EFFECT OF WATER ON THE ELECTRON-TRANSFER DYNAMICS OF 9-ANTHRACENECARBOXYLIC ACID BOUND TO TIO2 NANOPARTICLES - DEMONSTRATION OF THE MARCUS INVERTED REGION

The electron transfer dynamics for 9-anthracenecarboxylic acid bound t o nanometer-sized TiO2 particles has been examined by transient absorp tion and time-resolved anisotropy measurements. The results from these experiments show that the forward electron transfer reaction occurs w ithin the laser pulse, i.e., with a time constant of less than or equa l to 350 fs. In absolute ethanol solutions the reverse electron transf er reaction occurs on a 33 +/- 2 ps time scale. Addition of small amou nts of water to the TiO2/ethanol solutions produces a red shift in the absorbance spectrum of the TiO2 particles and increases the overall r ate of back electron transfer. This effect is attributed to the existe nce of oxygen vacancy defect sites at the surface of the TiO2 particle s. These defects produce Ti(III) centers which have an excess electron in a nonbonding t(2g) orbital When water is added to the sample, the Ti(III) surface atoms are converted to Ti(IV)-OH2 groups. This removes the excess electrons and allows the low-energy t(2g) orbitals to part icipate in the absorption of light, as well as in the back electron tr ansfer reaction. The observation that the back electron transfer react ion is faster when these lower energy states are available proves that the back-reaction is an example of a Marcus inverted region reaction. A reorganization energy of 0.75 +/- 0.05 eV was obtained for the back electron transfer reaction by using a two-state model, where the rela tive population of the lower energy state was determined by the amount of water added and the difference in energy between the states was ta ken from the UV/vis absorption spectrum. Both the forward and the reve rse reactions have faster time constants than the corresponding reacti ons in our previous study of 9-anthracenecarboxylic acid bound to TiO2 . This difference arises because the TiO2 samples in these two studies were prepared by different synthetic techniques. This leads to differ ent structures for the TiO2 surfaces and, therefore, different electro nic coupling elements with the adsorbed dye molecules.