Ultrafast study of interfacial electron transfer between 9-anthracene-carboxylate and TiO2 semiconductor particles

The excited state dynamics of 9-anthracene-carboxylic acid adsorbed onto the surface of ${\mathrm{TiO}}_{\mathrm{2}}$ 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 photo-excited dye molecules transfer an electron to the ${\mathrm{TiO}}_{\mathrm{2}}$ particles. The time scale for the forward electron transfer reaction was determined to be $\text{⩽1\hspace{0.17em}}\mathrm{ps}.$ The 9-anthracene-carboxylate radical cations produced by this reaction undergo back electron transfer on a 54 ps time scale. A more accurate estimate of the forward electron transfer reaction time is not possible, due to the contribution to the transient absorption signal from adsorbed dye molecules that do not transfer electrons to ${\mathrm{TiO}}_{\mathrm{2}}.$ These nonreactive species are deactivated by either nonradiative decay or fluorescence emission. The fluorescence spectrum from the dye molecules bound to the ${\mathrm{TiO}}_{\mathrm{2}}$ particles is very different to that of the free dye in solution. The free dye has a broad red-shifted spectrum, whereas, the adsorbed molecules have a structured 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. This 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 fluorescence spectrum. The dual behavior of the adsorbed dye molecules (electron transfer versus nonradiative decay/fluorescence) is attributed to the existence of two different sites for adsorption at the surface of the ${\mathrm{TiO}}_{\mathrm{2}}$ particles: electron transfer can occur from one site but not the other.