Part One. Thermal and Photosensitized Dimerizations of 1,3-Cyclohexadiene. Part Two. Photosensitized Isomerization of Stilbene: Further Studies

Citation

Valentine, Donald Herman (1966) Part One. Thermal and Photosensitized Dimerizations of 1,3-Cyclohexadiene. Part Two. Photosensitized Isomerization of Stilbene: Further Studies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8qsr-dn54. https://resolver.caltech.edu/CaltechETD:etd-09232002-104139

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Part One Thermal dimerization of 1, 3-cyclohexadiene gives 4:1 mixtures of endo and exo tricyclo[6.2.2.0(^2,7)]dodeca-4, 9-diene, 1 and 2. Photosensitized dimerization of cyclohexadiene gives 90% yields of three products in about 3:1:1 ratio. The major photodimer was shown in two ways to be trans-cis-trans-tricyclo[6.4.0.0(^2,7)]dodeca-3, 11-diene, 3. One of the minor photodimers was shown to be exo-dicyclohexadiene, 2; the other photodimer was assigned the structure cis-cis-cis-tricyclo[6.4.0.0(^2,7)]dodeca-3,11-diene,4. Photodimerization of 2-cyclohexenone and its photoaddition to cyclohexene are described. The mechanism of the photosensitized dimerization of cyclohexadiene is considered. Quantum yields for the sensitized dimerization are lower than the expected value by varying factors depending upon the particular sensitizer. This effect is interpreted in terms of formation of a sensitizer triple t-cyclohexadiene complex which can break apart either to yield sensitizer ground state plus diene triplet or the ground states of both species. The photoaddition of sensitizers to cyclohexadiene is discussed; it is concluded that in some cases at least the diene triplet is the reactive intermediate leading to sensitizer-diene adducts. Thermal rearrangements of the dicyclohexadienes are studied. At higher temperatures, 3 rearranges smoothly to give 1 as the only product, and 4 rearranges to give 2. Surprisingly, various catalysts are able to effect the reverse conversions at and below room temperature. Kinetic studies of these rearrangements are described as are heat of combustion measurements which show that the cyclobutanes 3 and 4 are less stable than the thermal dimers 1 and 2. A discussion of the significance of these results is also given. Part Two Reinvestigation of the Hammond-Saltiel description of the photosensitized isomerization of the stilbenes is reported. Most of the gross features of their theory are confirmed but many changes in detail are required by new data. Greater than 5 kcal/mole exothermicity in the energy transfer step is not, as previously reported, sufficient to insure that the process rate is diffusion controlled. A discussion of the meaning of a diffusion controlled process is given. Various factors are considered as possible sources of inefficiency in exothermic energy transfer. Quantum yield measurements for sensitized stilbene isomerization show there is only very limited wastage of quanta. Transfer of energy from sensitizers with triplet energies below 62 kcal/mole to the stilbenes and especially to cis-stilbene is governed not only by the sensitizer triplet energy but also by other factors which are characteristic of the sensitizer. These effects are discussed in terms of energy transfer to cis-stilbene via an intermediate complex of sensitizer triplet and stilbene. Results with sensitizers containing bromine indicate that these substances cause stilbene isomerization by light induced liberation of bromine atoms and not, in general, by electronic energy transfer. This finding casts doubt on some of the evidence adduced in support of nonvertical energy transfer to cis-stilbene. The nature and reactions of stilbene triplets are discussed with special reference to reversible energy transfer to trans-stilbene, azulene quenching effects and self quenching due to trans-stilbene. The implications of this study in the study of other photosensitized reactions are presented.

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