Halogen Substituent Effects on Concentration-Controlled Self-Assembly of Fluorenone Derivatives: Halogen Bond versus Hydrogen Bond

Self-assembled behaviors of three fluorenone derivatives substituted by different halogen atoms, 2-(pentadecyloxy)-6-bromo-fluorenone (Br-FC15), 2-(pentadecyloxy)-6-chloro-fluorenone (Cl-FC15), and 2-(pentadecyloxy)-6-fluoro-fluorenone (F-FC15), were investigated at the 1-phenyloctane/highly oriented pyrolytic graphite interface by scanning tunneling microscopy combined with density functional theory calculations in comparison with the self-assembly of 2-pentadecyloxy-fluorenone (H-FC15). It is found that different charge distributions on halogen substituents lead to a subtle change of the molecular packing nanostructures. By varying the solution concentrations, X-FC15 (X = Br, Cl, H) can self-assemble into polymorphic nanostructures, whereas only one pattern can be observed for the F-FC15 adlayer due to the stronger continuous C–H···F bonds. The intermolecular C–H···O═C hydrogen bonds are the main driving forces for all the self-assembled patterns. Particularly, the halogen-based hydrogen bonds and the t...