Effect of Structure on Electron Transfer Reactions between Anthracene Dyes and TiO2 Nanoparticles

Photoinduced electron transfer to TiO 2 nanoparticles has been examined for the 1-, 2-, and 9-isomers of anthracenecarboxylic acid. TiO 2 samples with either anatase or amorphous crystal structures were used for these experiments. The results from time-resolved transient absorption measurements show that the rates of the forward (dye-to-semiconductor) and reverse (semiconductor-to-dye) electron transfer reactions depend on the chemical structure of the dye and the method used to synthesize the particles. These effects arise from differences in both the energetics and the coupling elements for the reactions. Specifically, the reverse electron transfer reactions for the 1- and 2-isomers are significantly faster than that for the 9-isomer due to differences in the oxidation potentials of the dye molecules. In addition, both the forward and reverse electron-transfer times are faster for the anatase TiO 2 particles compared to the amorphous particles. For example, the forward electron transfer time for the anatase particles is ≤200 fs, whereas it is ca. 1.5 ps for the amorphous particles. This is due to a difference in the coupling elements for the forward electron transfer reaction. Finally, all the anthracenecarboxylate dyes examined show red shifts in their UV−vis absorption spectra when they are attached to the semiconductor particles. Experiments with ZrO 2 show that these shifts are not due to a charge-transfer band. The spectra are more strongly perturbed when the dye molecules are attached to the anatase particles, which shows that for this series of compounds there is a correlation between the spectral shifts and the time scale for electron transfer.