<title>Probing photoinduced electron transfer reactions at semiconductor-liquid interfaces</title>

Abstract

The electron transfer dynamics of 9-anthracene carboxylic acid bound to TiO<SUB>2</SUB> nanoparticles in ethanol has been examined by a combination of transient absorption and time- resolved anistropy measurements. The results from these experiments show that the forward electron transfer reaction is very fast, &lt;EQ 360 fs for anatase TiO<SUB>2</SUB> and &lt;EQ 1 ps for rutile TiO<SUB>2</SUB>. The back electron transfer reaction occurs on a slower time scale: 33 +/- 1 ps for the anatase particles and 54 +/- 1 ps for the rutile particles. The rate of the back electron transfer reaction for anatase TiO<SUB>2</SUB> is also very sensitive to the addition of small amounts of water to the ethanolic solutions. For example, adding 1 percent water by volume decreases the average back electron transfer time from 33 ps to 20 ps. The water also produces a red-shift in the absorption spectra of the TiO<SUB>2</SUB> particles. These observations show that the back electron transfer reaction is in the Marcus inverted region.