Photochemistry of alkylindenes in the gas phase
Abstract
The photochemistry of indene and several alkylindenes has been studied in the gas phase. This study has revealed rearrangements different from those operative in solution, as evidenced by the observation of photoproducts explicable in terms of net hydrogen and alkyl migrations. For example, gas phase irradiation of 1-methylindene produces 3-methylindene as the major product, in addition to 2-methylindene, while upon solution irradiation only 2-methylindene is produced. Compounds that are unreactive in solution, or that only result in dimerization or polymerization, such as 3-methylindene and 1,1-dimethylindene, respectively, are found to be reactive in the gas phase. Irradiation of 3-methylindene results in formation of 1-methyl and 2-methylindene, while 1,1-dimethylindene produces 2,3-dimethyl, 1,2-dimethyl and 1,3-dimethylindene. The reaction mechanism has been investigated, mainly via deuterium labelling studies and collisional deactivation by added inert gases. Irradiation of selectively deuterated indenes results in nearly statistical scrambling in the 5 membered ring of the indene skeleton. Reduced scrambling is observed when irradiation is performed in the presence of n-butane as a collisional quencher. These results are explained in terms of rearrangements via multiple 1,5 hydrogen and deuterium migrations, in addition to 1,5 alkyl migrations. The lack of triplet sensitization, quenching or enhancement indicates that the photochemistry is derived from a singlet state. Collisional deactivation with inert gases results in net quenching of all the photoproducts, with those exclusive to the gas phase quenched at a faster rate than the products observed in solution. In addition to causing a decrease in the photochemistry, collisional deactivation also results in an enhancement in emission intensity. Excitation with longer wavelengths also results in higher fluorescence efficiencies. The possible generation of the photoproducts from an S$\sb2$ state, upper vibrational levels of S$\sb1$, or upper vibrational levels of S$\sb0$ is discussed.
Degree
Ph.D.
Advisors
Morrison, Purdue University.
Subject Area
Organic chemistry
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