The function of SiO₂ colloids in photoinduced redox reactions. Interfacial effects on the quenching, charge separation, and quantum yields

The function of negatively charged colloidal SiO₂ particles in controlling photosensitized electron-transfer reactions by means of electrostatic interactions between the different components in the system has been studied under steady-state conditions of illumination and by flash photolysis. In particular, the photosensitized reduction of the zwitterionic electron acceptor, propyl viologen sulfonate, PVS⁰ (1), using tris(2,2′-bipyridinium)ruthenium(II), Ru(bpy)₃²⁺, as sensitizer has been investigated in the SiO₂ colloid and compared to the homogeneous system. Fluorescence quenching studies indicate that the quenching reaction of Ru(bpy)₃²⁺ with the neutral PVS⁰ is not affected by electrostatic interactions exerted by the negatively charged SiO₂ interface (k_q = 1.5 × 10⁹ M^-1 s^-1). In contrast, the quenching of Ru(bpy)₃²⁺ by the positively charged methylviologen, MV²⁺, is strongly enhanced by the SiO₂ colloid (k_q = 5 × 10¹⁰ M^-1 s^-1). This efficient quenching is attributed to electrostatic adsorption of the quencher on the negatively charged interface. Flash photolysis experiments indicate that the back-electron-transfer reaction between the intermediate photoproducts, Ru(bpy)₃³⁺ and PVS^-•, is substantially retarded in the SiO₂ colloid (k_b = 5.7 × 10⁷ M^-1 s^-1) as compared to the homogeneous system (k_b = 7.9 × 10⁹ M^-1 s^-1). This retardation is attributed to electrostatic repulsion of the reduced product, PVS^-•, from the oxidized sensitizer adsorbed on the SiO₂ particles. The back-electron-transfer process was examined at different temperatures and an activation energy of 4.2 kcal mol^-1 for the recombination reaction was estimated. The importance of the surface potential of the SiO₂ colloids in retarding back reactions was demonstrated by varying the ionic strength and the pH of the medium. The results confirm that the surface potential is the dominant factor in improving the quantum yield for PVS⁰ reduction.