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 of 1,10-phenanthroline (phen) adsorbed on silver colloid were studied using a flow cell. Phen adsorbed on fresh colloid is found to undergo almost no photochemistry while the same molecule adsorbed on aged colloid appears to photodecompose to a species in which the C−N ring bond in one or both of the heterocycles is broken to produce a C N moiety that binds to the silver surface. The contrast in the behavior of the two colloids is attributed to the differing bonding geometries of the adsorbate in the two cases. In the fresh colloid phen appears to bind with 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 bonding strategy is ascribed to the existence of many surface defects on the surface of the fresh colloid that are ideal sites for bidentate chelation. These defects presumably significantly decrease in number as the surface heals with time. The photoproduct which forms upon irradiating phen adsorbed on aged colloid is subsequently observed to photodesorb at a rate significantly less rapid than that of the photodecomposition reaction. Both reactions are found to be one-photon processes; hence, the large photoreaction cross section in the visible is likely due to a metal-to-molecule charge-transfer transition.