Part One. Photodimerizations of Coumarin. Part Two. Asymmetric Induction During Energy Transfer

Citation

Cole, Ronald Sinclair (1968) Part One. Photodimerizations of Coumarin. Part Two. 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

Abstract

Part I:

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.

Part II:

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.

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