Structural and electron-transfer characteristics of carbon-tethered porphyrin monolayers on Si(100).

Structural and electron-transfer characteristics are reported for two classes of zinc porphyrin monolayers attached to Si(100) surfaces via Si-C bonds. One class, designated ZnP(CH(2))(n)- (n = 2-4), contains an alkyl linker appended to the meso-position of the porphyrin, with the nonlinking substituents being p-tolyl groups. The other, designated ZnPPh(CH(2))(n)- (n = 0-3), contains a phenyl or phenylalkyl linker appended to the meso-position of the porphyrin, with the nonlinking substituents being mesityl groups. Both classes of zinc porphyrin monolayers on Si(100) were examined using Fourier transform infrared spectroscopy and various electrochemical methods. The studies reveal the following: (1) The structural and electron-transfer characteristics of the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- monolayers are generally similar to those of monolayers formed from porphyrins with analogous linkers, but anchored with an O, a S, or a Se atom. (2) The ZnP(CH(2))(n)-, ZnPPh-, and ZnPPhCH(2)- monolayers exhibit lower saturation coverages and have their porphyrin ring more tilted with respect to the surface normal than the ZnPPh(CH(2))(2)- and ZnPPh(CH(2))(3)- monolayers. (3) The electron-transfer rates for both the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- classes of monolayers monotonically decrease as the length of the linker increases. (4) For all the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- monolayers, both electron-transfer rates and charge-dissipation rates decrease monotonically as the surface coverage increases. Collectively, the studies reported herein provide a detailed picture of how the linker type influences the structural and electron-transfer characteristics of these general classes of monolayers.