Cross metathesis on olefin-terminated monolayers on Si(111) using the Grubbs' catalyst

This paper reports the functionalization and patterning of olefin-terminated monolayers on Si(111) through cross metathesis. A simple, one-step synthesis of a diolefin--CH2=CH(CH2)9O(CH2)9CH=CH2--was developed from commercially available starting materials. Mixed partially olefin-terminated monolayers of this novel diolefin and 1-octadecene on hydrogen-terminated Si(111) were obtained. The olefins are raised above the rest of the monolayer and thus sterically accessible for further functionalization. Olefin-terminated monolayers were reacted with the Grubbs' first generation catalyst and olefins in solution that were terminated with fluorines, carboxylic acids, alcohols, aldehydes, and alkyl bromides. Characterization of these monolayers using X-ray photoelectron spectroscopy and horizontal attenuated total reflection infrared spectroscopy demonstrated that olefins on the surface had reacted via cross metathesis to expose fluorines, carboxylic acids, aldehydes, alcohols, and bromides. Through calibration experiments, we demonstrated a simple 1:1 correspondence between the ratio of olefins in solution used in the assembly and the final composition of the mixed monolayers. Finally, these monolayers on silicon were patterned on the micrometer-size scale by soft lithography using microfluidic channels patterned into poly(dimethylsiloxane) (PDMS) stamps. Micrometer-wide lines of polymer brushes were synthesized on these monolayers and characterized by scanning electron microscopy. In addition, olefin-terminated monolayers were patterned into micrometer-sized lines exposing carboxylic acids by cross metathesis with olefins in solution. This method of patterning is broadly applicable and can find applications in a variety of fields including the development of biosensors and nanoelectronics.