High-Band-Gap Polycrystalline Monolayers of a 12-Vertex p-Carborane on Au(111)

Carborane cages are interesting materials for electronics because of their pseudoaromatic, wide-band-gap molecular structures. We have observed that p-carboranes tethered to gold by thiolate linkers form polycrystalline monolayers which are incommensurate with the Au(111) substrate. The incommensurability results from the strong interactions between adjacent p-carborane cages (1.6 eV/molecule) and from the adlayer lattice spacing, which is defined by the cage diameter. The high ionization potential of the carborane structures results in a significant shift in the position of the highest occupied molecular orbital (HOMO) of p-carborane (4.3 eV below the Fermi level) compared to the HOMO of oligophenylene derivatives (2.1 eV below the Fermi level). These findings suggest the combination of p-carboranes with established aromatic π-systems as a new design strategy for molecular electronic devices.