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.