Electronic spectroscopy and photochemistry of organic compounds: 1. Aryloxiranes and arylcyclopropanes; 2. Photochromic hydrazones



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  1. Aryloxiranes and Arylcyclopropanes In view of the considerable study recently devoted to the photochemistry of phenyl-substituted oxiranes, cyclopropanes, and other three-membered ring compounds, the present research was carried out, investigating the electronic spectroscopy and photochemistry of some aryloxiranes and arylcyclopropanes at -196[degrees]. Irradiation of the oxiranes into the 0-0 or higher energy absorption bands causes formation of a colored intermediate. Subsequently, the parent alkene, one or two arylcarbenes and one or two carbonyl compounds are formed depending on whether or not the oxirane is symmetrically substituted. A consistent mechanistic pathway is proposed to explain the photochemistry and spectroscopy of the oxiranes and their photoproducts. Unique spectroscopic and photochemical results were obtained for the phenylcyclopropanes. Excitation into the 0-0 or higher energy absorption band of the higher-substituted cyclopropanes resulted in a yellow phosphorescence. The remaining cyclopropanes exhibit a normal fluorescence. An exciton model was applied to explain the observation of the phosphorescence. In marked contrast to the aryloxiranes, the phenylcyclopropanes did not undergo irreversible photochemistry. 2. Photochromic Hydrazones It has been noted that the photochromicity of several classes of compounds could be explained via a mechanism centering around a six-atom portion of the molecule. This portion of the research was carried out to provide a brief study of photochromism of hydrazones in hydrocarbon solvents at -196[degrees], compare the results to reported results for the solid hydrazones, and attempt to determine whether the generalized mechanistic pathway might be applicable for these systems. Five phenylhydrazones were detected, by absorption spectroscopy, to photochemically yield a "colored" form. A decrease in the absorption intensity of the parent hydrazone was observed as the "colored" form is generated. In all cases, the "colored" form was observed to disappear upon warming the solution to room temperature, and for those systems checked, the absorption intensity was at least partially restored. Three mechanistic pathways were considered as possible explanations of these results; two were excluded based on comparison of information from this study with published information. Based on data from the literature, an analytic procedure was devised and applied to predict absorption values for model compounds closely related to the species postulated to be the "colored" forms. A good correlation was observed between these predicted values and the observed absorptions of the "colored" forms, providing strong support for the generalization of the involvement of a six-atom portion of the molecule in the photochromism process.