Orientational Order of Phenyl Rotors on Triangular Platforms on Ag and Au(111)

We investigated trioxatriangulenium functionalized with phenyl (phenyl-TOTA) on the (111) surfaces of Ag and Au using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). On Ag(111), the molecules form hexagonal arrays, and on Au(111), honeycomb patterns are also observed. The orientations of the phenyl moieties are resolved on both substrates. On Ag(111), the orientations are parallel within a row and they differ by approximately 60° between adjacent molecular rows, and STM images suggest dimerization of the molecules. DFT calculations for Ag(111) reveal that van der Waals interactions dominate this system. The optimized structure matches the experimental pattern, and the simulated STM images exhibit apparent dimerization. The dimerization results from an asymmetry of the phenyl wave function, which reflects intramolecular hydrogen bonding between the ligand and an oxygen atom within the triangulenium platform. The orientation of the phenyl moieties is explained by the interaction of each phenyl subunit with its triangulenium platform combined with the direct long-range interaction between phenyl moieties across molecules.