Adsorption of Aromatic and Anti-Aromatic Systems on Graphene through π-π Stacking

The adsorption of neutral (poly)-aromatic, antiaromatic, and more generally pi-conjugated systems on graphene is studied as a prototypical case of pi-pi stacking. To account for dispersive interactions, we compare the recent van der Waals density functional (vdw-DF) with three semiempirical corrections to density functional theory and two empirical force fields. The adsorption energies of the molecules binding to graphene predicted by the vdw-DF were found to be in excellent agreement with temperature desorption experiments reported in literature, whereas the results of the remaining functionals and force fields only preserved the correct trends. The comparison of the dispersive versus electrostatic contributions to the total binding energies in the aromatic and antiaromatic systems suggests that pi-pi interactions can be regarded as being prevalently dispersive in nature at large separations, whereas close to equilibrium bonding distance, it is a complex interplay between dispersive and electrostatic Coulombic interactions. Moreover our results surprisingly indicate that the magnitude of pi-pi interactions normalized both per number of total atoms and carbon atoms increases significantly with the relative number of hydrogen atoms in the studied systems.