Photoinduced Electron Transfer in Donor-acceptor Complexes of Ethylene with Molecular and Atomic Iodine
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 (λmax = 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, anti- and gauche-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, anti-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. © 2014 American Chemical Society.
A. Kalume et al., "Photoinduced Electron Transfer in Donor-acceptor Complexes of Ethylene with Molecular and Atomic Iodine," Journal of Physical Chemistry A, vol. 118, no. 34, pp. 6838-6845, American Chemical Society (ACS), Jan 2014.
The definitive version is available at http://dx.doi.org/10.1021/jp412212h
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