SURFACE CATALYZED ELECTRON-TRANSFER FROM POLYCYCLIC AROMATIC-HYDROCARBONS (PAH) TO METHYL VIOLOGEN DICATION - EVIDENCE FOR GROUND-STATE CHARGE-TRANSFER COMPLEX-FORMATION ON SILICA-GEL

Porous silica surfaces are shown to slowly catalyze the oxidation of a dsorbed polycyclic aromatic hydrocarbons (PAH) to the corresponding ra dical cation via Lewis acid sites present on the surface. When a good electron acceptor such as methyl viologen dication (MV++) is co-adsorb ed on silica surface, a red-shifted structureless absorption band char acteristic of a ground-state charge transfer (CT) complex formed betwe en the PAH and MV++ is observed. Oxygen efficiently competes with MV+ for the trapped electrons on the active sites of silica surface causi ng a significant decrease in the concentration of ground-state CT comp lex. The rate of this electron transfer process is enhanced dramatical ly at the solid/liquid interface when solution of PAH in a non-polar s olvent is added to dry silica containing adsorbed MV++. Room temperatu re electron paramagnetic resonance (EPR) spectra of PAHs adsorbed on s ilica 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 rest ored upon evacuation of the sample. The EPR measurements of mixed samp les containing PAH and MV++ co-adsorbed on silica show a composite sig nal with hyperfine structure that may be due to presence of two parama gnetic species corresponding to MV++ and possibly PAH radical cation. (C) 1998 Elsevier Science S.A. All rights reserved.