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1 May 2005 Photochemistry of Naphthalene Diimides: EPR Study of Free Radical Formation via Photoredox Process
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Earlier studies have shown that on exposure to UVA, hydroperoxynaphthalene diimide (IA) generates hydroxyl radicals, induces DNA strand scission, and kills cells.Here we employed electron paramagnetic resonance (EPR) and spin trapping to investigate the free radical photochemistry of IA and that of related naphthalene diimides, which are devoid of the hydroperoxyl moiety (N,N′-bis[2-methyl]-1,4,5,8-naphthaldiimide [IB], N,N′-bis[2-thiomethyl-2-methoxyethyl]-1,4,5,8-naphthaldiimide [IC]) and therefore are unable to generate hydroxyl radicals. It is shown that on UV irradiation (>300 nm) in air-free methanol or ethanol solutions all these naphthalene diimides undergo one-electron reduction to corresponding anion radicals, positively identified by EPR. With EPR and a spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO), we found that the photogeneration of the naphthalene diimide radicals is concomitant with the formation of radicals from the solvents, presumably through electron/hydrogen atom abstraction by photoactivated diimides. Irradiation of IA, IB or IC in the presence of oxygen generates superoxide, which was detected as a DMPO adduct. The high photoreactivity of IB and IC supports the notion that hydroperoxide IA can induce oxidative damage via photoprocesses that are independent of OH generation. These observations could be pertinent to the application of naphthalene diimides as selective photonucleases, PDT anticancer agents or both.

Krzysztof J. Reszka, Masami Takayama, Robert H. Sik, Colin F. Chignell, and Isao Saito "Photochemistry of Naphthalene Diimides: EPR Study of Free Radical Formation via Photoredox Process," Photochemistry and Photobiology 81(3), 573-580, (1 May 2005).
Received: 15 November 2004; Accepted: 1 February 2005; Published: 1 May 2005

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