Effects of the position and number of bromine substituents on the concentration-mediated 2D self-assembly of phenanthrene derivatives

The effects of the position and number of bromine substituents on the self-assembled patterns of phenanthrene derivatives by changing multiple weak intermolecular interactions were investigated at the 1-octanoic acid/graphite interface at different concentrations by scanning tunneling microscopy. Two Br substituted DBHP molecules (2,7-DBHP, 3,6-DBHP) and BHP without a Br group formed a linear lamellar pattern by the van der Waals interactions between the alkoxyl chains in each lamella at high concentrations, which forces the phenanthrene derivatives to self-organize in a p-p stacked edge-on conformation. On decreasing the solution concentration, owing to the molecule-molecule van der Waals force and Br center dot center dot center dot Br halogen bonds or the molecule-solvent cooperative Br center dot center dot center dot O (C=O) hydrogen and Br center dot center dot center dot HO-hydrogen bonds, 2,7-DBHP molecules were found to form two kinds of network structures, whereas 3,6-DBHP molecules formed only a zigzag pattern due to the intermolecular Br center dot center dot center dot Br van der Waals type interactions. One bromine substituted phenanthrene derivative (3-DBHP) formed a dislocated linear pattern by two C-H center dot center dot center dot Br hydrogen bonds in each dimer. These observations revealed that an important modification of the position and number of halogen substituents might dramatically change the self-assembly behaviors by different intermolecular interactions including Br center dot center dot center dot Br and Br center dot center dot center dot O halogen bonding, Br center dot center dot center dot Br van der Waals type interactions, and H center dot center dot center dot Br hydrogen bonding. DFT calculations were explored to unravel how slightly tuning the molecular structure defines the geometry of a 2D self-assembled nanoarchitecture through the different elementary structural units having Br center dot center dot center dot Br and Br center dot center dot center dot H interactions.