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 (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.

Department(s)

Chemistry

Research Center/Lab(s)

Center for High Performance Computing Research

Keywords and Phrases

Charge transfer; Chromophores; Electron transitions; Ethylene; Iodine; Back electron transfer; Charge transfer state; Charge-transfer bands; Donor-acceptor complex; Multireference methods; Photo-induced electron transfer; Single electron transfer; Spin-orbit couplings; Free radical reactions

International Standard Serial Number (ISSN)

1089-5639

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2014 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Aug 2014

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