Excitation Transfer in Organic and Inorganic Systems

Citation

Winterle, John Scott (1976) Excitation Transfer in Organic and Inorganic Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8h2g-v390. https://resolver.caltech.edu/CaltechETD:etd-03112009-140839

Abstract

PART I: Iron(II) is photooxidized to iron(III) in 0.100 N sulfuric acid in the presence of the photosensitizer tris(2,2'-bipyridine)ruthenium(II) dichloride. Control experiments show that the oxidation must be partly ascribed to a reactive oxygen species generated by interaction of molecular oxygen (dioxygen) with the excited photosensitizer because the oxidation quantum yield varies in strict proportion to the fraction of sensitizer excited states quenched by dioxygen. The reaction was found to be dramatically pH dependent in the range studied (1 to 0). However, the oxidation quantum yield was insensitive to changes in Fe[superscript +2](aq.) in the region 4 x 10[superscript -2] to 2 x 10[superscript -4] M. In 0.100 N H[subscript 2]SO[subscript 4] at 25 ± 1°C, the quantum yield for Fe[superscript +3] (aq.) production is 0.077 when extrapolated to 100% sensitizer quenching. Quenching of the emissive MLCT state of Ru(bipy)[subscript 3 superscript +2 ] is known to occur by two different mechanisms, energy and charge transfer. In the latter case electron transfer to dioxygen would result in superoxide anion and oxidized sensitizer, both of which rapidly oxidize Fe[superscript +2] (aq.) at pH 1. That this simple mechanism is operative is ruled out by the observed pH dependence since HO[subscript 2], O[...] radical oxidations of iron (II) are acidity independent in the range studied. In view of these considerations it is proposed that hydroperoxyl radicals are produced upon protonation of a transient sensitizer - dioxygen complex which otherwise decays without oxidation-reduction. PART II: Trans[underscored], trans[underscored]-2, 4-hexadiene is isomerized to its geometrical isomers when dilute benzene solutions are γ-irradiated. Because the diene receives negligible primary excitation, a solvent-to-solute excitation transfer mechanism must be invoked to explain the isomerization. A comparison of the photochemistry and the radiation chemistry of the diene in benzene, demonstrates that the kinetic behavior of trans[underscored], trans[underscored]-2,4-hexadiene under γ-irradiation is consistent with the transfer of triplet solvent electronic excitation. The triplet yield is consistent with recent careful determinations. Collateral photochemical experiments demonstrate that the [...] state of benzene does not efficiently isomerize trans[underscored], trans[underscored]-2,4-hexadiene. Thus, the excess isomerization must be contributed by another intermediate. By a process of elimination, an upper excited benzene singlet state is identified as the excitation donor. This choice is confirmed by an excellent concordance of the data with the model for energy transfer. Theoretical predictions make such transfer possible, because this energy migration rate in benzene far exceeds that predicted by simple mass diffusion.

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