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

IGNACIO MARTINI; JOSE´ H. HODAK; GREGORY V. HARTLAND

JOURNAL OF PHYSICAL CHEMISTRY B

Año: 1998 vol. 102 p. 9508 - 9517

1089-5647

Photoinduced electron transfer to TiO2 nanoparticles has been examined for the 1-, 2-, and 9-isomers ofanthracenecarboxylic acid. TiO2 samples with either anatase or amorphous crystal structures were used forthese experiments. The results from time-resolved transient absorption measurements show that the rates ofthe forward (dye-to-semiconductor) and reverse (semiconductor-to-dye) electron transfer reactions dependon the chemical structure of the dye and the method used to synthesize the particles. These effects arisefrom differences in both the energetics and the coupling elements for the reactions. Specifically, the reverseelectron transfer reactions for the 1- and 2-isomers are significantly faster than that for the 9-isomer due todifferences in the oxidation potentials of the dye molecules. In addition, both the forward and reverse electrontransfertimes are faster for the anatase TiO2 particles compared to the amorphous particles. For example,the forward electron transfer time for the anatase particles is e200 fs, whereas it is ca. 1.5 ps for the amorphousparticles. 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 spectrawhen they are attached to the semiconductor particles. Experiments with ZrO2 show that these shifts are notdue to a charge-transfer band. The spectra are more strongly perturbed when the dye molecules are attachedto the anatase particles, which shows that for this series of compounds there is a correlation between thespectral shifts and the time scale for electron transfer.