Effect of Water on the Electron Transfer Dynamics of 9-Anthracenecarboxylic Acid Bound to TiO2 Nanoparticles: Demonstration of the Marcus Inverted Region

Abstract

The electron transfer dynamics for 9-anthracenecarboxylic acid bound to nanometer-sized TiO2 particles has been examined by transient absorption and time-resolved anisotropy measurements. The results from these experiments show that the forward electron transfer reaction occurs within the laser pulse, i.e., with a time constant of ≤350 fs. In absolute ethanol solutions the reverse electron transfer reaction occurs on a 33 ± 2 ps time scale. Addition of small amounts of water to the TiO2/ethanol solutions produces a red shift in the absorbance spectrum of the TiO2 particles and increases the overall rate of back electron transfer. This effect is attributed to the existence of oxygen vacancy defect sites at the surface of the TiO2 particles. These defects produce Ti(III) centers which have an excess electron in a nonbonding t2g 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 t2g orbitals to ...