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

IGNACIO MARTINI; JOSÉ H. HODAK; GREGORY V. HARTLAND

JOURNAL OF PHYSICAL CHEMISTRY B

Año: 1998 vol. 102 p. 607 - 614

1089-5647

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 participate in the absorption of light, as well as in the back electron transfer reaction. The observation that the back electron transfer reaction 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 relative population of the lower energy state was determined by the amount of water added and the difference in energy between the states was taken from the UV/vis absorption spectrum. Both the forward and the reverse reactions have faster time constants than the corresponding reactions 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 different structures for the TiO2 surfaces and, therefore, different electronic coupling elements with the adsorbed dye molecules.