Photoinduced Electron Transfer in Photorobust Coumarins Linked with Electron Donors Affording Long Lifetimes of Triplet Charge-Separated States

Authors

  • Motonobu Murakami Dr.,

    1. Department of Material and Life Science, Graduate School of Engineering, Osaka University and SORST (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan), Fax: (+81) 6-6879-7370
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  • Kei Ohkubo Dr.,

    1. Department of Material and Life Science, Graduate School of Engineering, Osaka University and SORST (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan), Fax: (+81) 6-6879-7370
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  • Takashi Nanjo,

    1. Department of Material and Life Science, Graduate School of Engineering, Osaka University and SORST (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan), Fax: (+81) 6-6879-7370
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  • Kazunori Souma,

    1. Tokyo Chemical Industry Co., Ltd. 4-10-2 Nihonbashi-honcho, Chuo-ku, Tokyo 103-0023 (Japan), Fax: (+81) 1-3-5640-8868
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  • Nobuo Suzuki,

    1. Tokyo Chemical Industry Co., Ltd. 4-10-2 Nihonbashi-honcho, Chuo-ku, Tokyo 103-0023 (Japan), Fax: (+81) 1-3-5640-8868
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  • Shunichi Fukuzumi  Prof. Dr.

    1. Department of Material and Life Science, Graduate School of Engineering, Osaka University and SORST (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan), Fax: (+81) 6-6879-7370
    2. Department of Bioinspired Science, Ewha Womans University, Seoul, 120-750 (Korea)
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Abstract

Electron donor (D) substituted 3-ethoxycarbonylcoumarin (CM) derivatives [D–CM: D=4-diphenylaminophenyl (DPA), 4-diethylaminophenyl (DEA), 4-dimethylaminophenyl (DMA), and 2-methyl-4-dimethylaminophenyl (MeDMA)] are synthesized and characterized. Photoinduced electron transfer (ET) from the D moiety to the acceptor (CM) and back electron transfer (BET) are investigated by femtosecond and nanosecond laser flash photolysis measurements. Femtosecond laser excitation at 355 nm of a deaerated acetonitrile (MeCN) solution of D–CM shows generation of the singlet charge-separated (CS) state [1(D.+–CM.−)] by ET from D to the singlet excited state of the CM moiety (1CM*), and this is followed by rapid decay within 3 ns to afford the triplet excited state (D–3CM*). Nanosecond laser excitation of a deaerated MeCN solution of D–CM results in formation of the triplet CS state by ET from D to 3CM*. The quantum yield of formation of the triplet CS state [3(DPA.+–CM.−)] in the presence of iodobenzene (PhI) in deaerated MeCN increases with increasing concentration of PhI to reach 27 % at 0.5 M PhI. The triplet CS state decays by bimolecular BET because of the long CS lifetime by unimolecular BET. Formation of the long-lived triplet CS state was confirmed by electron spin resonance (ESR) measurements. The photorobust nature of DPA–CM is demonstrated by multiple laser pulse excitation (>1000 times) at 355 nm. The photoinduced ET and BET rate constants of a series of D–CM are thoroughly analyzed in light of the Marcus theory of electron transfer.

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