donor, EDTA('2-), prevented this energy wasting back reaction. The primary photoproduct Ru(bpy)(,3)('3+) was reduced by EDTA('2-) and the reduced quencher Pq.('+), paraquat radical cation, reduced water to H(,2) in the presence of platinum. A quantum yield of 0.05 was measured for the formation of Pq.('+).;The postulated ion pairing mechanism suggests that organization of the reactants makes the forward reaction competitive with the energy dissipating back reaction. In photosynthesis, reactants are organized in a heterogeneous environment. The Ru(bpy)(,3)('2+) / Pq('2+) / EDTA('2-) reaction system was adsorbed onto a glass matrix to determine the effect of non-homogeneity on the efficiency of the photoreduction of water. Preliminary experiments indicated that the photo-chemistry of Ru(bpy)(,3)('2+) on the glass was different than that observed in solution and a more detailed study of the photoredox chemistry of this complex on the glass was undertaken.;Photolysis of Ru(bpy)(,3)('2+) on glass induced spectral changes which were identical to those previously reported for the intense flash photolysis of aqueous solutions and photolysis of micellar solutions with high Ru(bpy)(,3)('2+) / micelle ratios. The striking difference was that the life-time of the photoproduct was increased from 5 seconds in fluid media to 91 days on the glass.;Lifetime measurements of the complex on the glass showed an enhancement of the radiative decay of ('*)Ru(bpy)(,3)('2+) comparable to that observed in micelles. At moderate concentrations, a two component decay was detected. A fast non-exponential decay (60 nsec) was followed by an exponential decay (680 (+OR-) 20 nsec) similar to that measured in homogeneous solution. At higher concentrations, only the fast decay was observed. This concentration dependence suggests that a direct disproportionation reaction competes with the.;2*Ru(bpy)(,3)('2+) Ru(bpy)(,3)('+) + Ru(bpy)(,3)('3+).;unimolecular decay pathways. The stoichiometry of the reaction in the glass, calculated from the absorbance changes at 510nm and 452nm, gave a value of 0.48 (+OR-) 0.05 for the ratio of the formation of Ru(bpy)(,3)('+) to the disappearance of Ru(bpy)(,3)('2+) compared to the theoretical value of 0.5. ESR spectra of photolyzed glasses revealed the formation of a radical species which increased in proportion to the 510nm absorbance. These data are in good agreement with a direct disproportionation reaction.;The disproportionation products, Ru(bpy)(,3)('+) and Ru(bpy)(,3)('3+), unlike the excited state, *Ru(bpy)(,3)('2+), react with water. Photodissociation of water via the photolysis of Ru(bpy)(,3)('2+) adsorbed onto glass may be a viable conversion system. Unlike the reaction schemes in homogeneous solution, no electron donor is consumed and a truly catalytic cycle may be developed. A limiting value of 2 x 10('-3) was measured for the formation of Ru(bpy)(,3)('+).
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
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Program
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Chemistry
