Photoinduced Electron Transfer in Donor–Acceptor Complexes of Ethylene with Molecular and Atomic Iodine

Abstract

Building upon our recent studies of radical addition pathways following excitation of the I₂ chromophore in the donor–acceptor complex of ethylene and I₂ (C₂H₄···I₂), in this article, we extend our studies to examine photoinduced electron transfer. Thus, irradiation into the intense charge-transfer band of the complex (λ_max = 247 nm) gave rise to a band at 366 nm that is assigned to the bridged ethylene–I radical complex on the basis of our prior work. The formation of the radical complex is explained by a mechanism that involves rapid back electron transfer leading to I–I bond fission. Excitation into the charge-transfer band of the radical complex led to regeneration of the parent complex and the formation of the final photoproduct, <i>anti</i>- and <i>gauche</i>-1,2-diiodoethane, which confirms that the reaction proceeds ultimately by a radical addition mechanism. This finding is contrasted with our previous study of the C₂H₄···Br₂ complex, where CT excitation led to only one product, <i>anti</i>-1,2-dibromoethane, a result explained by a single electron-transfer mechanism proceeding via a bridged bromonium ion intermediate. For the I₂ complex, the breakup of the photolytically generated I₂^–• anion radical is apparently sufficiently slow to render it uncompetitive with back electron transfer. Finally, we report a detailed computational examination of the parent and radical complexes of both bromine and iodine, using high-level single- and multireference methods, which provide insight into the different behaviors of the charge-transfer states of the two radicals and the role of spin–orbit coupling