EFFECT OF SURFACE GEOMETRY ON THE PHOTOCHEMICAL-REACTION OF 1,10-PHENANTHROLINE ADSORBED ON SILVER COLLOID SURFACES

The surface-enhanced Raman scattering spectra and the photochemistry o f 1,10-phenanthroline (phen) adsorbed on silver colloid were studied u sing a flow cell. Phen adsorbed on fresh colloid is found to undergo a lmost no photochemistry while the same molecule adsorbed on aged collo id appears to photodecompose to a species in which the C-N ring bond i n one or both of the heterocycles is broken to produce a C=N moiety th at binds to the silver surface. The contrast in the behavior of the tw o colloids is attributed to the differing bonding geometries of the ad sorbate in the two cases. In the fresh colloid phen appears to bind wi th its C-2 axis normal to the local surface while on the aged colloid the molecule seems to bind flat on the surface. This difference in bon ding strategy is ascribed to the existence of many surface defects on the surface of the fresh colloid that are ideal sites for bidentate ch elation. These defects presumably significantly decrease in number as the surface heals with time. The photoproduct which forms upon irradia ting phen adsorbed on aged colloid is subsequently observed to photode sorb at a rate significantly less rapid than that of the photodecompos ition reaction. Both reactions are found to be one-photon processes; h ence, the large photoreaction cross section in the visible is likely d ue to a metal-to-molecule charge-transfer transition.