Adsorption Structure and Reactivity of a Putative Asymmetric Molecular 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-isocyanophenyl disulfide (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 ∼130 K before transforming to the trans conformer on heating to ∼220 K and subsequently transforms to a more ordered structure at ∼270 K prior to multilayer desorption. Three regimes are identified for monolayer ICPD adsorption on Au(111) at 300 K. The first region is identified by XPS and RAIRS as adsorbing by S–S bond scission in a flat-lying geometry. In the second region, free isocyanide modes are detected and XPS results show the presence of surface-bound sulfur species indicating the formation of a more perpendicular structure. As the exposure increases further to saturation, 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 the isocyanide functional group.