Photocatalytic Tandem Protocol for the Synthesis of Bis(indolyl)methanes using Cu-g-C₃N₄–Imine Decorated on TiO₂ Nanoparticles under Visible Light Irradiation

In this article, the visible-light-assisted photocatalytic activity of TiO2 nanoparticles functionalized with Cu(II) g–C3N4–imine was exploited for aerobic oxidation of alcohols to aldehydes followed by condensation with indoles in the presence of 2,2,6,6-tetramethylpiperidinyloxy to present a one-pot tandem strategy for the synthesis of bis(indolyl)methanes (BIMs) under solvent-free conditions. The synergistic effect between the components to improve the photocatalytic activity of the as-prepared Cu-g-C3N4–imine/TiO2 nanoparticles resulting from electron–hole separation was approved by PL spectroscopy. Moreover, action spectra showed a light-dependent photocatalysis with effective visible-light responsivity of the photocatalyst. The present method includes different aspects of green chemistry: one-pot tandem synthesis of a variety of BIMs using alcohols that are less toxic, more available, more economical, and more stable than aldehydes; removing the byproducts resulting from overoxidation of alcohols and polymerization of aldehydes and indoles; the use of air as a safe oxidant; visible light as a safe energy source; and solvent-free conditions. A reusability test demonstrated that the catalyst retained its efficiency even after five runs.