TiO ₂-Catalyzed photodegradation of porphyrins: Mechanistic studies and application in monolayer photolithography

Patterned mixed monolayers of porphyrins on nanocrystalline TiO ₂ films were fabricated by substrate-catalyzed monolayer photolithography. Tin(IV) protoporphyrin IX (SnPP), zinc(II) protoporphyrin IX (ZnPP), and iron(III) meso-tetra(4-carboxyphenyl)porphine (FeTCP) were adsorbed to TiO ₂ through the carboxyl groups, yielding saturation surface amounts per projected area of approximately 10 ^-7mol/cm ². Illumination of SnPP- and ZnPP- functionalized TiO ₂ films with 355 nm light caused the desorption of the porphyrins, most likely through oxidative decarboxylation. SnPP was removed more rapidly than ZnPP. The faster kinetics was due, in part, to the contribution of other photochemical pathways including TiO ₂-catalyzed photoreduction and direct photodegradation reactions. Patterned binary monolayers were prepared by the photoinduced desorption of a protoporphyrin, followed by the adsorption of FeTCP to previously illuminated regions of the surface.