Kinetic Studies of Reactions of Hydroxyporphyrins on Silicate Surface

Construction of the monolayer film of semiconductive and redox-active porphyrins on the metal oxides such as silicate or titanates attracts interests for possible applications of the process in fabricating organic electronic devices, organic catalysts, and organic-inorganic hybrid materials. Porphyrins bearing linker functional groups such as hydroxy or bromo groups in the peripheral meso-phenyl groups were reacted with the surface silanol groups of silicate glass at 80 to 240 ^oC in a solid-solid reaction to obtain the monolayer film of porphyrin on the silicate glass. The rates of the reaction were determined by visible spectroscopic studies and they decreased in the order: primary alcohol > primary bromide ~ tertiary alcohol. In addition to the structures of the linker groups, melting points of porphyrins also affect the reactivity of the porphyrin. Porphyrins with long alkyl chains reacted faster than those without long alkyl chains, because the long alkyl groups would facilitate molecular motion to reach the transition state of the reaction. Hydrolysis of the porphyrin monolayer prepared from primary alcohol in 1 M aq. HCl at 50 ^oC proceeded in a similar rate to that prepared from tertiary alcohol. Base catalyzed hydrolysis in 1 M aq. NH₃ at 30 ^oC of the porphyrin monolayer prepared from tertiary alcohol proceeded 20 times slower than that prepared from the primary alcohol. In the literature, it is well established that the hydrolysis of the silicate ester bond proceeds via the Si-O bond cleavage, since the silicon atom can form a penta-coodinated structure, and much more reactive than carbon atom toward a substitution reaction. Hydrolysis of the porphyrin monolayer prepared from the tertiary alcohol in H₂¹⁸O afforded ¹⁸O-labelled tertiary alcohol, indicating that there was a covalent bond between the porphyrin and silicate glass, and the C-O bond was cleaved in the acid catalyzed hydrolysis reaction of the tertiary alkyl silicate ester on the silicate glass.