The importance and the complexity of biological electron-transfer catalytic systems such as photosynthesis and respiration have prompted the development of electron-transfer catalytic systems, which can mimic functions of redox enzymes [1]. We have developed simple donor-acceptor dyads to attain a long-lived and high energy charge-separated state without significant loss of excitation energy, by fine control of the redox potentials and of the geometry of donor-acceptor dyads that have small reorganization energies of electron transfer. Such simple molecular dyads, capable of fast charge separation but extremely slow charge recombination, have significant advantages with regard to synthetic feasibility, providing a variety of applications, which are presented herein [1].

A simple molecular electron donor-acceptor dyad, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes), capable of fast charge separation but extremely slow charge recombination [2], allows us to develop efficient photocatalytic oxygenation of anthracenes with oxygen [3]. Formation of 1,2-dioxetane of tetraphenylethylene (TPE) also occurs via formation of the electron-transfer state of Acr⁺-Mes under visible light irradiation, followed by electron transfer from TEP to the Mes^•+ moiety together with electron transfer from the Acr^• moiety to O₂, and the subsequent radical coupling between TEP^•+ and O₂^•- to yield the corresponding 1,2-dioxetane [4]. The dioxetane thus formed was isolated using column chromatography for the first time.

The electron-transfer state of Acr⁺-Mes is also capable of oxidizing DNA bases and the transient absorption spectra of four nucleotide radical cations have been successfully detected in the electron-transfer oxidation of the corresponding DNA bases with the Mes^•+ moiety of Acr^•-Mes^•+, which is produced upon photoexcitation of Acr⁺-Mes [5]. DNA is also efficiently oxidized by the Mes^•+ moiety of Acr^•-Mes^•+, leading to efficient DNA cleavage in the absence of O₂, as compared with acridinium ions without a donor moiety [5].

[1] a) S. Fukuzumi, In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001, Vol. 4, pp 3-67. b) S. Fukuzumi, Org. Biomol. Chem. 2003, 1, 609. c) S. Fukuzumi, Bull. Chem. Soc. Jpn. 2006, 79, 177.
[2] a) S. Fukuzumi, H. Kotani, K. Ohkubo, S. Ogo, N. V. Tkachenko, H. Lemmetyinen, J. Am. Chem. Soc. 2004, 126, 1600. b) H. Kotani, K. Ohkubo, S. Fukuzumi, Chem. Comm. 2005, 4520.
[3] H. Kotani, K. Ohkubo, S. Fukuzumi, J. Am. Chem. Soc. 2004, 126, 15999.
[4] K. Ohkubo, T. Nanjo, S. Fukuzumi, Org. Lett. 2005, 7, 4265.
[5] K. Ohkubo, K. Yukimoto, S. Fukuzumi, Chem. Comm. 2006, 2504.

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