Intermolecular interaction between anthraquinone dyes and TEMPO mediator in dye-sensitized photocatalytic systems

Intermolecular interactions of five anthraquinone dyes (1-hydroxyanthraquinone (1), 2-hydroxyanthraquinone (2), alizarin (3), sodium anthraquinone-2-sulfonate (4), and alizarin red S (5)) with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) species were investigated using density functional theory. Hydroxyl groups at position 1 of oxidized 1 and 3 were involved in the intermolecular interactions, but those at position 2 of oxidized 2 were not involved. For oxidized 4 and 5, most interactions occurred at the sulfonate substituent. Although the intermolecular interaction sites varied among the oxidized dyes, the atomic charges were completely transferred. Thus, we verified that the regeneration of the dyes by the TEMPO/TEMPO+ redox mediator occurred via a single-electron transfer process. Further, we discussed the influence of dyes on thioanisole oxidation reactivity over anthraquinone dye-sensitized TiO2 photocatalysts with TEMPO mediator (16% for 1, 63% for 2, 75% for 3, and 2% for 4) (Applied Catalysis B: Environmental 2019, 259, 118038) in terms of both the number of oxidized dye–TEMPO configurations and the interaction sites.