INTERFACIAL ELECTRON-TRANSFER IN COLLOIDAL SNO2 HYDROSOLS PHOTOSENSITIZED BY ELECTROSTATICALLY AND COVALENTLY ATTACHED RUTHENIUM(II) POLYPYRIDINE COMPLEXES

The complex [(H(2)Nphen)Ru(bpy)2](2+) (H(2)Nphen = 5-amino-1,10-phenan throline; bpy = 2,2'-bipyridine was covalently attached to the surface s of colloidal SnO2 particles (ca. 4-nm diameter) by using cyanuric ch loride and (3-aminopropyl)triethoxysilane as coupling agents. Electron injection from the MLCT excited state of the complex, RuL(3)(2+), in to the conduction band of SnO2 and recombination of RuL(3)(3+) with th e conduction band electron, e(cb)-(SnO2), were investigated by nanosec ond laser flash photolysis (532-nm excitation) and steady: state lumin escence techniques. These reactions were also investigated in the anal ogous system in which Ru(bpy)32+ was adsorbed to unmodified SnO2 parti cles by electrostatic attraction. Electron injection is rapid compared to recombination and competes effectively with the intrinsic decay of RuL(3)(2+), So that the quantum efficiency of formation of RUHL (Or Ru(bpy)(3)(3+)) and e(cb)-(SnO2) approaches unity. The lower quantum e fficiency observed in the covalently modified sol which was peptized w ith NH4OH (pH 10.7) compared to the one peptized with CH3COOH (pH 4.0) is attributed to the lower thermodynamic driving force for electron i njection at the higher pH. On the other hand, the faster rate of recom bination at the higher pH is attributed to electrostatic attraction be tween RuL(3)(3+) and the particle surface, rather than the difference in driving force. The recombination is a first-order reaction whose ti me dependence is satisfactorily described by the Kohlrausch relaxation function. The rate of decay of RuL(3)(3+) is insensitive to the prese nce of dissolved O-2 but is accelerated by phenol, which acts as a red uctant.