Diffusion-controlled charge transfer from excited Ru(bpy)(3)(2+) into nanosized TiO2 colloids stabilized with EDTA

Transparent colloidal TiO2 was prepared in aqueous EDTA solution. The avera ge particle size was 7 nm, measured by TEM. In the presence of EDTA, the co lloids were stable within a pH range from 2 to 10. In contrast to previous observations involving Ru(bpy)(3)(2+) and TiO2 where no emission quenching occurred up to pH 10, an efficient quenching of the excited Ru(bpy)(3)(2+) ((MLCT)-M-3) by the colloidal TiO2 was obtained in the presence of EDTA. A detailed kinetics investigation by laser flash photolysis showed the quench ing process to be diffusion-controlled. The fact that not all the excited-s tate population returned to the ground state in a simple exponential proces s led to the conclusion that Ru(bpy)(3)(3+) was being formed and the quench ing was by injection of electrons into the colloid. The overall quantum yie ld of formation of Ru(bpy)(3)(3+) was increased from 5.8% to 12.9% as the i onic strength increased from 0.1 to 1.5 M at pH 2.8. It was concluded that the increase in ionic strength weakens the electrostatic interaction betwee n the geminate redox photoproducts Ru(bpy)(3)(3+)... e(-)(TiO2) and hence p romotes the cage release of Ru(bpy)(3)(3+). With increases in temperature, both the charge-transfer (triplet quenching) rate and the release of the ge minate species Ru(bpy)(3)(3+)... e(-)(TiO2) were enhanced. The activation e nergy for the former process was measured as 22.0 +/- 0.3 kJ mol(-1). consi stent with the typical values (8-25 kJ mol(-1)) reported for the diffusion- controlled reactions in homogeneous solutions. For the separation of the ge minate species, a lower activation energy of 7.1 +/- 0.5 kJ mol(-1) was eva luated, implying a typical diffusion-controlled process. A detailed pH depe ndence study indicated that the charge-transfer rate increased as the pH de creased below 6, while at pH > 6 no charge transfer could be observed. This is surmised to be due to driving force dependence.