Photoinduced electron transfer in photorobust coumarins linked with electron donors affording long lifetimes of triplet charge-separated states

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 0.5m 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.