Thione Photochemistry. Mechanism of the Short-Wavelength Cycloaddition of Adamantanethione: Evidence for an Excimer Derived from S₂

Excitation of adamantanethione into the S₂ state leads to reaction from that state. Both dimerization to the 1,3-dithietane and insertion into the solvent (cyclohexane) occur. Neither process appears to involve long-lived radical intermediates. In the presence of ethyl vinyl ether, acrylonitrile, and fumaronitrile, cycloaddition to give the thietane occurs. Unlike the reaction occurring via the ³(n,π*) state, the reaction via S₂, ¹(π,π*), is stereospecific and nonregiospecific. The dimerization, insertion, and the addition to the first two olefins have been studied kinetically. Differential quenching experiments with olefins have shown that dimerization proceeds via an excimer derived from S₂: its collapse to dimer is quantitative. The lifetime of the thione in the S₂ state, obtained on the basis that concentration quenching or quenching by 1,1′-azoisobutane is diffusion controlled, is ∼0.13 ns.