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 π-π stacked edge-on conformation. On decreasing the solution concentration, owing to the molecule-molecule van der Waals force and Br Br halogen bonds or the molecule-solvent cooperative Br O (C&z.dbd;O) hydrogen and Br 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 Br van der Waals type interactions. One bromine substituted phenanthrene derivative (3-DBHP) formed a dislocated linear pattern by two C-H 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 Br and Br O halogen bonding, Br Br van der Waals type interactions, and H 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 Br and Br H interactions. Br Br halogen bonding exists in the self-assembly of 2,7-DBHP, whereas the driving force for the assembly of 3,6-DBHP is Br Br vdWs type interactions.