I. Photodimerizations of Coumarin. II. Asymmetric Induction During Energy Transfer
<p>Part I:</p> <p>Studies on the mechanism of the photodimerizations of coumarin are reported. Trans coumarin photodimers are shown to result from excited triplet coumarin. It seems likely that an intermediate triplet-ground state complex precedes a biradical intermediate which...
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| Format: | Others |
| Language: | en |
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1968
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| Online Access: | https://thesis.library.caltech.edu/278/1/Cole_rs_1968.pdf Cole, Ronald Sinclair (1968) I. Photodimerizations of Coumarin. II. Asymmetric Induction During Energy Transfer. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/KEEE-B108. https://resolver.caltech.edu/CaltechETD:etd-01222008-114929 <https://resolver.caltech.edu/CaltechETD:etd-01222008-114929> |
| Summary: | <p>Part I:</p>
<p>Studies on the mechanism of the photodimerizations of coumarin are reported. Trans coumarin photodimers are shown to result from excited triplet coumarin. It seems likely that an intermediate triplet-ground state complex precedes a biradical intermediate which closes to trans-dicoumarin. This dimer, formedby benzophenone sensitization is also produced by direct irradiation in benzene. Here coumarin has a low intersystem crossing yield of 2.2 x 10⁻². We account for 98% of the absorbed quanta by a photocleavage of coumarin singlets, competing with the other modes of unimolecular decay. No evidence supporting singlet self quenching was found. Singlet energy transfer from benzophenone to coumarin occurs by emission-reabsorption. The low efficiency of the latter process makes it insignificant compared to competing mechanisms.</p>
<p>Part II:</p>
<p>Studies on the detailed mechanism of an energy transfer are reported. Optically active photosensitizers induce optical activity into trans-1,2-diphenylcyclopropane, II, while effecting the trans → cis isomerization. The asymmetric induction is shown to result from differing rates of energy transfer to the two optical isomer of trans-II. A rather large steric effect operative during energy transfer is thus demonstrated. Quenching studies indicate that the reaction occurs by a singlet mechanism resulting in production of 1,3 -diphenyltrimethylene. The intermediate apparently has the same decays paths as that generated by triplet energy transfer to II. The asymmetric induction results from a number of sensitizers are useful in constructing a probable transition state for this energy transfer reaction.</p> |
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