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.

Structural and electron-transfer characteristics of O-, S-, and Se-tethered porphyrin monolayers on Si(100).

Monolayers of two classes of Zn porphyrins have been prepared and examined on Si(100). These molecules, designated as ZnPBzX- and ZnPCH2X-, contain either a benzyl (-Bz-) or a methylene (-CH2-) unit terminated with a Group VI atom (X = O, S, Se) appended to a meso-position of the porphyrin, with the nonlinking meso-substituents consisting of either mesityl (-Bz- class) or p-tolyl and phenyl (-CH2- class) units. The two series of ZnPBzX- and ZnPCH2X- monolayers on Si(100) were examined using a variety of techniques, including X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPBzX- and ZnPCH2X- monolayers. (1) Surface binding can be readily achieved to Si(100) with both relatively short (-Bz-) and very short (-CH2-) tethers regardless of the nature of the Group VI anchoring atom (O, S, Se). (2) The longer -Bz- tether affords monolayers with the porphyrin ring in a somewhat more upright orientation with respect to the surface than the shorter -CH2- tether. The more upright adsorption geometry of the porphyrins bearing the former type of linker leads to a higher packing density and more homogeneous redox thermodynamics. (3) The kinetics of electron transfer does not depend on the type of Group VI atom used for anchoring to the Si(100) surface. On the other hand, the type of linker does affect the electron-transfer rates, with the monolayers bearing the -CH2- linker exhibiting systematically faster rates than those bearing the -Bz- linker. Collectively, the studies reported herein provide a detailed picture of how the anchor atom and the linker type influence the structural and electron-transfer characteristics of these general classes of monolayers.

Characterization of self-assembled monolayers of porphyrins bearing multiple thiol-derivatized rigid-rod tethers.

A series of multithiol-functionalized zinc porphyrins has been prepared and characterized as self-assembled monolayers (SAMs) on Au. The molecules, designated ZnPS(n) (n = 1-4), contain from one to four [(S-acetylthio)methyl]phenylethynylphenyl groups appended to the meso-position of the porphyrin; the other meso-substituents are phenyl groups. For the dithiol-functionalized molecules, both the cis- and the trans-appended structures were examined. The ZnPS(n) SAMs were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPS(n) SAMs. (1) The ZnPS(n) molecules bind to the Au surface via a single thiol regardless of the number of thiol appendages that are available per molecular unit. (2) The porphyrins in the ZnPS(3) and ZnPS(4) SAMs bind to the surface in a more upright orientation than the porphyrins in the ZnPS(1), cis-ZnPS(2), and trans-ZnPS(2) SAMs. The porphyrins in the ZnPS(3) and ZnPS(4) SAMs are also more densely packed than those in the cis-ZnPS(2) and trans-ZnPS(2) SAMs. The packing density of the ZnPS(3) and ZnPS(4) SAMs is similar to that of the ZnPS(1) SAMs, despite the larger size of the molecules in the former SAMs. (3) The thermodynamics and kinetics of electron transfer are generally similar for all of the ZnPS(n) SAMs. The general similarities in the electron-transfer characteristics for all of the SAMs are attributed to the similar binding motif.