Subtle Fluorination of Conjugated Molecules Enables Stable Nanoscale Assemblies on Metal Surfaces

We elucidate the structure of assemblies formed by three conjugated molecules with very similar chemical structure, differing only by two fluorine atoms, on an Ag 111 surface to gain insight into the intricate interplay of attractive and repulsive interactions that govern the self assembly of molecules on a metal surface. With scanning tunneling microscopy we observe substantially different self assembled structures for sub monolayers of parasexiphenyl 6P and its two partially fluorinated derivatives ortho 2F 6P and meta 2F 6P see Fig. 1 , which we can fully rationalize only with state of the art density functional theory modeling. By deliberate discriminating all involved interactions, that is, electrostatic due to permanent charges, polarization and dispersion, as well as hydrogen bonding, we can provide new insights for advanced self assembly strategies. We demonstrate that fluorination at selected positions of conjugated molecules provides for sufficiently strong, yet nonrigid, H F bonding capability to circumvent dominating repulsive interactions at very low molecular surface coverage. This enables the realization and steering of individual and stable nanoscale molecular assemblies as well as their study, without the need to approach monolayer coverage, which could substantially alter the obtained structure. Furthermore, the insight provided here helps to understand how fluorine substitution in conjugated molecules and polymers contributes to thin film and bulk structures, which, in turn will enable realizing organic electronic materials with superior optical and charge transport properties for electronic and optoelectronic application