Various aspects of the constraints imposed on the photochemistry of systems in porous silica

This manuscript briefly reviews the photochemistry of organic molecules on porous silica (or SiO2). To gain an understanding of the chemistry on silic a, data are displayed and discussed with respect to studies in homogeneous solution. In particular, the exact dimensionality of kinetic processes on p orous SiO2 is a matter for debate. Hence, units of concentration of an adso rbate on the surface are expressed as moles per nanometer squared and as mo les per liter, in order to compare with solution. Many studies show that or ganic molecules adsorb to SiO2 via the surface silanol (or surface hydroxyl OH) groups. The adsorption is heterogeneous, due to various clusters of si lanol groups and to charge transfer (CT) sites. Photophysical studies clear ly show these effects. The photo-induced reactions on SiO2 may be described by 'fractal' approaches, but a 'Gaussian' approach is often more useful to the photochemist. Photo-induced reactions occur via movement of the reacta nts on the surface, as in the case of the Langmuir-Hinshewood (LH) mechanis m or, as in the case of the Eley-Rideal (ER) mechanism, by bombardment of a surface bound excited state by a gaseous reactant, such as O-2. Quenching of excited singlet states by O-2 produces excited triplet states, which in turn are quenched to give singlet molecular oxygen. At room temperature the O-2 quenching process on silica occurs by both mechanisms to approximately the same extent. However, the LH mechanism is dominant at lower temperatur es and the ER mechanism is dominant at higher temperatures. Some quenchers, including carbon tetrachloride and tetranitromethane only quench by the LI I mechanism giving rise to static quenching and chloro or nitro derivatives of the excited state. Photo-induced electron transfer between excited aren es and amines occurs readily, but the ionic products are short-lived compar ed to solution. This is due to the limited diffusion of the products on the surface, which in turn promotes back-electron transfer. Photoionization of arenes occurs on SiO2 via a two-photon process and gives very long-lived i ons compared to solution. This is due to trapping of the photo-produced ele ctrons by the SiO2 itself. Finally, the effects of co-adsorbants, including solvents, surfactants, and polymers, in photoreactions at the SiO2 surface are considered. The review ends with suggestions for future studies. (C) 2 001 Elsevier Science B.V. All rights reserved.