Surface catalyzed electron transfer from polycyclic aromatic hydrocarbons (PAH) to methyl viologen dication: evidence for ground-state charge transfer complex formation on silica gel

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Vol. 117; no. 3; pp. 223 - 233
• Main Authors: Dabestani, Reza, Reszka, Krzysztof J, Sigman, Michael E
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 15.09.1998; Elsevier Science
• Subjects: Chemistry; Electron transfer; Exact sciences and technology; General and physical chemistry; Methyl viologen dication; Polycyclic aromatic hydrocarbons; Solid-liquid interface; Surface physical chemistry; Polycyclic aromatic hydrocarbons; Electron transfer; Methyl viologen dication; Reaction intermediate; Liquid solid interface; Surface reaction; Hydrocarbon; Iminium compounds; Diffuse reflection; Active site; EPR spectrometry; Polycyclic aromatic compound; Experimental study; Condensed benzenic compound; Organic solvent; Apolar solvent; Cyclohexane; Charge transfer compound; Silica gel; Oxidation; Kinetics; Organic dication; Organic radical cation; Ultraviolet visible spectrometry
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/S1010-6030(98)00327-X

Porous silica surfaces are shown to slowly catalyze the oxidation of adsorbed polycyclic aromatic hydrocarbons (PAH) to the corresponding radical cation via Lewis acid sites present on the surface. When a good electron acceptor such as methyl viologen dication (MV ++) is co-adsorbed on silica surface, a red-shifted structureless absorption band characteristic of a ground-state charge transfer (CT) complex formed between the PAH and MV ++ is observed. Oxygen efficiently competes with MV ++ for the trapped electrons on the active sites of silica surface causing a significant decrease in the concentration of ground-state CT complex. The rate of this electron transfer process is enhanced dramatically at the solid/liquid interface when solution of PAH in a non-polar solvent is added to dry silica containing adsorbed MV ++. Room temperature electron paramagnetic resonance (EPR) spectra of PAHs adsorbed on silica show a broad unresolved signal ( g=2.0031–2.0045) due to PAH ⋅+ radical cation which disappears in the presence of air but can be restored upon evacuation of the sample. The EPR measurements of mixed samples containing PAH and MV ++ co-adsorbed on silica show a composite signal with hyperfine structure that may be due to presence of two paramagnetic species corresponding to MV ⋅+ and possibly PAH radical cation.