Halogen-bonded building block for 2D self-assembly: Triggered by hydrogen-bonding motifs relative to the terminal functions of the side chains

Owning to the excellent applications of halogen bonds in supramolecular chemistry, the recognition of halogen bonds is of significance during molecular self-assembly process. The 2D self-assembly of fluorenone molecules (BEDF, BCDF, and BHDF) characterized by different terminal groups have been studied at the 1-phenyloctane/ HOPG interface. The H-bonding motifs relative to the terminal groups determine the bonding motifs of the conjugated cores and serve as the trigger of halogen bonds. The COOC2H5-terminated groups of BEDF sever as hydrogen bond acceptors to bond with the cores, giving rise to the dimer configuration of cores linked by C-H center dot center dot center dot Br hydrogen bonds. By contrast, the COOH-terminated groups of BCDF and the OH-terminated groups of BHDF act as both the electrophile and nucleophile interacted with the cores. On account of the secondary hydrogen bond distinct in strength and the different intersection of the aggregates, the type-I bonds (Br center dot center dot center dot Br) and halogen bonds (Br center dot center dot O=C and Br center dot center dot center dot Br) are observed in the networks of BCDF and BHDF, respectively. The types of halogen bonds could be further modified by changing the concentrations. The work clearly demonstrates that an appreciation of terminal functions is critical to the recognition of halogen bonds, thus establishing the controllable self-assembly morphology.