Photoinduced Reactions in Porous Systems:  Reactions at the Solid−Liquid Interface¹

Abstract

Transient absorption spectroscopy is used to observe photoinduced reactions at a solid SiO 2 −liquid interface. Several different types of conditions are arranged for the reactants: (1) where both reactants are in the liquid contained in the SiO 2 pore, (2) where one reactant is adsorbed to the SiO 2 surface, with the other in the liquid, and (3) where both reactants are at the interface and where the surface captures one of the products of the photoreaction in the liquid. Pore size in the nanometer range plays a major role in the outcome of the photochemistry. Studies in category 1 show that the rate constants decrease with decreasing pore size. In category 2 the rate of approach of the liquid-borne reactants to the surface is efficient and can be explained by simple diffusion theory. On the contrary, the rate of capture by the surface of cations produced in the liquid phase is significantly less efficient than that of neutral species studied in category 2. Locating both reactants at the SiO 2 surface can lead to efficient reactions, but rapid back-reaction of the products lead to low yields of products compared to the bulk liquid phase. The results of the unique conditions imposed by a surface on conventional reactions are discussed in terms of what is established in bulk solution.