Adsorption Structure and Reactivity of a Putative AsymmetricMolecular Conductor; 4-Isocyanophenyl Disulfide on Au(111)

Both 1,4-phenlyene diisocyanide (PDI) and 1,4-benzenedithiol (BDT) have been found to oligomerize on Au(111)to form one-dimensional molecular conductors. Here, the structure of an asymmetric version of these molecules, 4-isocyanophenyldisulfide (ICPD), is studied on a Au(111) single crystal in ultrahigh vacuum using reflection-absorption infrared (RAIRS) and X-ray photoelectron spectroscopies (XPS). The vibrational modes of ICPD are assigned with the help of Gaussian 09 calculations.ICPD multilayer structures adsorb randomly in a cis conformation at similar to 130 K before transforming to the trans conformer on heatingto similar to 220 K and subsequently transforms to a more ordered structure at similar to 270 K prior to multilayer desorption. Three regimes areidentified for monolayer ICPD adsorption on Au(111) at 300 K. Thefirst region is identified by XPS and RAIRS as adsorbing by S-S bond scission in aflat-lying geometry. In the second region, free isocyanide modes are detected and XPS results show the presenceof surface-bound sulfur species indicating the formation of a more perpendicular structure. As the exposure increases further tosaturation, the XPS spectra indicates the presence of an intact S-S bond, while the RAIRS results show a bound isocyanide mode,implying the presence of an intact disulfide bond remote from the surface in a species that is bound to the surface through theisocyanide functional group.