Journal of the Chemical Society. Perkin transactions II p. 1867 - 1874 (1986)
Update date:2022-08-04
Topics:
Kamphuis, Johan
Bos, Hendrik J. T.
Visser, Robert J.
Huizer, Bert H.
Varma, Cyril A. G. O.
The photochemical reaction of xanthene-9-thione, thiobenzophenones, and thioxanthene-9-thione with allenes has been studied by initiating the reaction with a 308 nm laser flash and by following transient u.v.-visible optical absorptions on a nanosecond time scale.In the wavelegth range 320<λ<800 nm there is only a transient absorption arising from the thione in its lowest triplet state T1(n?*).The rate constant ka for quenching of the thione in this state by allenes is determined from Stern-Volmer plots.A 1,4-biradical which appears as a reaction intermediate could be scavenged with NN'-dimethyl-4,4'-bipyridinium dichloride (methyl viologen, MV).The yield of the methyl viologen radical cation MVRC resulting from scavenging has been determined as a function of the concentration of MV.A lifetime of 90 ps follows from this for the 1,4-biradical (8).The short lifetime is attributed to a large spin density on the sulphur atom from both unpaired electrons, causing efficient local spin-orbit coupling.Both the logarithm of the rate constant kt for the disappearance of the aromatic thione in the thermal reaction with an allene and log ka depend linearly on the ionization energy of the allene.Previous suggestions that the 1,4-biradical has a triplet state exciplex as precursor are rejected on the basis of the similarity of these relations and because of the fact that there is no difference in the effect on ka and kt caused by deuteriation of the allene.
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