Effect of structure on electron transfer reactions between anthracene dyesand TiO2 nanoparticles

Photoinduced electron transfer to TiO2 nanoparticles has been examined for the 1-, 2-, and 9-isomers of anthracenecarboxylic acid. TiO2 samples with e ither anatase or amorphous crystal structures were used for these experimen ts. The results from time-resolved transient absorption measurements show t hat the rates of the forward (dye-to-semiconductor) and reverse (semiconduc tor-to-dye) electron transfer reactions depend on the chemical structure of the dye and the method used to synthesize the particles. These effects ari se from differences in both the energetics and the coupling elements for th e 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 an atase TiO2 particles compared to the amorphous particles. For example, the forward electron transfer time for the anatase particles is less than or eq ual to 200 fs, whereas it is ca. 1.5 ps for the amorphous particles. This i s due to a difference in the coupling elements for the forward electron tra nsfer reaction. Finally, all the anthracenecarboxylate dyes examined show r ed shifts in their W-vis absorption spectra when they are attached to the s emiconductor particles. Experiments with ZrO2 show that these shifts are no t due to a charge-transfer band. The spectra are more strongly perturbed wh en the dye molecules are attached to the anatase particles, which shows tha t for this series of compounds there is a correlation between the spectral shifts and the time scale for electron transfer.