Hydrogen bonding vs. molecule-surface interactions in 2D self-assembly of [C60]fullerenecarboxylic acids

The adsorption of C₆₀-malonic derivatives C₆₁(CO₂H)₂ and C₆₆(CO₂H)₁₂ on Au(111) and a pentafluorobenzenethiol-modified Au substrate (PFBT@Au) has been investigated using scanning tunneling microscopy (STM) at a liquid-solid interface. Monofunctionalized C₆₁(CO₂H)₂ forms a hexagonal close-packed overlayer on Au(111) and individual aligned dimers on PFBT@Au(111). The difference is attributed to the nature of the substrateC₆₁(CO₂H)₂ interaction (isotropic π-Au bonding vs. anisotropic PFBTCOOH interactions). Surprisingly, in both cases, the directionality of the COOHCOOH motif is compromised in favor of synergistic van der Waals/H bonding interactions. Such van der Waals contacts are geometrically unfeasible in hexafunctionalized C₆₆(CO₂H)₁₂ and its assembly on Au(111) leads to a 2D molecular network controlled exclusively by H bonding. For both molecules, the "free" CO₂H groups on the monolayer surface can engage in out-of-plane H bonding interaction resulting in the epitaxial growth of subsequent molecular layers.