STM Investigate Self-Assembled Nanostructures Controlled By Intermolecular Halogen Bonds And Dipole-Dipole Interactions At The Solid-Liquid Interface

In recent years, halogen bonding has emerged as a powerful and broadly useful noncovalent interaction relevant to the fields of crystal engineering and supramolecular chemistry. Halogen bonding exhibits similar characteristics to hydrogen bonding in strength and directionality. So we designed a series of halogen substituents phenanthrene derivatives to get an insight into the self-assembled mechanism based on halogen bonding by scanning tunneling microscopy(STM). At the same time, Fluorenone derivatives are promising to become excellent components for OLEDs which have been received broad focus. To understanding the role of dipole-dipole interactions for the configuration of fluorine derivatives, we synthesized a series of fluorenone derivatives with different length of the alkyl chain to study the self-assembled process based on intermolecular dipolar interactions. The main innovative results are as follows:(1) We investigated the self-assembly of phenanthrene derivatives at the 1-octanoic acid/graphite interface by means of STM experiments and DFT calculations to explore the effect of the position and number of bromine groups on the molecular assembly. The self-assembly of 2,7-DBHP and 3,6-DBHP displays concentration dependence. At the saturated concentration, both kinds of molecules form a linear lamelle with an edge-on conformation resulting from intermolecular van der Waals interactions. Decreasing the solution concentration, the solvent coadsorption with 2,7-DBHP changes the intermolecular interactions, and the solvent–molecule halogen and hydrogen bonds dominate the structural formation. 3,6-DBHP molecules form a zigzag pattern which is stabilized by the Br…Br van der Waals type interactions. On the other hand, the 2D self-assembled pattern of 3-BBHP is exclusively driven by C–H…Br hydrogen bonds. These results successfully demonstrate that the position and number of Br substituents significantly affect intermolecular interactions and determine the final supramolecular architectures.(2) The self-assembly of 2,7-DBHP and 3,6-DBHP molecules have been investigated by STM at the 1-phenloctane/HOPG interface. The self-assembly of 2,7-DBHP and 3,6-DBHP also displays concentration dependence. At high and moderate concentrations, the self-assembly of 2,7-DBHP same with it in the 1-octanoic acid/graphite interface. However, no linear dimer pattern was observed without the molecule-solvent interactions at low concentrations. At high concentrations, owing to the intermolecular halogen bonds and van der Waals interactions, 3,6-DBHP could form a flat-on linear structure which is different from that in the 1-octanoic acid at high concentrations. At low concentrations, the self-assembly of 3,6-DBHP is the same with that in the 1-octanoic acid. In addition, in order to explore different halogen substituent on the self-assembly of phenanthrene derivatives. We investigated the self-assembly of 2,7-DDHP in 1-octanoic acid at different concentrations. Since the molecule-solvent I…O=C halogen bonding is stronger than the Br…O=C halogen bonding, only the linear arrangement and the linear dimer arrangement were observed. It was found that the intermolecular interactions and molecule–solvent interactions are the key factors to modify intermolecular halogen bonding at the solid–liquid interface.(3) The 2D self-assembly of various fluorenone derivatives with different chain length has been systematically studied by STM in 1-phenyloctane and 1-octanoic acid. All fluorenone derivatives tend to form a linear arrangement, in which the polar molecules in adjacent troughs arranged in an antiparallel fashion. When the number of carbon atom in the terminal of side chian was 1(n=1), the molecules only formed the linear pattern with the interdigitated side chains. When n=2, the linear structure and random flower-like pattern was formed. When n=3-7, in the linear patterns, the side chain packed in a tail-to-tail fashion due to the enhanced molecule-substrate van der Waals interactions, in which part of the side chains stretched into the solution. The paired arrangement induced by dipole-dipole interactions extinguished the polarity of single fluorenone derivatives and the whole dipole of the self-assembled pattern is zero.