Photo-induced charge separation in hydroxycoumarins on TiO2 and F–TiO2

Abstract

The efficiency of photocatalytic charge separation is much higher for 7-hydroxycoumarin (7-CN) and 6,7-dihydroxycoumarin (6,7-CN) adsorbed on the surface modified TiO₂ where the surface hydroxyl group was replaced by a fluorine atom (F–TiO₂) than on TiO₂. EPR measurements find 5- and 12-fold increases in free radical yields for 7-CN and 6,7-CN, respectively. DFT calculations for the coumarins on TiO₂ and F–TiO₂ were performed to investigate these phenomena. The calculations show that when the coumarins act as the H-bond donors, the driving force for photo-induced electron transfer from the dyes to TiO₂ is higher, and the dye's excited state mixes strongly with the TiO₂ conduction band states. This is attributed to the shorter distance between the coumarins and the surface of TiO₂ when the coumarins act as the H-bond donors. These calculations explain why the efficiency of charge separation of the coumarins is much higher on F–TiO₂ than on TiO₂.