Photoinduced intermolecular charge transfer processes on pure and modified silica surfaces

Abstract

This article briefly reviews some of our work over the past two decades devoted to the photoinduced electron-transfer processes and formation of bimolecular complexes–exiplexes between adsorbed donor pyrene and anthracene and acceptor N,N-dimethylaniline molecules on silica and silica–titania surfaces. Quenching of the fluorescence of acenes by N,N-dimethylaniline on silica surfaces is found to be diffusion controlled and results from exciplex formation at room temperature. The effect of silica–titania on the processes of photoinduced electron-transfer resulted vise-versa in increases of the fluorescence intensity from the adsorbed acenes. This phenomenon is explained by the selective adsorption of N,N-dimethylaniline on the acid “titanium” centers and a shift of the acenes to weaker adsorption sites located on the silica support. The excited molecular pair effectively yields reduced titanium ions due to an electron transfer process. This study demonstrates the important role that adsorption of organic molecules plays in photocatalytic processes on the surface of semiconductor composites. Sol–gel produced porous silica modified with benzophenone molecules leads to full photoreduction of silver and gold ions. Formation of stable Ag and Au nanoparticles embedded in porous silica films was carried out by irradiation of silver (gold) ions-modified silica in benzophenone–water–isopropanol solution. The triplet state of benzophenone takes part in the formation of the reducing agents, such as ketyl radicals and anion radicals of benzophenone and isopropanol. These systems can be used as bactericide materials, for extraction of metal ions from solution, for photocatalytic reduction of toxic transition metals and other applications.