PHOTOINDUCED CHARGE-TRANSFER REACTION BETWEEN PYRENE AND N,N'-DIMETHYLANILINE ON SILICA-GEL SURFACES

Photoinduced charge-transfer reactions between adsorbed pyrene and N,N '-dimethylaniline (DMA) in porous silica gel are examined by fluoresce nce quenching and transient absorption techniques. The quenching on si lica surfaces is found to be diffusion controlled and gives rise to ex ciplex formation on silica surfaces. Such a Langmuir-Hinshelwood type of surface reaction is described quantitatively using a two-dimensiona l kinetics model. Surface diffusion of DMA is found to be thermally ac tivated and related to the hydrogen-bonding interaction between DMA an d the surface hydroxyl groups. Properties of exciplex emission such as the position of the maximum and the quantum yield are used to charact erize the surface environment. Deactivation of the exciplex on silica surfaces via charge recombination is understood in terms of photoassis ted The low yield of ionic products from dissociation of the contact i on pairs of the exciplex (1)(Py(.-)DMA(.+)) is attributed to the lack of solvation and the low mobilities of ionic species on the surface. It is found that coadsorption of alcohols facilitates the charge separ ation.