An extremely short-lived 1,4-biradical as intermediate in the photo-cycloaddition reactions of triplet state aromatic thiones with allenes

Abstract

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 wavelength range 320 < λ < 800 nm there is only a transient absorption arising from the thione in its lowest triplet state T₁(nπ*). The rate constant k_a 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 k_t for the disappearance of the aromatic thione in the thermal reaction with an allene and log k_a 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 k_a and k_t caused by deuteriation of the allene.