Surface complexation for photocatalytic organic transformations

Photocatalysis constitutes an important research interest due to its capability for achieving important chemical reactions in an environmentally green and sustainable manner. The use of heterogeneous photocatalysts adds additional advantages such as ease of separation from reaction mixtures, reusability, as well as photo, thermal and chemical stability. In this account, we showed how the surface complexation of different key players on TiO₂ can be used control the reaction pathway to enable difficult organic transformations, as demonstrated by the selective aerobic oxidation of sulfides to sulfoxides. First, we designed a photocatalytic-surface complexation system comprising three fundamental components; visible-light-absorbing dye, TiO₂ and TEMPO as the redox mediator. Next, the said system was elegantly simplified into a visible-light-harvesting surface complex generated in-situ between TiO₂ and tertiary amines, which enabled O₂ to be selectively activated only in the presence of the target sulfide substrate. This was then expanded into the new concept of synergistic photocatalysis, which is based on the interplay of reactants (sulfides and benzylamines) via the aforementioned visible-light-harvesting surface complex to enable two seemingly irrelevant reactions in one photocatalytic system. Lastly, we briefly discussed how surface complexation on heterogeneous catalysts such as metal oxides can be further utilized for photocatalytic organic transformations.