Prevailing Intermolecular Bonding for Dinitrotoluene Self-Assembled Monolayers on Au(111)

The self-assembly of 2,4- and 2,6-dinitrotoluene was investigated on Au(111) surfaces using a combined experimental and density functional theory approach. Well-ordered monolayers were observed with rectangular unit cells commensurate with the Au substrate along the next-nearest-neighbor direction. The unit cell is made of two molecules per cell in an unusual vertical configuration, driven by a strong dipole-dipole interaction between molecules and with lesser interaction with the substrate. van der Waals density functional theory models of bonding in the molecular layer show strong intermolecular interactions that dominate over substrate interactions with cohesive energies near 24 kcal/mol. The calculations are corroborated by temperature-programmed desorption experiments and demonstrate that intermolecular interactions dominate the self-assembly of this molecular adsorption system. Both dipole-dipole and van der Waals interactions are significant contributors to the bonding, and the importance of van der Waals corrected density functional theory is shown. We argue that the molecule-molecule vs molecule-substrate interactions are controlled by the optimization of energy per unit surface area.