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 0.5 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. Photorobust donor–acceptor system: Photoexcitation of 6‐[4′‐(N,N‐diphenylamino)phenyl]‐3‐ethoxycarbonylcoumarin (DPA–CM) generates the long‐lived triplet charge‐separated (CS) state. The quantum yield of formation of the CS state of DPA–CM is increased by the heavy‐atom effect of iodobenzene (see picture). The photorobustness of DPA–CM is demonstrated by multiple laser pulse excitation to form the CS state (>1000 times).