Part I. Photodimerizations of Coumarin. Part II. Asymmetric induction during energy transfer

Abstract

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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 [...]. 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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