title: Photoinduced electron transfer in photorobust coumarins linked with electron donors affording long lifetimes of triplet charge-separated states.
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 ((1)CM*), and this is followed by rapid decay within 3 ns to afford the triplet excited state (D-(3)CM*). Nanosecond laser excitation of a deaerated MeCN solution of D-CM results in formation of the triplet CS state by ET from D to (3)CM*. 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.
authors.affiliation: Department of Material and Life Science, Graduate School of Engineering, Osaka University and SORST (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
authors.forename: Motonobu
authors.initials: M
authors.lastname: Murakami
authors.forename: Kei
authors.initials: K
authors.lastname: Ohkubo
authors.forename: Takashi
authors.initials: T
authors.lastname: Nanjo
authors.forename: Kazunori
authors.initials: K
authors.lastname: Souma
authors.forename: Nobuo
authors.initials: N
authors.lastname: Suzuki
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
mesh_terms: D003374:Coumarins / Q000138:chemical synthesis / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004579:Electron Transport; D004583:Electrons; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010777:Photochemistry; D010782:Photolysis
chemical_list: D003374:Coumarins