Studies of Photochemical and Photophysical Processes: I. The Photochemistry of 3-Methyl-1-Phenoxybut-2-Ene. II. The Role of Charge Transfer Interactions in the Quenching of 1,4-Dimethoxybenzene Fluorescence. III. A New Method for the Determination of Intersystem Crossing Quantum Yields

Citation

Carroll, Felix Alvin (1973) Studies of Photochemical and Photophysical Processes: I. The Photochemistry of 3-Methyl-1-Phenoxybut-2-Ene. II. The Role of Charge Transfer Interactions in the Quenching of 1,4-Dimethoxybenzene Fluorescence. III. A New Method for the Determination of Intersystem Crossing Quantum Yields. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/WSAF-0X70. https://resolver.caltech.edu/CaltechETD:etd-02102009-101143

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. PART I: The photochemical reactions 3-methyl-I -phenoxybut-2-ene [(8)] have been investigated in solution and in the gas phase. Irradiation of [8] results in the formation of phenol [(9)], 3-methy1-3-phenoxybutene[(10)], 1-(2-hydroxypheny1)-3-methylbut-2-ene [(11)], 1-(4 -hydroxypheny1)-3-methylbut-2-ene [(12)], 3-(2-hydroxyphenyl)-3-methylbutene [(13)], and 3-(4-hydroxyphenyl)-3-methylbutene [(14)]. Formation of [10-14] in the gas phase is efficiently quenched by nitric oxide. The results suggest that all products are formed by a dissociation-radical recombination mechanism and that none are formed by concerted processes. The absorption spectrum of [8] and its quantum yields of photochemical reaction and fluorescence are used to estimate the rates of the processes which deactivate its lowest-lying excited singlet state. The results suggest that O-C bond homolysis need not occur via intersystem crossing from an S[subscript 1](π,π*) to a [superscript 3]σ(C-O) state. Fragmentation to radicals, and perhaps nonradiative decay without chemical reaction, probably involves excitation to high vibrational levels of the C-O bond. Weak electronic interaction between the aromatic and olefinic chromophores may also be important. PART II: The fluorescence of 1,4-dimethoxybenzene (DMB) in acetonitrile is quenched by a number of organic compounds having lowest-lying excited singlets higher in energy than that of DMB. The rate constants show a structure-reactivity pattern which implicates charge transfer as an integral process, but they do not agree qualitatively with rate constants predicted from a kinetic formulation involving the intermediacy of radical ion pairs. The results suggest that charge transfer provides the binding energy of the exciplex, while the internal conversion of electronic excitation involves a vibronic mechanism which depends primarily on the type of quencher (organic chloride, acetate, or mesylate) and not on its reduction potential. PART III: A method is described for the determination of intersystem crossing quantum yields which is especially useful for benzene derivatives. The experimental procedure involves measurement of relative fluorescence intensities of the aromatic compound and relative yields of isomerization of a "triplet counter" in solutions with varying concentrations of a heavy atom fluorescence quencher such as xenon. Intersystem crossing quantum yields for anisole, mesitylene, and toluene in isooctane solution are 0.74, 0.60, and 0.52, respectively.

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