Blue light-powered hydroxynaphthoic acid-titanium dioxide photocatalysis for the selective aerobic oxidation of amines.

Solar photocatalysis is the key to resolve many environmental challenges but is usually hard to achieve over a metal oxide semiconductor. Therefore, assembling π-conjugated molecules onto semiconductors becomes an efficient approach to solar conversion via ligand-to-metal charge transfer. Here, a rational design of ligands for titanium dioxide (TiO ₂ ) is presented to produce robust visible light photocatalysts. Three hydroxynaphthoic acids (HNAs) were selected as ligands by extending an extra benzene ring of salicylic acid (SA) at 3,4 or 4,5 or 5,6 positions. These ligands could regulate the performance of TiO ₂ in which 2-hydroxy-1-naphthoic acid (2H1NA) endows the best outcome. In detail, blue light-powered cooperative photocatalysis of 2H1NA-TiO ₂ with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 5 mol%) inaugurates the expeditious formation of imines by oxidation of amines with atmospheric oxygen (O ₂ ). Interestingly, the increase of the O ₂ pressure from 1 atm to 0.4 MPa promoted the selective oxidation of benzylamine but thereafter declined with a further boost to 0.6 MPa. Notably, an electron transfer between the oxidatively quenched 2H1NA-TiO ₂ and TEMPO is established, offering a new pathway for environmental applications. This work presents a strategy in designing cutting-edge visible light photocatalysts via altering semiconductors with surface ligands.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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