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Laser Flash Photolysis Studies on Radical Monofluoromethylation by (Diarylamino)naphthalene Photoredox Catalysis: Long Lifetime of the Excited State is Not Always a Requisite

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journal contribution
posted on 07.10.2020, 18:37 by Naoki Noto, Keigo Takahashi, Shion Goryo, Akira Takakado, Koichi Iwata, Takashi Koike, Munetaka Akita
Organic photoredox catalysis has become a useful tool for the development of metal-free radical reactions. Recently, we have reported that 1,4-bis­(diphenylamino)­naphthalene N serves as an efficient photoredox catalyst for radical monofluoromethylation with N-tosyl-S-monofluoromethyl-S-phenylsulfoximine 2. In this paper, we report the preparation and photo- and electrochemical properties of (diarylamino)­naphthalene derivatives, 1,4-bis­(di­(p-tert-butylphenyl)­amino)­naphthalene 1a, 1,5-bis­(di­(p-tert-butylphenyl)­amino)­naphthalene 1b, and 1-(di­(p-tert-butylphenyl)­amino)­naphthalene) 1c, as supported by density functional theory (DFT) and time-dependent-DFT calculations. In addition, their performance of photocatalysis has been evaluated by means of methoxy-monofluoromethylation of aromatic alkenes. Laser flash photolysis shows that the fluorescence of 1a in the excited state is efficiently quenched by 2 (quenching rate constant kq = ca. 2 × 109 M–1 s–1). Transient absorption spectroscopic analyses reveal that the excited species of 1a in the presence of 2 starts decreasing in ca. 100 ps, suggesting the occurrence of fast electron-transfer processes. These results lead to the unconventional concept for the catalyst design, that is, long lifetime of the excited state is not always a requisite for efficient photoredox catalysts.