| Self-assembled monolayers (SAMs) of n-alkanethiols on gold have been thoroughly investigated in recent years as useful model systems to which a wide variety of functional groups can be added. The large band gap of the alkane backbone, however, renders it essentially optically and electronically inactive.;To address these issues, a new self-assembling system was designed, based upon oligo(para-phenylene-ethynylene)thiols (arenethiols). The conjugated structure of these rigid, linear molecules was predicted to greatly increase the rate of electron transfer across the monolayer. A straightforward synthetic methodology for producing a variety of functionalized arenethiols was formulated, allowing the rapid production of these compounds in high yield.;The arenethiols were found to form highly ordered, commensurate SAMs on Au (111), as observed by macroscopic and microscopic techniques. Furthermore, the electron transfer rate across these SAMs was determined to be up to three orders of magnitude faster than for alkanethiols of comparable length. In addition, arenethiol SAMs exhibit rectifying behavior in current-voltage measurements made with a scanning tunneling microscope.;Arenethiol SAMs were observed to undergo heterogeneous exchange when immersed in an n-alkanethiol solution. This exchange process was determined to occur through an activated exchange mechanism, rather than by unimolecular elimination. The heterogeneous SAMs resulting from the exchange process had significantly fewer defects than homogeneous arenethiol SAMs.;Additionally, the ability of terminally substituted arenethiols to modify the work function of the gold substrate was studied by Kelvin probe. The change in work function was found to vary with the calculated dipole of the arenethiol adsorbate, as predicted by a simple dipole model, and was proportionately largest for the shortest arenethiols. Based upon this result, the para-substituted thiophenols are candidates for further testing as components in thin-film electronic devices.;In another development, gold nanocrystals encapsulated in n-alkanethiol shells were found to self-assemble on the surface of a phase-separated diblock copolymer. The extension of this self-assembly process to alkylammonium-passivated palladium colloids demonstrated the generality of the process. Significantly, the patterned palladium colloids were used as catalysts for electroless deposition of copper, creating copper features 25 nm in width, twice as small as those accessible by optical lithography. |
