ULTRAFAST STUDY OF INTERFACIAL ELECTRON-TRANSFER BETWEEN 9-ANTHRACENE-CARBOXYLATE AND TIO2 SEMICONDUCTOR PARTICLES

The excited state dynamics of 9-anthracene-carboxylic acid adsorbed on to the surface of TiO2 semiconductor particles were examined with ca. 250 fs time resolution. A combination of transient absorption and time -resolved anisotropy measurements show that approximately 76% of the p hoto-excited dye molecules transfer an electron to the TiO2 particles. The time scale for the forward electron transfer reaction was determi ned to be less than or equal to 1 ps. The 9-anthracene-carboxylate rad ical cations produced by this reaction undergo back electron transfer on a 54 ps time scale. A more accurate estimate of the forward electro n transfer reaction time is not possible, due to the contribution to t he transient absorption signal from adsorbed dye molecules that do not transfer electrons to TiO2. These nonreactive species are deactivated by either nonradiative decay or fluorescence emission. The fluorescen ce spectrum from the dye molecules bound to the TiO2 particles is very different to that of the free dye in solution. The free dye has a bro ad red-shifted spectrum, whereas, the adsorbed molecules have a struct ured spectrum that displays a small Stokes shift. The red shift in the free dye fluorescence spectrum is due to stabilization of the excited electronic state through torsional motion of the carboxylate group. T his motion cannot occur when the dye is bound to the particle surface. Thus, the excited molecules emit from a nuclear configuration that is similar to the ground-state geometry, producing a structured fluoresc ence spectrum. The dual behavior of the adsorbed dye molecules (electr on transfer versus nonradiative decay/fluorescence) is attributed to t he existence of two different sites for adsorption at the surface of t he TiO2 particles: electron transfer can occur from one site but not t he other. (C) 1997 American Institute of Physics.