Extending the π-conjugated molecules on TiO2 for the selective photocatalytic aerobic oxidation of sulfides triggered by visible light

Abstract

The assembly of TiO₂ surface with π-conjugated molecules can give rise to an apparent red-shift of its optical absorption band, and subsequently engender ligand-to-metal charge transfer (LMCT) triggered by visible light. However, the application of LMCT remains elusive due to the vulnerability of these π-conjugated molecules at the vicinity of reactive centers. Therefore, extending the π-conjugation of these molecules could be envisioned to overcome this bottleneck with the oxidation of sulfides as a probe reaction. With salicylic acid (SA) as the starting molecule, extending the benzene ring at 5,6 or 3,4 positions with another one afforded 2-hydroxy-1-naphthoic acid (2H1NA) and 1-hydroxy-2-naphthoic acid (1H2NA). Compared to SA, 2H1NA showed a moderate improvement, while 1H2NA caused a significant enhancement for modifying anatase TiO₂ to conduct the selective aerobic oxidation of sulfides triggered by blue light with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as a redox mediator. Ultimately, the aerobic oxidation of organic sulfides to corresponding sulfoxides in CH₃OH could be achieved by the 1H2NA–TiO₂ photocatalyst in cooperation with TEMPO. This study introduces the assembly of extended π-conjugated molecules on TiO₂, which may pave the way to construct more efficient visible-light photocatalysts.