Electrochemically grafted single molecule junctions exploiting a chemical protection strategy

Diazonium grafted oligo(phenylene ethynylene) (OPE) monolayers suitable for single molecule conductance studies have been prepared. The surface-bound OPE is prepared from the amino tolan, 4-(2-(4-(2-(triisopropylsilyl)ethynyl)phenyl)ethynyl)benzamine, which is converted to the diazonium derivative in situ, and subsequently attached to a gold substrate using conventional cathodic electrochemical grafting. Multilayer formation is avoided by the presence of the bulky triisopropylsilyl (TIPS) protected ethynyl group. After removal of the TIPS group by treatment with fluoride, the film, now bearing exposed terminal acetylene moieties, is further functionalised by reaction with 1-iodo-4-acetylthiobenzene using Sonogashira coupling chemistry, to create sites bearing a strongly aurophilic top-contacting group. The success of these surface reactions is confirmed using Raman spectroscopy (gap-mode and SHINERS, employing gold nanoparticles and SiO2 coated gold nanoparticles, respectively). It is shown that such thiolate-modified, diazonium-grafted monolayers are suitable for STM-based molecular junction conductance measurements.