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

Yunlong Gao; Molly Lockart; Lowell D Kispert; Michael K Bowman

doi:10.1039/c9dt01455a

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

Dalton Trans. 2019 Aug 7;48(29):10881-10891. doi: 10.1039/c9dt01455a. Epub 2019 May 20.

Authors

Yunlong Gao¹, Molly Lockart², Lowell D Kispert², Michael K Bowman²

Affiliations

¹ College of Sciences, Nanjing Agricultural University, Nanjing, 210095, China. yunlong@njau.edu.cn.
² Department of Chemistry, BOX 870336, University of Alabama, Tuscaloosa, Alabama 35487, USA.

PMID: 31107487
DOI: 10.1039/c9dt01455a

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.