Enhancing Selective Photooxidation through Co–Nx-doped Carbon Materials as Singlet Oxygen Photosensitizers

Singlet oxygen ( 1 O 2 ) is considered one of the most effective and selective oxygen agents. However, it is always obtained with the help of heavy atoms in the photosensitizers to sensitize 3 O 2 . Herein, metal–nitrogen (M–Nx) doped 1 O 2 photosensitizers were readily prepared from metal–nitrogen complex. Their relative metal centers (e.g., Co) chelated with the N/C moiety (Co–Nx/C) provide the primary active sites for 1 O 2 generation and selective oxidation. The structures of Co–Nx active sites are investigated by scanning and transmission electron microscopy and X-ray photoelectron, Fourier transform infrared, and X-ray absorption fine structure spectroscopy. Their functions for 1 O 2 generation are confirmed by electrons spin resonance, 1 O 2 emission, KSCN poisoning test, and H 2 SO 4 etching test. These Co–Nx photosensitizers show excellent selective photooxidation abilities for 1,5-dihydroxynaphthalene after irradiation by a light-emitting diode lamp. After simple concentration and filtration, it is easy to obtain the pure product (juglone), which is confirmed by 1 H NMR spectroscopy. On the basis of density functional theory calculations, metal (e.g., Co) chelated with N/C moiety, especially for the Co–pyridinic N structure, could effectively reduce the singlet–triplet energy gap (Δ E ST ). It is speculated that this strategy for lowering Δ E ST could benefit intersystem crossing from the singlet state to the triplet state and efficient sensitization of 3 O 2 (triplet state) into 1 O 2 for selective photooxidation.