Permethylated 12-Vertex p -Carborane Self-Assembled Monolayers

closo-Carborane cages and their adsorption onto metal surfaces is a subject of growing interest because of the distinct structural and electronic properties of boron carbide derivatives. Nevertheless, the extent to which the substitution of the cage alters structural and spectroscopic characteristics of closo-carborane monolayers has not been investigated so far. In this paper, core level shifts and valence electronic structure of two different p-carboranes, the parent and the (2,11)-deca-methylated 12-vertex closo-1,12-dicarbadodecaborane, are compared using photoemission spectroscopy, whereas the lateral arrangement of the molecules on gold is investigated by scanning tunneling microscopy. Both compounds present a significant energy gap between the molecular highest occupied molecular orbital (HOMO) and the Fermi level of Au (similar to 4 eV), attributed to the high ionization potential of boron carbides. Permethylation of the p-carborane cage induces a charge redistribution resulting in a shift of the boron is core level signal (Delta E = 0.25 eV) and in a change of the molecular dipole moment; the HOMO level is concomitantly shifted by similar to 0.5 eV to lower binding energies. The dense molecular packing (intermolecular distance of 7 angstrom for closo-1,12-dicarbadodecaborane and 10 angstrom for its permethylated counterpart) is defined by the physical diameter of the carborane cage and by intermolecular dispersion forces. For both carboranes, the overlayer structure shows incommensurism with the Au (111) substrate, but due to the higher intermolecular forces acting between the parent closo-1,12-dicarbadodecaboranes a polycrystalline phase is observed with a significantly higher coherence length than for the permethylated analogue.